Olefin epoxidation with tert-butyl hydroperoxide catalyzed by MoO2X2L complexes: a DFT mechanistic study

DFT calculations suggest that the catalytic epoxidation of olefins by Mo(vi) complexes, modeled by MoO2Br2(MeN=C(H)-C(H)=NMe), in the presence of MeOOH, the model for tert-butyl hydroperoxide, starts with a hydrogen transfer from the peroxide to one of the terminal Mo=O oxygen atoms and the remaining MeOO anion binds as a seventh ligand, forming a five-membered Mo-O(alpha)-O(beta)(Me)...H-O-Mo ring held together by a hydrogen bond. In the second step, a concerted approach of ethylene to the Mo-O(alpha) bond gives rise to an intermediate containing a seven-membered Mo-C-C-O(alpha)-O(beta)(Me)...H-O-Mo ring. In the final step, decomposition of the intermediate leads to the starting complex, alcohol and the epoxide. The activation energy for the addition of the olefin (second step) is the highest one, in agreement with available kinetic studies showing that the catalyst formation is not always a rate-limiting step. DFT calculations also show that the alcohol by-product (MeOH) can react with the starting complex, competing with ROOH and hence leading to the progressive catalyst poisoning, which has been observed experimentally.