1H NMR and EPR spectroscopic monitoring of the reactive intermediates of (Salen) MnIII catalyzed olefin epoxidation

Using H-1 NMR and EPR spectroscopy, manganese species formed in the catalytic systems 1 + iodosobenzene (PhIQ) and it meta-chloroperoxybenzoic acid (m-CPBA), where 1 is (R,R)-(-)-N, N'-bis(3,5-di-tert-butylsalicylidene)-1,2 cyclohexanediamino-manganese(III) chloride ([(Salen)Mn-III]) were studied. Three types of manganese complexes were characterized in the catalytic system 1 + PhIO (4-6). Complex 4 is very unstable and reacts with styrene at - 20 degrees C to afford styrene oxide. It exhibits three signals of tBu groups at 1.68, 1.64 and 1.42 ppm. This pattern closely resembles that for a model complex [(Salen)Mn-V = N]. Based on these data, 4 was identified as d(2) low-spin oxomanganese(V) complex [(Salen)Mn-V = O](+). Complexes 5 and 6 are relatively stable at -20 degrees C and poorly reactive towards styrene at this temperature. They display H-1 NMR spectra characteristic for antiferromagnetically coupled mu-oxo-dinuclear Mn-IV species and are identified as dinuclear complexes [(Salen)LMnIV-O-Mn-IV(Salen)L'] with L, L' = Cl- and PhIO. It was found by EPR that the acylperoxo complex (Salen)Mn-III(OOCOAr) (7) was formed at the first stage of the interaction of 1 with m-CPBA in CH2Cl2. Complex 7 is unstable and converts into manganese(IV) oxo complex [(Salen)Mn-IV(O)] (8). The evaluated first order rate constant of this conversion is 0.25 +/- 0.08 min(-1) at -70 degrees C. Complex 7 reacts with styrene with the rate constant 1.1 +/- 0.4 M-1 min(-1) at -70 degrees C to give epoxide and restore 1. Complex 8 is inert towards styrene at low temperature. The effect of donor ligand N-methylmorpholine-N-oxide (NMO) on the epoxidation of styrene by the system 1 + m-CPBA was studied. Addition of NMO (2-5 equiv.) to the solution of 1 in CH2Cl2 before interaction with m-CPBA was found to dramatically increase the rate of undesirable transformation of the reactive acylperoxo complex 7-NMO into relatively inert oxo complex-8-NMO. However, in the presence of styrene, such undesirable conversion is entirely suppressed by very rapid reaction of 8-NMO with styrene to afford styrene oxide and restore 1-NMO. (C) 2000 Elsevier Science B.V. All rights reserved.