A kinetic study of the reaction between boldine and singlet oxygen [O2(1Äg)]

Abstract

The sensitized photo-oxidation of boldine and its permethylated derivative, glaucine, has been studied in several solvents using steady-state kinetic methods. Values of kB, the total rate constant for the reaction between boldine and O2(1Äg), vary from 3.44 × 107 M21 s21 in benzene to 150.00 × 107 M21 s21 in water. For glaucine, similar values were determined. Quenching is faster in solvents with high capacity to stabilize a charge or dipole (higher ð* parameter values). From these results, a charge-transfer quenching mechanism, to account for the interaction between boldine and O2(1Dg), is proposed. Rate constants for the chemical reaction are at least two orders of magnitude smaller than the total rate constants, which also show a large dependence on the ð* parameter. This last result can be explained in terms of a more polar transition state leading to products.