Carbon-chain homologations mechanism

From reaction (b), a new carbon-carbon bond is formed by the intermediacy of the transition metal. Several chemical processes in which new carbon-carbon bonds are formed (e.g., hydroformylation, olefin polymerization, homologation of alcohols, the Fischer-Tropsch reaction) are rationalized by a common mechanism of carbon chain growing. Much discussion and scientific work is under way to ascertain the general applicability of these findings to specific chemical or biochemical processes however, a basic understanding of the elementary steps involved in insertion reactions will lead to a better understanding of known reactions and to the development of new reactions. 

A homologous series of a,co-perfluorodicarboxylic acids (n= 1-16) were recently synthesized by a controlled free radical polymerization of tetrafluoroethylene using aqueous potassium peroxy-disulfate with either an iron or copper promoter. The dominant products were the even-numbered members, as demonstrated by the proposed mechanism in Scheme 26. It was shown that the odd-numbered carbon chains result from oxidative decarboxylation. 

In 1981, Texaco announced the rutheiuum/H2/CO-catalyzed homologation of carboxyhc acids. Homologation refers to a chain-extension reaction that increases the carbon number of the carboxyhc acid see Homologation Reactions). The particular reaction that was initially studied was the conversion of acetic acid to propioiuc acid. The proposed mechanism shown in Scheme 22 is based on a Ru /Ru cycle, similar to that suggested by Knifton and coworkers at Texaco. 

Besides isomerization and dehydrocyclization, substituted benzenes also undergo hydrogenolysis of the side chain and homologation. This very interesting reaction, which consists in the growth of the side chain by one carbon atom, was demonstrated unambiguously by Csicsery and Burnett (160). o-Ethyltoluene leads to o-n-propyl- and o-isopropyltoluenes, o-di-ethylbenzene and cymenes, and the occurence of this reaction is, in our opinion, an argument in favor of carbene mechanisms. ... 

The conclusions reveal a rather general mechanism which can explain all of the results reported to date. For the homologous series under study, we consider two situations determined by the CMC value of the surfactant, i.e., the alkyl chain length (n = 8, 12 or 16 carbon atoms). 

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