KINETICS AND MECHANISM OF THE OXIDATION OF SECONDARY HYDROXYLAMINES TO NITRONES WITH HYDROGEN-PEROXIDE, CATALYZED BY METHYLRHENIUM TRIOXIDE

Secondary hydroxylamines, (RCH2)(2)NOH and (R2CH)(2)NOH, were converte d to nitrones, RCH2N(O)=CHR and R2CHN(O)=CR2, in >94% yield with hydro gen peroxide as an oxygen donor and methylrhenium trioxide (MTO) as a catalyst. High concentrations of hydrogen peroxide were used so that t he methylrhenium diperoxide, CH3Re(O)(eta(2)-O-2)(2)(H2O), was the dom inant 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 (PhCH2) in methanol at 25.0 degrees C. There is no H/D kin etic isotope effect on the rate constant for this step. The data are i nterpreted to infer the intervention of an oxygenated intermediate, (R CH2)(2)N(O)OH, which then rapidly dehydrates to yield the nitrone. Two products are formed from unsymmetrical hydroxylamines, the ratio of w hich establishes the reactivities of the intermediate toward the compe ting elimination reactions: (RCH2)(R'CH2)NOH-->((RCH2)(R'CH2)N(O)OH)-- >chi RCH2N(O)=CHR'+(1-chi)R'CH2N(O)=CHR.