Regioselective control of allyl anions with cadmium chloride: α vs. γ condensation with aldehydes

✍ Scribed by Lei Bo; Alex G Fallis

• Publisher: Elsevier Science
• Year: 1986
• Tongue: French
• Weight: 192 KB
• Volume: 27
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(00)85166-7

✦ Synopsis

Summery:

A general method for directing the condensation of ally1 anions such as 2 with aldehydes preferentially to the y position of the unsaturated nucleophile is described.

The procedure employs cadmium chloride to control the regioselectivity and will facilitate the extended use of vinylogous anions in synthesis.

There is considerable current interest in

the synthetic application of allylic carbanions such as those employed in the vinylogous Reformatsky reaction, and related species derived from ketene dithioacetals. l-3 The control of Q vs. y substitution in heteroatom-stabilized allylic anions and resonance-stabilized enolates depends upon the complex interplay between the nature of the heteroatom(s), charge delocalization, steric effects, solvation, the type of electrophile, and the counterion. These difficulties are compounded by the observation that halides and carbonyl systems often exhibit opposite regioselectivities.3 We wish to report that organocadmium reagents generated ill otiu from the corresponding alkyl lithium derivatives condense preferentially with aldehydes at the y position of the allylic anion.

This is in contrast to the reported behaviour of the "crotyl-cadmium" reagent prepared from crotyl magnesium bromide, which afforded exclusively the methylallyl adducts. 4 ,:,