Leaving groups 1,2-metallate rearrangements

Most electrophilic allylic rearrangements involve loss of hydrogen, but they have also been observed with metallic leaving groups. Sleezer, Winstein, and Young found that crotylmercuric bromide reacted with HCl 10 times faster than n-butyl-mercuric bromide and the product was >99% 1-butene. " These facts point to an SeI mechanism (lUPAC designation cyclo- l >ID k nk ... 

Another consequence of the high electron density around a cyclopropane is that attack by metal ions or protons can initiate cationic, formal cyclopropyl to allyl rearrangements in which a leaving group X is not a prerequisite (see also Houben-Weyl, Vol.4/3, pp 651-654). 

Other leaving groups like the enantiomerically pure imidate85 (Table 8, entry 11) can also be used to give the substituted product with net retention of configuration. Without an external nucleophile, transition metal-catalyzed Claisen rearrangement occurs for allyl imidates. 

In general, [M(Con)]" and [M(ODC)] react with electrophiles at C-5 (or C-15) under mild or stoichiometric conditions, and at C-5 and C-15 under more forcing conditions. Nitration appears to be less selective. Meso substitution is more or less reversible, and the substituents may be removed under acidic conditions if they are reasonably good leaving groups (Schemes 89 and For [Ni(ODC)] , nucleophiles also react at the meso positions (Scheme 91). Thermal rearrangement of Co "(ODC)(CN)2 accompanies metal reduction and Co (5-CN-ODC) and Co (5,15-(CN)20DC) are obtained, probably via radical reaction. ... 

Solanesol and other prenyl alcohols are important as metabolites in mulberry and tobacco leaves and in the synthesis of isoprenoid quinones. Hence, Sato and collaborators107 have developed a stereoselective synthesis of all-trans-polyprenol alcohols up to C50. Construction of the requisite skeletons was accomplished by the alkylation of a p-toluenesulphonyl-stabilized carbanion, followed by reductive desulphonylation of the resulting allylic sulphonyl group. This was achieved most efficiently by the use of a large excess of lithium metal in ethylamine (equation (43)), although all reaction conditions led to mixtures. The minor product results from double bond rearrangement. 

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