Energy of Concert and Origins of Regioselectivity for 1,3-Dipolar Cycloadditions of Diazomethane.

Mechanisms and regioselectivities in 1,3-dipolar cycloadditions of diazomethane with alkenes have been investigated with density functional theory (DFT) and DLPNO-CCSD(T) computations. The concerted but asynchronous pathway is shown to be favored by at least 6 kcal/mol over the stepwise diradical pathway in all diazomethane-alkene cycloadditions. Frontier molecular orbital calculations indicate a dominant HOMO → LUMO interaction in diazomethane cycloadditions involving electron-deficient alkenes, whereas for electron-rich alkenes, both frontier interactions contribute significantly, and the regioselectivity is governed by frontier coefficients.