Oxidation of ethers, alcohols, and unfunctionalized hydrocarbons by the methyltrioxorhenium/H2O2 system: a computational study on catalytic C-H bond activation
2009 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 15, no 8, 1862-1869 p.Article in journal (Refereed) Published
A concerted mechanism that does not involve an ionic intermediate was revealed by a DFT study on oxidation of ethers, alcohols, and unfunctionalized hydrocarbons by methyltrioxorhenium/H2O2. Instead, CH insertion occurs through hydride transfer and then turns into a hydroxide transfer/rebound in a concerted fashion. The picture shows selected frames from an intrinsic reaction coordinate scan from the transition state to the product for the oxidation of cis-1,2-dimethylcyclohexane.
The potential-energy surfaces (PESs) of methyltrioxorhenium (MTO)-catalyzed CH insertion reactions in the presence of hydrogen peroxide were studied by accurate DFT methods for a series of substrates including unsaturated hydrocarbons, an ether, and an alcohol. Based on the comprehensive analysis of transition states and intrinsic reaction coordinate (IRC) scans, CH insertion was found to proceed by a concerted mechanism that does not require, as previously thought, a side-on or a butterfly-like transition state. We found that a typical transition state follows requirements of the SN2 reaction instead. Furthermore, by exploring the PESs of several CH insertion reactions, we discovered that no ionic intermediate is formed even in a polar solvent. The latter was modeled within the self-consistent reaction field approach in a polarizable continuum model (PB-SCRF/PCM). According to our study, CH insertion occurs by a concerted but highly asynchronous mechanism that first proceeds by hydride transfer and then turns into hydroxide transfer/rebound. For the oxidation of alcohols, CH bond cleavage occurs without formation of alkoxide intermediates on the dominant pathway. The computed deuterium kinetic isotope effect of 2.9 for the hydride-transfer transition state for alcohol oxidation is in good agreement with the experimental kH/kD ration of 3.2 reported by Zauche and Espenson. As confirmed by IRC and PES scans in different solvents, the OH-rebound phase of the CH insertion pathway demonstrates strong similarities with the rebound mechanism that was previously proposed for cytochrome P450 and metalloporphyrin-catalyzed oxidations.
Place, publisher, year, edition, pages
2009. Vol. 15, no 8, 1862-1869 p.
CH activation, density functional calculations, oxidation, reaction mechanisms, rhenium
IdentifiersURN: urn:nbn:se:su:diva-29462DOI: 10.1002/chem.200801493ISI: 000263400200010OAI: oai:DiVA.org:su-29462DiVA: diva2:233245