Kinetics and Mechanism of the Oxidation of Secondary Hydroxylamines to Nitrones with Hydrogen Peroxide, Catalyzed by Methylrhenium Trioxide

Secondary hydroxylamines, (RCH 2 ) 2 NOH and (R 2 CH) 2 NOH, were converted to nitrones, RCH 2 N(O) CHR and R 2 CHN(O) CR 2, in >94% yield with hydrogen peroxide as an oxygen donor and methylrhenium trioxide (MTO) as a catalyst. High concentrations of hydrogen peroxide were used so that the methylrhenium diperoxide, CH 3 Re(O)(η 2 -O 2 ) 2 (H 2 O), was the dominant and reactive form of the catalyst. Representative rate constants are as follows: k /L mol - 1 s - 1 = 150 (R = Me), 52 (Et), 13.8 (Pr i ), and 3.33 (PhCH 2 ) in methanol at 25.0 °C. There is no H/D kinetic isotope effect on the rate constant for this step. The data are interpreted to infer the intervention of an oxygenated intermediate, (RCH 2 ) 2 N(O)OH, which then rapidly dehydrates to yield the nitrone. Two products are formed from unsymmetrical hydroxylamines, the ratio of which establishes the reactivities of the intermediate toward the competing elimination reactions: (RCH 2 )(R‘CH 2 )NOH → {(RCH 2 )(R‘CH 2 )N(O)OH} → χRCH 2 N(O) CHR‘ + (1−χ)R‘CH 2 N(O) CHR.