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Review

The Reims Journey Towards Discovery and Understanding of Pd-Catalyzed Oxidations

by
Jean Le Bras
and
Jacques Muzart
*
Institut de Chimie Moléculaire de Reims, UMR 7312, CNRS-Université de Reims Champagne-Ardenne, B.P. 1039, 51687 Reims Cedex 2, France
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(1), 111; https://doi.org/10.3390/catal10010111
Submission received: 9 December 2019 / Revised: 7 January 2020 / Accepted: 7 January 2020 / Published: 12 January 2020
(This article belongs to the Special Issue Palladium-Catalyzed Reactions)
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Abstract

:
This review recounts the development by the authors of the Pd-catalyzed procedures devoted to various kinds of oxidation. Starting with reactions assisted with UV light, the research has explored reactions under light-free conditions: allylic oxidation, alcohol oxidation, etherification, Wacker oxidation and dehydrogenations with, always, accompanying efforts towards mechanism determination.

Graphical Abstract

1. Introduction

Our initial studies on Pd-catalyzed oxidations results from our observation in the early 1980s of the formation of unsaturated carbonyl compounds from irradiation with UV light of bis(µ-chloro)bis(η3-allyl)dipalladium complexes in oxygenated acetonitrile (Scheme 1) [1]. That result urged us to look for such reactions under catalytic conditions. This activity progressively led us to study a variety of Pd-catalyzed oxidations under light-free conditions. The aim of the present account is to highlight the main results that we obtained in the area over the years.

2. UV-Light-Assisted Oxidation

2.1. Ethylenic Compounds

The easy formation at room temperature of η3-allylpalladium complexes from alkenes and Pd(OCOCF3)2 reported by Trost and Metzner [2] urged us to use this Pd salt as a catalyst for the photo-assisted oxidation of alkenes [3,4]. For an example, the reaction of 1-eicosene (1) in MeCN/CH2Cl2 afforded a mixture of saturated and unsaturated ketones 2 to 4 (Equation (1)). Some migration of the double bond of 1 was a competing reaction [5,6]. In contrast to the isomerization, ketones 2 to 4 were not produced in the absence of light. Switching to acetone as the solvent increased the ketone yields. Similar results were obtained using [(η3-CH2CHCHC3H7)Pd(OCOCF3)]2 as the catalyst whereas the turnover number was inferior to 1 with Pd(OAc)2 [4]. Under alkene-free conditions, monitoring the irradiation of an acetone solution of Pd(OCOCF3)2 showed the adsorption of oxygen [4]. That led us to suspect the formation of peroxydic species. Nevertheless, ketones 2 to 4 were also produced from the reaction of 1 in the presence of radical and 1O2 traps [4].
Catalysts 10 00111 i001
The process was used for the oxidation of allylsulfones [7]. Under the above conditions, 5 led to a mixture of unsaturated aldehyde 6 and alcohol 7 (Equation (2)). The yields were improved with a cocatalyst such as Cu(OCOCF3)2 or Co(OCOCF3)2.
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Under both Pd(OCOCF3)2 catalysis and UV light, allylsulfones substituted with a trimethylsilyl group underwent cleavage of the allyl-Si bond. Thus, full conversion of 8 occurred in 38 h leading to 6 in 95% yield (Equation (3)) [8]. Lower conversions were mediated by other Pd catalysts.
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The mechanism of the reaction of allylsilanes was studied using l-phenyl-3-(trimethylsilyl)-l-propene (9) and l-phenyl-1-(trimethylsilyl)-2-propene (10) as substrates, and comparison with the reactivity of corresponding allylpalladium chloride 11 [9]. The latter would be dissymmetric due to the different substitution of the allyl unit extremities [10,11,12]. Cinnamaldehyde (12) was selectively obtained from irradiation of either 9 and 10 in the presence of catalytic Pd(OCOCF3)2, or 11 (Scheme 2). That contrasts from oxidations of the free phenylallyl radical which gave mixtures of oxidation products in 1- and 3-position [9,13,14]. Consequently, the regiospecificity of the reactions of 9, 10 and 11 excluded the formation of the free phenylallyl radical. The formation of η3-allylpalladium complexes from allylsilanes and PdII salts is known [15]. Thus, a common intermediate, which would be a dissymetric η3-allylpalladium complex, has been proposed (Scheme 3). Light-mediated cleavage of the longer C–Pd bond would lead to intermediate 3A [16,17] which reacts with oxygen to afford a peroxopalladium complex. The latter evolves towards aldehyde 12, liberating HOPdX which would be involved in the subsequent catalytic cycle.

2.2. Alkanes

Irradiation with UV light of a MeCN solution of adamantane (13) containing trifluoroacetic acid and catalytic Pd(OCOCF3)2 afforded adamantanyl acetamides 14 and 15 in quantitative yields versus the amount of palladium (Equation (4)) [18]. The reaction became catalytic with Cu(OCOCF3)2 as the cocatalyst and provided traces of adamantanyl trifluoroacetates 16. The mechanism of these reactions remains obscure, the %14/%15 ratio indicating a radical or electrophilic process [19].
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Oxidation of cyclohexane (17) and 3-methylhexane (18) was performed using light and peroxopalladium complexes [20]. Thus, [(t-BuOO)Pd(OCOCF3)]4 in air led, from 17, to cyclohexanol and cyclohexanone with a slight catalytic character (Equation (5)). A similar result arose with [(t-BuOO)Pd(OCOCH3)]4. The oxidation of 18 mainly occurred at the level of the tertiary C–H bond.
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3. Allylic Oxidation

3.1. 1-(p-Toluenesulfonyl)-2-Propene and 1-(Trimethylsilyl)-1-(p-Toluenesulfonyl)-2-Propene

The PdII-catalyzed oxidation of allylsulfone 5 was carried out with t-BuOOH or oxygen in conjunction with either CuCl or benzoquinone (BQ) (Equations (6) and (7)) [21]. Under the former conditions, the main products were alcohol 7 and peroxide 19, which could be produced from a η2-olefin palladium complex formed from 5 and t-BuOOPdOCOCF3 (Scheme 4) [22].
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Catalysts 10 00111 i007
Submitting allylsilane 8 to the PdCl2/CuCl or BQ system under oxygen mainly led to the desilylated compound 5. (Equation (7)) [21]. Aldehyde 6 and alcohol 7 were concomitantly produced.

3.2. Terminal Alkenes

Various conditions have been reported for the synthesis of allylic carboxylates via PdII-catalyzed allylic oxidation of olefins in carboxylic acids [23,24]. After the observation of the improvement of the Pd(OAc)2-catalyzed allylic acetoxylation of allylbenzene with BQ as the stoichiometric oxidant in the presence of base but with inconsistent yields, we performed the efficient, reproducible and regioselective allylic acyloxylation of terminal alkenes using lithium hydroxide as the additive and propionic acid as the solvent (Equation (8)) [25].
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The reaction of 1-decene was less selective (Equation (8)). The regioselectivity increased with the more-hindered pivalic acid but to the detriment of the conversion. Improved results were finally obtained using a BQ/MnO2 mixture as the oxidant (Equation (9)) [25]. This modified procedure was well adapted to the oxidation of homoallylic alcohols but led to lower yields with allylarenes (compare Equations (8) and (9)).
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Careful analysis by Electrospray Ionization Mass Spectrometry (ESI–MS) of the allylic acyloxylation under the above conditions led to identification of different clusters, especially those corresponding to intermediates having the Pd atom (in mauve color) in the proposed catalytic cycle depicted in Scheme 5. Palladium acetate, which is a trimer in the solid state [26], reacts with the in-situ formed salt of the carboxylic acid to afford 5A. Coordination of the substrate to 5A leads to η2-alkenyl intermediate 5B, which evolves towards an η3-allyl complex 5C. Subsequent intramolecular acetoxylation delivers the product and Pd0. The reoxidation of Pd0 completes the catalytic cycle.

4. Alcohol Oxidation

The transformation of alcohols into the corresponding carbonyl compounds with metal oxides and metal salts may occur through three pathways (Scheme 6). Instead of the term “oxidation” used for reactions following paths a and b, those arising via path c are often called “dehydrogenation” or “oxidative dehydrogenation”. Most Pd-catalyzed oxidation of alcohols occur via paths b and c [27]. As shown below, we have developed procedures for such reactions using various species to regenerate the catalyst.

4.1. With Sodium Percarbonate

Despite its name, sodium percarbonate (SPC) is not a persalt. SPC, which is a versatile oxidizing agent for organic synthesis [28,29], is the association of sodium carbonate with hydrogen peroxide with the formula Na2CO3,1.5 H2O2. In the course of the screening of metal chlorides for the catalytic oxidation of 1-indanol (20) by SPC in 1,2-dichloroethane (DCE) in the presence of Adogen 464 [30], we discovered that the reaction with PdCl2 effectively occurred even in the absence of SPC. That led us to the procedure documented in Section 4.2. In contrast, SPC is required with solvents such as acetonitrile, hexane and benzene, leading selectively to 1-indanone (21) from 20 (Equation (10)) [31].
Catalysts 10 00111 i010

4.2. With 1,2-Dichloroethane

The PdCl2-catalyzed oxidation of alcohols in DCE containing sodium carbonate and catalytic amounts of Adogen 464 (Equation (11)) involves the regeneration of active Pd species by the solvent, leading to the formation of ethylene. The formation of the latter has been highlighted by its reaction with iodine, giving 1,2-diiodooethane [32]. The method is efficient for saturated and benzylic secondary alcohols. Some overoxidation of primary alcohols occurred leading to acids which react with the solvent to afford esters. Isomerization of secondary allylic alcohols to saturated ketones may compete with the oxidation (Equation (11)).
Catalysts 10 00111 i011
As the plausible mechanism, we initially suspected the insertion of Pd0 into a C–Cl bond of DCE to afford ClCH2CH2PdCl which would undergo β-Cl elimination leading to ethylene and PdCl2 [32]. According to a theoretical study, ClCH2CH2PdCl would rather be the active species reacting with the alcohol [33]. The process is, however, carried out in the presence of Adogen 464 which reacts with PdCl2 to afford the soluble palladium salt [PdCl4]2− [34]. These remarks led us to propose the catalytic cycle depicted in Scheme 7. Hydridopalladium 7A formed after the first alcohol oxidation leads to Pd0 species 7B via elimination of HCl. Insertion of 7B into DCE affords 7C. Coordination of the alcohol to the latter gives 7D. Subsequent elimination of ethylene and HCl leads to an alkoxypalladium intermediate.
The PdCl2/Adogen 464/DCE procedure effected the lactonisation of various 1,4- and 1,5-diols except that of cis-endo-2,3-bis(hydroxymethyl)bicyclo [2.2.1]hept-5-ene (22) which led to lactol 23 (96% selectivity) (Equation (12)) although the corresponding saturated diol 25 provided lactone 26 (Equation (13)) [35]. Oxidation of lactol 23 into lactone 24 did not occur under the Pd conditions, but arose using pyridinium dichromate in CH2Cl2 (83% yield [36]) [37] or the Swern oxidation method [38]. The PdCl2/Adogen 464/DCE procedure is however able to oxidize α lactol such as 27 (Equation (14)).
Catalysts 10 00111 i012
Catalysts 10 00111 i013
Catalysts 10 00111 i014
The above results clearly demonstrated that the lack of formation of 24 from 22 under Pd-conditions was attributable to the C=C bond. Analysis of both the plausible intermediates of the process and literature [39,40,41] led to the proposal of Scheme 8. The reaction of hydroxyaldehyde 8B obtained via 8A leads to alkoxypalladium intermediate 8C. Subsequent intramolecular reaction occurs through the stereoselective approach of the alkoxypalladium moiety to one face of the aldehyde to afford 8D. In contrast to the intermediate obtained from 25, 8D undergoes a ligand exchange leading to palladacycle 8E. The syn relationship between O-Pd and C-H bonds, which would allow a β-H elimination leading to the carbonyl unit [42,43], is prevented in 8E. That favors alkoxyde exchange with diol 22 to afford 8F. The latter is in equilibrium with 8B which evolves towards 23, that is, the more stable isomer [44,45,46]. The apparent absence of 23 evolution under the PdCl2/Adogen 464/DCE conditions would be a “no reaction” reaction [47,48], which involves 8B, 8C, 8D, 8E, alcoholysis and equilibration regenerating the starting substrate.
As depicted in Equation (11), the PdCl2/Adogen 464/DCE procedure efficiently oxidizes 1-indanol (20) into 1-indanone (21). The oxidation was also effective using the soluble catalyst (n-Bu4N)2PdCl4.0.5 H2O (92% conversion, 90% yield) instead of the PdCl2/Adogen 464 association [31]. Surprisingly, soluble (MeCN)2PdCl2 produced di(1-indanyl) oxide (28) in high yields, even in the absence of the base (Equation (15)) [49].
Catalysts 10 00111 i015
We rationalized the formation of 21 and 28 through two competitive pathways catalyzed with L2PdCl2 (L = Cl− or RCN), that is with [PdCl4]2− and (RCN)2PdCl2, respectively (Scheme 9) [49]. In contrast to the anionic catalyst (L = Cl-), the neutral catalyst (L = RCN) is electrophilic [50,51,52,53]. Exchange of ligand between L2PdCl2 and 20 affords 9A. The evolution of 9A depends on the electrophilicity of the L2PdCl2. Transition metals having Lewis acid properties mediate the formation of ethers from alcohols [31,54]. Consequently, 9A formed from the anionic catalyst evolves towards 21 via the ketonisation pathway [32,33], while 9A formed from (RCN)2PdCl2 undergoes heterolytic cleavage of the C–OH bond leading to ionic species 9B. The latter reacts with 20 to give ether 28, water and the starting catalyst.

4.3. With Aryl Bromide

The Yoshida procedure of oxidation of alcohols used a Pd catalyst with an aryl halide as hydrogen acceptor and a base (Scheme 10) [55,56]. We used this procedure for the oxidation with high yields of benzyl-protected sugar hemiacetals into lactones (Equation (16)) [57].
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4.4. Dehydrogenation

Over the last thirty years, growing attention has been devoted to the use of ionic liquids and molten salts as solvents for organic synthesis [58,59,60]. The strong interest for catalyzed reactions is due to the immobilization of the catalyst in the ionic liquid or molten salt that would allow the recycling of the tandem catalyst/solvent. These unusual solvents have been used for various catalytic oxidations [61,62,63]. Our above studies with (n-Bu4N)2PdCl4.0.5 H2O (see Section 4.2) and Heck reaction in molten n-Bu4NBr [64] urged us to carry out Pd-catalyzed oxidations in this medium.
Initial experimentation using 20, catalytic PdCl2 and n-Bu4NBr at 120 °C led to a mixture of 21 and indane. The reductive cleavage of the C–OH bond of 19 indicated in-situ formation of hydrogen [65] and/or [Pd]H2 species [66]. Addition of cyclohexene as a hydrogen acceptor increased the selectivity towards 21. Finally, the best result was obtained under a gentle flow of argon which removes hydrogen gas (Equation (17)) [67]. Under these conditions, the recycling of both catalyst and n-Bu4NBr was relatively efficient. Secondary benzylic alcohols provided the corresponding ketones in good yields. The method is less selective for primary benzylic alcohols and is ineffective from allylic and saturated alcohols. Subsequent experiments showed that recycling was more efficient with Pd(OAc)2 than with PdCl2, the yield obtained from 4th reuse of the catalyst/n-Bu4NBr association being 92% (94% conversion) with the former and 64% (67% conversion) with the latter [68].
Catalysts 10 00111 i017

5. Allylphenols Oxidation

Green chemistry has led the chemical community to intensify research on aqueous procedures [69,70,71,72,73,74,75], and we previously used the hydrophilic ligand [(HOCH2CH2NHCOCH2)2NCH2]2 (LH) for copper-catalyzed allylic oxidations in water [76]. The intramolecular Wacker oxidation of allylphenol (29) leads to 2-methylbenzofuran [77,78] or 2H-chromene [79] depending on the reaction conditions. Thus, we were interested in performing such a reaction in aqueous media with a PdII/LH catalytic system.
Treatment of 29 with aqueous H2O2 and catalytic amounts of both Pd(OCOCF3)2 and LH at 50 °C in water afforded diol 30 instead of the cyclization products (Equation (18)) [80]. Reaction in a mixture of water and methanol led to 30 and hydroxyl methyl ether 31Me. Similar compounds were produced in H2O/EtOH and H2O/i-PrOH, or using 2-allyl-4-methylphenol and 2-allyl-6-methylphenol.
Catalysts 10 00111 i018
The report of Jacobs’ team about the phenol-mediated epoxidation of alkenes by H2O2 under metal-free conditions [81] led us to propose in 2005 the mechanism depicted in Scheme 11 [80]. Pd-catalyzed isomerization of 29 affords 32 [82]. Activation by the phenolic OH of the epoxidation of 32 provides 11A. The high instability of such a compound [83] brings on spontaneous ring opening leading to 30 and 31R. However, the strong acceleration of the palladium-catalyzed reaction of 32 (Equation (19)) indicates some participation of Pd(OCOCF3)2/LH in the process [80]. In fact, epoxides are very sensitive to Pd catalysis [84]. Complementary mechanistic experiments and ESI-MS studies supported the proposed reaction pathway [85].
Catalysts 10 00111 i019
Recycling of the Pd(OCOCF3)2/LH catalytic system led to gradual loss of activity, the 4th reuse in H2O/MeOH yielding 35% and 24% of 30 and 31Me, respectively, from 29 [85].

6. Wacker Oxidation

Tsuji’s conditions of the Wacker reaction use PdCl2 catalyst in DMF/H2O and oxidants such as copper salts/O2 or benzoquinone [86]. According to investigations through isotope effects, kinetic, stereochemical and theoretical studies [87,88,89,90], the mechanism involves alkene coordination to PdCl2, followed by hydroxypalladation and β-hydride elimination leading to a palladium–enol η2-complex. The latter evolves towards the ketone, liberating HCl and Pd0. The catalyst is regenerated from Pd0 through reaction with BQ. Mechanistic details remain however matter of debate [87].
Our interest in the Pd-catalyzed oxidations and in the synthetic properties of DMF [91,92,93] and BQ [94] urged us to investigate the Wacker reaction of terminal alkenes 33 with ESI-MS, using BQ as the terminal oxidant (Equation (20)) [95].
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ESI–MS monitoring of the reactions showed that dinuclear palladium complexes were more involved as active catalytic intermediates than mononuclear species. Tests of complexation suggested a reoxidation of Pd occurring before the decoordination of the product. These studies associated to kinetic experiments led us to propose the catalytic cycle depicted in Scheme 12, in which intermediates having the Pd atom in mauve color correspond to clusters detected by ESI–MS.
Compared with the usual mechanism, the most important differences are the involvement of mainly dinuclear Pd species and the reoxidation of a Pd-hydride complex before decomplexation of the ketone.

7. Cyclohexanone Dehydrogenation

In 1982, we disclosed the room temperature Pd(OCOCF3)2-catalyzed dehydrogenation of cyclohexanones under oxygen atmosphere (Equation (21)) [96]. Pd procedures were previously reported but using mainly stoichiometric amounts of PdII [97].
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Cyclohexenone was selectively produced at low conversion. Increase of the latter led to over-oxidation giving phenol. The proposed catalytic cycle (Scheme 13) maintains the formal oxidation state of PdII throughout the reaction. Coordination of cyclohexanone or its enol form to Pd(OCOCF3)2 provides 13A, which led to oxo-η3-allyl palladium complex 13B in liberating CF3CO2H. Hydrogen abstraction by palladium provides 13C which undergoes insertion of oxygen and ligand exchange giving 2-cyclohexenone and hydroperoxy complex 13D. The latter leads to H2O2 and either 13B (path a) or 13A by reacting with CF3CO2H (path b).
This oxidation process was, in 1982, one of the first reports on the regeneration of active PdII species using only oxygen [98,99,100]. The pathway leading to PdOOH from PdH and O2, that is 13D from 13C remains however a matter of debate [98,99,100,101].
A number of procedures are now available for the Pd-catalyzed dehydrogenation of carbonyl compounds [102]. Moreover, such a dehydrogenation may be a step of a domino reaction involving the Heck reaction, decarboxylative Heck reaction or dehydrogenative Heck reaction [103].

8. Dehydrogenative Heck Reaction

Disclosed in 1970, the Heck reaction is traditionally the synthesis of an arylalkene from the Pd0-catalyzed cross-coupling of an aryl halide with an alkene [104]. Previously, Fujiwara’s team reported the synthesis of stilbene from the reaction of benzene with styrene and PdCl2, leading to two turnovers of palladium [105]. Such a cross-coupling, for which we adopted the name “dehydrogenative Heck reaction” (DHR) [106], may be more respective of the atom economic principle [107], and has been intensively studied over the last twenty years [108].
Our studies focused on the coupling of furan 34 with styrene. Screening various experimental conditions initially led to the best results with catalytic Pd(OAc)2 and the BQ/Cu(OAc)2/O2 oxidizing system (Equation (22)). The method was used for the DHR of various furans and styrenes with high regio- and stereoselectivities (30 examples, 50%–78% yields) [109].
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Kinetic investigations showed an induction period which depends on the nature of the furan, the transformation being faster with electron-rich furans. This led us to investigate the influence of ligands and solvents on the activity of the catalyst. The use of DMSO/AcOH as solvent mixture and BQ as oxidant led to a catalytic system showing no induction period at room temperature, leading to cross-coupling of furans and thiophenes with styrenes, and compatible with halogenated substrates such as 35 and 36 (Equation (23)) [110]. The positive influence of DMSO on the efficiency of Pd(OAc)2-catalyzed oxidations is largely documented in the literature [111,112]. Ligation of DMSO to the trimer [Pd(OAc)2]3 affords the dimeric active species Pd(OAc)2(DMSO)2 [113,114], which may react with the arene leading to ArPdOAc(DMSO)2. Subsequent ligand exchange with BQ could give the less electron-rich species ArPdOAc(BQ)(DMSO), which would be susceptible to easily coordinate to electron-rich styrenes [110].
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The mechanism of the reaction of 34 with tert-butyl acrylate under these conditions (Equation (24)) has been investigated by ESI–MS leading to identification of clusters corresponding especially to PdII and Pd0 intermediates 14A, 14B and 14C, leading us to propose the catalytic cycle shown Scheme 14 [115].
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Catalytic amounts of metallic co-oxidants are often required for efficient DHRs under oxygen [108]. We observed, however, that such additives are not always beneficial. Indeed, the room temperature Pd(OAc)2-catalyzed reaction of 34 with styrene in oxygenated DMSO/AcOH afforded the cross-coupling product in higher yield in their absence (Equation (25)). Thus, these mild experimental conditions were used for the efficient DHR of furans, thiophenes and indoles with styrenes (29 examples, 42%–95% yields) [116]. ESI–MS studies of mixtures of Pd(OAc)2 and AgOAc have shown the formation of mixed species; which could be inactive towards the DHR.
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The reaction of thiophene 37 with a hindered alkene such as methyl cinnamate under conditions of Equation (24) occurred with 5% conversion leading to traces of DHR product 38 [117]. With AcOH as the solvent instead of the AcOH/DMSO mixture, the conversion increased to 20%. Testing various ligands led to an efficient DHR with 4,5-diazafluorenone (Equation (26)). Moreover, increase of the temperature to 60 °C with O2 instead of BQ led to 38 in 90% isolated yield. Consequently, these conditions have been used for the cross-coupling of furans and thiophenes with various hindered alkenes (19 examples, 51%–96% yield). According to kinetics and competitive experiments as well as ESI–MS studies, 4,5-diazafluorenone influences the C–H bond activation, the alkene insertion, the stereoselectivity and the regeneration of the catalyst [117].
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Under conditions of Equation (24) except the presence of DMSO, a low yield of the DHR product 39 was obtained from the sluggish reaction of 34 with allylbenzene (Equation (27)) [118]. Surprisingly, addition of MeCN as a co-solvent suppressed the formation of 39 and increased the conversion leading to a 1:1 mixture of difurylalkanes 40 and 41. Moreover, switching to Pd(OCOCF3)2 as the catalyst increased the 40 + 41 yield to 94% (Equation (27)).
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Suspecting the formation of 41 from prop-1-en-1-ylbenzene via isomerization of allylbenzene [82], the reaction was repeated with styrene [118]. Difurylalkanes were also obtained (Equation (28)). Labelling experiments led to assignment of the hydrogen shifts shown in Equation (28) and, associated ESI–MS studies, to propose the catalytic cycle depicted in Scheme 15 [118].
Catalysts 10 00111 i028

9. Conclusions

Starting at the end of the 1970s with oxidations of CH or CH2 units under UV light, our research evolved towards light-free reactions in various media: organic solvents, water, molten salts, leading to alcohol oxidation, dehydrogenation, etherification or formation of C–C bonds. We have always been strongly focused on the mechanisms; these lead us to various proposals, especially those based on ESI–MS results. Some reactions and mechanisms have been serendipitously discovered [119], but nevertheless, in most cases, they were the fruit of deep investigations and reflection, rather than good fortune.

Funding

This research received no external funding.

Acknowledgments

We are grateful to colleagues and students from the Universities of Grenoble, Lille, Marseille, Moscow, Prague, Reims and York whose names are cited in the references, for their involvement in the above reactions. We thank CNRS for the freedom in the choice of our research activities.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 10 00111 sch001 550
Scheme 1. Photocatalyzed oxidation of bis(µ-chloro)bis(η3-allyl)dipalladium complexes
Scheme 1. Photocatalyzed oxidation of bis(µ-chloro)bis(η3-allyl)dipalladium complexes
Catalysts 10 00111 sch001
Catalysts 10 00111 sch002 550
Scheme 2. Three photochemical reactions leading to cinnamaldehyde.
Scheme 2. Three photochemical reactions leading to cinnamaldehyde.
Catalysts 10 00111 sch002
Catalysts 10 00111 sch003 550
Scheme 3. Plausible reaction pathways leading to cinnamaldehyde from allylsilanes 9 and 10.
Scheme 3. Plausible reaction pathways leading to cinnamaldehyde from allylsilanes 9 and 10.
Catalysts 10 00111 sch003
Catalysts 10 00111 sch004 550
Scheme 4. Pd(OCOCF3)2-catalyzed oxidation of allylsulfone 5 with t-BuOOH.
Scheme 4. Pd(OCOCF3)2-catalyzed oxidation of allylsulfone 5 with t-BuOOH.
Catalysts 10 00111 sch004
Catalysts 10 00111 sch005 550
Scheme 5. Proposed mechanism of the Pd-catalyzed allylic acyloxylation.
Scheme 5. Proposed mechanism of the Pd-catalyzed allylic acyloxylation.
Catalysts 10 00111 sch005
Catalysts 10 00111 sch006 550
Scheme 6. The three pathways of metal-mediated oxidation of alcohols.
Scheme 6. The three pathways of metal-mediated oxidation of alcohols.
Catalysts 10 00111 sch006
Catalysts 10 00111 sch007 550
Scheme 7. PdCl2-catalyzed oxidation of alcohols with 1,2-dichloroethane.
Scheme 7. PdCl2-catalyzed oxidation of alcohols with 1,2-dichloroethane.
Catalysts 10 00111 sch007
Catalysts 10 00111 sch008 550
Scheme 8. The “no reaction” reaction of lactol 23.
Scheme 8. The “no reaction” reaction of lactol 23.
Catalysts 10 00111 sch008
Catalysts 10 00111 sch009 550
Scheme 9. Dependence of the reaction pathway on the electrophilicity of the catalyst.
Scheme 9. Dependence of the reaction pathway on the electrophilicity of the catalyst.
Catalysts 10 00111 sch009
Catalysts 10 00111 sch010 550
Scheme 10. PdII-catalyzed oxidation of alcohols with aryl halides.
Scheme 10. PdII-catalyzed oxidation of alcohols with aryl halides.
Catalysts 10 00111 sch010
Catalysts 10 00111 sch011 550
Scheme 11. Domino reaction of 2-allylphenol.
Scheme 11. Domino reaction of 2-allylphenol.
Catalysts 10 00111 sch011
Catalysts 10 00111 sch012 550
Scheme 12. Proposed catalytic cycle for the Wacker reaction in DMF/H2O in the presence of benzoquinone.
Scheme 12. Proposed catalytic cycle for the Wacker reaction in DMF/H2O in the presence of benzoquinone.
Catalysts 10 00111 sch012
Catalysts 10 00111 sch013 550
Scheme 13. Pd(OCOCF3)2-catalyzed dehydrogenation of cyclohexanone.
Scheme 13. Pd(OCOCF3)2-catalyzed dehydrogenation of cyclohexanone.
Catalysts 10 00111 sch013
Catalysts 10 00111 sch014 550
Scheme 14. PdII-catalyzed C-H activation.
Scheme 14. PdII-catalyzed C-H activation.
Catalysts 10 00111 sch014
Catalysts 10 00111 sch015 550
Scheme 15. Diaddition of 2-methylfuran to styrene.
Scheme 15. Diaddition of 2-methylfuran to styrene.
Catalysts 10 00111 sch015

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Le Bras, J.; Muzart, J. The Reims Journey Towards Discovery and Understanding of Pd-Catalyzed Oxidations. Catalysts 2020, 10, 111. https://doi.org/10.3390/catal10010111

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Le Bras J, Muzart J. The Reims Journey Towards Discovery and Understanding of Pd-Catalyzed Oxidations. Catalysts. 2020; 10(1):111. https://doi.org/10.3390/catal10010111

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Le Bras, Jean, and Jacques Muzart. 2020. "The Reims Journey Towards Discovery and Understanding of Pd-Catalyzed Oxidations" Catalysts 10, no. 1: 111. https://doi.org/10.3390/catal10010111

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