SUBSTITUENT EFFECTS ON THE REGIOCHEMISTRY OF ENONE-ALKENE 2-PHOTOCYCLOADDITIONS - EXPERIMENTAL RESULTS AND FMO ANALYSIS(2)

The regiochemical selectivities (i. e., head-to-head (hh) and head-to- tail (ht)) of 2+2-photocycloaddition reactions of cyclic alpha,beta-un saturated carbonyl compounds (1, 2, 5, 8, 9) with substituted alkenes have been determined experimentally. The observed preferences, associa ted with a two-step mechanism via biradical intermediates for addition of ground state alkenes to triplet enones, were analyzed by use of fr ontier molecular orbital (FMO) methods using molecular orbital energie s and coefficients of corresponding species calculated by the PM3-Cl m ethod. Regiochemical selectivities for the two-step 2+2-photocycloaddi tions were then analyzed in terms of two-center HSOMO-LUMO and LSOMO-H OMO interaction factors Sri corresponding to bonding between the enone triplet and alkene leading to formation of biradical precursors of th e respective head-to-head and head-to-tail adducts. In addition, intra molecular interaction factors Sr2, corresponding to cyclization of the biradical intermediates were also determined. Good correlations were seen between the calculated interaction factor differences(e.g. Srl(3 beta)-Srl(2 beta)) and logarithims of the experimentally determined ad duct ratios (e.g. log hh/ht). This observation suggests that the excit ed state 2+2-cycloaddition regioselectivities are mainly determined in the initial biradical producing step of this process but that they ca n be altered py partitioning of the biradical intermediates to product vs. ground state reactants. (C) 1997 Elsevier Science Ltd.