Transannular interaction in bifunctional olefins and ketones: A semiempirical electron density analysis

Electron densities on the olefinic carbons of some dimethylene compounds ar e calculated by semiempirical model PM3, and the values are compared with t hose of the corresponding monomethylene compounds. A relationship is observ ed between the change of electron density Delta(C) and the change of C-13 N MR chemical shift Delta delta. A correlation is also found between Delta(C) and Delta(O), i.e. the carbon and oxygen atoms, of methylene-ketones. The electron density differences are induced by three types of transannular int eractions: electrostatic force, orbital interactions through bond (TB) and orbital interactions through space (TS). Electrostatic force is the most pr ominent factor for ketones in which the high polarity of carbonyl group can apply a strong field effect onto nearby atoms. For nonpolar methylene grou ps, orbital interactions can be transmitted either TS or TB more effectivel y than electrostatic force. A conformational analysis, performed on a dimet hylene compound 4e, confirms the existence of TS interaction between the do uble bonds. (C) 1999 Elsevier Science Ltd. All rights reserved.