,
Dramatic Enhancement of Enone Epoxidation Rates in Nonionic Microemulsions
Chemistry – A European Journal, 12 (29): 7565-7575 (2006)
DOI: 10.1002/chem.200600550
Аннотация
The ability of microemulsions to dissolve polar and non-polar components with a huge internal interface can overcome the reagent incompatibilities frequently encountered in organic reactions. We investigated model epoxidation reactions of α,β-unsaturated enones and alkaline hydrogen peroxide in different nonionic microemulsions, both in the presence and absence of a phase-transfer agent (PTA). The obtained reaction profiles were compared with those for the corresponding surfactant-free two-phase systems. In addition, we defined a time constant τ as a measure for the rate of turnover. The epoxidation of trans-chalcone using an n-alkyl-polyoxyethylene surfactant based microemulsion was fastest in the system with the PTA (τ=66 min) and slightly slower without the PTA (τ=77 min). It was still slower in the two-phase system with a PTA (τ=114 min) and extremely sluggish without a phase-transfer agent. With n-alkyl β-D-glucopyranoside as the surfactant the conversion was twice as fast than in the former microemulsion systems, but the PTA did not accelerate the reaction further (τ=35 and 33 min). The epoxidation of vitamin K3, the second model system, was extremely accelerated. It proceeded a factor of approximately 35 faster in the microemulsion (τ=1.44 min) than in the corresponding two-phase system (τ=57 min).
