Kinetics and mechanism of the oxidation of alkenes and silanes by hydrogen peroxide catalyzed by methylrhenium trioxide (MTO) and a novel application of electrospray mass spectrometry to study the hydrolysis of MTO
Robert S. Houk
Conjugated dienes were oxidized by hydrogen peroxide with methylrhenium trioxide as catalyst. As is true for other MTO-catalyzed reactions, methylrhenium bis-peroxide (CH3Re(O)(eta2-O2)2(H 2O)) was the major reactive catalyst present. The rate constants between it and the dienes, increase or decrease as substituents add or remove electron density from the double bond, suggesting a concerted mechanism in which the peroxide oxygen attacks the double bond electrophilically. The H2O 2/MTO oxidation system was also used in the regioselective cyclization of hydroxyalkenes to tetrahydrofurans and unsaturated carboxylic acids to lactones. In these reactions MTO acts as a bifunctional catalyst for both epoxidation and cyclization reactions. The reactions of trisubstituted silane with hydrogen peroxide catalyzed by MTO result in its quantitative convertion to silanol. An oxene mechanism is proposed for this reaction based on the kinetics and isotope ratio study;The full kinetics pH profile for the base-promoted decomposition of MTO to CH4 and ReO4-- was examined with the inclusion of new data at pH 7--10. Spectroscopic and kinetics data gave evidence for mono-and dihydroxo complexes: MTO(OH--) and MTO(OH--)2. Some kinetic data were acquired with electrospray mass spectrometry to monitor the build up in the concentration of perrhenate ions.