Photocycloaddition of cyclohex-2-enones to acrylonitrile

Abstract

The photocycloaddition of cyclohex-2-enones 1a, 1b, and 5a–5c to acrylonitrile (2) in benzene has been studied. The reactions proceed with moderate regioselectivity and with preferential formation of diastereomeric mixtures of endo- and exo-5-oxobicyclo[4.2.0]octane-7-carbonitriles 3, 4, and 6–8. Very similar relative rates of conversion to products are observed in the photocycloadditions of 1b, 5a, and 5b to 2 and methacrylonitrile (9), respectively. In contrast, 5,5-dimethylcyclohex-2-enone (1b) forms cycloadducts with 2,3-dimethylbut-2-ene (12) as efficiently as with the unsaturated nitriles, but the 2,5,5-trialkylcyclohex-2-enones 5a and 5b do not react with this alkene at measurable rates of conversion. A dual path mechanism is presented that involves either exciplex formation between triplet-excited cyclohexenones and electron-rich alkenes or radical addition of the former to electron-deficient alkenes bearing a substituent, exercising a resonance effect.Key words: [2+2]-photocycloaddition, cyclobutane, cyclobutanecarbonitrile, radical addition to unsaturated nitriles.