Is phenyl a good migrating group in the rearrangement of organoborates generated from sulfur ylides?

Abstract

Calculations show that the unexpected low phenyl migratory aptitude observed in reactions of mixed alkyl–aryl boranes with benzylic sulfur ylides can be attributed to (1) a conformational issue, (2) the reduction of the usual neighbouring effect of the phenyl in the transition state by the benzylic nature of the migrating terminus, (3) steric hindrance suffered by the larger phenyl group migrating to the hindered migrating terminus and this despite (4) the increase in the barrier to alkyl migration by the presence of a ‘non-migrating’ phenyl on the boron atom.