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

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 k^B, the total rate constant for the reaction between boldine and O₂(¹ω_g), vary from 3.44 × 10⁷M^-1 s^-1 in benzene to 150.00 × 10⁷M^-1 s^-1 in water. For glaucine, similar values were determined. Quenching is faster in solvents with high capacity to stabilize a charge or dipole (higher π* parameter values). From these results, a charge-transfer quenching mechanism, to account for the interaction between boldine and O₂(¹Δ_g), 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 π* parameter. This last result can be explained in terms of a more polar transition state leading to products.