Role of MnIV species in Mn(salen) catalyzed enantioselective aerobic epoxidations of alkenes:: an EPR study

Using EPR spectroscopy, high valence Mn(salen) complexes were monitored in Mn-III(salen)/isobutyraldehyde/O-2 and Mn-III(salen)/pivalaldehyde/O-2 catalytic systems in the absence and in the presence of N-methylimidazole (N-Me-Imd), where Mn-III(salen) (1) is (SS)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-manganese(III) chloride. Interaction of 1 with aliphatic aldehydes in the presence of 0, was found to give a mixture of oxo manganese(IV) species L-1(salen)Mn-IV = O (2) with various axial ligands L-1 (L-1 = Cl-, RCOO-). They display EPR signals having close g values of ca. 5.2 with resolved hyperfine structures from one manganese (A = 75 G Mn-55, I = 5/2). In the presence of N-Me-Imd, new manganese(IV) complexes were detected. The first one was shown to be an antiferromagnetically coupled mixed-valence [(salen)Mn-IV(mu(2)-O)(2)Mn-III(salen)](-) dimer having 16-line EPR signal at g = 2. The second one is probably (salen)Mn-IV(OOCOR)N-Me-Imd acylperoxo complex (2) (g = 4.2 (R = CH(CH3)(2)), or g 4.7 (R = C(CH3)(3)), unresolved manganese hyperfine structure). Possible roles of each complex in the enantioselective alkene epoxidations are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.