Synthesis of meso- and (±)-3,4-dimethylhexa-1,5-diene and their acetylacetonatorhodium(I) complexes

Abstract

Controlled addition of dry hydrogen chloride to a mixture of propionaldehyde and trans-crotyl alcohol at –5° gives cis- and trans-prop-1-enyl trans-but-2-enyl ether which are separable by spinning-band fractional distillation. At 142° the cis-isomer is cleanly converted into erythro-2,3-dimethylpent-4-enal, the trans-isomer giving the threo-isomer. The aldehydes are separately transformed into meso- and (±)-3,4-dimethylhexa-1,5-diene respectively by reaction with methylenetriphenylphosphorane in dimethyl sulphoxide; the isomeric purity of the resulting olefins is 95%(meso) and 96·5%[(±)]. The dienes react with acetylacetonato(bis-ethylene)rhodium(I) to form the corresponding hexadiene complexes; evidence pertaining to their stereochemistry, relative stability, and thermal stability is presented.