Carboxylation electron-deficient arenes

Electron-deficient rhodium(III) complexes participate in the electrophilic metalation of arenes, as originally shown by Aoyama, Ogoshi, and co-workers [60,61], Highly electrophilic rhodium(III) complexes have also been employed in other reactions that involve the C-H functionalization of arenes, such as the carboxylation of... 

Electron-deficient alkenes can be alkylated under similar conditions. In this procedure, a mixture of alkene 560 and [bis(acyloxy)iodo]arenes 561 [prepared from PhI(OAc)2 and the respective carboxylic acid or monoalkyl esters of oxalic acid] is irradiated with a high-pressure mercury lamp in dichloromethane in the presence of 1,4-cyclohexadiene to give a reductive addition product (562) (Scheme 3.222) [610,611]. 

Rh-Catalyzed Reactions Zhao and coworkers have shown that the reaction of ortho-substituted arene carboxylic acids with electron-deficient olefins in the presence of ahydrorhodium(I) complex as the catalyst gave the Mizoroki-Heck products (8) and the conjugate addition products (9) with high selectivity (Scheme 22.18a) [13, 26]. The selectivity depends on the content of water in the reaction mixture higher water content favored the hydrolysis of the enolato intermediate 10, while lower water content favored P-H elimination. This reaction works only for ortho-suhstituted arene carboxylic acids and with electron-deficient olefins (Scheme 22.18h). 

Coupling Reactions. Palladium pivalate is an effective catalyst for mild and efficient direct arylation reactions. One account described the intramolecular arylation phenolic ethers (eq 1). Initial optimization with Pd(OAc)2 in conjunction with electron-deficient phosphines led to the desired biphenyl in low yield. Upon the addition of carboxylic acid additives, the yield improved markedly with the optimal additive being pivalic acid. Indeed it was determined that the additive was not needed when Pd(OPiv)2 was errqtloyed as a catalyse although improved yields were observed when the title compound was used in conjunction with the acid additive. The role of the pivalate is believed to be that of a proton shuttle in a concerted metallation-deprotonation (CMD) sequence. A further advantage of using Pd(OPiv)2 was the rate enhancement of the arylation with most reactions complete in less than 6 h (vs. 12 or more hours with Pd(OAc)2). The catalyst was applied to a range of electron-rich and -deficient arenes with good to excellent yields. 

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