CrIII(salen)Cl Catalyzed Asymmetric Epoxidations: Insight into the Catalytic Cycle

Intermediates of chromium-salen catalyzed alkene epoxidations were studied in situ by EPR, 1H and 2H NMR, and UV-vis/NIR spectroscopy (where chromium-salens were (S,S)-(+)-N,Nʹ-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino chromium(III) chloride (1) and racemic N,Nʹ-bis(3,4,5,6-tetra-deuterosalicylidene)-1,2-cyclohexanediamino chromium(III) chloride (2)). High-valence chromium complexes, intermediates of epoxidation reactions, were detected and characterized by EPR and NMR. They are the reactive mononuclear oxochromium(V) intermediate (A) CrVO(salen)L (where L = Cl- or a solvent molecule) and an inactive chromium-salen binuclear complex (B) which acts as a reservoir of the active species. The latter complex demonstrates an EPR signal characteristic of oxochromium(V)-salen species and 1H NMR spectra typical for chromium(III)-salen complexes, and it is identified as mixed-valence binuclear L1(salen)CrIIIOCrV(salen)L2 (L1, L2 = Cl- or solvent molecules). The intermediates CrVO(salen)L and L1(salen)CrIIIOCrV (salen)L2 exist in equilibrium, and their ratio can be affected by addition of donor ligands (DMSO, DMF, H2O, pyridine). Addition of donor additives increases the fraction of A over that of B. The same two complexes can be obtained with m-CPBA as oxidant. Reactivities of the CrVO(salen)L complexes toward E-(beta)-methylstyrene were measured in DMF. The L1(salen)CrIIIOCrV(salen)L2 intermediate has been proposed to be a reservoir of the true reactive chromium(V) species. The chromium-salen catalysts demonstrate low turnover numbers (ca. 5), probably due to ligand degradation processes.