Alkenyl groups conjugate additions

Another type of mixed cyanocuprate has both methyl and alkenyl groups attached to copper. Interestingly, these reagents selectively transfer the alkenyl group in conjugate addition reactions.16 These reagents can be prepared from alkynes via hydrozirconation, followed by metal-metal exchange.17... 

A diastereoselective Rh(I)-catalysed conjugate addition reaction of aryl- and alkenyl-boronic acids to unprotected 2-phenyl-4-hydroxycyclopentenone (207) has been investigated. The free OH group on the substrate was found to be responsible for the (g) stereochemistry, which is cis for arylboronic derivatives (208). In the case of the alkenylboronic compounds, the stereochemistry can be tuned to either a cis (with a base as additive) or trans addition (209) (with CsF as additive), without the need for protecting groups.249... 

This reaction illustrates a stereoselective preparation of (Z)-vinylic cuprates, 5 which are very useful synthetic Intermediates. They react with a variety of electrophiles such as carbon dioxide,5,6 epoxides,5,6 aldehydes,6 allylic halides,7 alkyl halides,7 and acetylenic halides 7 they undergo conjugate addition to a,6-unsaturated esters,5 6 ketones,6 aldehydes,6 and sulfones.8 Finally they add smoothly to activated triple bonds6 such as HCSC-OEt, HC3C-SEt, HC=C-CH(0Et)2. In most cases these cuprates transfer both alkenyl groups. The uses and applications of the carbocupration reaction have been reviewed recently.9 The configurational purity in the final product 1s at least 99.951 Z in the above transformations. 

Primary, secondary, and even tertiary alkyl groups undergo the addititsi reaction, as do aryl and alkenyl groups. Alkynyl groups, however, react poorly ill the conjugate addition process. 

Unsubstituted vinyl-lithium and vinyl Grignard 14 reagents can be made directly from the halide by oxidative insertion of Li(0) and Mg(0). Vinyl-lithium is available as a 2M solution in THF from Alfa and vinyl magnesium bromide, which must be prepared in THF, is available in THF from Aldrich. These are quite stable o-complexes because alkenyl anions are more stable than saturated alkyl anions. They add as nucleophiles to carbonyl groups, e.g. cyclobutanone to give 15 and prefer direct to conjugate addition with enones to give e.g. 13. We have already used them in enone synthesis (chapter 5). 

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