The surprising nitrogen-analogue chemistry of the methyltrioxorhenium-catalyzed olefin epoxidation.
Density functional theory calculations at the B3LYP level have been carried out to investigate the mechanism of the reaction of ethylene with [Re(O)2(O-NH)Me], a formal hydroxylamine derivative of the industrial epoxidation catalyst methyltrioxorhenium(VII) (MTO). A variety of reaction pathways has been considered, including the concerted heteroatom-transfer mechanism postulated by Sharpless and the stepwise mechanism via a five-membered "organometallacycle" postulated by Mimoun. Ethylene has been found not to coordinate directly at the metal. The calculations reveal similar activation free enthalpies for the concerted nitrene-transfer event (aziridination) and for the formation of an organometallic rhena-2,3-oxazolidine via [2+2] addition of ethylene across the Re-N bond of the metallaoxaziridine moiety. The fragmentation of the organometallacycle is faster than its formation and gives ethylideneazane rather than aziridine. An additional pathway has a lower activation free enthalpy and leads to a rhena-3,2-oxazolidine. The formation of this organometallacycle proceeds via an intermediate ring-opening product, [Re(O)2(eta1-O-NH)Me], which undergoes [3+2] cycloaddition across the C=C bond of ethylene. Analysis of its electronic structure reveals that the eta1 species should be considered a metalla-analogue nitrosonium ylide rather than a metalla-analogue imine oxide. Fragmentation of the rhena-3,2-oxazolidine liberates acetaldehyde. The discovery of favorable pathways leading to organometallacycles upon reaction of C=C bonds with [Re(O)2(O-NH)Me] stands in sharp contrast to the strong preference of the concerted mechanism in the olefin epoxidation with rhenium peroxo complexes. The calculations show the multiple mechanisms to be distinguishable by four different products, calling for further experimental studies. The successful search for the five-membered organometallacycles parallels the computational prediction of four-membered organometallacycles derived from d0 metal oxo complexes (Deubel, D. V.; Frenking, G. Acc. Chem. Res. 2003, 36, 645) and the indirect observation of metalla-2-oxetanes in recent gas-phase experiments (Chen, X.; Zhang, X.; Chen, P. Angew. Chem., Int. Ed. 2003, 42, 3798).
Deubel
Dirk V
DV
Swiss Center for Scientific Computing, ETH Zürich, 6928 Manno, Switzerland. deubel@cscs.ch