o-Quinonoid compounds. Part 16. 1,5-Shift of vinyl groups in 1,3-dimethylindenes; product studies and migratory aptitudes of substituted vinyl groups

Abstract

Several 1-vinylindenes (1; X = substituent) undergo clean thermal rearrangement to 2-vinylindenes (2; X = Me, Ph, C₆H₄NO₂-p, CHO, CO₂Et, CN, CO₂Ph, COPh, CONHMe, and COMe). Racemisation rates for the optically active indenes E-(1) provide migratory aptitudes for E-substituted-vinyl groups in the 1,5-shift leading to the 2H-indene intermediates (2). The migratory aptitude of CHCH₂ is much greater than that of Ph, and E-substituents on a vinyl group produce much larger effects than p-substituents on a phenyl ring. Migratory aptitudes of E-(CHCHX) correlate with σ_R- values of the substituent X, as well as with the rates of morpholine addition to the olefins CH₂CHX and the pK_a ofthe carbon acids CH₃X. The activating effect of X on the 1,5-vinyl shift is similar to its effect on the dienophilic character of olefins (COCI > COPh > COMe > CN > CO₂R > H > Me). Z-Vinyl groups migrate faster than the corresponding E-vinyl groups in accord with preferred migration via an exo-arrangement (4).