Retention of configuration at the migrating centre in both photochemical and thermal [1,3]-sigmatropic shift of a benzyl group. Relaxation of orbital symmetry control in an unsymmetrical allyl system

Abstract

Photoisomerisation of (+)-1,1-dicyano-2-methyl-4-phenylpent-1-ene (VII) to 3,3-dicyano-2-methyl-4-phenylpent-1-ene (VIII) took place with ca. 85% retention of configuration at the migrating carbon atom and 15% inversion; thermal reversal of the rearrangement occurred with >90% retention and <10% inversion, rather than with the complete inversion expected for a concerted [1,3]-sigmatropic shift in the ground state: this apparent violation of orbital symmetry is attributed to the very unsymmetrical nature of the (dicyano) allyl group across which the α-phenylethyl group migrates.