The relative sign of through-bond and through-space interactions; “sigma assistance” of cyclization and intramolecular hydrogen transfer

Abstract

A perturbational frontier MO model indicates that the relative sign of the through-bond and through-space interaction in nonconjugated bifunctional molecules alternates with the number (N) of intervening sigma bonds. The qualitative predictions from the FMO model are in full accord with quantitative computational and experimental literature data, the latter being mainly available for N<4. It is suggested that alternation of the relative sign of through-bond and through-space interactions with N effect—for which the name “sigma assistance” is proposed—may account for the preference of radical cyclizations to yield odd membered instead of even membered rings and for the preference of hydrogen transfer to occur via an even membered transition state.