ALLENES AND ACETYLENES .22. MECHANISTIC ASPECTS OF THE ALLENE-FORMING REDUCTIONS (SN2' REACTION) OF CHIRAL PROPARGYLIC DERIVATIVES WITH HYDRIDE REAGENTS

Four chiral 3-decyn-2-yl derivatives (la-d, Scheme I) were treated in THF with various aluminum hydride reagents selected to give high yields of 2,3-decadiene. The preferred mode of substitution was deduced from the known absolute configurations of starting material and product. The effect of the temperature on the stereoselectivity was also studied and the thermodynamic parameters were calculated. The use of hydroxy, tertiary amine, or bromide as the leaving group (in compounds la, Ic, and Id, respectively) yielded the allene in a preferred overall syn mode of substitution, the degree of which increased with temperature. The mesylate (lb) with lithium trimethoxyaluminum hydride yielded the allene in an anti displacement which was more preferred at lower temperatures. The mechanisms of these reductions are discussed. The chiral allenic alcohols (11 and 12) were also prepared via lithium aluminum hydride reductions of chiral acetylenic derivatives (Schemes III and IV). © 1979, American Chemical Society. All rights reserved.