Homocoupling reactions arene coupling

Application in organic synthesis of pentacoordinated triorganodifluorosilicate anions, such as [Bu4N][Ph3SiF2] 825, have been extended to palladium(0)-catalyzed cross coupling reactions (solvents DMF, TFIF, dioxane) with arene halides (Scheme 111).825 This method is tolerant to various palladium(O) catalysts and provides excellent yields of mainly heterocoupled products and only small amounts of homocoupled byproducts. 

Intermolecular dehydrogenative oxidative homocouplings of (hetero)arenes turned out to be among the most important methods for the synthesis of symmetrically substituted biaryls [122]. A recent illustrative example is oxidative coupling reactions of 2-naphthols, which were accomplished in an asymmetric fashion employing an inexpensive iron catalyst (Scheme 9.47) [123]. 

Based on studies directed towards chelation-assisted oxidative homocouplings of 2-arylpyridines, a protocol was developed for intermolecular coupling reactions [132]. Thus, benzo h quinoline (186) was efficiently arylated with a variety of arenes using silver salts and benzoquinone as additives (Scheme 9.53). 

A very important series of reactions are those in which two hydrocarbons such as arenes and alkenes are homocoupled with loss of two hydrogens in the presence of a stoichiometric amount of palladium(II) compounds such as PdCl2 and Pd(OAc)2, or more usefully with a catalytic amount of palladium catalysts and a stoichiometric amount of oxidantsJ f The palladium(II)-promoted coupling reaction was first reported by Hiittel and Bechter in 1959, who found that a-substituted styrenes were coupled by PdCl2 in acetic acid in the presence of NaOAc (Scheme... 

It is not common, however, probably because it requires high catalyst loading and yields both hetero- and homocoupled products. The amount of homocoupled product can be decreased by using excess (such as 3/1) PhSi(OMe)3 and reduced temperature (60°C). For instance, the bulky NHC (above) was used to couple aryl- and heteroarylbromides in high yields without formation of homocoupled arene product. Even activated chloroarenes such as para-chloroacetophenone and pura-chloroben-zonitrile could be coupled. Vinyltrimethoxysilane also quantitatively yields the substituted styrene upon reaction with para-chloroacetophenone for 18 h at 80°C in 1,4-dioxane/THF in the presence of... 

Carbon Nucleophiles. Palladium-mediated homocoupling of substituted arenes generally yields mixtures of all possible coupling products. If the reaction is carried out with a catalytic amount of palladium diacetate and with thallium(III) trifluoroacetate as stoichiometric oxidant (eq 67), aryls carrying substituents such as alkyl or halide afford mainly the 4,4 -biaryls in yields ranging from 60% (R = ethyl) to 98% (R = H). Biaryls can also be formed without the palladium catalyst. 

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