THERMAL [1,5] SIGMATROPIC ALKYL SHIFTS OF ISOINDENES

2, 2-Dialkylisoindenes are generated by the thermal extrusion of N2O from bicycloazoxy compounds at 180 °C. Such isoindenes undergo facile 1, 5-alkyl shifts which allow the determination of relative migratory aptitudes of a number of alkyl groups : Me : Et : i-Pr : cyclopropylcarbinyl : benzyl = 1 : 6.2 : 5.3 : 7.9 : 55.6. Isolation of di-methylisoindene allowed the determination of activation parameters for its methyl-shift process : log A = 11.0, Ea = 26.1 kcal/mol. The results are discussed in terms of a pericyclic mechanism with the migrating group taking on a significant degree of radical character in the transition state. © 1979, American Chemical Society. All rights reserved.