Axial Coordination Dichotomy in Dirhodium Carbenoid Catalysis: A Curious Case of Cooperative Asymmetric Dual-Catalytic Approach toward Amino Esters

One of the most recent developments in asymmetric catalysis is to employ two or more catalysts under one-pot reaction conditions. This article presents some interesting mechanistic insights on a cooperative dual-catalytic protocol relying on the catalytic ability of dirhodium carbenoid (derived from rhodium(II) tetracarboxylate and a diazo compound) and a chiral spirophosphoric acid ((R)-SPA) in an asymmetric NH insertion reaction. We have employed DFT(M06 and B3LYP) computational methods to identify the stereocontrolling transition states wherein a chiral (R)-SPA protonates a dirhodium-bound enol intermediate. A true cooperative action elicited by both catalysts has been noted in the enantioselective protonation. More importantly, whether the second axial ligand on the remote rhodium atom could influence the energetic features of the reaction has been probed for the first time. In all steps (such as nitrogen extrusion, addition of amine to the dirhodium carbenoid, and the enol formation), except that in the stereocontrolling event, no major effect of axial ligation has been noticed. However, the presence of the axial ligand helps in stabilizing the protonation transition state and reduces the activation barrier for protonation, suggesting a vital role in stereoselectivity. The predicted sense of stereoselectivities is in good agreement with the experimental results.