o-Quinonoid compounds. Part 17. Evidence for exo-selectivity in the 1,5-sigmatropy of acyl and vinyl groups

Abstract

Arguments and experiments are presented which suggest that in the 1,5-sigmatropic shift of acyl groups in 1-acylindenes the acyl group prefers to adopt a transition state (t.s.) arrangement with the acyl-oxygen directed away from the indene ring (3-exo) rather than the arrangement (3-endo) with the acyl-oxygen over the indene ring. The order of migratory aptitude HCO Me₂CHCO > MeCO > Bu^tCO is consistent with this view, as is the slow racemisation of the cyclic ketones (11) and (18) constrained to adopt t.s. arrangements of the 3-endo type. Similar exo-selectivity in the migration of vinyl groups is indicated by comparing the racemisation rates of the olefins (20) and (21) at 250 °C; (21), which can give the transient and symmetric 2H-indene (23)via a t.s. of exo-type, racemises at least 50 times faster than (20), which must rearrange [to (22)] by a t.s. of endo-type.