Importance of charge-transfer effects in regiochemistry of 1,3-dipolar cycloadditions between azides and substituted ethylenes

The self-consistent charge and configuration method for subsystems (SCCCMS) is used to study the cycloaddition reactions of 1,3-dipoles such as R-N-3 (R = H and CH3) azides with H2C=CHX (X = H, F, Cl, CH3, OH, and CN) ethylen es acting as dipolarophiles. All calculations are performed for reactive sy stems (1,3-dipole/dipolarophile) in transition-state (TS) geometries. Speci al emphasis is put on the components of interaction energy and on DFT-based reactivity indices. It is shown that charge-transfer (CT) energy is not re sponsible for the observed trends in regioselectivity though its stabilizin g influence cannot be disregarded. The CT energy is the only term, which is very sensitive to the type of substituent in azide and ethylene molecules. The main factor governing the orientation in 1,3-dipolar cycloaddition is the Heitler-London energy. Present analysis also confirms adequacy of a sim ple two-reactant regional softness matching rule, called the maximum comple mentarity rule, for predicting more stable configurations. (C) 2001 Elsevie r Science B.V. All rights reserved.