KINETIC STUDIES OF SINGLET OXYGEN [4+2]-CYCLOADDITIONS WITH CYCLIC 1,3-DIENES IN 28 SOLVENTS

摘要

The [4 + 2]-cycloaddition of singlet oxygen with 1,4-dimethylnaphthalene (DMN) and derivatives has been studied in 28 solvents by laser flash photolysis and steady state photolysis. The bimolecular rate constants of singlet oxygen quenching by DMN, via a physical process (k(q)) and via a chemical reaction (k(r)), are solvent-dependent and increase by more than 2 orders of magnitude from cyclohexane to formamide. This significant solvent dependence is in contrast with previous investigations conducted in six or seven solvents. It is discussed in terms of the solvatochromic properties of the different solvents. Moreover, for seven water-soluble 1,3-dienes, the overall rate constant k(0) (= k(r) + k(q)) is much higher in water than in methanol. Our results are consistent with a two-stage mechanism implying as a first step an equilibrium producing an exciplex with charge transfer character. A shift toward the formation of this exciplex leading to the cycloaddition product occurs by an increase (i) in the Hildebrand solubility parameter delta(H), (ii) in the dipolarity-polarizability parameter pi* of the solvent, and (iii) in the solvophobicity of the diene. [References: 40]