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Review

Syntheses, Structures and Reactivity of Metal Complexes of Trindane, Trindene, Truxene, Decacyclene and Related Ring Systems: Manifestations of Three-Fold Symmetry

by
Philippa E. Lock
,
Nada Reginato
,
Julia Bruno-Colmenárez
and
Michael J. McGlinchey
*
School of Chemistry, University College Dublin, Belfield, D04 V1W8 Dublin, Ireland
*
Author to whom correspondence should be addressed.
Molecules 2023, 28(23), 7796; https://doi.org/10.3390/molecules28237796
Submission received: 24 October 2023 / Revised: 20 November 2023 / Accepted: 23 November 2023 / Published: 27 November 2023
(This article belongs to the Section Organometallic Chemistry)
Browse Figures
Review Reports Versions Notes

Abstract

:
The triple condensation of cyclopentanone or indanone to trindane (C15H18) or truxene (C27H18), respectively, provides convenient access to molecular skeletons on which major fragments of the prototypical fullerene C60 can be assembled. In particular, early approaches (both organic and organometallic) towards sumanene, as well as the final successful synthesis, are described. Organometallic derivatives of trindane have been prepared in which Cr(CO)3, Mo(CO)3, [Mn(CO)3]+ or [(C5H5)Fe(CO)2]+ are η6-bonded to the central arene ring. The debromination of hexabromotrindane yields trindene, which forms a tri-anion to which as many as three organometallic fragments, such as Mn(CO)3, W(CO)3Me, or Rh(CO)2, may be attached. Truxene forms complexes whereby three metal fragments can bind either to the peripheral arene rings, or to the five-membered rings, and these can be interconverted via η6 ↔ η5 haptotropic shifts. Truxene also forms a double-decker sandwich with Ag(I) bridges, and decacyclene, C36H18, forms triple-decker sandwiches bearing multiple cyclopentadienyl-nickel or -iron moieties. The organic chemistry of trindane has been investigated, especially with respect to its unexpectedly complex oxidation products, which were only identified unambiguously via X-ray crystallography. The three-fold symmetric trindane framework has also been used as a template upon which a potential artificial receptor has been constructed. Finally, the use of truxene and truxenone derivatives in a wide range of applications is highlighted.

1. Introduction

The cyclopentadienide fragment is a component of many polycyclic systems, and their metal complexes are very numerous [1]. We focus here on the syntheses, structures and reactivity of three-fold symmetric ligands, such as trindane or trindene, and related frameworks whereby the molecular periphery has been augmented via the incorporation of additional benzo rings, as in truxene or decacyclene.
Trindane, tris(cyclopenteno)benzene, 1, first reported by Wallach in 1897 [2], is commonly prepared via the acid-catalysed condensation of three molecules of cyclopentanone [3], and was characterised via X-ray crystallography in 1964 [4]. Other trimerisation routes include the use of SiCl4 in ethanol at room temperature [5], TiCl4 in refluxing ethanol [6], or aqueous NH4Cl at high temperatures [7]. The bromination of trindane via the photolysis of Br2 in CCl4 gave hexabromotrindane, 2, [8,9], which upon debromination with zinc furnished both isomers of dihydro-1H-trindene, 3. Subsequent multiple deprotonation (Scheme 1) generated the trindene trianion, 4, ideally poised for reactions with a range of organometallic fragments.

2. Organometallic Derivatives of Trindane

2.1. Trindane Complexes of Chromium, Molybdenum, Manganese and Iron

The earliest report of an organometallic derivative of 1 appears to describe the formation of a series of cationic complexes of the type [99mTc(arene)2]+,, where the arene was C6H6, C6Me6, C6Et6, indane, trindane, etc. [10]. These materials were injected into rats to study the biodistribution of radioactive technetium to test their viability as myocardial imaging agents. These studies were, out of necessity, carried out on trace quantities of material, and no analytical or spectroscopic data were reported for [99mTc(trindane)2]+ [PF6]−, 5. In terms of the first fully characterised derivatives, complexes of the type (η6-trindane)MLn, where MLn = Cr(CO)3, 6, Mo(CO)3, 7, [Mn(CO)3]+, 8, or Fe(C5H5)+, 9, were each prepared via the reaction of trindane with an appropriate organometallic precursor (Scheme 2) [11].
(η6-Trindane)Cr(CO)3, 6, forms yellow crystals, the structure of which appears in Figure 1, and reveals that the tripodal moiety is oriented such that the three carbonyl ligands are staggered with respect to the cyclopentenyl rings. These five-membered rings adopt envelope conformations such that the three methylene “wingtips” are folded in an endo fashion relative to the metal [11]. This may be compared to the structures of the previously known molecules [tris(cyclohexene)benzene]MLn, where MLn = Cr(CO)3 or [Mn(CO)3]+, 10, which also crystallise in a staggered tripodal orientation, but their peripheral six-membered rings exhibit conventional half-chair conformations [12].
It was noteworthy that in the mass spectra of the neutral complexes of (η6-trindane)M(CO)3, where M = Cr or Mo, peaks with m/z values corresponding to those of [(trindane)2M2(CO)3]+ were observed, leading to the speculation of the formation of triple-bridged systems of the type [(η6-trindane)M(μ-CO)3M(η6-trindane)]+, 11, entirely analogously to known 30-electron systems such as (η5-C5Me5)Re(μ-CO)3Re(η5-C5Me5) [13].
Molecules 28 07796 i001
Trindane itself exhibits a very simple 1H NMR spectrum, i.e., a triplet (12H) for the benzylic protons and a quintet (6H) for the wingtip methylene groups. The incorporation of a π-complexed organometallic fragment renders the faces of the trindane ligand inequivalent, thus giving rise to four 1H NMR environments in complexes such as 6 through 9. The benzylic protons (each 6H) are readily distinguished from the resonances for the wingtip methylenes (each 3H) by their relative intensities, but their assignment to exo or endo positions is less trivial. However, the X-ray data for the chromium complex, 6, provide a rational means of assigning these resonances. Figure 2 illustrates the dihedral angles between a pair of benzylic hydrogens and those of the neighbouring wingtip methylene group within a five-membered ring. In all cases, the endo-benzylic hydrogen and its adjacent exo-wingtip hydrogen make a dihedral angle of approximately 90°, which is typical of di-equatorial interactions, which, in accordance with the Karplus relationship relating dihedral angles to 3JH-H values, exhibit a rather small vicinal coupling constant (4–5 Hz). In contrast, the diaxial interaction between the exo-benzylic hydrogen and its endo-wingtip counterpart leads to a noticeably larger 3JH-H value (~12 Hz), thus allowing a straightforward assignment of all the resonances.

2.2. Trindane Complexes of Ruthenium

An arene exchange reaction between [(p-cymene)RuCl2]2 and molten trindane was attempted and, gratifyingly, [(trindane)RuCl2]2 was produced in a 85% yield; its structure appears in Figure 3. In the solid state, the complex adopts the arrangement whereby the two ruthenium atoms are linked by two bridging chlorines, as in 12a, and each also possesses a terminally bonded chlorine. However, in solution, 1H-1H and 1H-13C two-dimensional NMR data clearly reveal the existence of a second species, the triple-bridged ionic isomer [(η6-trindane)Ru(μ-Cl)3(η6-trindane)]Cl, 12b, formed via the loss of a chloride ligand. These isomers are in a temperature-dependent equilibrium such that the ratio of 12a to 12b is 30:70 at −50 °C in CD2Cl2, but at room temperature in nitromethane CD3NO2 (which would surely favour the ionised species), this ratio changes dramatically to 1:10 [14].
We note parenthetically that hexaethylbenzene (HEB) may be thought of as an “unrestricted” trindane in which the wingtip methylene groups of the cyclopentenyl rings are now untethered, thus leaving them free to rotate. It was shown that the analogous ruthenium complex of HEB, i.e., [(HEB)RuCl2]2, also exists as an interconverting double- and triple-bridged species, but in this case both the double-bridged neutral species, 13a, analogous to 12a, and also the D3h-symmetric cation, 13b, analogous to 12b, together with its [C5(CO2Me)5]− counter-anion, have been characterised via X-ray crystallography (Figure 4) [15,16,17].
When [(trindane)RuCl2]2, 12, was allowed to react with excess trindane in the presence of AgBF4, the sandwich compound [(η6-trindane)2Ru]2+ 2[BF4]−, 14, was isolated and fully characterised spectroscopically [14]. This molecule is, of course, the ruthenium analogue of the [99mTc(trindane)2]+ [PF6]− complex, 5, claimed, but never unambiguously identified, in the medicinal study of potential radio-labelled myocardial imaging agents noted above [10]. Finally, it was found that treatment of 12 with trimethyl phosphite delivered the anticipated monomeric species (η6-trindane)RuCl2[P(OMe)3], 15, the structure of which is shown in Figure 5.

3. Organometallic Derivatives of Trindene

As noted above, Katz and Ślusarek prepared the trindene trianion, 4, which was then allowed to react with ferrous chloride to form bis(trindene)diiron, 16, as a red-brown material, along with traces of a tri-iron complex [8]. NMR data suggested that that the hydrocarbon rings in 16 are disposed as anti, as depicted in Scheme 3.
Almost four decades later, the first triple-metallocene derivative of trindene was prepared via the exchange reaction of the potassium salt of 4 with [Fe(C5H5)(η6-fluorene)]PF6 to yield the syn,syn,syn- and syn,syn,anti-isomers of (η5:η5:η5-trindenyl)[Fe(C5H5)]3, 17a and 17b, respectively, in a 1:3 ratio. The latter molecule was characterised via X-ray crystallography (Figure 6), which clearly showed a notable deviation of two five-membered rings in the trindene skeleton from planarity caused by the need to relieve repulsive non-bonding interactions between the syn ferrocenyl groups [18].
The organometallic chemistry of trindene was considerably extended by Lynch and Rheingold, who successfully characterised a series of trimetallic derivatives bearing manganese- or rhenium-tricarbonyl fragments. The treatment of dihydro-1H-trindene, 3, with KH and either Mn(CO)3(py)2Br (py = pyridine) or [Re(CO)3(THF)Br]2 delivered (η5:η5:η5-trindenyl)[M(CO)3]3, where M = Mn or Re, 18 or 19, respectively (Scheme 4). As was found in 17b, the X-ray crystal structure of 19 (Figure 7) revealed that the metals were situated in a syn,syn,anti-fashion such that the five-membered rings bearing the cis rhenium units were bent away from each other and out of the central plane by ~10°, and that the Re(CO)3 groups exhibited maximal staggering of their carbonyl ligands [19]. It was also established that when only two metal carbonyl moieties were introduced, they adopted a trans geometry, 20, thus facilitating the introduction of a third different substituent, thereby forcing the cis disposition of the heterometals, as in the (trindenyl)Re2Rh(CO)8 complex, 21.
In these systems, two of the metal fragments are necessarily positioned proximate to each other on the same face, and this feature has been exploited in a series of tri-molybdenum or tri-tungsten complexes. When the trindene trianion, 4, was treated with Mo(CO)6 and then with methyl iodide, the major product was syn,syn,anti-(η5:η5:η5-trindenyl)[Mo(CO)3Me]3, 22, along with a lower quantity of a material, 23, in which the two adjacent molybdenum atoms had lost their methyl substituents and formed a metal–metal bond. The tungsten congener behaved similarly, and the structure of the corresponding benzyl analogue, 24, possessing a tungsten–tungsten linkage was verified via X-ray crystallography, as shown in Figure 8 [20].
The electrochemical behaviour of a number of these trimetallic derivatives of trindene was studied via cyclic voltammetry (CV). It was found that the complexes (trindenyl)(MLn)3, where MLn = Mn(CO)3, 18, or Rh(1,5-cyclooctadiene), 25, showed three well-defined, reversible one-electron oxidation reactions. In the rhodium case, the observed formal potentials are −0.33, −0.16 and +0.43 V, for the couples 25–25+, 25+–252+ and 252+–253+, respectively, and this abnormally large separation in potentials has been interpreted in terms of a Class III (totally delocalised) system [21]. There is also a report of the tri-rhodium complex, (η5:η5:η5-trindene)[Rh(1,5-COD)]2Rh(CO)2, 26, whose structure is shown in Figure 9 [22]. Since the two Rh(COD) fragments occupy anti positions, one can assume that they were coordinated initially, and the Rh(CO)2 unit was added subsequently.
The CV scan of the triferrocenyl system 17b proceeds in two well-defined and reversible (chemically and electrochemically) one-electron steps with values of 0.23 and 0.61 V, for the first and second oxidations; however, this is followed by a chemically irreversible process (0.94 V) showing that the trication precipitates on the Au electrode [18]. Finally, we note that the charge transfer properties of multi-ferrocenyl dihydro-1H-trindenes, 27, have been investigated [23].
Molecules 28 07796 i002

4. Metal Complexes of Truxene

Using the analogy of the self-condensation of cyclopentanone to form trindane, the analogous reaction of indanone yields C3h-symmetric truxene, C27H18, 28. It was first obtained accidentally by Hausmann in 1889 [24] during the synthesis of indanone from 3-phenylpropanoic acid, but the mechanism involving the intermediate tetracyclic ketone shown in Scheme 5 was only elucidated many years later [25].
The deprotonation of truxene with KH in the presence of Mn(CO)3(py)2Br yielded truxene–manganese adducts with one, two or three Mn(CO)3 units bound to five-membered rings of the truxenyl ligand, whereby in (η5:η5:η5-truxenyl)[Mn(CO)3]3, only the syn,syn,anti-isomer, 29a, was obtained. However, these products can also be accessed via η6-to-η5 haptotropic shifts of arene-coordinated [Mn(CO)3]+ fragments in [(η6:η6:η6-truxenyl){Mn(CO)3}3]3+, which is formed as both the syn,syn,anti- and all-syn isomers, 30a and 30b, respectively; this provides the only route to the all-syn isomer 29b (Scheme 6) [26].
Truxene also reacts with silver perchlorate to form the dimeric sandwich compound [(μ2:μ2-truxene)2(η1-toluene)(H2O)Ag2(ClO4)2], 31, in which the truxenes are linked by Ag(I) bridges, each bonded in a dihapto fashion to peripheral arene rings, as depicted in Figure 10 [27].

5. Metal Complexes of Decacyclene

Decacyclene, C36H18, 32, was first obtained by Dziewonski in Krakow, Poland, in 1903 upon the dehydrogenation of acenaphthene with elemental sulphur at 205–295 °C [28]. It differs from truxene in terms of the incorporation of additional peripheral benzo rings, thus apparently recovering the D3h symmetry of its trindenyl central core. However, the X-ray structure of decacyclene revealed it to be a shallow molecular propeller of D3 symmetry as the result of non-bonded repulsions between hydrogens of the peripheral naphthalene groups. This feature is clearly illustrated in the side view of 32 depicted in Figure 11. Interestingly, crystals of decacyclene also have a helical morphology, which is most pronounced when grown from solutions in organic solvents [29].
A more elegant route (Scheme 7) has been reported that takes advantage of the palladium-catalysed trimerisation of strained cycloalkynes. The caesium fluoride-initiated elimination of fluorotrimethylsilane and triflate from the disubstituted acenaphthene, 33, brought about the in situ formation of acenaphthyne, 34, which underwent cyclisation to form decacyclene in a 23% yield [30].
Several triple-decker metal complexes of decacyclene have been reported from the Schneider group in Essen, Germany. The reduction of decacyclene in THF with potassium metal led to a red-brown solution containing polyanionic species that were allowed to react with organometallic reagents. Treatment with (η5-C5Me4Et)Ni(η2-acac) furnished black crystals of [(η5-C5Me4Et)Ni]2(μ-η3:η3)decacyclene], 35, in which the two organo-nickel fragments were each bonded in an η3-fashion to opposite faces of the central arene ring (Figure 12) [31].
The analogous reaction with (η5-C5Me4Et)FeCl(tmeda), where tmeda is N,N,N′,N′-tetramethylethylenediamine, yielded two black crystalline products with the structures [(η5-C5Me4Et)Fe]2(μ2-η6:η6)decacyclene], 36, and [(η5-C5Me4Et)Fe]4(μ2-η6:η6:η6:η6)decacyclene], 37. In the former case, the organo-iron fragments were attached to opposite faces of the same naphthalene unit, as shown in Scheme 8. Metal complexation in 37 occurs in an alternating up/down fashion on two naphthalene moieties, and the molecule adopts a gently twisting propeller geometry in accordance with the topology of the decacyclene skeleton (Figure 13). It is notable that the Fe-C distances to the bridging carbon atoms are longer than those to the others since the two electrons connecting them must be shared between the two metals, thereby satisfying the 18-electron rule [32].

6. Towards Sumanene

6.1. The Pyrolytic Approach

Although C60, with its “soccer-ball” icosahedral (Ih) symmetry, is preparable via the evaporation of graphite under appropriate conditions, and is now commercially available, attempts to develop logical stepwise synthesis continue to be explored. Two major components of the C60 skeleton are corannulene C20H10, 38, which possesses a central pentagon surrounded by five six-membered rings, and sumanene C21H12, 39, which has alternating five- and six-membered rings around a central hexagon; both are illustrated in Figure 14. Corannulene is now available in kilogram quantities, thanks to the pioneering contributions of Larry Scott (in Nevada) and Jay Siegel (in Zurich), and this work has been comprehensively reviewed [33,34,35,36]. However, in 1997, when the organometallic chemistry of trindane was still in its early stages, no route to sumanene had been reported, and success was only achieved some years later (see below).
The bowl-shaped C3v-symmetric molecule sumanene, with its central six-membered ring surrounded by alternating five- and six-membered rings, was first postulated by Mehta in 1993; its name is derived from the Sanskrit word suman, which translates into “flower”, whereby the ring edges are considered to resemble petals. The earliest synthetic approach (Scheme 9) involved the palladium-mediated thermolysis of C3h-symmetric 1,5,9-trimethyltriphenylene, 40, in an attempt to bring about multiple dehydrogenations and subsequent ring closures; however, only the mono-bridged product, 41, was formed. Under even more forceful conditions, the flash vacuum pyrolysis (FVP) of 1,5,9-tri(bromomethyl)triphenylene, 42, led to the double-bridged species 43, which was characterised via X-ray crystallography [37].
These experimental observations validate the computational results of Priyakumar and Sastry, which revealed that all these cyclopentenation steps lead to an increase in strain, with the third endothermic step leading to an increase of more than 50 kcal/mol, making the final ring closure almost impossible. In contrast, starting from trindane (thereby incorporating the five-membered rings early on in the procedure) and then generating the six-membered peripheral rings subsequently is a thermodynamically favoured process [38,39].

6.2. The Proposed Organometallic Approach

Retrosynthetic analysis (Scheme 10) suggested a strategy starting from the cationic species [(trindane)Mn(CO)3]+, 8, whereby multiple deprotonation at the benzylic positions with subsequent attack by an electrophile could lead to a system, 44, bearing six bromomethyl substitutuents. The attachment of the organometallic fragment to the central arene ring would be expected to block that face and thereby favour exo attack by incoming electrophiles. Ring closure to form the bromosulfide 45, oxidation to the corresponding sulfone, followed by a Ramberg–Bäcklund rearrangement (a standard technique for the formation of small rings [40]) of the triple halosulfone, 46, should result in the elimination of HBr and SO2 and the direct formation of the new alkene moieties, leaving only a final dehydrogenation step.
This approach was predicated on earlier work by Astruc [41] on the multiple successive deprotonations at benzyl positions in [(η6-C6Me6)Fe(C5H5)]+, 47, to initially form (η5-Me5C6=CH2)Fe(C5H5), 48, which reacted with a range of electrophiles. This concept was further developed by Eyman [42] using the isoelectronic manganese system [(η6-C6Me6)Mn(CO)3]+, 49, as exemplified in Scheme 11. Moreover, it was found that substitution of a carbonyl ligand by a phosphine, as in [(η6-C6Me6)Mn(CO)2PMe3]+, increases the electron density on manganese and enhances the nucleophilicity of the exocyclic methylene group [43].
To test this approach, [(trindane)Fe(C5H5)]+, 9, was deprotonated by t-BuOK in the presence of excess methyl iodide and yielded a product mixture in which up to twelve benzylic positions had been substituted, as shown in Scheme 12. The analogous reaction with allyl bromide also delivered the dodeca-substituted material.
In an attempt to hinder the approach by an electrophile to the endo face of the polycyclic ligand, the cyclopentadienyl-iron fragment was replaced by the more voluminous tricarbonylmanganese moiety. However, when [(trindane)Mn(CO)3]+, 8, was deprotonated by t-BuOK in the presence of excess allyl bromide, a red crystalline product, obtained in 17% yield after chromatographic separation, was identified, surprisingly, as (trindane)Mn(CO)2Br, 50 (Scheme 13). This material is also readily prepared, in a 93% yield, via the reaction of 8 with trimethylamine N-oxide (to bring about the loss of a carbonyl ligand) and a subsequent reaction with n-Bu4N+Br−. Likewise, the reaction of 8 with t-BuOK and methyl iodide produced (trindane)Mn(CO)2I, 51 (15%). The X-ray crystal structures of 50 and 51, shown in Figure 15, reveal that, as in (trindane)Cr(CO)3, 6, the tripodal moiety is oriented such that its ligands are staggered with respect to the cyclopentenyl rings, which adopt envelope conformations with endo-folded wingtip methylene groups [44].
This contrasts the behaviour of [(C6Me6)Mn(CO)3]+, for which the displacement of a carbonyl ligand to form (C6Me6)Mn(CO)2X, where X = Cl, Br or I, requires either photolysis or a reaction with Me3NO in the presence of NaX [45]. It is also noteworthy that the treatment of (C6Me6)Mn(CO)2Cl with t-BuLi yields (C6Me6)Mn(CO)2H, 52, apparently via the elimination of isobutene from the presumed tert-butyl intermediate (Scheme 14). However, this hydride, which is sufficiently stable for its structure to be corroborated via X-ray crystallography [46], is more conveniently prepared via the reaction of (C6Me6)Mn(CO)2I with (n-Bu)4N+ [BH4]−; furthermore, 52 reacts readily with CCl4 or CHCl3, but not with CH2Cl2 to form (C6Me6)Mn(CO)2Cl [47].
In light of these data, one can postulate a mechanism for the formation of (trindane)Mn(CO)2I upon the treatment of 51 with t-BuOK in the presence of methyl iodide. Since [(C6Me6)Mn(CO)3]+ is known to react with methanol to form the rather unstable ester (C6Me6)Mn(CO)2CO2Me [48], one can readily envisage the formation of the tert-butyl ester, 53, which readily eliminates isobutene and carbon dioxide via a very favourable six-membered transition state to produce (trindane)Mn(CO)2H, 54, as in Scheme 15. Treatment of 8 with t-BuOK in CH2Cl2 furnished (trindane)Mn(CO)2Cl, suggesting that 54 is more reactive than (C6Me6)Mn(CO)2H towards alkyl halides.
Since numerous attempts to prepare 54 via the treatment of (trindane)Mn(CO)2Br with (n-Bu)4N+ [BH4]−, or to detect the hydride signal via NMR were unsuccessful, an attempt was made to trap the purported metal hydride either as (trindane)Mn(CO)(PR3)H, or as the formyl complex (trindane)Mn(CO)(CHO)(PR3). When the cation 8 and tert-BuOK were treated with trimethyl phosphite in THF and kept at 40 °C for 20 h, the products isolated after chromatographic separation were yellow crystalline materials 55 and 56, the 1H and 13C NMR data of which indicated that the three-fold symmetry of the trindane ligand had been broken, as shown in Scheme 16. These products were unambiguously identified via X-ray crystallography as η5-indenyl complexes (Figure 16) in which the manganese had migrated from the central arene onto a five-membered ring that had evidently lost three hydrogens. The reaction with triphenylphosphine behaves analogously to give 57 [49].
In seeking a precedent for such behaviour, we note that King reported the reaction of [(η6-indane)Mn(CO)3]+ with t-BuOK and a phosphine to yield (η5-indenyl)Mn(CO)2L, where L = P(OMe)3 or PPh3 [50]. Furthermore, Crabtree and Parnell found that [(η6-indane)Ir(PPh3)2]+ underwent dehydrogenation and an η6-to-η5 haptotropic shift to yield [(η5-indenyl)Ir(PPh3)2(H)]+ [51]. Moreover, Ustynyuk and co-workers have shown that (η5-indenyl)Cr(CO)3Me, 58, undergoes a “ricochet reaction” in which the methyl is delivered to the five-membered ring and the tricarbonylchromium moiety migrates onto the six-membered ring, probably via an isoindene intermediate, 59, (Scheme 17) [52].
To account for the formation of the η5-bonded rearrangement complexes 55 and 56, we suggest that, in the absence of an alkyl halide, the initially generated (trindane)Mn(CO)2H, 54, undergoes hydrogen migration from an endo-benzyl site onto the metal, thus producing the cyclohexadienyl complex, 60, which in turn loses dihydrogen to give 61. A second round of endo-benzyl hydrogen migration to yield the isoindene framework, 62, is followed by the final round of hydrogen migration, producing 63. The loss of dihydrogen and the incorporation of either a carbonyl or phosphite ligand would give the observed products, 55 and 56, respectively, as depicted in Scheme 18 [49].

7. The Synthesis of Sumanene

The synthesis of sumanene (Scheme 19) was finally achieved by Sakurai, Daiko and Hirao, who prepared a bromo-lithio derivative of norbornadiene, 64, that was converted into the corresponding tin compound 65. The copper-mediated trimerisation of 65 gave a 3:1 mixture of anti and syn benzotris(norbornadiene), 66, the latter of which underwent metathesis with the Grubbs catalyst to produce hexahydrosumanene, 67. The process was completed upon oxidation with DDQ to deliver sumanene, 39, with no need for a flash vacuum pyrolysis step [53]. This work has since been comprehensively reviewed recently [54,55].

8. Towards C60

8.1. From Decacyclene

As we know, the first well-characterised fullerene, C60, is now commercially available, but efforts towards a rational stepwise synthetic procedure continue to unfold [36]. As noted above, the chemistry of corannulene and sumanene have been well explored and numerous derivatives have been reported. Nevertheless, other elegant approaches based on three-fold symmetry have been disclosed. In particular, we note work by Scott, a major pioneer in the fullerene field, starting from decacyclene (Scheme 20). Having successfully prepared 8-chloro-1(2H)-acenaphthylenone, 68, this was trimerised using TiCl4 to produce 3,9,15-trichlorodecacyclene, 69, which upon FVP treatment at 1100 °C delivered a geodesic dome in a 27% yield with the formula C36H12, representing 60% of C60, and was given the trivial name Circumtrindene, 70 [56]. The structure was confirmed via X-ray crystallography [57], and Figure 17 illustrates this molecule as a component of the C60 framework.

8.2. The Designed Stepwise Synthesis of C60

The three-fold cyclisation of ketones, such as that of cyclopentanone to trindane, or indanone to truxene, is mirrored by the route to trichlorodecacyclene, 69, from 8-chloroacenaphthylenone, 68. Continuing with this approach, an even more audacious experiment by Scott and de Meijere sought to prepare C60 directly via the trimerisation of a C20 ketone. As shown in Scheme 21, the treatment of the pentacyclic ketone 71 with TiCl4 in refluxing o-dichlorobenzene led to the C3h-symmetric molecule, 72, with the formula C60H27Cl3. They were hoping to effect multiple dehydrohalogenations and dehydrogenations to bring about a “stitching together” of the three arms of the molecule via FVP at 1100 °C, thus forming C60 directly. Gratifyingly, this strategy was successful, and the only fullerene formed was C60; though in a very low yield, it was sufficient to be unambiguously characterised via mass spectrometry [58].

9. Organic Chemistry of Trindane

9.1. Oxidation of Trindane

The reported organic chemistry of trindane has focused primarily on its reactivity when treated with different oxidants. Ruthenium trichloride and sodium periodate, in situ, generate a ruthenium(VIII) species, “RuO4”, which reacted with trindane to bring about an unanticipated carbohydrate-like product, 72, the structure of which was elucidated via X-ray crystallography. One can envisage a mechanism whereby one of the double bonds in the aromatic ring is doubly hydroxylated to yield 73, and the subsequent cleavage of a second double bond forms a diketone that upon further oxidation and hydrolysis rearranges to deliver the final product (Scheme 22) [59].
In contrast, the ozonolysis of trindane led to 74, which again was only unequivocally characterised via X-ray crystallography (Scheme 23). Under these conditions, the initial cleavage of two double bonds of the central aromatic ring forms the tetraketone, 75, that is in equilibrium with its enol tautomer, 76, and cyclisation leads to the final product. The retention of the C15 periphery in 74 resembles the structure of some natural products, such as ginkgolides, and demonstrates that the cleavage of the π-bond endo to the cyclopentane present in trindane provides a simple route to complex natural products [60].
More recent work has revealed that the result of the Ru(VIII) oxidation of the next higher homologue of trindane, dodecahydrotriphenylene, 77, which is readily prepared via the cyclisation of cyclohexanone, differs from that found with trindane. In this case, the central aromatic ring remains intact and the products arise from benzylic oxidation to the mono-, di- and tri-ketones 78, 79 and 80, respectively, depending on the length of the reaction time (Scheme 24) [61]. Interestingly, the products maintain their directionality such that the ketones are arranged unidirectionally (note that the cyclohexanone rings adopt their normal non-planar geometry).

9.2. Synthesis of a Potential Artificial Receptor

In a particularly elegant example that takes advantage not only of the three-fold symmetry of trindane, but also of the ability of an organometallic fragment to block one face of the arene, these concepts were exploited to investigate the construction of artificial receptors. As illustrated in Scheme 25, the hexaester 81 was prepared from 1,3,5-tris(bromomethyl)-2,4,6-tris(chloromethyl)benzene via a reaction with the sodium enolate of diethyl malonate, whereupon saponification, decarboxylation and esterification produced a mixture of cis,cis,cis- and cis,cis,trans-trindane-2,5,8-tricarboxylic ester, 82. However, the treatment of the triester with either Cr(CO)6 or Mo(CO)6 to form 83 allowed the isolation of the all-syn isomer in a good yield. A subsequent reaction with LDA and benzyl bromide proceeded exclusively via exo attack to form the desired C3v-symmetric product. The subsequent removal of the metal carbonyl tripod with iodine to form 84, and further elaboration led to the urea derivative, 85, which was investigated for its anion-binding capability [62].

10. Applications of Truxene and Truxenone

Truxene, 28, with its planar C3h symmetry, is an important building block for dendrimers, organic frameworks, and star-shaped, cage-like and porous molecules [63,64,65]. An early example of its application was its use in liquid crystals [66], but it has more recently found widespread utility in materials science [67], and in polymer chemistry [68], as a component of the stationary phase in capillary gas phase chromatography [69], as an organic sensitiser, or as a potential hole transport material in perovskite solar cells [70,71].
Truxenone, 86, was originally prepared directly via the acid-catalysed triple condensation of indane-1,3-dione [72,73], but the more recent five-step synthesis starting from 2-methylacetophenone (Scheme 26) proceeds under milder conditions and is more tolerant to sensitive substituents [74].
The ready availability of truxenone has led to the syntheses of a very large number of functionalised derivatives, many of which have been characterised via X-ray crystallography. A common feature that emerges in such structural determinations is the loss of planarity caused by steric crowding between the dicyanomethylene substituents at positions 5, 10 and 15 and the neighbouring peripheral aromatic rings. Typically, as shown in Figure 18, truxene maintains its C3h symmetry [75], whereas in the tris(dicyanomethylidene) system, 87, prepared via Knoevenagel condensation from truxenone (Scheme 27), the horizontal plane was broken, and the propeller-type molecule adopted a C3 symmetry and was chiral [76]. Truxenone has found applications in many areas ranging from organic voltaics [77], and n-type semiconductors [78], to serving as the framework for the cathode in a solid-state lithium-ion battery [79].

11. Concluding Remarks

The three-fold D3h symmetry of trindane, 1, or of the trindene trianion, 4, provides a versatile framework in which chemical structure and reactivity can be controlled. When an organometallic fragment, such as M(CO)3, is π-complexed to one face of trindane, it not only enhances the acidity of the exo-benzylic hydrogens, but also protects that face from approach by electrophiles. However, nucleophilic attack on the metal carbonyl ligands, with the elimination of specific fragments (such as isobutene or CO2), can bring about the formation of a metal–hydride linkage that opens up other reaction possibilities, as exemplified in Scheme 16.
When the trindene system bears three π-complexed organometallic units, at least two of them must be sited adjacently on the same face, thereby enhancing the likelihood of electronic interaction between the metals, or even the development of a formal metal–metal bond, as in Figure 8. The addition of three peripheral benzo rings, as in truxene, maintains the three-fold symmetry of the system but also offers different types of coordination sites whereby the organometallic moiety can bind in an η5 or η6 fashion. Moreover, protonation–deprotonation sequences can bring about haptotropic shifts between these two situations, as exemplified in Scheme 6.
The formation of decacyclene apparently regains the D3h symmetry of trindenyl, but this is not in fact the case. Non-bonded repulsions between hydrogens of neighbouring naphthyl groups cause the molecule to adopt a chiral (C3) propeller geometry that is even more evident when organometallic units are attached to opposite faces of the molecule.
It has long been hoped that functionalised versions of these polycyclic frameworks could lead towards a rational stepwise route to fullerenes, or at least to sizeable portions of the C60 skeleton. This approach was initially used in early attempts to prepare sumanene, and one spectacular example of success has been the controlled coupling of adjacent peripheral naphthyl rings in decacyclene to generate circumtrindene, C36H12, which contains 60% of the C60 framework.
Finally, one can only marvel at the complexity of the oxidation products of a molecule as simple as trindane, and the multitude of important applications, such as liquid crystals, optoelectronics, solar cell technology or solid-state batteries, in which truxene or truxenone plays such a crucial role.

Author Contributions

Conceptualisation, M.J.M.; writing—original draft, P.E.L., N.R., J.B.-C. and M.J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded for many years by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Petroleum Research Fund (PRF), administered by the American Chemical Society, and by Science Foundation Ireland (SFI).

Acknowledgments

The authors thank University College Dublin and the UCD School of Chemistry for additional financial support, and the Centre for Synthesis and Chemical Biology (CSCB) for the use of analytical facilities.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 28 07796 sch001
Scheme 1. Syntheses of trindane, 1, and trindene, 3, via hexabromotrindane, 2 (CSD ID: KIKNIC).
Scheme 1. Syntheses of trindane, 1, and trindene, 3, via hexabromotrindane, 2 (CSD ID: KIKNIC).
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Molecules 28 07796 sch002
Scheme 2. Syntheses of selected (η6-trindane)–metal complexes: (i) Cr(CO)6/Bu2O/THF, (ii) Mo(CO)6/THF, (iii) BrMn(CO)5/AgBF4/CH2Cl2, and (iv) (C5H5)Fe(CO)2I/AlCl3/CHCl3/AgBF4.
Scheme 2. Syntheses of selected (η6-trindane)–metal complexes: (i) Cr(CO)6/Bu2O/THF, (ii) Mo(CO)6/THF, (iii) BrMn(CO)5/AgBF4/CH2Cl2, and (iv) (C5H5)Fe(CO)2I/AlCl3/CHCl3/AgBF4.
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Molecules 28 07796 g001
Figure 1. Molecular structure of (η6-trindane)Cr(CO)3, 6 (CSD ID: NEVVOZ), showing (left) the folding of the central wingtip methylene groups towards the metal, and (right) the staggered orientation of the tripod.
Figure 1. Molecular structure of (η6-trindane)Cr(CO)3, 6 (CSD ID: NEVVOZ), showing (left) the folding of the central wingtip methylene groups towards the metal, and (right) the staggered orientation of the tripod.
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Molecules 28 07796 g002
Figure 2. View along the C(2)-C(1) bond of (trindane)Cr(CO)3, 6, showing the dihedral angles between the hydrogens of the central (wingtip) and benzylic methylenes.
Figure 2. View along the C(2)-C(1) bond of (trindane)Cr(CO)3, 6, showing the dihedral angles between the hydrogens of the central (wingtip) and benzylic methylenes.
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Molecules 28 07796 g003
Figure 3. Dynamic exchange between (trindane)RuCl((μ-Cl)2RuCl(trindane), 12a (NOMXOC), and [(trindane)Ru(μ-Cl)3(η6-trindane)]Cl, 12b.
Figure 3. Dynamic exchange between (trindane)RuCl((μ-Cl)2RuCl(trindane), 12a (NOMXOC), and [(trindane)Ru(μ-Cl)3(η6-trindane)]Cl, 12b.
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Molecules 28 07796 g004
Figure 4. Structures of (HEB)RuCl((μ-Cl)2RuCl(HEB), 13a (CSD ID: RXB7), and of the cation [(HEB)Ru(μ-Cl)3(HEB)]+, 13b (PENTEI).
Figure 4. Structures of (HEB)RuCl((μ-Cl)2RuCl(HEB), 13a (CSD ID: RXB7), and of the cation [(HEB)Ru(μ-Cl)3(HEB)]+, 13b (PENTEI).
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Molecules 28 07796 g005
Figure 5. [(η6-Trindane)2Ru]2+ 2[BF4]−, 14, and (η6-trindane)RuCl2[P(OMe)3], 15 (NOMXUI).
Figure 5. [(η6-Trindane)2Ru]2+ 2[BF4]−, 14, and (η6-trindane)RuCl2[P(OMe)3], 15 (NOMXUI).
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Molecules 28 07796 sch003
Scheme 3. Reaction of the trindene trianion, 4, with FeCl2, to form bis(trindene)diiron, 16.
Scheme 3. Reaction of the trindene trianion, 4, with FeCl2, to form bis(trindene)diiron, 16.
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Molecules 28 07796 g006
Figure 6. Reaction of trianion, 4, with [Fe(C5H5)(η6-fluorene)]PF6 to form 17a and 17b (NODZUC).
Figure 6. Reaction of trianion, 4, with [Fe(C5H5)(η6-fluorene)]PF6 to form 17a and 17b (NODZUC).
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Molecules 28 07796 sch004
Scheme 4. Manganese, rhenium and rhodium complexes of trindene.
Scheme 4. Manganese, rhenium and rhodium complexes of trindene.
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Molecules 28 07796 g007
Figure 7. Structure of (η5:η5:η5-trindenyl)[Re(CO)3]3, 19, showing the syn,syn,anti-configuration of the metal carbonyl groups, and the maximal staggering of the tripods (CSD ID: KARKAH).
Figure 7. Structure of (η5:η5:η5-trindenyl)[Re(CO)3]3, 19, showing the syn,syn,anti-configuration of the metal carbonyl groups, and the maximal staggering of the tripods (CSD ID: KARKAH).
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Molecules 28 07796 g008
Figure 8. Trimetallic molydenum or tungsten complexes of trindene, 22, 23, and 24 (CESWII).
Figure 8. Trimetallic molydenum or tungsten complexes of trindene, 22, 23, and 24 (CESWII).
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Molecules 28 07796 g009
Figure 9. A trimetallic complex of trindene, 26 (WUXWUJ), bearing different rhodium fragments.
Figure 9. A trimetallic complex of trindene, 26 (WUXWUJ), bearing different rhodium fragments.
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Molecules 28 07796 sch005
Scheme 5. Hausmann’s original (1889) synthesis of truxene.
Scheme 5. Hausmann’s original (1889) synthesis of truxene.
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Molecules 28 07796 sch006
Scheme 6. Neutral (η5:η5:η5) and tricationic (η6:η6:η6) manganese tricarbonyl complexes of truxene.
Scheme 6. Neutral (η5:η5:η5) and tricationic (η6:η6:η6) manganese tricarbonyl complexes of truxene.
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Molecules 28 07796 g010
Figure 10. Side view (left) and bird’s eye view (right) of (truxene)2Ag2, 31 (OZOYUZ), with hydrogens and peripheral ligands removed for clarity.
Figure 10. Side view (left) and bird’s eye view (right) of (truxene)2Ag2, 31 (OZOYUZ), with hydrogens and peripheral ligands removed for clarity.
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Molecules 28 07796 g011
Figure 11. Molecular structure of decacyclene, 32 (TONLEN); (left) bird’s eye view, (right) side view.
Figure 11. Molecular structure of decacyclene, 32 (TONLEN); (left) bird’s eye view, (right) side view.
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Molecules 28 07796 sch007
Scheme 7. Palladium-catalysed trimerisation route to decacyclene.
Scheme 7. Palladium-catalysed trimerisation route to decacyclene.
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Molecules 28 07796 g012
Figure 12. Molecular structure of the decacyclene-dinickel triple-decker 35 (QOWKIW).
Figure 12. Molecular structure of the decacyclene-dinickel triple-decker 35 (QOWKIW).
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Molecules 28 07796 sch008
Scheme 8. Formation of decacyclene di-iron, 36, and tetra-iron, 37, complexes.
Scheme 8. Formation of decacyclene di-iron, 36, and tetra-iron, 37, complexes.
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Molecules 28 07796 g013
Figure 13. Bird’s eye and side views of [(η5-C5Me4Et)Fe]4(μ2-η6:η6:η6:η6)decacyclene], 37 (SEDHUF).
Figure 13. Bird’s eye and side views of [(η5-C5Me4Et)Fe]4(μ2-η6:η6:η6:η6)decacyclene], 37 (SEDHUF).
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Molecules 28 07796 g014
Figure 14. Corannulene, 38, and sumanene, 39, shown as components of the C60 framework.
Figure 14. Corannulene, 38, and sumanene, 39, shown as components of the C60 framework.
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Molecules 28 07796 sch009
Scheme 9. Mehta’s pioneering work on a potential route to sumanene.
Scheme 9. Mehta’s pioneering work on a potential route to sumanene.
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Molecules 28 07796 sch010
Scheme 10. Proposed route from trindane to sumanene, via an organometallic intermediate.
Scheme 10. Proposed route from trindane to sumanene, via an organometallic intermediate.
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Molecules 28 07796 sch011
Scheme 11. Substitutions at the benzylic position in (arene)iron or (arene)manganese cations.
Scheme 11. Substitutions at the benzylic position in (arene)iron or (arene)manganese cations.
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Molecules 28 07796 sch012
Scheme 12. Substitution at multiple benzylic positions in [(trindane)Fe(C5H5)]+.
Scheme 12. Substitution at multiple benzylic positions in [(trindane)Fe(C5H5)]+.
Molecules 28 07796 sch012
Molecules 28 07796 sch013
Scheme 13. Formation of (trindane)Mn(CO)2X derivatives.
Scheme 13. Formation of (trindane)Mn(CO)2X derivatives.
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Molecules 28 07796 g015
Figure 15. Structures of (left) (trindane)Mn(CO)2Br, 50 (MOKROT), and (right) (trindane)Mn(CO)2I, 51 (YOQFEQ).
Figure 15. Structures of (left) (trindane)Mn(CO)2Br, 50 (MOKROT), and (right) (trindane)Mn(CO)2I, 51 (YOQFEQ).
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Molecules 28 07796 sch014
Scheme 14. Formation of (C6Me6)Mn(CO)2H, 52, via elimination of isobutene.
Scheme 14. Formation of (C6Me6)Mn(CO)2H, 52, via elimination of isobutene.
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Molecules 28 07796 sch015
Scheme 15. Proposed mechanism for the formation of (trindane)Mn(CO)2H, 54.
Scheme 15. Proposed mechanism for the formation of (trindane)Mn(CO)2H, 54.
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Molecules 28 07796 sch016
Scheme 16. The haptotropic (η6 to η5) migration of the Mn(CO)3 tripod upon treatment with t-BuOK.
Scheme 16. The haptotropic (η6 to η5) migration of the Mn(CO)3 tripod upon treatment with t-BuOK.
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Molecules 28 07796 g016
Figure 16. Molecular structures of η5-complexes (left) 55 (YOQDEO) and (right) 56 (MOKRUZ).
Figure 16. Molecular structures of η5-complexes (left) 55 (YOQDEO) and (right) 56 (MOKRUZ).
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Molecules 28 07796 sch017
Scheme 17. Ricochet reaction of (η5-indenyl)Cr(CO)3Me.
Scheme 17. Ricochet reaction of (η5-indenyl)Cr(CO)3Me.
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Molecules 28 07796 sch018
Scheme 18. Proposed mechanism for the formation of the rearrangement products 55, 56 and 57.
Scheme 18. Proposed mechanism for the formation of the rearrangement products 55, 56 and 57.
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Molecules 28 07796 sch019
Scheme 19. The Sakurai–Daiko–Hirao route to sumanene, 39.
Scheme 19. The Sakurai–Daiko–Hirao route to sumanene, 39.
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Molecules 28 07796 sch020
Scheme 20. Scott’s synthetic route to circumtrindene, 70.
Scheme 20. Scott’s synthetic route to circumtrindene, 70.
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Molecules 28 07796 g017
Figure 17. Circumtrindene shown as 60% of the C60 skeleton; (left) the purple bonds indicate the final ring closures, and (right) the X-ray crystal structure of circumtrindene 70 (CSD ID: NARZEL).
Figure 17. Circumtrindene shown as 60% of the C60 skeleton; (left) the purple bonds indicate the final ring closures, and (right) the X-ray crystal structure of circumtrindene 70 (CSD ID: NARZEL).
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Molecules 28 07796 sch021
Scheme 21. Stepwise synthesis of C60 via ketone trimerisation and FVP ring closures.
Scheme 21. Stepwise synthesis of C60 via ketone trimerisation and FVP ring closures.
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Molecules 28 07796 sch022
Scheme 22. Ru(VIII) oxidation of trindane to form 72.
Scheme 22. Ru(VIII) oxidation of trindane to form 72.
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Molecules 28 07796 sch023
Scheme 23. Ozonolysis of trindane to form 74 (CSD ID: OZ4).
Scheme 23. Ozonolysis of trindane to form 74 (CSD ID: OZ4).
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Molecules 28 07796 sch024
Scheme 24. Ru(VIII) oxidation of dodecahydrotriphenylene to form benzylic ketones.
Scheme 24. Ru(VIII) oxidation of dodecahydrotriphenylene to form benzylic ketones.
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Molecules 28 07796 sch025
Scheme 25. Construction of a C3v-symmetric potential artificial receptor.
Scheme 25. Construction of a C3v-symmetric potential artificial receptor.
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Molecules 28 07796 sch026
Scheme 26. The multistep synthesis of truxenone under mild (room-temperature) conditions. (i) SiCl4, EtOH, (ii) NBS, AIBN, CCl4, (iii) NaOEt, Me2CHNO2, EtOH, (iv) gaseous BF3, benzene, (v) PCC, and CH2Cl2.
Scheme 26. The multistep synthesis of truxenone under mild (room-temperature) conditions. (i) SiCl4, EtOH, (ii) NBS, AIBN, CCl4, (iii) NaOEt, Me2CHNO2, EtOH, (iv) gaseous BF3, benzene, (v) PCC, and CH2Cl2.
Molecules 28 07796 sch026
Molecules 28 07796 sch027
Scheme 27. A Knoevenagel reaction to form the hexacyanomethylidene derivative 87.
Scheme 27. A Knoevenagel reaction to form the hexacyanomethylidene derivative 87.
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Molecules 28 07796 g018
Figure 18. Molecular structures of truxene, 86 (DEQPIC), and the hexacyanomethylidene derivative, 87 (MATWEJ).
Figure 18. Molecular structures of truxene, 86 (DEQPIC), and the hexacyanomethylidene derivative, 87 (MATWEJ).
Molecules 28 07796 g018
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Lock, P.E.; Reginato, N.; Bruno-Colmenárez, J.; McGlinchey, M.J. Syntheses, Structures and Reactivity of Metal Complexes of Trindane, Trindene, Truxene, Decacyclene and Related Ring Systems: Manifestations of Three-Fold Symmetry. Molecules 2023, 28, 7796. https://doi.org/10.3390/molecules28237796

AMA Style

Lock PE, Reginato N, Bruno-Colmenárez J, McGlinchey MJ. Syntheses, Structures and Reactivity of Metal Complexes of Trindane, Trindene, Truxene, Decacyclene and Related Ring Systems: Manifestations of Three-Fold Symmetry. Molecules. 2023; 28(23):7796. https://doi.org/10.3390/molecules28237796

Chicago/Turabian Style

Lock, Philippa E., Nada Reginato, Julia Bruno-Colmenárez, and Michael J. McGlinchey. 2023. "Syntheses, Structures and Reactivity of Metal Complexes of Trindane, Trindene, Truxene, Decacyclene and Related Ring Systems: Manifestations of Three-Fold Symmetry" Molecules 28, no. 23: 7796. https://doi.org/10.3390/molecules28237796

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