The reactivity of carbenes from photolysis of diazo-compounds towards carbon–hydrogen bonds. Effects of structure, temperature, and matrix on the insertion selectivity

Abstract

Direct and/or sensitized photolyses of diazo-acetate (1a) and -malonate (1b) in hydrocarbons and ether were investigated at various temperatures in order to learn more about the nature of the C–H insertion process and the structural factors governing positional selectivity within the matrix. Photolysis of the diazo-compounds in a rigid matrix resulted in a marked decrease in the insertion selectivity, which may be interpreted as indicating that the matrix imposes severe steric demand especially on the direct C–H insertion process of the singlet carbene. The addition of a sensitizer in matrix photolysis causes a marked increase in the selectivity in the case of (1a), as is observed in the comparable liquid-phase experiment, but it causes a decrease in the case of (1b). This is interpreted as suggesting that the excited triplet (1b) itself is involved in C–H insertion under these conditions. More extensive temperature studies show that, as the temperature decreases, the C–H insertion selectivity of :CHCO₂R decreases regularly regardless of the reaction phase, whereas that of :C(CO₂R)₂ increases in the liquid phase but decreases in the solid phase. This difference in the temperature dependence is explained by assuming that the singlet carbene is responsible for the C–H insertion of :CHCO₂R throughout the temperature range studied, while both singlet and triplet are involved in the insertion of :C(CO₂R)₂.