Alkenes methylene homologation

Ketone methylenation is usually effected with Ph,P=CH2, scarcely an atom-efficient process. Tu-Hsin Yan of National Chung-Hsing University reports (Organic Lett. 2004,6,4961) that reduction of CHjClj with Mg powder in the presence of TIC14 leads to a reagent that efficiently homologates even easily-enolizable ketones such as 1. The reagent also converts esters such as 3 to alkenes, albeit more slowly. 

A pyridinium methylide can undergo a novel reaction with an electron-deficient alkene to provide the next higher homolog in which the original double bond is saturated and a methylene group is added. This reaction fails with hindered alkenes, and apparently requires DME as solvent. A [3 + 2] cycloaddition may be the initial step. 

Examples of alkylation, dealkylation, homologation, isomerization, and transposition are given in Sections 1, 17, 33, and so on, lying close to a diagonal of the index. These sections correspond to such topics as the preparation of acetylenes from acetylenes carboxylic acids from carboxylic acids and alcohols, thiols, and phenols from alcohols, thiols, and phenols. Alkylations that involve conjugate additions across a double bond are given in Section 74E (Alkyls, Methylenes, and Aryls from Alkenes). 

Oxidative methylenation Alkyl halides and mesylates are homologated to alkenes. Similarly, epoxides give allylic alcohols with one more carbon atom. 

Interestingly, Z- and E-non-3-ene and Z- and -non-2-ene could not be identified as reaction products of Z- and -l-methyl-2-n-pentylcyclopropane. These products would correspond to the fission of the most substituted central cr-bond of the cyclopropane. Furthermore, methylene-carbene elimination leading to the lower homologous alkenes seems to be an unfavourable process with those catalysts. This is in contrast to the behaviour of bicyclo[2.1.0]pentane, which reacts in the presence of PhWCl3/EtAlCl2 to cyclobutene in 70% yield . 

The B-alkyl-9-BBN undergoes an interesting reverse reaction to afford the parent alkene when treated with benzaldehyde. Consequently, the reaction is uniquely employed for the synthesis of exocyclic olefins (Chart 24.3). The hy-droboration of cyclic olefins with an internal double bond, followed by homologation with carbon monoxide in the presence of lithium trimethoxyaluminum hydride afford B-(cycloalkylmethyl)-9-BBN. This intermediate on treatment with benzaldehyde leads to an exocyclic methylene compound (Chart 24.3) [16]. Since the synthesis proceeds from the cycloalkene, thus it provides a valuable alternative to the customary methylenation of carbonyl compounds by Wittig and related procedures. The method also provides a clean synthesis of deuterium-labeled compounds (Eq. 24.10) [16], without positional scrambling or loss of label. Consequently, methylmethylene-d -cyclopentane in 52% isolated yield is obtained. 

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