Regiochemistry of 1,3-dipolar cycloadditions between azides and substituted ethylenes: a theoretical study

The cycloaddition reactions of 1,3-dipoles such as the simple azides (R-N-3, R = H, CH3) with substituted ethylenes (H2C=CHX, X = H, F, Cl, CH3, OH and CN) as dipolarophiles have been studied by a density functional procedure using the hybrid B3LYP exchange correlation functional and 6-31G(d,p) basis functions. For each system, two distinct transition states have been located corresponding to the relative orientation of two molecules. The energy difference between the two transition states varies from 0.2 to 5 kcal mol(-1). The addition thus shows a certain regioselectivity. This observed regiochemistry is explained using recently developed DFT-based reactivity descriptors, such as local softness and Fukui functions.