T. Suishu et al., SUBSTITUENT EFFECTS ON THE REGIOCHEMISTRY OF ENONE-ALKENE 2-PHOTOCYCLOADDITIONS - EXPERIMENTAL RESULTS AND FMO ANALYSIS(2), Tetrahedron, 53(10), 1997, pp. 3545-3556
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.