Catalysis homocoupling reaction

There has been a review of palladium-catalysed carbonylative coupling reactions of aryl halides with carbon nucleophiles in the presence of carbon monoxide. It has been shown that rhodium is an efficient catalyst for the homocoupling reaction of arylzinc compounds in the presence of 1 atm of carbon monoxide to give diaryl ketones. Under similar conditions, palladium and nickel catalysts yield biaryls. The beneficial catalysis by rhodium is likely to derive from the ease of migration of the aryl ligand to carbon monoxide in the rhodium(III) intermediate. A rhodium catalyst has also been used in the formation of indole-3-carboxylates by reaction of indoles with alcohols in the presence of carbon monoxide. The catalytic cycle. Scheme 5, is likely to involve metallation of the indole at the 3-position, followed... 

Figure 4.9 The proposed mechanism of catalysis of Au NPs-rGO composites towards Ullmann homocoupling reaction of aryl bromide. [Reproduced with permission from Ref. [54], Copyright 2014, RSC Publication, RSC Advances, 2014, VoL 4, pp. 5243-5247.]...

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

Radical intermediates and transition metal-catalyzed reactions are in principle ideally suited to be linked together. A prerequisite to perform successful radical reactions is that the concentration of radicals has to be kept low to promote the desired reaction and to avoid competing homocoupling and disproportionation, which occur often diffusion-controlled. Including radical intermediates in the regime of a transition metal catalyzed process is thus ideal to keep their concentrations low, since their maximum concentration cannot exceed that of the metal catalyst. On the other hand, radicals are much more reactive than closed-shell organotransition metal intermediates. Thus, the involvement of radicals in transition metal catalysis often leads to a strong acceleration of the reactions compared to a process where only closed-shell intermediates are involved [101]. 

Keywords C-Cl activation, Ar-Cl oxidative addition, Chloroarenes, Homogeneous catalysis with metal complexes, Reductive dechlorination, Aromatic nucleophilic substitution, Heck reaction, Homocoupling, Cross-coupling, Carbonylation... 

Itami went on to report a new synthesis of [12JCPP based on the shotgun homocoupling of dihalide 56 with bis(cyclooctadiene)nickel(0) in the presence of 2,2 -bipyridyl. This new approach circumvented the synthesis of a boronate and also the need for palladium catalysis, albeit stoichiometric nickel is required (Fig. 27). With these synthetic advantages, Itami was able to synthesize 61, the macrocyclic precursor to [12JCPP on a gram scale. In addition, the authors report that with this synthesis they were able to produce 0.5 g in total of [12JCPP from combined reaction products [36]. 

To date, most research on NHCP transition-metal catalysis has been devoted to cross-couplings [13] or related reactions such as hydroarylation of alkenes [18], direct arylation of alkynes [17], or oxidative homocoupling of terminal alkynes [19]. All NHCP systems used in these studies feature one or two... 

Cross-coupling Reactions. Triphenyl(difluoro)silicates arise from the reaction of Phs SiH and quaternary onium hydrogendiflu-oride. These species can then be subject to cross-coupling phenyla-tion reactions under Pd-catalysis. Usefully, no homocoupled product formation was observed under these conditions (eq 32). ... 

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