Activation of molecular dioxygen at a molybdenum(IV) imido compound led to the isolation and full characterization of a remarkably stable transition-metal imidoperoxido complex.

New molybdenum complexes of the type [CpMo(CO) 3 X] containing ligands of the formula X = CHR 2 CO(OR 1 )where R 1 = ethyl (1), menthyl (4), and bornyl (5) and R 2 = H; R 1 = ethyl and R 2 = methyl (2) and phenyl (3) have been synthesized and characterized by NMR and IR spectroscopy and X-ray crystallography.These compounds have been applied as catalyst precursors for achiral and chiral epoxidation of unfunctionalized olefins with tert-butyl hydroperoxide (TBHP) as the oxidant at 22°C (in CH 2 Cl 2 ) and 55°C (in CHCl 3 ). The substrates cis-cyclooctene, 1-octene, cis-and trans-stilbene, and trans-β-methylstyrene were selectively and quantitatively converted into their epoxides using a catalyst :substrate : oxidant ratio of 1 : 100 : 200 within 4 h at room temperature in CH 2 Cl 2 and within 15 min at 55°C in CHCl 3 . Complexes 1-5 are precursors of active epoxidation catalysts and turnover frequencies (TOFs) of ca. 1200 h −1 are obtained with cis-cyclooctene as the substrate. No enantioselectivity is observed with trans-β-methylstyrene as the substrate despite the application of enantiomerically pure precatalysts. In situ monitoring of catalytic epoxidation of cis-cyclooctene with complex 5 by 1 H and 13 C NMR spectroscopy suggests that the chiral alkyl ester side chain is retained during oxidation with TBHP. During epoxidation, the primary catalytic species is the dioxo complex [CpMoO 2 X]. After near complete conversion of cis-cyclooctene to its epoxide, further oxidation of the dioxo complex to oxo-peroxo complex [CpMo(η 2 -O 2 )(O)X] takes place. The oxo-peroxo complex is also an active epoxidation catalyst.

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