6818-10-6Relevant articles and documents
Carbenes in a Rigid Matrix. Substituent Effects on the Temperature Dependence of α-Carbonylcarbene Reactions
Tomioka, Hideo,Okuno, Hiroshi,Izawa, Yasuji
, p. 1636 - 1641 (2007/10/02)
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 deg 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 supressed in a rigid matrix at -196 deg C, whereas that of (2b) was not appreciably supressed even at -196 deg 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.
