Suprafacial [1,3]-sigmatropic benzyl shift in 1,4-dibenzyl-1,4-dihydro-2,6-diphenylpyrazine

Abstract

The previously postulated intermediacy of 1,4-dibenzyl-1,4-dihydro-2,6-diphenylpyrazine (Ia) in the rearrangement to 1,2-dihydropyrazine (IIa) is demonstrated and the reaction proceeds in 95 ± 2% yield with first-order kinetics: crossover recombination experiments show only 12 ± 6% intermolecular contribution from a radical dissociation-recombination process which is prevented with butanethiol scavenger, and chiral (Id) rearranges in the presence of the scavenger with 95% stereospecificity and with inversion of the migrating group indicating an 88 ± 6% component of a concerted [1,3]-singmatropic shift with suprafacial allylic utilisation.