Mechanism of the Chromium-Catalyzed Epoxidation of Olefins. Role of Oxochromium(V) Cations

Article abstract of DOI:10.1021/ja00311a064

The catalytic epoxidation of various olefins with iodosylbenzene is efficiently carried out by a series of chromium(III) cations Cr^III(salen)⁺ (I) which are promoted by pyridine N-oxide (pyO) and related oxygen donors as the cocatalyst.Analysis of the catalytic rate profile and products establish the oxochromium(V) derivative O=Cr(salen)⁺ (II) and its donor adduct O=Cr(salen)(pyO)⁺ (III) as the reactive intermediates in the catalytic cycle.The successful isolation as well as the complete spectral analysis and structural characterization by X-ray crystallography of both II and III reveal the basis for oxygen activation in the O=Cr^V functionality.The mechanism of oxygen atom transfer involves the rate-limiting attack on the olefin by the electrophilic oxochromium(V) cation.The observation of benzaldehyde as a byproduct derived from the pyO-promoted C=C cleavage of styrene provides a method for unequivocally proving the existence of a transient intermediate during oxygen atom transfer.Thus the rate of olefin oxidation is found to be completely independent of the product-forming steps leading to epoxide and benzaldehyde, as they are modulated by added pyO.Steric effects, isotopic 18O tracers, stereochemistry, skeletal rearrangement, and substituent effects all provide mechanistic probes for the structure of the metastable intermediate.