Kinetics of reaction of the Fe-II-cyclam complex with H2O2 in acetonitrile and the mechanism of catalyzed epoxidation of cyclohexene

The Fe-II complex with macrocycle cyclam (1,4,8,11 -tetraazacyclotetradecane) reacts with H2O2 in acetonitrile to give a mixture of products which result from oxidation both at the metal center and the ligand. At 25 degrees C in the presence of 0.05 M Bu4NBF4 the reaction occurs with three kinetically distinguishable steps. The first two steps are first order with respect to both H2O2 and the metal complex. The values of the second order rate constants are k(1) = 3.7 M-1 s(-1) and k(2) = 0.83 M-1 s(-1). EPR spectra suggest that intermediates formed in these two steps are low spin Fe-III complexes. The kinetics of the third step is more complicated, with a dependence on the concentration of H2O2 of the form k(3)[H2O2] + k(4), with k(3) = 2.72 x 10(-3) M-1 s(-1) and k(4) = 2.18 x 10(-4) s(-1). This rate law is interpreted in terms of two-parallel pathways leading to Fe-III and extensively dehydrogenated cyclam. NMR experiments suggest that the active catalyst in the iron-cyclam catalyzed epoxidation of cyclohexene is the intermediate formed in the first step. However, this intermediate is not able to transfer an oxygen atom directly to the substrate and requires the participation of additional H2O2, in a mechanism very different to that proposed for porphyrin complexes. (C) 1997 Elsevier Science Ltd.