Selectivity and charge transfer in photoreactions of donor-acceptor systems. Part 10. meta Photocycloadditions of m-bis(trifluoromethyl)benzene and trifluoromethyl benzonitriles to olefins

Abstract

Photocycloadditions of m-bis(trifluoromethyl) benzene and meta- and para-trifluoromethylbenzonitriles to cyclopentene and cyclohexene yield preferentially 2,4-(from meta arene) and 2,5-(from para arene) disubstituted meta cycloadducts. This is in accordance with the correlation between reaction mode and free enthalpy of electron transfer, calculated by means of the Weller equation. The high regioselectivities of these meta cycloadditions support the involvement of dipolar intermediates which are stabilized in position 1′ and/or 3′ by the acceptor substituents. Products formed from p-trifluoromethylbenzonitrile usually have the cyclopentane ring in the ‘normal’endo configuration whilst those from cycloadditions of m-bis(trifluoromethyl)benzene to cyclopentene or cyclohexene give rise to products with an exo configuration.‡ This is discussed in terms of steric hindrance with substituents in position 2 and 4 of the meta adducts (position 1′ and 3′ of the dipolar intermediate). m-Trifluoromethylbenzonitrile leads to products with diverse stereoselectivities; i.e. location of CN(CF₃) in position 2 of the meta adduct prefers the exo(endo) configuration of the cyclopentane ring. This effect may indicate an unsymmetrical approach of cyclopentene onto m-trifluoromethylbenzonitrile during the 4′,6′attack.