Absolute configuration of allenes, conversion of molecular dissymmetry into centrodissymmetry by heterogeneous catalytic hydrogenation

Abstract

Palladium-catalysed hydrogenation of 4-methylhexa-2,3-dienoic acid and its methyl ester has been studied, by product analysis, over the complete range of hydrogen absorption. 4-Methyl[1-¹⁴C]hex-cis-2-enoic and 4-methyl[1-¹⁴C]hex-3-enoic acid and methyl ester have been employed in co-hydrogenations with the allene, which show that the substantial amount of 4-methylhex-trans-2-enoic acid (or ester) formed in the first stage arises neither directly from the allene, nor by isomerisation of the 3-enoic acid, but by stereomutation of 4-methylhex-cis-2-enoic acid on the catalyst. The conversion of molecular dissymmetry into centrodissymmetry by catalytic hydrogenation can therefore be used to determine the absolute configuration of allenes.

(–)-Methyl 4-methylhexa-2,3-dienoate gave (–)-(R)-methyl 4-methylhexanoate on catalytic semi- and full-hydrogenation and, from analysis of steric effects in the transition state for adsorption, the (–)-allene ester has the R-configuration. Semihydrogenation of the (+)-allenic acid, followed by esterification, gave methyl (+)-4-methylhex-2-enoate, oxidised and esterified to (+)-(S)-methyl 2-methylbutyrate: this provides a variant of the method. The assumption of cis-addition of hydrogen from the catalyst surface is formally proved by hydrogenation of (+)-methyl 4-phenyl-penta-2,3-dienoate of established S-configuration.