ORGN 81 |
| Valerian Dragutan1, Ileana Dragutan1, and Mihai Dimonie2. (1) Institute of Organic Chemistry, Romanian Academy, 202B, Spl. Independentei, P.O. Box 15-254, 71141 Bucharest, Romania, (2) Department of Chemistry, Polytechnic University of Bucharest, 1, Polizu, 78126 Bucharest, Romania |
| Due to unprecedented developments of the molybdenum and ruthenium alkylidene catalysts of Schrock and Grubbs types, olefin metathesis has newly become a versatile method for synthesis in organic and polymer chemistry. It has been unanimously accepted that in metathesis reactions catalyzed by such organometallics the initiation and propagation steps occur via a metallacarbene-metallacyclobutane pathway, though the Grubbs catalysts promote as well radical reactions such as ATRP or Kharash addition on vinyl compounds. Recent reports of Rooney et al. concerning olefin and cycloolefin metathesis triggered by Ru-based catalysts indicated involvement of certain radical species, in the initiation and possibly also in the propagation step. Our studies focused on the generation (from WCl6 and organoaluminums) and monitoring of W(V) and W(III) paramagnetic species in the course of metathesis polymerization of cycloolefins, e.g. cyclopentene, cyclooctene, cyclooctadiene, cyclododecene. ESR spectroscopy carried out during the interaction of the tungsten catalysts with cycloolefins, in the presence or absence of an organoaluminum compound, revealed that carbon-centered radicals are formed, along with tungsten paramagnetic species [W(V) and W(III)]; characteristic g-factors have been determined in the case of each cycloolefin. Based on ESR data and parallel ceriometric determinations a radical mechanism for olefin metathesis induced by tungsten catalysts in a low oxidation state, that is similar to that reported earlier by Rooney et al. for the ruthenium-based catalysts, is now being proposed.
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New Reactions and Methodology, Metal-Mediated Reactions, Heterocycles and Aromatics
8:00 PM-10:00 PM, Sunday, March 28, 2004 Anaheim Convention Center -- Hall A, Poster
Sci-Mix
Division of Organic Chemistry |