Base catalysed rearrangements involving ylide intermediates. Part 10. The effects of torsional strain in the transition state upon [1,2] and [3,2] rearrangements

Abstract

The [3,2] and [1,2] sigmatropic rearrangements of the bicyclic ammonium ylides (8), (11), and (14) are inhibited by the bicyclic framework which prevents correct orbital alignment for π-bonding in the transition state for a concerted [3,2] sigmatropic rearrangement or in the vinylogous nitroxide component of the radical pair required as an intermediate in a [1,2] sigmatropic rearrangement. The ylides (14a) and (14d), based upon the 1-azabicyclo-[3.3.1]nonane system, are the only bicyclic allylic ylides which undergo a clean [3,2] rearrangement, which involves the exo-transition state either exclusively (14a) or predominantly (14d).