Quenching of singlet oxygen by tertiary aliphatic amines. Structural effects on rates and products

A kinetic and product study of the reaction of a series of alpha-methyl-substituted N-methylpiperidines with thermally generated O-1(2) in MeCN was carried out. It was found that as the number of alpha-methyl groups (Me in alpha-position relative to the N-atom) increases, the rate of O-1(2) quenching (physical plus chemical) slightly decreases. This finding shows that, with respect to the reaction rate, steric effects are much more important than electronic effects as the latter should have produced the opposite result. The opposite outcome was instead found for the chemical quenching that leads to the N-demethylation products and N-formyl derivatives. The same trend was observed for the ratio between N-demethylation and formation of the N-formyl derivatives (NH/NCHO ratio). All these results are consistent with the mechanism reported in Scheme I where an exciplex is first formed that by a H-atom transfer process produces an alpha-amino-substituted C-radical. The latter forms the product of N-demethylation by one electron oxidation, or affords the N-formyl derivative by radical coupling (Scheme 1). Similar results were obtained with N,N-dimethylcyclohexanamine. However, this 'acyclic' amine exhibited behaviors quite distinct from those of the N-methylpiperidines series, with respect to reaction rate, extent of chemical quenching, and NH/NCHO ratio.