Influence of Quaternary Onium Salts, Crown Ethers and Cryptands on Olefin Epoxidations Promoted by HOCl/ClO- in the Presence of Mn(III)-tetrakis(2,6-dichlorophenyl) porphyrin Chloride

Abstract

Reaction rates of alkene epoxidations, promoted by aqueous NaOCI and catalyzed by Mn(III)-tetrakis(2,6-dichlorophenyl)porphyrin chloride 1 (P) in the presence of a lipophilic axial ligand (L) (e.g. N-hexylimidazole) and carried out under H2O/CH2 CI2 two phase conditions at 0°C, are strongly enhanced by lowering the pH of the aqueous phase from 12.7 to 9.5. Under these conditions, a further relevant increase in the reaction rates can be achieved by adding phase-transfer catalysts (PT), e.g. quaternary ammonium salt 3, lipophilic crown ether 4 or cryptand 5, provided that the amount of L is very small (L/P = I for very reactive alkenes, e.g. cyc1ooctene, and 10 for poorly reactive ones, e.g. l-dodecene). In the case of cyclooctene epoxidation, the use of 0.006-0.03 mol. equiv. of PT completes the reaction in 1-10 min., the initial rates being up to 600 turnovers/min. with (2.2.2,C10) cryptand. In the absence of the axial ligand, the quaternary ammonium salt 3 and cryptand 5 show an inhibitory effect. Such an effect is due to the formation of the poorly reactive Mn(P)CI species, by Cl- extraction to the organic phase. However, dibenzo crown ether 4 does not show this effect. In the presence of 4, and with LIP = I, the I-dodecene epoxidation reaches 94% in 1 min. The ‘unique’ behavior of crown ethers can be explained by their ineffectiveness in extracting alkali chlorides, providing a very low concentration of CI- in the organic phase and thus avoiding the Mn(III)-porphyrin deactivation.