OXIDATIVE ADDITION OF CARBON OXYGEN AND CARBON NITROGEN DOUBLE-BONDS TO WCL2(PMEPH2)4 - SYNTHESIS OF TUNGSTEN METALLAOXIRANE AND TUNGSTEN OXO-ALKYLIDENE AND IMIDO ALKYLIDENE COMPLEXES

WCl2L4 (1, L = PMePh2) reacts rapidly with a variety of ketones and aldehydes to form bis(η2-ketone) or bis(η2-aldehyde) complexes WC12(η2-O=CRR′)2L2 (2, 3). When the ketone is part of a five-membered ring (cyclopentanone, 2-indanone, etc.), 2 rearranges with loss of ketone to give tungsten(VI) oxo-alkylidene products, W(O)(CRR′)C12L2 (4) in high yield. The net reaction is insertion of the tungsten center into the ketone carbon-oxygen double bond, a four-electron oxidative addition. Insertion into the carbon-nitrogen double bond of A-cycIopentyl-p-toluidine yields the analogous imido-alkylidene complex W(N-Tol)[C(CH2)4]Cl2L2. With other ketones, an oxo-alkylidene product is observed only in the reaction of 1 with 1 equiv of ketone and is not seen on decomposition of 2. This is due to further reaction of the alkylidene complex with free ketone. Aromatic ketones such as benzophenone react with 1 apparently by a radical pathway to give W(O)C12L3 and the olefin derived from two ketones (Ph2C=CPh2), without formation of an observable bis(ketone) intermediate. The two-electron oxidative addition of heterocumulenes is also observed: for example CO2 is cleaved to form the oxo-carbonyl complex W(O)(CO)C12L2. These are remarkable reactions because of the strength of the carbon-oxygen double bonds being cleaved under very mild conditions. A substantial driving force for the reactions is the formation of a tungsten-oxygen multiple bond, which is estimated to be >138 kcal/mol. The mechanism of double-bond cleavage is proposed to involve an η2-ketone or cumulene adduct. Based on the spectroscopic data for compounds 2 and 3 and the X-ray crystal structure of the bis(acetone) adduct 2g, the η2-ketone intermediates are best described as three-membered rings (metallaoxiranes). The oxo-alkylidene products are apparently formed upon opening of thisring. The preference for these two- or four-electron double-bond oxidative addition reactions versus one-electron (radical) chemistry is discussed. © 1990, American Chemical Society. All rights reserved.