Arylcyclopropane photochemistry. Part 3. The photochemistry of 1-phenyl-4,4,5-trimethyl-3-oxabicyclo[3.1.0]hexan-2-one
The photochemistry (wavelength of irradiation 254 nm) of the 3-oxabicyclo[3.1.0]hexan-2-ones (2a and b) in benzene and acetonitrile in the presence and absence of an added alkanol has been studied. Product formation is explained in terms of the formation of two intermediate ketens. One, containing an oxiranyl group, is trapped by trimeric methanol (if present) with formation of ester (3b) with simultaneous loss of a carbonyl compound. In the absence of alkanols this oxiranyl keten intermediate reacts in two ways: (i) thermal ring closure with reformation of (2a or b) and (ii) photolysis with formation of carbon monoxide. The other keten intermediate is proposed to be formed from a vibrationally excited 1S state of (2a and b) by loss of a carbonyl compound. It is trapped by an alkanol (if present) with formation of ester (4). Without added alkanol it is photolysed with formation of carbon monoxide. It is concluded from the ratio of the photoproducts (3) and (4) that the cyclopropane bond of a 1-phenyl-3-oxabicyclo[3.1.0]hexan-2-one to be cleaved is preferentially that which is more highly substituted and which has the better orbital overlap with the π-system of the carbonyl group.