Acetylene in Organic Synthesis: Recent Progress and New Uses

Recent progress in the leading synthetic applications of acetylene is discussed from the prospect of rapid development and novel opportunities. A diversity of reactions involving the acetylene molecule to carry out vinylation processes, cross-coupling reactions, synthesis of substituted alkynes, preparation of heterocycles and the construction of a number of functionalized molecules with different levels of molecular complexity were recently studied. Of particular importance is the utilization of acetylene in the synthesis of pharmaceutical substances and drugs. The increasing interest in acetylene and its involvement in organic transformations highlights a fascinating renaissance of this simplest alkyne molecule.


Introduction
Since the discovery of acetylene, new areas of acetylene chemistry have been continuously developed. The rich scope of chemical transformations available for a C≡C triple bond can be exemplified by coupling [1][2][3][4][5] and addition reactions [6,7]. The removal of trace acetylene from ethylene has led to another important process, selective hydrogenation of acetylene [8]. Recently, several opportunities of controlled accommodation of acetylene in microporous materials have appeared [9]. New functional materials allow separating, sensing and storing acetylene and other small molecules [10,11].
Acetylene, or ethyne, with the molecular formula C 2 H 2 , is the simplest alkyne. Due to its structural simplicity and high reactivity, acetylene represents a versatile building block for organic synthesis, and its chemistry has evolved rapidly [12]. In recent reviews [13,14] acetylene is compared with ethylene. Acetylene is commonly obtained from coal and natural gas and these sources will last longer than current oil sources. Acetylene chemistry is therefore of high importance in various aspects [15].
Acetylene can be produced by several well-established methods. One of them is based on the reaction of calcium carbide (obtained from coal and lime) with water [16]. The other methods include partial combustion of hydrocarbons, electrothermal cracking in an electric arc furnace, and thermal cracking with heat carriers [14,16].
Pure acetylene is a colorless gas with no odor. It is lightweight and flammable, with a high combustion temperature [17]. Due to this, acetylene is widely used in welding [16,18]. It is also This review discusses recent advances in acetylene chemistry by describing the most important processes at various levels of chemical complexity. Recent literature within the time period of 2013-2018 was considered to highlight emerging directions and applications in organic chemistry. Representative examples in each research direction are provided to cover most important trends in synthetic acetylene chemistry. It should be emphasized that the present review is focused on the reactions of acetylene (C 2 H 2 )-a fascinating molecule with significant practical importance and future potential. Reactions involving substituted alkynes have been described in details elsewhere and these discussions were not repeated here.

Acetylene in Organic Synthesis
Versatile and ubiquitously used synthetic transformations of acetylene involve functionalization of the C≡C bond; these processes are considered first (Section 2.1). Next, access to acrylic acid derivatives (Section 2.2) and preparation of substituted alkynes (Section 2.3) are discussed, followed by preparation of heterocyclic molecules (Section 2.4). Selected applications of acetylene in the synthesis of drugs and pharmaceutical substances is highlighted in a separate section (Section 2.5). Synthetic organic and organometallic reactions are also considered (Sections 2.6 and 2.7), followed by industry-related reactions involving hydrogenation, hydrochlorination reactions and their mechanisms (Sections 2.8 and 2.9). Finally, useful emerging concepts from the chemistry of alkynes are briefly mentioned (Section 2.10).

Functionalization of Triple Bond
Polymerization of vinyl-containing substrates has been a subject of close attention for many years. Polyvinyl compounds constitute a basis of many composites including paints, adhesives, varnishes, packaging [15], electroluminescent materials [21], pill coatings and many more [22]. Despite the widest range of applications, their chemistry is still in progress. Vinyl ethers, thioethers, sulfur-and nitrogen analogs become increasingly involved in synthesis of various cyclic compounds [45][46][47][48][49][50], natural substances, and medicinal chemicals [51,52], hence the demand for their development, modification, and systematization. A number of studies have been devoted to metathesis [53,54] and cross-coupling reactions of vinyl derivatives [55,56]. This section presents a review of the latest achievements in this field.

Synthesis of Vinyl Ethers
Addition of alcohols to acetylenes was introduced by Favorskii [57,58]. While studied isomerization of alkynes in the presence of alcohols and alkalis, he found that the alcohols react with acetylenes to give ethers. Later on, this reaction was turned into an industrial method for vinyl ether synthesis based on the addition of alcohols to acetylene, known as vinylation [14,20]. Under industrial conditions, it is carried out in autoclaves at elevated pressures and high temperatures in the presence of alkaline catalysts [13,14]. It can be performed on a smaller scale in special laboratory reactors or standard flasks and tubes. In the laboratory practice of the last decade, gaseous acetylene is frequently and effectively replaced with calcium carbide as an acetylene source. Calcium carbide readily reacts with water and produces gaseous acetylene [59]. Here we will consider the recent examples of acetylene and carbide usage in vinyl ether synthesis.
The reaction of alcohols with acetylene can be classified as nucleophilic addition to carbon-carbon triple bond (Scheme 2). This process requires the presence of a base as a catalyst. The base interacts with an alcohol molecule 1 giving alcoholate anion 2. The alkoxide ion reacts with acetylene forming the corresponding carbanion 3. The latter acts with the molecule of alcohol or water, thus giving vinyl ether 4 [20,60].
The reaction was carried out under the standard vinylation conditions [20]: superbasic KOH-DMSO system as a reaction media, high temperature and elevated acetylene pressure. The desired vinyl derivative was isolated in 85% yield (Scheme 3). Vinylation of triterpenoid 7 under atmospheric acetylene pressure resulted in vinyl derivative 10 in good yield (64%) [62]. Formation of the oxygen bridge was thought to occur through intramolecular formation of vinyl ether 8 with its subsequent devinylation-cyclization and vinylation of the last OH-group (Scheme 4). The side product 9 with a free hydroxyl group can be easily converted to the desired ether 10.

Scheme 4. Triterpenoid vinylation.
Another recently published example of superbase-catalyzed (KOH-DMSO system-mediated) vinylation involves hydroxymethyl-and α-hydroxyethylferrocenes 11 [63]. Vinyl ethers 12 were obtained in excellent yields (Scheme 5) and represented a promising route for development of new polymeric materials with specific physical and chemical properties. Despite the outstanding chemical versatility of acetylene and the widest range of possibilities for its application, it is a highly flammable gaseous substance [64]. Safety precautions should be taken to avoid its leakage and explosion. Besides, gaseous materials usage requires additional measuring equipment and high-pressure furnishing. The development of alternatives for gaseous acetylene is an important problem of modern chemistry. A reasonable solution of these complications is provided by utilization of calcium carbide as an in situ source of acetylene, which is immediately released upon reaction of CaC 2 with water (Scheme 6) [59]. A small admixtures of water does not interfere with the vinylation, and sometimes is even favorable [60,65]. Scheme 6. Reaction of calcium carbide with water.
Successful vinylation of benzyl alcohols 13 with calcium carbide was performed without any complicate equipment [66]. The reactions were conducted in standard reaction tubes or in a flask for gram-scale synthesis. The desired vinyl ethers 14 were obtained in good to excellent yields (Scheme 7). Using CaC 2 instead of gaseous acetylene provides certain economy, as only 2.5 to 3.3-fold excess of it is required to complete the reaction. Similar reactions with gaseous acetylene (see  were carried out under high acetylene pressures (10-13 atm) or in acetylene flow [61][62][63]. Apart from benzyl alcohols, the successful vinylation of dodecyl alcohol (15a), 3-phenylpropanol (15b) and 3-(pyridine-2-yl)propanol (15c) was also performed under the same conditions (Scheme 8) [66]. Vinyl ethers 16a-c were obtained in very good yields. For phenethyl alcohol (17), the unexpected formation of styrene 19 instead of vinyl ether 18 was observed (Scheme 8). The reaction of phenols 20 with calcium carbide as an acetylene source was carried out at similar conditions [67,68]. Apparently, Cs 2 CO 3 is also capable of providing a superbasic system with DMSO. The method allows obtaining vinyl derivatives 21 in up to 96% yields (Scheme 9A). Using the developed technique, diphenyl carbonate 22 was also transformed to phenyl vinyl ether (23) in 73% yield (see Scheme 9B) [67]. Scheme 9. The synthesis of aryl vinyl ethers with calcium carbide.
The vinylation of 1,5-pentanediol, ethylene glycol, glycerol and erythritol with carbide was also performed in a superbasic Cs 2 CO 3 -DMSO system [67]. The corresponding vinyl derivatives 24-27 were obtained in moderate to excellent yields (Scheme 10). Another synthetic procedure for vinylation of alcohols and phenols was recently developed [69,70]. It employs KOH-DMSO-KF and K 2 CO 3 -DMSO-KF ternary mixtures instead of the renowned KOH-DMSO and Cs 2 CO 3 -DMSO systems. Potassium fluoride significantly accelerates the reaction [70]. The KOH-DMSO-KF mixture works well for aliphatic alcohols, particularly steroids and carbohydrates, giving vinyl ethers 28 in good to excellent yields (Scheme 11) [69,70]. It also favors the reaction with ferrocenyl alcohol and allows obtaining its vinylated derivative 28g in 76% yield. The alternative K 2 CO 3 -DMSO-KF system is perfect for vinylation of phenols.
In general, both acetylene and calcium carbide as an acetylene source can be used in vinyl ether synthesis. The reaction of alcohols with acetylene or CaC 2 is applicable to a wide variety of substrates. It can effectively proceed with aliphatic substrates, phenols, naphthols, ferrocene-derived alcohols, nitrogen-containing substrates, carbohydrates, etc. Using calcium carbide in a laboratory is preferable for its convenience and safety, as CaC 2 is a solid material that can be easily quantified and handled without special equipment. Direct addition of carbide into the mixture allows carrying out the reaction under milder conditions. When using gaseous acetylene, the expenditure is considerably higher, even under low pressures, because the constant gas flow must be maintained until the end of the reaction. Scheme 11. Representative scope of alcohols vinylation with CaC 2 in KOH-DMSO-KF system.

Synthesis of S-, Se-and Te-Vinyl Derivatives
Organochalcogen compounds occupy an important niche in contemporary organic synthesis due to their availability, rich chemistry [71][72][73] and the biological activities [74]. Among the organochalcogen compounds, vinyl chalcogenides have attracted a considerable attention in recent years [75][76][77]. Vinyl sulfides are of particular interest as intermediates for various organic transformations. Sulfur-containing polymers derived from vinyl monomers have attracted a great deal of attention, due to their potential applications in numerous areas in polymer research. For example, they have a wide variety of applications in plastic lenses, optical disc substrates, optical adhesives or encapsulants, antireflective coatings, photoresists [75][76][77][78][79].
Addition of thiols and selenols to acetylene is the simplest route to vinyl sulfides and vinyl selenides. It is chemically similar with the nucleophilic addition of alcohols to acetylene described previously and it is also require basic catalysis [20], and is accordingly called thiovinylation.
The common approach to this reaction based on the use of gaseous acetylene was proposed recently. The simplest alkyl vinyl sulfides 31a-d and phenyl vinyl sulfide 31e were synthesized in excellent yields [80]. On the first step, generation of potassium thiolate 30 from thiol 29 occurred (Scheme 12). Potassium thiolate further reacted with acetylene and water giving vinyl ether 31. This approach can be considered as green, because the reactions were performed in aqueous media. It is probably applicable to a wide range of thiols. Its serious disadvantage is the high acetylene pressure requirement. Scheme 12. Addition of thiols to acetylene.
Phenyl selenol (33) was also modified by calcium carbide. Vinyl derivative 34 was obtained in 62% yield (Scheme 14) [81]. Another reaction, which is also acetylene-dependent, allows obtaining vinyl chalcogenides from disulfides, diselenides, and ditellurides; it leads to vinyl sulfides, vinyl selenides, and vinyl tellurides, respectively. It has several reported implementations [61,83,84]. The first resulted from interaction of a dichalcogenide 35 with gaseous acetylene in the presence of a reducing agent, e.g., hydrazine-KOH mixture or sodium borohydride. To complete the reaction, the presence of the reducing agent is required (Scheme 15) [61,84]. Using this methodology, the corresponding vinyl chalcogenides 36 were obtained in excellent yields. The reaction proceeds in two steps. At first, dichalcogenide 35 interacts with the reducing-basic mixture giving potassium chalcogenide 37 (Scheme 16), which further reacts with acetylene and water to produce vinyl derivative 36 [84]. Another variation on the theme was recently proposed [61,83,84]. In this setting, alkyl or aryl halide was directly exposed to sulfur, selenium or tellurium one-flask with a reducing agent and acetylene. Heating of this mixture under high acetylene pressure promoted formation of vinyl chalcogenides in moderate yields (52-68%) [83,84].
The third protocol for synthesis of vinyl chalcogenides from dichalcogenides and acetylene is based on the in situ generation of the acetylene itself. As a source of acetylene calcium carbide can be used [85]. Vinyl sulfides 38a and phenyl vinyl selenide (38b) were obtained in up to 93% yields (Scheme 17), although in a number of cases the yields are diminished by competitive formation of bis-chalcogenide derivatives 39. Scheme 17. Reactions of carbide-induced acetylene with dichalcogenides.
Thus, reactions of thiols and selenols (or dichalcogenides) with acetylene (or calcium carbide as its source) provide convenient access to the wide range of vinyl chalcogenides. Availability of both acetylene and calcium carbide associates with good prospects of development in this direction.

Synthesis of N-Vinyl Derivatives
N-Vinylated compounds are versatile building blocks for polymerization. The nitrogen-containing polymers showed exceptional electroluminescent properties and photoconductivity and widely used in the pill coating or excipient formulations [21,22,[86][87][88][89]. The synthesis of N-vinyl derivatives is a demanded goal of modern chemistry.
A simplest synthetic route to N-vinyl derivatives is provided by addition of nitrogen compounds to acetylene. For example, reaction of indole (40) with acetylene in the presence of potassium hydroxide gives N-vinylindole (41) in 94% yield (Scheme 18) [90]. This reaction illustrates common route to vinylated nitrogen derivatives due to the high availability of acetylene and the wide scope of potentially suitable nitrogen substrates [20]. Replacement of gaseous acetylene with solid calcium carbide, proposed in several recent publications [68,91], seems reasonable. Calcium carbide facilitated obtaining of N-vinylindoles 43 from the corresponding substituted indoles 42 in good to excellent yields (Scheme 19) [68]. The reaction was performed in a Cs 2 CO 3 -DMSO mixture, similarly with the above-mentioned protocol for alcohol vinylation. In one case, reaction of 1H-indol-5-ol (44) with CaC 2 reproducibly resulted in formation of a divinyl derivative 45.

Scheme 19. Vinylation of indoles with carbide-induced acetylene.
A new protocol was developed for the carbide-mediated nitrogen core vinylation in superbasic KOH-KF-DMSO system (Scheme 20) [91]. Its application to various indoles, carbazoles, diaryl amines, pyrroles, pyrazoles, and other nitrogen substrates results in moderate to good yields of the target products, additionally increased by the presence of fluoride. Thus, both acetylene and calcium carbide effectively promote the synthesis of vinyl indoles. The reaction has a wide substrate scope, as it also works for diarylamines, indoles, carbazoles, and other nitrogen heterocycles.

Synthesis of C-Vinyl Derivatives
Vinylation of carbon atoms with acetylene affords a number of important polymers, e.g., polystyrenes for manufacturing of plastic tableware, packaging, building blocks and accessories (insulation, tapes, plugs, cases), and other useful items [92].
Acetylene gas was successfully used in the synthesis of fluorinated styrene compounds. The transition metal-catalyzed reaction was accelerated by a stop-flow micro-tubing reactor developed by the authors [93]. Within this device, fluorinated aryl bromides 46 reacted with acetylene in the presence of iridium photocatalyst 47 and diisipropyl ethylamine giving the corresponding vinyl derivatives 48 in 37-70% yields (Scheme 21). Application of this approach is compromised by formation of side products.

Scheme 21. Styrenes synthesis with acetylene.
A recently published example of a modification of N-benzyl-3-hydroxy-2-oxindole (49) with acetylene involves ruthenium carbonyl catalysis [94]. The reaction proceeds in the presence of 1,3-bis(diphenylphosphino)propane and adamantanecarboxylic acid and allows obtaining vinyl derivative 50 in 69% yield (Scheme 22). This product is a potential spirooxindole precursor with specific arrangement of functional groups in its structure [94]. The examples of C-vinylation are of certain theoretical importance and immense practical relevance. Further progress in discovering their potential is expected in the near future.

Synthesis of Acrylic Acid Derivatives
Acrylic acid (AA) and its derivatives are important components of plastics, rubbers, superabsorbent polymers, and paints. In industry, AA is prepared by partial oxidation of propene [95], which is refined from petroleum. Carbonylation of acetylene, introduced by Reppe [96], represents an alternative route to AA, which is better in terms of atom economy and independence from oil sources (Scheme 23). Reppe used the nickel carbonyl as a catalyst to yield AA under 313.15 K and 0.1 MPa. Ni-Catalysts, especially nickel halides, are commonly used as homogeneous catalysts for AA production [14,97]. Ni 2 O 3 is active as a heterogeneous catalyst in the presence of copper salts [98]. It was experimentally compared with NiO and other common homogeneous catalysts, including NiBr 2 , NiCl 2 , Ni(NO 3 ) 2 , and Ni(OAc) 2 . Under fixed conditions (4 MPa, 235 • C, and CuBr 2 as a promoter), Ni 2 O 3 showed the best average activity with 764.6 (mol AA) (mol Ni) −1 h −1 . It could be reused at least five times without loss of activity and regenerated through calcination within 400 • C. The other catalysts' activities were related as Ni(OAc) 2 > NiCl 2 ≈ NiBr 2 > NiO > Ni(NO 3 ) 2 > Ni(OAc) 2 /PPh 3 / methanesulfonic acid.
Molecular sieves (zeolites) are one of the most common and versatile heterogeneous catalysts [99]. Zeolite-based Ni catalysts were tested in acetylene carbonylation [97,100]. Comparison of the catalysts prepared by using different supports and nickel introduction procedures (incipient wetness impregnation and aqueous ion exchange) revealed the best catalytic activity of Ni-exchanged Y zeolites in the presence of Cu salt as a promoter (62 g AA/(gcat.·h)). Multiple reuse of NiY in a batch reactor confirmed its stable activity at 235 • C and 3.6 MPa of initial total pressure.
An alternative acetylene carboxylation approach is based on using formic acid (FA) instead of toxic CO. FA provides an efficient in situ source of CO for hydrocarboxylation and hydroformylation of olefins [101].
Hydrocarboxylation of acetylene with FA in the presence of palladium catalysts and a catalytic amount of benzoic or acetic anhydride is shown in Scheme 24 [28].     The same combined catalytic system is suitable for effective hydrocarboxylation of other terminal alkynes. Its application to internal alkynes provides 2,3-substituted derivatives of AA in up to 97% yields.
In comparison with traditional homogeneous Pd(OAc) 2 / 2 -PyPPh 2 / P-toluenesulfonic acid catalysts, the copolymer supported palladium catalyst (Pd-PyPPh 2 -SO 3 H@POPs) exhibits higher activity in alkoxycarbonylation of terminal alkynes under the same conditions. The advantage is attributed to synergistic action of the single-site Pd centers, 2-PyPPh 2 ligands, and SO 3 H groups, as well as the swelling properties and high enrichment of the reactant concentrations by the porous catalyst.

Cross-Coupling Reactions
Sonogashira reaction was discovered in 1975 [105] and represents a fascinating route to various alkynes. Sonogashira cross coupling of acetylene with iodo-and bromoarenes can be successfully applied to obtain symmetric diaryl ethynes [42,[106][107][108]. In recent studies [37,42,106], calcium carbide was directly used as an acetylene source in cross-coupling reactions to synthesize symmetric diaryl ethynes. Sonogashira coupling of generated from CaC 2 and water acetylene with aryl halides 53 gave rise to substituted diphenylacetylenes 54 (Scheme 28) [42]. The reaction was carried out in a multiphase one-pot system with fluorinated solvent Galden HT135. The solvent formed a liquid membrane, which separates CaC 2 from the water phase and facilitates reagents transport in a controllable manner. Sufficiently good yields were obtained with aryl iodides; the yields for aryl bromides were significantly lower. Using di(4-trifluoromethylphenyl)acetylene resulted in moderate yields of the product (Scheme 28), and 1-bromo-4-methylbenzene showed no reactivity under these conditions [42]. Tetra-n-butyl ammonium fluoride (TBAF) effectively promoted Sonogashira cross coupling reactions between the in situ released acetylene and aryl bromides [106]. For example, yield of diphenylacetylene 55 was increased up to 97% by using TBAF as an additive (Scheme 29) [106]. Palladium-catalyzed cross coupling of arylboronic acids or arylboronic pinacol esters with the carbide-derived acetylene led to corresponding diaryl ethynes 56 (Scheme 30) [37]. The synthesis was initiated by simple mixing of the components and afforded the desired products in satisfactory yields. Catalytic cross coupling with acetylene allows facile production of terminal alkynes, versatile building blocks for drug development and material synthesis. However, constructing terminal alkynes directly from acetylene is problematic due to the low selectivity between terminal and symmetric disubstituted alkynes. This obstacle can be successfully resolved by using flow reactors [93,109]. Significant improvements of the reactivity and selectivity result from the distinguished gas/liquid interfacial contact. Sonogashira coupling between acetylene and aryl iodides 57 in a stop-flow microtubing reactor convincingly affords terminal arylacetylenes 58 as major products (Scheme 31) [93].
Mono-vs. disubstituted product selectivities of the TBAF-promoted Sonogashira coupling of arylhalides 60 with carbide-derived acetylene under various conditions have been examined in a pilot study [106]. The best monosubstitution selectivity was achieved when extra amounts of TBAF and water were gradually added to the reaction mixture during the reaction. The ratio of calcium carbide and water is also important; the best results were obtained at H 2 O:CaC 2 molar ratio of 1.3:1. Reactions with 4-methyl-, 4-phenyl-, 4-(dimethylamino)phenyl bromides and 3,5-dimethylphenyliodide gave the corresponding monosubstituted products 61 in 28-57% yields. Using other aryl halides afforded mixtures of mono-and di-coupled products (with 61:62 ratio of 1:1 to 4:1) in 57-90% yields (Scheme 32). A three-component aldehyde-alkyne-amine coupling reaction was also possible with calcium carbide as an acetylene source [42]. The reaction has been originally performed with cyclohexanone (instead of aldehydes) in the multiphase one-pot system with fluorous liquid membrane Galden HT135. The system allows obtaining up to 75% yields of the desired products (Scheme 33). A copper-catalyzed Sonogashira-type reaction provides the means for allylation of acetylene with allyl halides [110]. Mono-coupled product 64 can be obtained in up to 80% yield by cross-coupling of acetylene with allyl halides 63 under atmospheric pressure in the presence of copper(I) iodide, a base, and a reducing agent. Excess amounts of allyl halide over acetylene lead to predominant formation of disubstituted product 65 in up to 72% yield (Scheme 34). A recently reported catalytic system promoted oxidative homocoupling of acetylene under oxygen atmosphere in the presence of carbon-supported gold nanoparticles [111]. The reaction resulted in 70% yield of corresponding dimer 66 (Scheme 35) [111].

Favorskii Reaction
Introduced in the early 20th century by Favorskii [112] the reaction for obtaining propargyl alcohols by alkynylation of carbonyl compounds is still a heavy hitter [34,[113][114][115][116][117][118][119][120]. Convenient and mild synthetic procedures for alkynylation of various aldehydes and ketones have been recently reported by Trofimov et al. [34,113,114]. The catalytic system KOH-H 2 O-DMSO [34,113] was appropriate for the synthesis of secondary propargyl alcohols 68 from aromatic and heteroaromatic aldehydes 67 and acetylene (Scheme 36). The reaction was performed under atmospheric pressure, at constant bubbling of acetylene through a reaction mixture. The target products were obtained in satisfactory yields. The Bu 4 NOH-H 2 O-DMSO [115] catalytic system was suitable for alkynylation of both aromatic and aliphatic aldehydes and ketones. The reaction was carried out in a flow reactor under atmospheric pressure of acetylene. This protocol was efficient for different types of substrates: propargylic alcohols were prepared in high yields from aromatic, heteroaromatic, cyclic and acyclic aliphatic carbonyl compounds (Scheme 38). The synthesis of ferrocene-derived propargylic alcohol demonstrates the versatility of Favorskii reaction. 1-Phenyl-1-ferrocenyl-2-propin-1-ol (72) was formed through passing of acetylene into a KOH suspension in THF with benzoylferrocene 71 (Scheme 39) [116]. Alkynylation of carbonyl compounds with metal acetylides represents an alternative to the direct base-catalyzed alkynylation with gaseous acetylene [117,118]. Reaction of aldehydes and ketones with in situ generated ethynylmagnesium bromide is carried out in a commercially available falling film microreactor (FFMR) [117]. Consistent gas-liquid contact in the continuous microflow setting is a prerequisite for effective generation of ethynyl-Grignard reagent from EtMgBr and acetylene. The protocol affords a number of propargyl alcohols in high yields and short reaction times, under atmospheric pressure and low reaction temperature (Scheme 40). Lithium acetylide was used for the preparation of decalin-derived acetylenic alcohol 74, an intermediate for the multistep synthesis of racemic forskolin [118]. Selective addition of monolithium acetylide to one of the two carbonyl groups in cis-decalin 73 and subsequent epimerization in the presence of aluminum oxide afforded trans-decalin 74 in overall yield 82% (Scheme 41). Replacement of gaseous acetylene with solid calcium carbide is frequently proposed in recent studies of Favorskii reaction [119,120]. Reactions can be performed for a wide substrate scope including aromatic and aliphatic aldehydes and ketones (Scheme 42). TBAF [119] or Cs 2 CO 3 [120] were used as catalysts, and using of TBAF provided higher yields. Activation of calcium carbide by fluoride is thought to generate an acetylide "ate"-complex that readily adds to carbonyl groups [119]. Both Sonogashira cross coupling and Favorskii alkynylation with acetylene are currently in demand. Mild and environmentally benign protocols for these reactions, including those with the in situ generated acetylene, continuously evolve.

Scheme 42.
Favorskii reaction with in situ generated acetylene.
1,2,3-Triazoles are produced in the copper-catalyzed click-reactions of organic azides with acetylene [44,[122][123][124]. These reactions can be performed in aqueous media under atmospheric pressure [44,122]. A handy procedure for obtaining 1-substituted 1,2,3-triazoles involves CuI-Et 3 N as a catalyst and water as a solvent [44]. Its application to a variety of substrates facilitates transformation of substituted phenyl, benzyl, and octyl azides into corresponding substituted triazoles 75 in up to 96% yield (Scheme 43). A water soluble catalyst 77 based on N-heterocyclic carbenes-Cu complex for [3 + 2]-cycloaddition of azides 76 and acetylene was proposed [122]. Various alkyl and benzyl triazoles 78 were obtained in high yields using 77 as catalyst. The reactions were carried out in water or 1:1 DMSO/water mixture (Scheme 44). Triazole-fused benzoheterocycles were prepared via Cu-catalyzed click-reaction of o-iodo-azides 79 with acetylene and subsequent Pd-catalyzed intramolecular coupling [123]. The triazole-fused isoindole derivative 80 can be obtained in one-pot manner. Obtaining of six-, seven-and eight-membered heterocycles 81 requires a two-step procedure (Scheme 45). Identical Cu-catalyzed cycloadditions can be performed with calcium carbide instead of gaseous acetylene [42]. Triethylamine serves as a solvent and base simultaneously, while Galden HT135 effectively separates CaC 2 from the organic phase. Benzyl azides with variously substituted phenyl rings undergo cyclization with the released acetylene to afford good yields of the corresponding triazoles (Scheme 46). Scheme 46. Cycloaddition with CaC 2 . Similar 1,3-dipolar cycloadditions of the calcium carbide-derived acetylene to diazo compounds [126], nitrile oxides [41] and nitrile imines [43] were reported.
Calcium carbide was used as an acetylene source to react with a wide range of N-tosylhydrazones 82 derived from aldehydes or ketones, affording various substituted pyrazoles in good yields with high regioselectivities [126]. When N-tosylhydrazones were derived from aldehydes, the starting materials were smoothly transformed into the corresponding products 83. Ketone-derived N-tosylhydrazones afforded the pyrazoles 84 and 85 with regioselectivities of more than 5:1 (Scheme 47).

Scheme 47. Synthesis of NH-pyrazoles from N-tosylhydrazones.
The mechanism of these transformations plausibly involves [3 + 2]-cycloadditions of in situ generated diazo compounds followed by [1,5]-sigmatropic rearrangements (Scheme 48) [126]. Initial deprotonation of N-tosylhydrazone 82 in the presence of Cs 2 CO 3 gives cesium salt A, which undergoes rapid transformation into reactive diazo compound B. Its [3 + 2]-cycloaddition with acetylene affords a 3H-pyrazole compound C. When R 1 is H, 83 is formed via [1,5-H]-shift and tautomerization. When R 1 is aryl and R 2 is alkyl, migration of R 1 or R 2 to C4 occurs. Subsequent multiple [1,5]-sigmatropic rearrangements result in formation of 84 or 85. Since the migration ability of electron-rich aryl group is higher than that of alkyl group [126], migration of R 1 to C4 is favored, and pyrazoles 84 predominate as major products. This proposed mechanism was supported by deuterium-labeling experiments [126]. A convenient synthetic methodology for the one-pot preparation of isoxazoles 87 directly from the reaction of calcium carbide with aldoximes 86 was developed [41]. This method is based on a 1,3-dipolar cycloaddition of nitrile oxides to acetylene. Nitrile oxides precursors, chloroaldoximes 88 were formed in situ by aldoxime chlorination with N-chlorosuccinimide (NCS). Reaction of CaC 2 with the water provides the sources of both acetylene and Ca(OH) 2 base to enable generation of nitrile oxides 89. Various 3-substituted isoxazoles 87 were synthesized from the corresponding aldoximes 86 in up to 95% yields (Scheme 49).

Scheme 49. CaC 2 -mediated synthesis of isoxasoles from aldoximes.
An efficient protocol for the preparation of 1,3-disubstituted pyrazoles 91 from [3 + 2]-cycloaddition reactions of in situ generated nitrile imines 92 with CaC 2 -derived acetylene was reported [43]. The reactions were performed in a two-chambered reactor in order to separate the water-sensitive nitrile imines from the acetylene-generating mixture. One part of the reactor was loaded with hydrazonoyl chloride precursors 90 of active nitrile imine species and Et 3 N as a base. The other part was used to generate acetylene from CaC 2 and water. Escaping from the carbide chamber, acetylene readily dissolves in the reaction mixture with nitrile imines freshly derived from hydrazonoyl chlorides and Et 3 N as a base. A variety of 1,3-disubstituted pyrazoles 91 were produced in up to quantitative yields by this approach (Scheme 50). Utilization of carbide and water as an acetylene source in cycloaddition reactions is not just safe and convenient, but allows to obtain deuterium-labeled heterocycles by using deuterated water, as exemplified by syntheses of 4,5-dideuteroisoxazoles and 4,5-dideuteropyrazoles [41,43].

Scheme 60. Reaction of acetylene and ketones.
A proposed mechanism for this reaction involves the following cascade transformations [145]. A superbase-promoted addition of the initial ketone to acetylene yields an intermediate 2-propenyl ketone, which undergoes prototropic isomerization into 1-propenyl ketone A. The next step involves Michael addition of a second initial ketone molecule to A thus affording 1,5-diketone B. Further on, an acetylene unit is added to a carbonyl group of B to form acetylenic ketoalcohol C. Subsequent cascade cyclizations include reversible formation of hemiacetal D, which undergoes nucleophilic addition to the triple bond and becomes the exocyclic enol ether moiety in the final product (Scheme 61). A recently reported synthesis of methyl-substituted benzofuran rings resulted from combination of calcium carbide-derived acetylene with salicylaldehyde p-tosylhydrazones 113a and 2-hydroxyacetophenone p-tosylhydrazones 113b [151]. The reactions were performed with a copper chloride catalyst. Various 2-methylbenzofurans 114a and 2,3-dimethylbenzofurans 114b were obtained in satisfactory yields (Scheme 63). The reactions were performed in the presence of catalytic amounts of Pd(PPh 3 ) 2 Cl 2 in a DMSO/H 2 O solvent mixture. This copper-free one-pot Sonogashira-type synthesis provided high yields of 2,3-disubstituted pyrrolo[2,3-b]quinoxalines (Scheme 65)A proposed reaction mechanism [152] includes the following steps. Initially, calcium acetylide A is generated from CaC 2 and water.  In summary, acetylene is available and flexible starting material for heterocycles synthesis. The classical protocols for 1,3-dipolar cycloaddition and Trofimov reaction are constantly modified, and new methodologies based on cascade reactions of acetylene continuously develop, as they allow obtaining fine and versatile heterocyclic compounds from simple and inexpensive chemicals.

Applications in Medicinal Chemistry and Drug Discovery
Acetylene is involved in drug synthesis by two chemical approaches: reactive species vinylation and Favorskii reaction.
Vinylation of trifluoroethanol 120 with acetylene gave an inhalation anesthetic fluoroxene 122 (Scheme 67) [153]. The transformation was performed in two steps: first, the starting alcohol was treated with potassium, and then the resulting alkoxide 121 was heated under the elevated pressure in a metal pressure reactor giving the vinyl ether 122. Fluroxene (122) was also prepared from acetylene by an alternative two-step procedure (Scheme 68) [154]. First, trifluoroethanol (120) reacted with acetylene in the presence of mercury oxide and boron trifluoride diethyl etherate giving di(2,2,2-trifluoroethyl) acetaldehyde acetal. The latter was heated with montmorillonitrile producing 122. Scheme 68. Two-step fluoroxene synthesis.
The Favorskii reaction is usually performed either by using a Grignard method, or in the presence of a base (Scheme 69) [155]. The first procedure involves a preliminary reaction of acetylene with Grignard reagent or lithium, sodium, potassium in liquid ammonia or lithium-organic compounds [155][156][157]. The resulting acetylide reacts with the ketone giving a tertiary acetylenic alcohol. The second procedure is equally widespread [155,158]. Both procedures afford acetylenic alcohols from aldehydes and ketones with arbitrarily chosen substituents in moderate to excellent yields [155][156][157][158]. Combining the Favorskii reaction with other synthetic methods, some terpenes were synthesized [156]. Starting from ketone 125 and sodium acetylide, generated by bubbling of acetylene throught sodium-ammonia mixture, the precursor 126 of isophytol 127 was obtained (Scheme 71). Isophytol 127 can be converted to rac-vitamin K 1 128 in some steps. Linalool 129 and nerolidol 130 can be obtained by similar procedures, starting from acetone and subsequently involving Favorskii reaction with partial hydrogenation and condensations [156]. The wide class of steroids ethynyl derivatives was synthesized by Favorskii reaction. These substances are used for the improvement of elderly human health and well-being quality (hormonal therapy), the treatment of rheumatic diseases, to improve sports performance and in cancer therapy [159,160].

Scheme 74. Synthesis of desogestrel.
Eplerenone, which is used in the treatment of congestive heart failure and hypertension, can be synthesized by Favorskii reaction. The key step in its synthesis is provided by a reaction of acetylene with TMS-protected precursor 137 (Scheme 75). This step gave an acetylene-derived alcohol 138, which was successively converted into eplerenone 139 [165]. Favorskii reactions with acetylene, acetylenides, or calcium carbide provide a neat and effective means for the synthesis of ethynyl-substituted steroids [159,160,166]. The approach allows obtaining a wide range of active pharmaceutical ingredients for hormone medications, anti-rheumatic and other anti-inflammatory drugs, medications for prevention and treatment of heart disorders, and many more [167].

Organometallic Complexes of Acetylene
Transition metal alkynyl complexes are potent intermediates in supramolecular design and organometallic synthesis [168][169][170][171]. Their tunable nature makes them excellent candidate molecules in non-linear optics [172,173] and polymer chemistry [174,175], as antitumor drugs [176,177] and luminescent materials [178,179]. In most cases, acetylene is utilized as a bridge and possesses a high potential due to possibility of Sonogashira-like reactions.
A symmetrical bis(alkynyl) derivative 141 is obtained essentially in compliance with the established Sonogashira mechanism (Scheme 76) [180]. The reaction is carried out under atmospheric acetylene pressure; it is initiated by mixing acetylene with metal cluster 140 in 2:1 molar ratio. Excess of acetylene results in equilibrium between four-and five-coordinate complexes. Facile substitution of the π-bonded acetylene with free olefins or CO results, respectively, in alkene-alkenyl 145 or carbonyl-alkenyl 146 complexes (Scheme 77). In some cases, it is possible to isolate Pt-containing intermediates with acetylene to study kinetics and other properties [182]. An unusual heterobimetallic Au(I)/Pt(II) complex with acetylide (-C≡CH) bridge was formed in reaction between Au(I) and Pt(0) complexes 147 and 148 [183]. These complexes with bulky ligands terphenyl phosphine and tri-tert-butylphosphine, respectively, act as a Lewis acid and a Lewis base. The metal pair acquires a remarkable capacity of activating acetylene molecule. Upon exposure of benzene solutions of complexes 147 and 148 to acetylene their color instantly changes from bright yellow to intense orange. According to NMR data, the Au and Pt complexes were converted into two structurally distinct products (Scheme 78). In complex 149, the triple bond bridge is π-bonded to a cationic Au(I) center and σ-bonded to a Pt(II) hydride fragment. In complex 150, which lacks the triple bond, the metals are linked by a µ-ethene-1,2-diyl moiety. Hydroarylation of alkynes provides a valuable route to various alkenes, although chemo-, regio and stereoselectivities of this reaction may be compromised by collateral formation of diaryl alkanes, dialkenyl arenes, and aryl dienes. An in situ generated dicationic complex [Pt(2,6-bis(dephinylphosphinomethyl)pyridine)C 2 H 2 ](BF 4 ) 2 151 reacts with arenes [184]. Only electron-rich arenes are incorporated into arylalkenyl products 152 (Scheme 79). Another interesting study of unsaturated Pt(II) complex showed tautomerization of acetylene to vinylidene [185]. In this case, C-C bond formation occurs via isomeric methyl vinylidene structure instead of the classical migratory insertion reactivity. Vinylidene formation is also characteristic of the iridium pyridylidenes (R = H, Me, Ph) reaction with acetylene [186] The reaction afforded four-membered iridacycles with IrC(=CH 2 )N moiety. Selective insertion of alkynes into Mo-H and W-H bonds was catalyzed by Pd(0) complex and resulted in alkenylmolybdenum and alkenyltungsten complexes [189]. Interestingly, no incorporation of deuterium into trans and geminal positions of vinyltungsten complex 157 was observed in the reaction of acetylene with tungsten complex WDCp(CO) 3 (Scheme 82). Tungsten complexes with diphosphines [Tp*W(CO)(I){η 2 -C 2 (PPh 2 ) 2 }] (Tp* = hydrido-tris(3,4,5-trimethyl-pyrazolyl)borate) showed highly reversible redox potentials [190]. A variety of substituted bisphosphine acetylene bridged dinuclear complexes with a side-on bound Tp*W(CO)(I) moiety and a chelate-like coordinated PtCl 2 unit was reported [191]. Thus, promotion of acetylene insertion by metal atoms results in modified metal complexes, which can be isolated. Various metal complexes are used as catalysts or building blocks and also, notably, in luminescence applications. Here we descried a few representative examples of reactions with organometallic complexes, where a number of other possibilities are available as well as other terminal alkynes react with metal complexes in a similar manner [192].
Compounds with 2-bromovinyltellanyl group are used in stereoselective syntheses of alkenes via consecutive replacement of bromine and tellurium atoms by organic groups in cross-coupling reactions [194].
The reaction of iodomethane and acetylene in the presence of Na 2 PtCl 4 and NaI yielded a mixture of iodoethylenes (Scheme 84) [195]. A mechanism proposed by the authors involved intermediate formation of a Pt(IV) methyl vinyl derivative by iodoplatination of acetylene with a reversibly formed Pt(IV) methyl complex. Another example of catalytic addition to acetylene, preparation of 1,2-bis(diphenylphosphino)ethane dioxide (dppeO 2 ), proceeded in the presence of t-BuOLi (Scheme 85) [196]. A possible mechanism for this reaction includes deprotonation of Ph 2 P(O)H by t-BuOLi. The resulting formation of anion A directs the catalytic cycle toward dppeO 2 .

Scheme 85.
A mechanism proposed for the formation of dppe oxide.

Selective Semi-Hydrogenation of Acetylene
Naphtha cracking generates streams of ethylene, which may be directly used in polymerization or partial oxidation to ethylene oxide. Acetylene impurities constitute 0.5-2 vol% of the primary ethylene streams. Their amount should be diminished to 5 vol% prior to polymerization. Acetylene, which is poisonous to the downstream polymerization catalysts, is removed from the gas mixture by hydrogenation. This step involves hydrogenation of acetylene to ethylene (i), hydrogenation of ethylene to ethane (ii), acetylene polymerization into green oil (iii), and carbonaceous deposition (iv, Scheme 93). The aim of the purification step is to achieve (i) avoiding (ii), (iii), and (iv). Selective hydrogenation of acetylene requires π-bonding of its molecules to the surface of a catalyst. The process is hindered when the adsorption centers are placed too densely; its performance can be enhanced by the active-site-isolation approach. As for the selectivity, all improvements fall into three groups. The first type of improvements results from modification of electronic properties of the catalyst is caused by alloying and/or support influence. The ligand effect [204] is caused by interaction of two metals and results in shift of their d-band center, which affects the adsorption strength and, the activation energy. Concomitant changes in structure and composition of the surface have modulatory effect on the number and geometry of the adsorption sites (the so-called ensemble effect). The emerging A sites are mainly responsible for selective acetylene hydrogenation, whereas the larger Pd ensembles, E sites, favor the ethylene-to-ethane conversion. The second type of selectivity improvements results from modification of the active site geometry and morphology. The third type of improvements is achieved by dilution or blocking of large active sites caused by reducing, deposition technique and ligands addition.
The catalysts used for selective acetylene hydrogenation can be divided into Pd-based (Pd/Ga [29,30], Pd/Ag [31][32][33], Pd/Cu [205]) and Pd-free, further subdivided into monometallic (Cu [206], Ag [207], Fe [208], Ni [209,210], Au [211]), bimetallic (Au/Ni [212,213], Au/Ag [214], Ni/Zn [8], Cu/Al hydrotalcite [215]), tri-metallic (Cu-Ni-Fe [216]), and metal free (CeO 2 [217,218]). Today, Pd-based catalysts are often used for selective removal of acetylene [219,220]. However, they are prone to contamination with Pd carbides [221][222][223] and Pd hydrides [31,224], which worsen the performance and lead to over-hydrogenation [225,226]. Addition of carbon monoxide improves the selectivity, because CO competes for adsorption sites with ethylene but not acetylene [32,227]. However, CO co-feeding is inconvenient, as it requires continuous control of CO amounts in order to maintain the full acetylene conversion. In addition, CO promotes the formation of so-called green oil, which is composed of acetylene oligomers [210]. Although the activity of pure Pd is high, its selectivity is limited to 20% at 200 • C [228]. The optimum temperatures for such catalysts are much higher than those accepted in industry; besides, they are really expensive. Addition of the second metal improves the selectivity for two reasons: by changing electronic properties of Pd and by dilution of adsorption sites and thereby reducing the number of E sites.
An alternative approach is based on addition of selective surface modifiers, that is, ligands with specific affinity to Pd surface. The ligands can be used during both liquid and gas phase hydrogenation. The nitrogen- [229,230], sulfur- [231,232], and phosphorus-containing [233,234] modifiers of this type usually enhance the selectivity by adsorption to the metal surface, which causes the decreasing the number of large Pd ensembles sites.
The third approach involves reducible oxide support modifications and deposition technique. TiO 2 , La 2 O 3 , Nb 2 O 5 , and CeO 2 migrate onto the Pd surface during reduction, diluting the multiply coordinated sites and enriching the electron density of Pd [205,[235][236][237] Dispersion across the surface of graphene [238] and insertion into zeolite frameworks [239] may also be considered as support modifications.
When pure palladium catalyzes acetylene hydrogenation, conversion and selectivity of the reaction depend on size and morphology of the metal particles. Acetylene hydrogenation is sensitive to the surface structure of Pd NPs. The most selective facet for acetylene hydrogenation is Pd(100). Cubic palladium nanoparticles (NPs) (100) facets can be prepared under controlled conditions [240][241][242][243][244]. Quite unluckily, the low-index facets of Pd [245,246] and Pt [247] are unstable and changed under the reaction conditions. After the reaction, the flat facets turn into sunken facets, and the cubic shape becomes twisted ( Figure 5). The selectivity is maintained high for 27 h of the reaction and then sharply decreases [248]. Microstructural changes on the surface of the well-defined (100) facets result in rough (100) facets with steps and corners. These changes are induced by multiple repetitive adsorptions-desorptions. Cubic NPs with Pd(100) facets show better results than spherical NPs of a similar size, which comprise significant portion of Pd(111) facets [249]. The temperatures of Pd hydride decomposition and hydrocarbon desorption for the cubic NPs are lower than for the spherical NPs. They also show higher acetylene conversion rates and better ethylene selectivity than Pd(222)-faceted spherical particles ( Figure 6) or smaller Pd(I) NPs prepared by using the incipient wetness method. Surface acidity and basicity effect on the status of metals and on catalytic performance. Palladium supported hydrotalcite (Pd/HT) exhibits higher conversion of acetylene compared with aluminum oxide and magnesium oxide supports [250]. HT is a synthetic anionic clay, where M stands for metal cations and A represents an interlayer anion. After heat treatment, the interlayer anion and water are removed, leaving a mixture of metal oxides, which determines the acidity/basicity of the surface. Full conversion of acetylene over Pd/HT is reached at 55 • C (Pd/MgO and Pd/Al 2 O 3 are left behind with 31% and 22%, respectively, Figure 7). Ethylene selectivity of the system is 88% at 80% isoconversion, which is respectively 1.8 and 2.1 times higher than for Pd/MgO and Pd/Al 2 O 3 . It is well known that changing the support affects the parameters of catalytic reduction. Atomic Pd samples deposited on nanocarbons of differential morphology (representing different proportions of nanotubes, nanofibers, and other nanocarbon species) show differential conversions and selectivities [251]. Pd is usually present as a skin on the surface of a support layer. Dispersion of Pd active sites over the skin favors acetylene hydrogenation and inhibits over-hydrogenation and oligomer formation. Tunable thickness of a support layer allows effective control of Pd penetration range and results in uniform lateral distribution of Pd active sites. Egg-shell Pd/α-Al 2 O 3 @SiC catalysts with the controlled thickness of support layer (50-300 µm, Figure 8) were obtained by a specifically developed procedure and tested in acetylene hydrogenation [252]. The catalysts had the same acetylene conversion but higher ethylene selectivity as compared with the conventional Pd-Ag/α-Al 2 O 3 catalyst. A more than 10 times increase in surface area can be achieved by using porous hollow silica NPs as a coating in combination with monolithic cordierite as a ceramic substrate (Figure 9) [253]. Catalytic activity of the samples is enhanced by increasing reaction temperature, although by the cost of ethylene selectivity. The best results among the series are achieved for PdO/PHSNs-Al 2 O 3 /MC with 100% acetylene conversion and 47% ethylene selectivity. This study is an example of successful development of a new monolithic catalyst with increased surface area and maintainable mesoporous structure of the coating layer. Ionic liquids stabilize nanoscale structures by forming the protective anionic and cationic layers [254,255]. Ionic liquid supports in gas phase hydrogenation, catalyzed by Pd, Ru, Ir, and Rh NPs, resulted in formation of 1,3-butadiene, C6-olefins, and benzene [256][257][258]. In the case of acetylene hydrogenation, ionic liquids with [BF 4 ] anion promote higher conversion and lower selectivity, whereas the effects of ionic liquids with [PF 6 ] anion are quite the opposite [259]. The Pd/BmimBF 4 /SiO 2 stability is slightly decreasing after 6 h. The Ar plasma/H 2 -treated Pd/EmimBF 4 /SiO 2 is more stable than the corresponding product of conventional reduction, due to minimized Pd/IL/SiO 2 fluctuation phenomena. Noteworthy, the size of Pd NPs and ionic liquids are stable under plasma conditions [260]. The catalytic properties of Pd/TiO 2 catalysts depend on the way of preparation (impregnation vs electroless deposition) and the type of atmosphere used for calcination-O 2 /N 2 [261] or H 2 [262]. Dispersion of Pd increased in the presence of Ti 3+ significantly. TiO 2 support prepared in N 2 atmosphere under electroless conditions possessed similar average crystallite size, surface area and pore diameter resulted in improved catalytic performance. Full acetylene conversion with high ethylene selectivity (92%) is achievable with 1%Pd/TiO 2 -N 2 . At the same time, H 2 treatment of sol-gel derived TiO 2 increases the number of Ti 3+ defective sites on its surface.
Treatment of a Pd/TiO 2 catalyst with triphenylphosphine (TPP) improves the selectivity and limits over-hydrogenation under low pressures [263]. Acetylene and ethylene have similar rates of hydrogenation over the unmodified Pd catalyst (Figure 6, label 1). Adding CO (3 ppm) has little impact on the results; although it diminishes the ethylene conversion rate (Figure 6, label 2), continuous addition of CO is required to maintain the gain in selectivity. Adding TPP suppresses ethylene hydrogenation (Figure 10, label 3) and promotes a 10-fold decrease in formation of the gas phase-specific byproducts (Table 1). Probably, acetylene more readily adsorbs to the modified catalyst, whereas ethylene adsorption is hindered. Stability of TPP under both low and high pressures makes its superiority especially relevant.  Double additives (Ti and La oxides) are moderate improvers of Pd catalysis [264]. La oxide is beneficial to the performance, but its reduction requires high temperatures. Ti oxide partially covered by La oxide maintained the modified geometric and electronic structures of the catalyst even after low-temperature reduction. It also confers higher ethylene selectivity than the single La oxide. Probably, Ti oxide prevents the La species from re-migrating during high-temperature reduction to their initial positions after air contact. In total, the Pd catalyst modified with Ti and La simultaneously exhibited the best results compared with pure Pd and monomodified catalysts (Figure 11).

Bimetallic Pd-based Catalysts: Pd and Ga/In
As compared with ethylene, acetylene is much more soluble in acetone, dimethylformamide, N-methylpyrrolidone (NMP) and some ionic liquids ( Figure 12) [265]. At conversions near 100%, ethylene selectivity becomes unevenly distributed between the liquid and gas phases. For instance, it may constitute 80-96% in the NMP liquid phase and 70% in gas phase. For this reason, NMP augments the hydrogenation, which a nonpolar solvent (e.g., decane) would be incapable of. NMP was also used for acetylene hydrogenation in the presence of 1% Pd/Ga 2 O 3 catalyst [266]. Another study with a similar catalyst was less successful, providing only 53% acetylene conversion and 36% ethylene selectivity in liquid phase, possibly due to the low solubility of hydrogen in NMP in the presence of alloys with 0.5%Pd-0.08%Ga [267]. Low selectivity at full conversion was also reported for Pd/Ga 2 O 3 -Al 2 O 3 catalysts in NMP as a liquid phase [268]. A bimetallic Mg-Ga-Al-layered double hydroxide (LDH) Pd catalyst is more effective than a monometallic Pd/MgO-Al 2 O 3 system [269]. Although bimetallic Pd/Ga catalysts are just behind the pure Pd catalyst in activity, the bimetallic system shows much better selectivity in comparison with the monometallic Pd/MgO-Al 2 O 3 catalyst without Ga. High dispersion of bimetallic Pd/Ga nanoalloy, with the average particle size of 2.6 nm, is due to the uniform distribution of Ga and the electrostatic interaction between Pd and support layer ( Figure 13). Strong adhesion of MgGaAl-LDH to Al 2 O 3 is a consequence of direct growth of the catalyst on the spherical Al 2 O 3 . Pure Pd/Ga alloys (GaPd 2 and GaPd) are also moderately active, but there was no rate limitation of acetylene concentration and occupation on the catalyst surface [270]. Inkjet printing allows to obtain Pd/Ga catalysts with thin, uniform and homogeneous wall coatings over the entire channel surface [271]. But even in this case the best ethylene selectivity is only 76%. Using Ni instead of Ga in the NiMgAl-LDH catalysts is not particularly beneficial [272], although the strong interaction between Pd and Ni modulates the affinity of hydrogen to the catalyst surface in comparison with similar monometallic systems.

Bimetallic Pd-based Catalysts: Pd and Zn/Co
A series of Pd/ZnO single-atom catalysts shows 80% ethylene selectivity at nearly full acetylene conversion [274]. Decreasing the Pd loading from 1% to 0.1% and reduction of the samples at relatively low temperatures of~100 • C leads to smaller Pd NPs with distributed single-atom Pd sites for the 0.01-Pd/ZnO catalyst ( Figure 15). These high-valent Pd active sites promote electrostatic interactions with acetylene, but restrain ethylene hydrogenation due to the inability of σ-bond formation. Using Zn as an intermetallic catalyst instead of ZnO improves the performance to 90% selectivity at full conversion [275]. Nanostructured Pd-Zn-Pd ensembles have moderate σ-bonding mode for acetylene with two neighboring Pd sites and weak π-bonding of ethylene adsorption on the single Pd site. Well-defined hollow Zn/Co ZIF rhombic dodecahedron was obtained using special modification of ZIF-67@ZIF-8 structures in the presence of Co 2+ [276]. A noble metal@MOF yolk-shell composite was prepared and tested as a catalyst for acetylene hydrogenation. The high-angle annular dark-field scanning TEM (HAADF-STEM) analysis showed that Pd@H-Zn/Co-ZIF was composed of hollow coordination polymer with embedded Pd NPs (Figure 16). The obtained catalysts were compared with Pd NPs in selective acetylene hydrogenation and demonstrated high activity and ethylene selectivity at mild temperatures. Polyols (tri-ethylene glycol and ethylene glycol) were used as a medium for Pd/Co bimetallic system preparation [277]. CoPd NPs were deposited on active carbon, MgO, and Al 2 O 3 using poly(N-vinylpyrrolidone) as a stabilizing agent. Addition of Co led to formation of bimetallic NPs with better dispersion, as compared with monometallic Co or Pd NPs ( Figure 17). Spherically shaped Co/Pd NPs of 3 nm diameter were catalytically active in hydrogenation and showed significantly higher ethylene selectivity, as compared with Pd NPs (Figure 18). The decreases in acetylene and ethylene adsorption heat were due to effects of Co on the electronic structure of Pd, whereas the increase in ethylene selectivity was caused by separation of individual Pd sites by Co incorporation.    [277]. Copyright 2013, Elsevier Inc.).

Pd-Based Bimetallic Catalysts: Pd-Cu/Ag/Au
Pd single-atom alloyed structures supported on silica, with ppm content of Pd, were alloyed with gold NPs and tested in hydrogenation [278]. Interestingly, the bimetallic systems showed a 1000% better ethylene selectivity at 160 • C as compared to monometallic Pd/SiO 2 with similar Pd loadings ( Figure 19). The role of Pd in this case is to increase the conversion and to decrease the reaction temperatures, whereas Au provides isolation between Pd atoms and prevents acetylene over-hydrogenation. An Au/Pd alloy with TiO 2 support, prepared with the 2-nozzle system, exhibited the highest conversion (50%) with high ethylene selectivity (95%) at only 40 • C [279]. Partial coating of the most active bimetallic Au/Pd particles with the Ti precursor afforded less active catalysts due to the formation of Ti-O species. Various configurations of the particles were achieved by using different types of feeding system (single-vs. double-nozzle). Formation of bimetallic NPs was minimized by separated feeding and enhanced by co-feeding.
The results of acetylene hydrogenation over Ag-and Au-Pd/SiO 2 bimetallic catalysts suggest the possibility of two alternative pathways [280]. High coverage of Pd with Ag or Au leaves small ensembles of Pd sites, which adsorb acetylene as a π-bonded complex thus promoting its semi-hydrogenation to ethylene. The lower coverage leaves much larger, contiguous and merging Pd surface sites, where acetylene is adsorbed as a multi-σ-bonded complex preferably resolved by ethane formation.
The alumina phases of Al 2 O 3 support and the mode of metal deposition on TiO 2 support dramatically affect the Pd/Ag catalyst performance. Specific combinations of electrostatic properties with acidity determine differential strength of active sites: decreasing acidity of support is caused by a decrease in site strength [281]. A core-shell Pd@Ag bimetallic catalyst of this type, showing 73.5% selectivity at 98.5% conversion, is prepared by photodeposition of Pd and Ag, with the support acting as a photocatalyst during the preparation procedure [33]. The formed species have superficial Pd core and support specific growth of Ag on the Pd particle surface during the photocatalytic deposition process. Blockage of the high coordination Pd sites by the Ag shell, as well as the increased number of isolated Pd sites, results in better performance.
A defect-rich Pd/Ag catalyst on NiTi-layered double hydroxide support was tested in acetylene hydrogenation [282]. The compromise values of selectivity (82%) and conversion (90%) were achieved by virtue of the small size and high dispersion of Pd/Ag NPs and the abundance of Ti 3+ defective sites. Withdrawing of electron density from Ti 3+ by Pd enhanced the Pd-support interaction. Using ZnAl instead of TiO 2 significantly improved the selectivity/conversion and also the oligomer selectivity [283]. Decreased amounts of carbonaceous deposits were observed for PdAg/ZnO-Al 2 O 3 in comparison with Pd/Al 2 O 3 , PdAg/Al 2 O 3 , and Pd/ZnO-Al 2 O 3 ( Figure 20). The results were further improved to full acetylene conversion with 83.8% ethylene selectivity at 70 • C by using mixed Mg-Ti supports [284]. Presumably, the moderate acidic sites of Mg 0.5 Ti 0.5 O y support accelerate hydrogen-spillover effect followed by facilitated hydrogen activation and dissociation. The selectivity is additionally improved by electron transfer from basic sites and Ti 3+ species of Mg 0.5 Ti 0.5 O y support. A silica gel-supported single-atom Pd/Ag catalyst demonstrated 92.3%/92.6% selectivity/conversion due to extremely low content of Pd (ppm) [285]. The isolated electron-rich Pd atoms are capable of achieving the best results even in an ethylene-rich stream. A bimetallic Pd/Cu (1:50) catalyst on alumina showed a good 70%/99% selectivity/conversion at 423 K [286]. The catalyst combines the properties of Cu and Pd; its high ethylene-selective activity at moderately low temperatures is attributed to facile dissociation of H 2 on Pd with the immediate spillover onto Cu sites where the reaction proceeds. At lower temperatures, the ability of Cu to convert acetylene is poor (9.59%, Table 2). At higher temperatures, the acetylene conversion increases and the ethane selectivity is kept constant, but the ethylene selectivity is limited to 72% because of oligomerization. A modified Pd/Al 2 O 3 catalyst shows better acetylene conversion at lower temperatures, but predominantly to ethane. Efficient and selective acetylene hydrogenation over Pd/Cu catalysts proceeds at higher temperatures. Copper generally acts as a surface diluent decreasing the size of the palladium ensembles. After the hydrogenation over Pd/Cu(111) catalyst, the products were simultaneously analyzed in gas phase [287], and reflection absorption infrared spectroscopy and Auger electron spectroscopy confirmed the presence of a carbonaceous layer on the surface (Figure 21). Palladium alters the nature of the carbonaceous species by promoting hydrogenation of polyacetylene, the product of copper-mediated acetylene coupling.  A silica gel-supported alloyed Pd/Cu single atom catalyst showed 85% ethylene selectivity at full acetylene conversion [288]. The mechanistic investigations confirmed the role electron transfer from Cu to Pd in promoting hydrogen ( Figure 22). Pd-doped Cu/Al 2 O 3 catalysts with 50:1 Cu/Pd ratio, prepared by co-impregnation, sequential impregnation, or colloidal approach, show 80%/98% selectivity/conversion at only 353 K [289]. It is important to note that Cu/Pd bimetallic catalysts with fixed Pd content and varied Cu loadings are not as sensitive to the reductive pretreatment as the Pd/Ag catalysts [290]. However, being reduced at 400 • C and 250 • C, these catalysts show similar behaviors.

Pd-Free Monometallic Catalysts: Mo
A rare reported example of a non-precious metal catalyst for acetylene hydrogenation is based on tri-, tetra-, and hexanuclear mixed-valence molybdenum clusters [291]. The liquid phase reaction was carried out in the presence of Na or Eu amalgams, the [Mo 4 Cl 4 O 2 (OCH 3 ) 6 (CH 3 OH) 4 ] complex, and methanol as a medium. After the catalyst reduction, ethylene and ethane were detected in the parallel reactions simultaneously. After addition of tributylphosphine to Mo-complex, both ethylene and ethane yields increased more than 50% ( Figure 23).

Pd-Free Monometallic Catalysts: Au
Gold catalysts, obtained by photo-dependent synthesis, showed 80% ethylene selectivity at full acetylene conversion after 12 h at 225 • C [292]. The best of them, 1% Au/C-TiO 2 and 5% Au/C-TiO 2 , were tested for different H 2 /C 2 H 2 ratios. At low H 2 /C 2 H 2 , both catalysts showed higher ethylene selectivities at limited acetylene conversion. Reciprocally, the excess of C 2 H 2 favored its high conversion with low ethylene selectivity. After reduction at 250 • C, the 1% Au/C-TiO 2 stably supported full acetylene conversion with 80% ethylene selectivity.

Pd-Free Monometallic Catalysts: In
Indium oxide catalysts showed extraordinary good results at 550 K, with 85% ethylene selectivity at full conversion even for the ethylene excess conditions [293]. The most stable surface configuration, In 2 O 3-y (111), was chosen to investigate the mechanisms of the main reaction and side reactions. Formation of a unique In 3 O 5 site with O vacancy was predicted by theoretical calculations. Acetylene and H 2 molecules co-adsorb to the catalytic site composed of In trimer and adjacent O atoms. This is an example of single-ensemble heterogeneous catalysis, which requires several particular atoms surrounding the empty site for the reaction to occur.

Pd-Free Bimetallic Catalysts: Ni/Zn
Nickel-zinc catalysts specifically suppress the oligomeric species formation by decreasing acetylene adsorption energy. This effect contributes to the higher ethylene selectivity [294]. Isotope labeling studies in a batch reactor confirmed that ethane was produced from both acetylene and ethylene, while oligomeric species were produced from acetylene only. Partial hydrogenation of acetylene led to adsorbed vinyl, which immediately reacted with adsorbed acetylene yielding C 4 H 5 , whose reaction with adsorbed hydrogen yielded butadiene as a final product (1-butene, cis-2-butene, and trans-2-butene can also be formed depending on the hydrogen availability). The carbohydrates can desorb from the catalyst surface and further react with acetylene molecules to form higher molecular weight oligomers. Transition states for vinyl hydrogenation and C-C bond formation on Ni and NiZn are shown in Figure 24.

Pd-Free Bimetallic Catalysts: Fe/Mo
Nitrogenases (Nases) are able to reduce nitrogen to ammonia and acetylene to ethylene [295][296][297][298][299][300][301]. Inorganic Fe/Mo clusters with a [Fe 2 MoOS 3 ] core resemble a part of FeMo-cofactor, the active center of nitrogenases [302]. The dihydride complex reduced acetylene to ethylene via the vinyl monohydride complex; the detailed mechanism, verified by using various combinations of reductants, substrates, and heavy water, is shown in Figure 25. Even very small (2.0 ± 0.5 nm) and probably planar Fe 2,3+ oxide NPs on slightly acidic inorganic oxide supports (TiO 2 , ZrO 2 , or ZnO) are capable of mimicking the natural enzyme activities [303]. Surprisingly, these catalysts selectively and almost totally reduce acetylene and other alkynes not only in batch reactors, but also in flowing streams. Mixed Fe-oxides dissociate and transfer hydrogen onto alkynes in a chemo-and stereoselective manner. Cheap, nontoxic, ligand-free, and ambient condition-resistant, the solid iron catalysts ensure cis-selective semi-hydrogenation of different alkynes, including acetylene, in high yields and with good selectivities. The scale of horizontal axis represents oxidation states 0 to -4, with the initial oxidation state of 1 corresponding to 0. Accordingly, the two and four electron-deprived species correspond to -2 and -4 oxidation states, respectively (reproduced with permission from [302]. Copyright 2016 the Royal Society of Chemistry.).

Pd-Free Bimetallic Catalysts: Pt/Sn
Colloidally prepared platinum and platinum-tin nanocrystals with controllable structural properties were tested in the acetylene semi-hydrogenation [304]. The ligand-free and ligand-capped bimetallic catalysts were separately compared with Pt NPs to sort out possible specific effects of Sn and organic ligands (Figure 26). Using dodecylamine (DDA) promoted a decrease in the unwanted ethylene hydrogenation. Organic ligands are thought to act by diluting or blocking the large Pt ensembles (E sites), while diminishing the surface enrichment of Sn, and thereby suppress ethylene hydrogenation. The bimetallic dodecylamine-capped Pt/Sn catalyst showed the best selectivity in this series, especially relative to the ligand-free and pure Pt catalysts.

Pd-Free Bimetallic Catalysts: Cu/Au
The synergistic effect of bimetallic Cu/Au catalyst is provided by a combo of the electronic effect and the sintering resistance [305]. As it is, the bimetallic 1:3 Cu/Au catalyst shows an upgraded performance ( Figure 27). Probably, the gold renders the copper more electron-deficient. The added amounts of Au reduce the size of Cu ensembles, which suppresses the C-C coupling reactions and results in better ethylene desorption. When Cu dominates in the alloy, the selectivity is very low; the addition of Au leads to an increase in selectivity.

Non-Metallic Catalysts
A one-atom-thick layer of carbon atoms effectively catalyzed the selective acetylene semi-hydrogenation in the absence of a metal [306]. This graphene-based material was catalytically active at room temperature, but achieved its utmost performance at 110-120 • C, with 99% acetylene conversion and 21% ethylene conversion. GO was able to convert 50.6% of acetylene with no measurable ethylene hydrogenation at 150 • C, while rGO converted all acetylene with only 5% ethylene hydrogenation. These results indicate a very high potential of graphene-based catalysts ( Table 3). Given that the graphene oxides were contaminated with ppm traces of metals (Fe and Mn) due to the presence of these metals in the parent graphite, the results were validated in additional experiments. Selected GO samples were purposefully contaminated with two known amounts of MnCl 2 and PdCl 2 before testing. The results confirmed the intrinsic catalytic potential of graphene materials. Multiwalled carbon nanotubes (MWCNTs) showed 90% acetylene conversion at 350 • C [307]. The ethylene selectivity constituted~90% and loosely depended on the acetylene concentration. The higher temperatures led to acetylene decomposition and oligomerization, while the lower temperatures provided no support to acetylene conversion (3% at 80 • C). The nanotubes had cylindrical shape of 10 to 60 nm in diameter ( Figure 28). The thicker tubes had thick walls and no sharp bends; the thinner tubes comprised thin walls and a large number of bends. Metal NPs contaminated the inner space of the tubes, and it was impossible to remove them even by heating at reflux in nitric acid. The high number of defects, the low oxygen content, and basicity of the surface compromised the MWCNT performance in selective acetylene hydrogenation. In summary, the unique properties of Pd-based complexes and nanostructures make them the catalysts of choice for the ethylene-selective acetylene hydrogenation. Huge variety of the available Pd-based materials allows fine tuning of the catalytic process to optimize relative values of selectivity, conversion, and other parameters. Nevertheless, the Pd-based catalysts have certain drawbacks, hence the prospective relevance of their non-Pd and metal-free counterparts.

Synthesis of Vinyl Chloride
Polyvinyl chloride is a commodity chemical, which is employed in manufacturing of electric wires, water pipes and fittings, plastic membranes, and many other items. The monomer, vinyl chloride (VCM), is produced by two alternative routes. The first is based on chlorination and oxychlorination of ethylene, the second is the direct catalytic hydrochlorination of acetylene (Scheme 94). In industry, the hydrochlorination proceeds at temperatures 130-180 • C with a catalyst, mercuric chloride HgCl 2 , supported on activated carbon (AC). However, HgCl 2 is highly toxic and causes environmental pollution. An active search for environment-friendly alternative catalytic systems is currently under way.
Noble (Au, Pt, Pd) and some non-noble metal catalysts have been intensively studied as an alternative to catalize the hydrochlorination reaction. Hutchings and co-workers [308] was the first who demonstrated the high activity of AuCl 3 and dynamic nature of catalyst caused Au 3+ to Au 0 reduction during the reaction. Treatment of the catalyst with aqua regia was proposed as an effective regeneration method for this system [309]. Specific effects of the aqua regia constituents, HCl/HNO 3 , as well as the metal/support modifications in the course of deactivation/regeneration of Au catalysts, were also demonstrated [310][311][312]. Despite HCl and HNO 3 being able to affect the carbon support prior to deposition, the enhancement effect of aqua regia was observed only in the presence of Au during the impregnation step of the catalyst preparation. This was considered to be due to the oxidizing effect of HNO 3 on both the carbon support and gold, in combination with the nucleation effect of HCl on gold particles over the carbon surface. The activity depends on the presence of Au 3+ species in the catalyst, and their spatial distribution in particular (not only the concentration matters).
Despite the glorious history of the catalysis by gold, its mechanistic understanding is still incomplete. A series of gold catalysts was prepared by using the aqueous HCl/HF pretreated activated carbon at 300 • C (Cat 300 ), 600 • C (Cat 600 ), and 900 • C (Cat 900 , Figure 29) [313]. The C 2 H 2 temperature-programmed desorption of the catalysts was also measured. Compared with Cat original and Cat HF , the Cat HCl catalyst showed an increased initial activity with the same decrement over time. The results indicate that dispersibility of gold positively correlates with the stability of the catalyst. Accordingly, the C 2 H 2 adsorption/activation capacity negatively correlates with both the catalyst stability and gold dispersibility. Which means, that the active site is not composed entirely of gold dispersed on the carbon surface, and that the carbon itself works as a constituent part of the site responsible for the acetylene activation.
Stability of Au-catalysts can be enhanced by inhibiting Au 3+ reduction [314], which can be achieved by strengthening the adsorption of hydrogen chloride to the catalyst. The enhanced catalytic activity and stability of AuCl 3 /PPy-MWCNTs in acetylene hydrochlorination, as compared with AuCl 3 /MWCNT, is explained as follows. During the reaction, a lone pair on the nitrogen atom of pyrrole is transferred to the Au 3+ center via the metal-support interaction. This event increases the electron density of the Au 3+ center to strengthen the bond between HCl and AuCl 3 . Other N-doped material-based catalysts, such as carbon [315], carbon nanotubes [316], SiC [317], graphene [318] were studied with some interesting results; however, the low conversion efficiencies and high energy demands do not contribute to their commercial applicability. The nitrogen-containing ligands are able to inhibit the reduction of Au 3+ to Au 0 [319], although the synthesis of AuCl 3 complex with 1,10-phenanthroline [AuCl 2 (phen)]Cl and its successful application in acetylene hydrochlorination have been reported [320].
Catalytic stability of Au catalysts can be improved by addition of a second metal component, for example, CoCl 3 [321] or CsCl [322,323]. Currently available Au catalysts for hydrochlorination contain more than 1 wt % of Au. Addition of Cu as a third metallic component to an AC-supported AuCs catalyst allowed obtaining of a trimetallic catalyst with enhanced activity and stability at~0.25 wt % Au loading [324]. Particularly remarkable activity enhancement in this series was shown by a catalyst with 1:1:4 Au:Cu:Cs weight ratio, at turnover frequency of 73.8 min −1 based on Au. The synergy between Au, Cu, and Cs is associated with enhancements in dispersion of Au particles, stabilization of Au in the state of Au 3+ , and adsorption kinetics of the C 2 H 2 substrate molecules.
ACs have micropores and no interconnected porous network. Mesoporous carbons with regularly interconnected 2-50 nm mesopores could provide an excellent alternative support for the acetylene hydrochlorination catalysts. Mesoporous carbon materials with controllable pore sizes within the range of 5.6-40.2 nm were synthesized using colloidal silica as hard templates and boric acid as a pore expanding agent. The Au catalysts on mesoporous carbon showed excellent performances with increased acetylene conversion. Another material, γ-Al 2 O 3 , was used as support for an AuCl 3 -CuCl 2 bimetallic catalyst. The AuCl 3 -CuCl 2 /γ-Al 2 O 3 catalyst promoted 97% acetylene conversion, which is close to AuCl 3 -CuCl 2 /AC. The advantages of using γ-Al 2 O 3 as a support are provided by its mechanical strength and mesoporous structure. Some zeolite-supported Pd catalysts [326][327][328] are also good for acetylene hydrochlorination.
Another important trend in the acetylene hydrochlorination catalysis involves ruthenium. The calculated activation barriers for acetylene hydrochlorination over HgCl 2 , AuCl 3 and RuCl 3 constitute 16.3, 11.9, and 9.1 kcal mol −1 , respectively [329]. Certain advantages of Ru catalysts make them a proper alternative to other metal catalysts.
Catalytic performance of Ru-catalysts including monometallic Ru, bimetallic Ru-Cu and Ru-Co with spheric active carbon (SAC) as the support was studied [330]. C 2 H 2 conversion and the selectivity to vinyl chloride (VCM) over monometallic and bimetallic Ru-catalysts are depicted at   The 1%Ru/SAC catalyst is capable of 91% acetylene conversion at 48 h on stream. Addition of cobalt significantly enhances its catalytic activity leading to 95% acetylene conversion at 48 h on stream, whereas addition of copper results in lower acetylene conversion. A remarkable 99.9% VCM selectivity at 95.0% acetylene conversion has been shown by RuCl 3 /MWCNT catalyst under 170 • C, C 2 H 2 GHSV= 90 h −1 , and V(HCl)/V(C 2 H 2 ) = 1.1 for 10 h [331].
Among a variety of non-noble metals, Cu occupies a worthy position due to its low cost and good thermostability. A catalyst with 400 ppm Ru and 4.24 wt % Cu on carbon nanotubes was highly active (TOF = 1.81 min −1 ) due the synergistic effect between Cu and Ru [333]. Despite the slower acetylene hydrogenation rates in comparison with AuCl 3 catalysts, this catalyst is superior to other metal complexes known to catalyze this reaction ( Figure 32). Nitrogen-doping of AC [334] and its chemical modification by consecutive nitration, amination and reaction with pyridine [335] were also performed to improve the activity and stability of Ru-based catalysts.

Emerging Fundamental Concepts for Alkynes Transformations
As has been shown in the previous sections (2.1-2.9), the chemistry of acetylene is diverse and occupies a unique position in organic synthesis. Undoubtedly, the synthetic utility of acetylene will be further expanded to a number of new opportunities. Effective translation of new concepts from the emerging areas of alkyne chemistry plays an important role and essentially increases synthetic potential of acetylene. Examples of useful concepts from the area of substituted alkyne chemistry will be briefly discussed in this section. It should be noted, that no detailed coverage of alkyne chemistry will be provided in this section. Instead, it provides a representative selection of recent examples as conceptual highlights. The key emphasis is made on efficient increase in molecular complexity, which is a priority goal in modern organic chemistry [337].
The orthogonality of the two π-orbitals in alkynes can be used for the metal-free cascade transformations [338]. The existing examples include ionic chemistry of neutral hydrocarbons, preparation of radicals without radical initiators, generation of excited states without light, "1,2-dicarbene reactivity" of alkynes in "boomerang" radical processes, selective conversion of alkynes into carbonyl compounds, and full disassembly of the alkyne moiety ( Figure 33) [338]. This feature provides an access to a wide scope of organic molecules.  Cascade reactions with polyacetylenic derivatives have been recently described as an effective one-pot route to increased molecular complexity [338,339]. Interestingly, a minor activating influence on a triple bond of the polyacetylene molecule is sufficient to initiate the avalanche-like process with all the other reactive centers (Figure 35). The use of metal catalysis or a radical initiator triggers the cascade process and directs the pathway toward polyaromatics [340,341].
The methodology is advantageous since it requires the minimal quantities of a catalyst or other initiator. This advantage is explained by the specific properties of triple bond mentioned above (see Figure 34); the mechanism has been discussed in the literature [341]. Compatibility of the cascade reactions with Sonogashira cross-coupling and other transformations [342,343] makes them relevant for fine organic synthesis. Although this concept of increasing molecular complexity was not yet applied to the unsubstituted acetylene, we believe that its potential should be thoroughly evaluated. Synthesis of macrocyclic polyethynes is another way of rapid assembling of complex molecular structures, especially in combination with the involvement of energy-reach macrocycles in further transformations. In a recent article, an example of successive phenyl acetylene 168 polymerization was described [344]. By subsequent propagation-insertion on a catalyst 169, a cyclic polyethyne 170 of M n = 45,600 Da was obtained in a high yield (Scheme 95). Since the cyclic polymers have no terminal groups, they show a number of unique physical properties [344]. For example, the density [345], refractive index [346,347], glass-transition temperature [348], viscoelasticity [349] and surface properties [350] of cyclic polymers differ from those of their linear analogues. Partial replacement of alkynes with acetylene allows obtaining a unique cyclic copolymer material, which is worth studying. Other potential applications of acetylene are related to additive manufacturing technologies. These technologies have been dramatically upgraded in recent years, due to the progress in research and industrial development. 3D printing provides a rapid link from computer-designed digital models to prototyping/production of materials and objects of desired shape and composition. 3D printing at the molecular level seems promising for direct construction of complex molecules; however the idea is highly challenging. To pump it up to the level of technology, it would be necessary to select a scope of simple molecules (versatile, accessible and modifiable) to be used as building blocks. The simplest low-molecular weight hydrocarbons-methane, ethylene and acetylene-can be certainly considered. The key restrictions for molecular 3D printing with hydrocarbons are related to the limited possibilities of the C-H-activation and atom-economical modification. By contrast with more saturated candidates, acetylene is capable of being engaged in two atom-economical addition reactions; it has a relatively reactive hydrogen atom at each of the carbons and no unreactive C-H bonds (Table 4) [351]. Hence, it is highly probable that alkyne molecules will serve as a main building material for molecular 3D printing [351]. Table 4. Potential chemical building blocks for a molecular 3D printer [351]. In this section, only few examples are shown to illustrate some important directions of acetylene chemistry. Given the rapidity of contemporary progress, many other possibilities of acetylene utilization may arise in the nearest future.

Conclusions
Chemistry of alkynes is a well-developed area, which represents a fundamental tool for organic synthesis. In contrast with other alkynes, the use of acetylene in research practice is rather limited, mostly due to the technical difficulties of handling gaseous acetylene and for the concomitant safety reasons. Circumvention of these obstacles would undoubtedly boost the use of acetylene in organic synthesis. A corresponding rapid trend observed in recent years favors a renaissance of acetylene chemistry, with full actualization of its rich potential.
Literature analysis carried out in the present review shows a wide scope of chemical transformations, which relate acetylene with such processes as vinylation, cross-coupling reactions, access to substituted alkynes and synthesis of heterocycles. These transformations provide an access to new drugs, pharmaceutical substances, biologically active molecules and valuable building blocks (synthones). Safe and reliable procedures for preparation of vinyl derivatives are in high demand, as they allow production of monomers for the synthesis of polymers and copolymers with unique properties. Particularly tempting are applications of acetylene-derived functionalized molecules in materials science.
Besides the noticeable progress in acetylene chemistry, several problems still remain a challenge. Of particular importance is the development of convenient practical procedures with high safety standards to utilize acetylene-based syntheses in everyday laboratory practice. Another challenging topic concerns the development of Green reactions involving acetylene, directed towards minimization of wastes and increased cost-efficiency.
Exceptional reaction capacities of acetylene are associated with several different reaction channels and complex mechanisms. Comprehensive understanding of the mechanisms of organic transformations of acetylene is one of the primary challenges. Even the well-known acetylene transformations (for example, selective hydrogenation or hydrochlorination) have rather complex mechanisms, which must be taken into account when developing chemical procedures and industrial technologies.
Considered in the present review, the current research trends clearly show that the great potential of acetylene molecule is far from being completely utilized, and a number of fascinating studies can be anticipated in the nearest future. Translation of new basic findings from alkyne chemistry to applied fields is of special importance. Cutting-edge research projects reinforce a dedicated search for effective routes of transforming acetylene into the products of increasing molecular complexity. This direction will remain a top priority in the nearest future.