Kinetics, Mechanisms, and Catalysts of Oxygen Atom Transfer Reactions of S-Oxide and Pyridine N-Oxide Substrates with Molybdenum(IV,VI) Complexes: Relevance to Molybdoenzymes

Article abstract of DOI:10.1021/ja00215a022

The kinetics and mechanism of the oxygen atom transfer reactions MoO2(L-NS2) + (R_F)3P -> MoO(L-NS2)(DMF) + (R_F)3PO (1) and MoO(L-NS2)(DMF + XO -> MoO2(L-NS2) + X, with X = (R_F)2SO (2) and 3-fluoropyridine N-oxide (3), heve been investigated in DMF solutions (L-NS2 = 2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine(2-), R_F = p-C6H4F).The following rate constants (297.5 K) and activation parameters were obtained: reaction 1, k₂ = 9.7 (4) X 10^-3 M^-1 s^-1, ΔH(excit.) = 11.7 (6) kcal/mol, ΔS(excit.) = -28.4 (1.6) eu; reaction 2, k₁ = 14.0 (7) X 10^-4 s^-1, ΔH(excit.) = 22.1 (1.3) kcal/mol, ΔS(excit.) = 2.6 (1.6) eu; reaction 3, k₁ = 16.0 (8) X 10^-4 s^-1, ΔH(excit.) = 23.4 (1.4) kcal/mol, ΔS(excit.) = 7.2 (2.0) eu. reactions 2 and 3 exhibit saturation kinetics, under which the rate-determining step is intramolecular atom transfer.Mechanisms and transition states are proposed.The activation parameters are the first measured for oxo transfer from substrate; the small activation entropies suggest a transition state structurally similar to the complex MoO(L-NS2)(XO) formed in a labile equilibrium prior to oxo transfer to Mo.Coupling of reaction 1 with reaction 2 or 3 affords the catalytic reaction 4, (R_F)3P + XO -> (R_F)3PO + X; no reaction occurs in the absence of the Mo catalyst.The kinetics of catalysis were examined by monitoring the concentrations of reactants and products by 19F NMR spectroscopy.After 15 h, each system showed ca. 100 turnovers.Reaction 4 with XO = (R_F)2SO has a catalytic rate constant of 7 X 10^-3 M^-1 s^-1, close to the value for reaction 1.This and other considerations show that the catalytic rate is limited by the rate of oxo transfer from the Mo(VI) complex MoO2(L-NS2). An attempt to establish the catalytic mechanism led to detection of inhibition; the inhibitory species could not be identified.These results provide the most detailed information on the kinetics and mechanisms of Mo-mediated oxygen atom transfer and demonstrate the efficacy of 19F NMR for detecting and monitoring catalysis and determining catalytic velocities and rate constants.The relation of these results to the enzymatic reduction of N-oxides ans S-oxides is briefly discussed.