Temperature dependence in the sensitized photoreaction of 3,3-dimethyloxepin-2(3H)-one and 2,2,7,7-tetramethylcyclohepta-3,5-dien-1-one

Abstract

The triplet-sensitized photolysis of the title compounds (1) and (4) has been studied at various temperatures. 3,3-Dimethyloxepin-2(3H)-one (1) gave rise to the cycylisation product 4,4-dimethyl-2-oxabicyclo[3.2.0]hept-6-en-3-one (2)(major), and the 1,2-acyl shift product 7,7-dimethyl-3-oxabicyclo[4.1.0]hept-4-en-2-one (3)(minor, < 5%), at room temperature. It was found that the (3) : (2) ratio depends upon the temperature applied for the photoreactions; the amount of (3) increased at higher temperatures (28% at 70 °C). The photoreaction of 2,2,7,7-tetramethylcyclohepta-3,5-dien-1 -one (4) was also temperature dependent; an increasing amount of the cyclisation product 2,2, 4,4-tetramethylbicyclo[3.2.0]hept-6-en-3-one (5), which is a hitherto unobserved product in the experiment run at room temperature, was produced at lower temperatures (34% at –60°C). The results indicate that the thermodynamic activation parameters of each substrate in such systems play an important role in the preference of either of two competing photoprocesses, cyclisation or 1,2-acyl shift: A similar phenomenon was also observed in the photoreaction of 2,2,7-trimethyl-7-phenylcyclohepta-3,5-dien-1-one (11).