Carbon-chain homologations reaction

The Arndt-Eistert synthesis allows for the conversion of carboxylic acids 1 into the next higher homolog 4. This reaction sequence is considered to be the best method for the extension of a carbon chain by one carbon atom in cases where a carboxylic acid is available. 

In the absence of a proton donor, the alkoxide ion generated by carbanion addition to the carbonyl function can interact with a carbon-halogen bond in the 8 2 displacement reaction. Reactions of this type have led to some novel carbon chain forming processes. Ketones are converted to homologated enones in good yield by... 

Reactions of alkane-1,1-dicarboxylic acids, e.g. la, alkane-1,4-dicarboxylic acids, e.g. lb, and their longer-chain homologs with sulfur tetrafluoride afford mixtures of bis(trifluoromethyl)-alkanes, bis(pentafluoroalkyl) ethers and small amounts of polyethers.118 Total yields, generally, decrease with increasing length of the carbon chain and in the presence of C = C bonds. 

Most of the investigations into disproportionation reactions have mainly concentrated on chlorofiuoro derivatives of methane and ethane. When trichlorofluoromethane is refluxed with aluminum trichloride or aluminum tribromide, dichlorodifluoromethane and carbon tetrachloride are obtained. Dichlorofluoromethane yields chlorodifiuoromethane and chloroform chlorofiuoro derivatives of ethane and longer chain homologs exhibit a tendency towards isomerization as well as disproportionation, i.e. intramolecular halogen atom exchange. In this case, both types of reaction take place simultaneously. In other words, disproportionation of l,l,2-triehloro-1.2,2-trifiuoroethane (1) forms l,l,1.2-tetrachloro-2,2-difluoroethane (2) and... 

Yur ev and Magdesieva41 found a very convenient procedure to obtain selenophene and its homologs by the reaction of butylenes with metallic selenium at 580°C. The selenophene ring closure is most favored when the four-carbon chain of the initial hydrocarbon has a central double bond and the doubly bonded carbons carry as many substituents as possible. 

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. 

A more general hallmark of the FTT reaction is the presence of homologous series aldehydes, alcohols, nitriles, and cyanoacetylenes (Table 2), The FTT reaction builds carbon chains by successive addition of Cj or Cj units, and so the lightest member of a series is always accompanied by its heavier homologues. A good test case is provided by the cyanoacetylenes the most numerous family, with the largest member (HC N, with 11 atoms). 

Within this scope, homologation reactions are all variants of enlarging the carbon chain in a oxygen-containing molecule by one C atom with the use of syngas. 

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. 

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