Mechanistic insights into catalyst-dependent divergent cycloaddition reactions via discrimination between diazo compounds

Abstract

Catalyst-based divergent synthesis has emerged as a promising strategy since it enables the synthesis of structurally distinct products from the same starting reactants. In this regard, the combination of vinyldiazo reagents and metal catalysts provides highly selective methods for the effective synthesis of cyclic compounds. Herein, the mechanisms of the divergent cycloaddition reactions of two different diazo compounds catalyzed by copper(I) or dirhodium(II) were investigated using density functional theory (DFT) calculations. The underlying discrimination mechanism and the origins of catalyst-dependent selectivity are disclosed. The Cu(I)-catalyzed [3 + 3]-cycloaddition occurs via an unexpected mechanism that involves [3 + 2]-cycloaddition and formation of a free carbene intermediate. In the case of the dirhodium(II) catalyst, the exclusive generation of the ketene-N,O-acetal complex proceeds via water-assisted keto–enol tautomerization rather than the direct 1,4-H transfer. Dispersion interactions play a key role in ligand-controlled regioselectivity for the dirhodium(II) catalyst. The mechanistic understandings provide insights into the unique reactivity of metallo-enolcarbenes.