Hiroshi G. Okuno

Carbenes in a rigid matrix. Substituent effects on the temperature dependence of α-carbonylcarbene reactions

The temperature dependence of methoxycarbonyl-(1a and b) and benzoyl-carbenes (2a and b) reactions in alcohols has been examined in order to elucidate the scope and limitation of low-temperature photolysis as a tool for detecting triplet carbenes. The results reveal that the method cannot be applied to all carbenes but gives important information on the reactivity and/or multiplicity of ground-state carbenes. Low-temperature photolysis of PhCN2CO2Me in an ethanol matrix, for example, resulted in a dramatic increase in C–H insertion products, probably derived from the triplet (1a)via an abstraction–recombination mechanism, at the expense of the singlet product, i.e. the O–H insertion compound, which was shown to be the main product of photolysis at ambient temperature. In marked contrast, (1b) generated in a propan-2-ol matrix at –196 °C did not result in a major increase in the C–H insertion product. Similar and more contrasting substituent effects on the temperature dependence were observed in the benzoylcarbene system (2). Thus, the Wolff rearrangement of (2a) was almost completely suppressed in a rigid matrix at –196 °C, whereas that of (2b) was not appreciably suppressed even at –196 °C. These differences were explained by considering the effects of the substituent on the ground-state multiplicity of carbene and/or on the relation of the activation energy differences of the singlet and triplet reactions with energy differences between two states.