Efficient Catalytic Oxidation of Alkanes by Lewis Acid/[OsVI(N)Cl4]− Using Peroxides as Terminal Oxidants. Evidence for a Metal-Based Active Intermediate

The oxidation of alkanes by various peroxides ((t)BuOOH, H2O2, PhCH2C(CH3)2OOH) is efficiently catalyzed by [Os(VI)(N)Cl4](-)/Lewis acid (FeCl3 or Sc(OTf)3) in CH2Cl2/CH3CO2H to give alcohols and ketones. Oxidations occur rapidly at ambient conditions, and excellent yields and turnover numbers of over 7500 and 1000 can be achieved in the oxidation of cyclohexane with (t)BuOOH and H2O2, respectively. In particular, this catalytic system can utilize PhCH2C(CH3)2OOH (MPPH) efficiently as the terminal oxidant; good yields of cyclohexanol and cyclohexanone (>70%) and MPPOH (>90%) are obtained in the oxidation of cyclohexane. This suggests that the mechanism does not involve alkoxy radicals derived from homolytic cleavage of MPPH but is consistent with heterolytic cleavage of MPPH to produce a metal-based active intermediate. The following evidence also shows that no free alkyl radicals are produced in the catalytic oxidation of alkanes: (1) The product yields and distributions are only slightly affected by the presence of O2. (2) Addition of BrCCl3 does not affect the yields of cyclohexanol and cyclohexanone in the oxidation of cyclohexane. (3) A complete retention of stereochemistry occurs in the hydroxylation of cis- and trans-1,2-dimethylcyclohexane. The proposed mechanism involves initial O-atom transfer from ROOH to [Os(VI)(N)Cl4](-)/Lewis acid to generate [Os(VIII)(N)(O)Cl4](-)/Lewis acid, which then oxidizes alkanes via H-atom abstraction.