Further Studies on the Allylic Diazene Rearrangement

Former graduate student Wei Qi and Professor Matt McIntosh have reported diastereoselective reductive 1,3-transpositions of acyclic alpha, beta-unsaturated tosyl hydrazones to afford substrates with a 1,4-syn or 1,4-anti relationship between alkoxy and methyl groups that proceed via an ADR (Qi, W.; McIntosh, M. C. Org. Lett. 2008, 10, 357; Qi, W.; McIntosh, M. C. Tetrahedron 2008, 64, 7021). In these reports, silica gel was employed to accelerate the reduction. We have found that acetic acid gives the same results with high diastereoselectivity in the reaction. We further optimized the reaction by lowering the amount of catecholborane to 3 eq. Effects of hydrazone E/Z geometry and implication for reaction mechanism were also investigated