Photochemistry of 1-cyclohexenyl phenyl ketone in enol ether solvents. Trapping of the primary photoproduct by formation of [4+2] adducts in high yields

Abstract

Photolysis (>300 nm) of 1-cyclohexenyl phenyl ketone ( 1) in ethyl vinyl ether (EVE) or in the methyl homologues of EVE, ethyl E-1-propenyl ether and ethyl Z-1-propenyl ether, in each case gives exactly two stereoisomers of the [4 + 2] head-to-head adducts of the enol ether double bond to the olefin-carbonyl moiety of 1. The two stereoisomers obtained from the E-propenyl ether are different from the two stereoisomers obtained from the Z-propenyl ether; all four of them arise from suprafacial addition to the enol ether double bond. As shown in some detail in the case of EVE, besides the two [4 + 2] adducts smaller amounts of other products are formed, some of which have been known before from the enol ether-free system, which stem from the oxyallyl 3 formally derived from 1 by cyclisation. Absolute and relative quantum yields of formation of the diverse products in dependence on EVE concentration allow the conclusion that the photo-intermediate A that gives the [4 + 2] adducts with EVE, does so in competition with both reversion of A to ground state 1 and with ring closure of A to 3. It is concluded that A is the highly strained ground-state trans double-bond isomer of 1, viz. 2, formed from 1 with a quantum yield of ca. 0.36 in toluene–EVE mixtures. Even in neat EVE, reversion to 1 accounts for about half of the 2. The cyclisation of 2 to 3 is about three times faster in the polar acetonitrile than in the nonpolar toluene, relative to reversion to 1.