Trapping of photoenols from o-tolualdehydes in gas matrices. Dependence of photoenol formation on the nature of the carbonyl function

Abstract

o-Methylbenzaldehyde (1) and o-methylacetophenone (2), matrix-isolated in Ar, N₂, Xe, or carbon monoxide, were irradiated with u.v. light. In contrast to the ketone (2) which was unreactive, the benzaldehyde gave a photoenol in all gas matrices. Irradiation of 2,5- or 2,4-dimethylbenzaldehyde (9) and (10), respectively, also produced photoenols under the above conditions. In propan-2-ol under cryogenic conditions, o-methylacetophenone (2) can be photoisomerized to an enol, probably the consequence of stabilization of the Z-enol by hydrogen bonding. 5,8-Dimethyltetralone (11) does not give a photoenol in gas matrices, although it is reported to give one at 77 K in EPA.^‡ It is proposed that cage effects in gas matrices can significantly alter rates of internal rotation and are determinants in the production of E-enols in this environment. I.r. spectral evidence for photoenols is presented for the first time.