The Effect of Strain on the RhI-Catalyzed Rearrangement of Allylamines

A multitude of challenges facing modern synthesis have an enamine component as part of a solution. Transition-metalcatalyzed rearrangement of allylamines to enamines has occupied a prominent place among catalytic transformations. Similar to more conventional synthetic approaches to enamine intermediates, this chemistry typically leads to the formation of (E) isomers. Recently, a Rh-I-catalyzed rearrangement of N-allylaziridines to (Z)-N-alkenylaziridines was reported with kinetic stereoselectivities between 75: 25 and 95: 5 (Z/E). This exciting result warranted a mechanistic explanation. Herein we describe the results of quantum chemical calculations [B3LYP/6-31+G(d,p)-LANL2DZ] aimed at evaluating competing mechanisms for this isomerization. We have compared and contrasted two mechanisms for the rearrangement: one that proceeds through an azametallocyclo-pentene intermediate and another through a hydrometalation/beta-hydride elimination. We find that the latter mechanism corresponds to the lower energy pathway, which, counterintuitively, exhibits a kinetic preference for the formation of products with (Z) C=C double bonds. A close correspondence between product ratios determined by simple Boltzmann distribution calculations based on these theoretical results and the previously reported ratios is observed. Lastly, we have examined whether the observed stereocontrol is exclusive to strained amines, and we find that the unique characteristics of the aziridine ring, compared to other amines, prove to be essential. Given the privileged status of the enamine functional group in synthesis, application of the "strain effect" in a range of metal-catalyzed processes is expected to have broad consequences.