Experimental Evidence for Multiple Oxidation Pathways in the (salen)Mn-Catalyzed Epoxidation of Alkenes

摘要

The substrate electronic effects on the selectivity in the catalytic epoxidation of para-substituted cis stil-benes 2a-i wre investigated by using (R,R)-[N,N'-bis(3,5-di-tBu-salicylidene)-1,2-cyclohexanediamine]manganese(III) chloride 1 in benzene as the catalyst with iodosobenzene as the terminal oxidant. A Hammett study of the selectivity results reveals a stronger electrophilic character than previously assumed in the (salen)Mn-catalyzed reaction. In general, the best correlations with the experimental values were obtained by using the Hammett #sigma#~+ values, which gave #rho# = - 1.37 for the rate of cis-epoxide formation and #rho# = - 0.43 for the rate of stepwise process leading to the corresponding trans product. The reaction invovles two separate pathways as indicated also by the competitive breakdown of the intermediate on the path to trans epoxide for methoxy-substituted substrates. The asynchronicity in the concerted pathway leading to cis epoxide is apparent for 4-methoxy-4'-nitrostilbene, which yields cis epoxide with 75% ee entirely as a result of electronic effects.