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

Using H-1 NMR and EPR spectroscopy, manganese species formed in the catalyt ic 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 c yclohexanediamino-manganese(III) chloride ([(Salen)Mn-III]) were studied. T hree types of manganese complexes were characterized in the catalytic syste m 1 + PhIO (4-6). Complex 4 is very unstable and reacts with styrene at - 2 0 degrees C to afford styrene oxide. It exhibits three signals of tBu group s at 1.68, 1.64 and 1.42 ppm. This pattern closely resembles that for a mod el 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 a re relatively stable at -20 degrees C and poorly reactive towards styrene a t this temperature. They display H-1 NMR spectra characteristic for antifer romagnetically 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 P hIO. 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 CH 2Cl2. Complex 7 is unstable and converts into manganese(IV) oxo complex [(S alen)Mn-IV(O)] (8). The evaluated first order rate constant of this convers ion is 0.25 +/- 0.08 min(-1) at -70 degrees C. Complex 7 reacts with styren e with the rate constant 1.1 +/- 0.4 M-1 min(-1) at -70 degrees C to give e poxide and restore 1. Complex 8 is inert towards styrene at low temperature . The effect of donor ligand N-methylmorpholine-N-oxide (NMO) on the epoxid ation 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 th e reactive acylperoxo complex 7-NMO into relatively inert oxo complex-8-NMO . However, in the presence of styrene, such undesirable conversion is entir ely suppressed by very rapid reaction of 8-NMO with styrene to afford styre ne oxide and restore 1-NMO. (C) 2000 Elsevier Science B.V. All rights reser ved.