Insights into the Structure and Reactivity of Acylperoxo Complexes in the Kochi−Jacobsen−Katsuki Catalytic System. A Density Functional Study

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Abstract
Structural properties of the acylperoxo complexes [(Salen)MnIIIRCO3] (2) and [(Salen)MnIVRCO3] (3), the critical intermediates in the Kochi−Jacobsen−Katsuki reaction utilizing organic peracids or O2/aldehydes as oxygen source, have been studied with the density functional theory. Four distinct isomers, cis(O,N), cis(N,O), cis(N,N), and trans, of these complexes have been located. The isomer 2- cis(O,N) in its quintet ground state, and nearly degenerate isomers 3- cis(O,N) and 3- cis(N,O) in their quartet ground states are found to be the lowest in energy among the other isomers. The O−O bond cleavage in the cis(O,N), cis(N,O), and trans isomers of 2 and 3 has been elucidated. In complex 3, the O−O bond is inert. On the contrary, in complex 2, the O−O bond cleaves via two distinct pathways. The first pathway occurs exclusively on the quintet potential energy surface (PES) and corresponds to heterolytic O−O bond scission coupled with insertion of an oxygen atom into an Mn−N(Salen) bond to form 2-N-oxo species; this pathway has the lowest barrier of 14.9 kcal/mol and is 15.6 kcal/mol exothermic. The second pathway is tentatively a spin crossover pathway. In particular, for 2- cis(O,N) and 2- cis(N,O) the second pathway proceeds through a crucial minimum on the seam of crossing (MSX) between the quintet and triplet PESs followed by heterolytic O−O cleavage on the triplet PES, and produces unusual triplet 2- cis(O,N) - and 2- cis(N,O) -oxo ([(Salen)MnV(O)RCO2]) species; this pathway requires 12.8 kcal/mol and is 1.4 kcal/mol endothermic. In contrast, for the 2- trans isomer, spin crossing is less crucial and the O−O cleavage proceeds homolytically to generate 2- trans -oxo [(Salen)MnIV(O)] species with RCO2 radical; this pathway, however, cannot compete with that in 2- cis because it needs 21.9 kcal/mol for activation and is 15.3 kcal/mol endothermic. In summary, the O−O cleavage occurs predominantly in the 2- cis complexes, and may proceed either through pure high spin or spin crossover heterolytic pathway to produce 2- cis -oxo and 2- N -oxo species.

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