Chromyl Complexes in the Direct Epoxidation of Alkenes

Article abstract of DOI:10.1021/ja00346a044

Alkenes such as (E)- and (Z)-β-methylstyrene are converted stereospecifically by dinitratodioxochromium(VI), or chromyl nitrate, to the corresponding epoxide with high selectivities in aprotic media under rather mild conditions.The presence of the cosolvents, N,N-dimethylformamide (DMF), acetone, pyridine, etc., is critical for effective epoxidation with this reagent.In DMF and pyridine, the epoxide remains generally intact but in acetone it is transformed to the corresponding alkene ketal which can also be isolated in high yields.The rates of oxidation are evaluated by the competition method, and the relative reactivities of various alkenes toward chromyl nitrate are found to generally parallel those previously determined for other chromyl reagents such as chromic acid, chromyl acetate, and chromyl chloride.Under optimum conditions for epoxidation, chromyl nitrate effects exclusive oxidation of 1,2-diphenylethanol to deoxybenzoin, which is uncontaminated by the usual cleavage products benzaldehyde and benzyl alcohol.Since the latter is known to derive from chromium(IV) intermediates, we conclude that the active species in chromyl epoxidation is oxochromium(V) formed in situ by the prior one-electron oxidation of solvent.The latter is in accord with the efficient transfer of the oxygen atom from macrocyclic oxochromium(V) species previously observed by Grovers and co-workers.The ESR spectra of the transient chromium(V) intermediates derived from chromyl nitrate and chromyl acetate by reduction with cosolvent are reported.