Bromide ions and methyltrioxorhenium as cocatalysts for hydrogen peroxide oxidations and brominations

Oxidation of alcohols by hydrogen peroxide is negligible; even when catalyzed by methyltrioxorhenium (MTO), the process requires a long reaction time. The addition of a catalytic quantity of bromide ions, as HBr or NaBr, greatly enhances the rate. Some of the reactions were carried out on a larger scale in glacial acetic acid, and others at kinetic concentrations. The data establish that Br-2 is the active oxidizing agent in the system, because the catalytic rates under suitable circumstances match those for the independently measured Br-2 reaction with alcohol (benzyl alcohol, in particular). At much lower levels of MTO, however, Br-2 formation plays a role in the kinetics. Certain other reluctant transformations are conveniently carried out with the MTO/H2O/Br- combination: aldehydes to methyl esters; 1,3-dioxolanes to glycol monoesters; and ethers (with cleavage) to ketones (mostly), but in fair yield only. When Br was used in stoichiometric quantity, certain bromination reactions occur. Thus, phenyl acetylenes (PhC2R, R = H, Me, Ph) are converted to dibromoalkenes that are entirely or largely formed as the trans isomer, and phenols are brominated. The latter reaction shows the preference para > ortho > meta. Kinetic studies of benzyl alcohol oxidation with MTO/H2O2/Br- were carried out in aqueous solution. With sufficient (normal) levels of MTO, the rate constant for the formation of benzaldehyde agreed with the independently determined value for Br-2 + PhCH2OH, k = 4.3 x 10(-3) L mol(-1) s(-1) at 25.0 degrees C; for sec-phenethyl alcohol, k = (9.8 +/- 0.4) x 10(-3) L mol(-1) s(-1). Bromine is formed from the known oxidation of Br- with H2O2, catalyzed by MTO. This reaction results in BrO-/HOBr, which is then rapidly converted to Br-2. However, with substantially lower concentrations of MTO, the buildup of benzaldehyde is ca. 4-fold slower, reflecting the diminished rate of Br- oxidation.