Deoxygenative coupling of ketones and alkenes by tungsten(II) compounds

Article abstract of DOI:10.1021/om00060a010

Tungsten(II) compounds such as WCl₂(PMePh₂)₄ (1) react with acetone and ethylene to give a good yield of the tungsten(IV)-oxo complexes W(O)(CH₂=CH₂)Cl₂(PMePh₂)₂ (4) and a moderate amount of 3-methyl-1-butene. Cyclopentanone and ethylene plus 1 yield 4 and vinylcyclopentane; methyl vinyl ketone and ethylene give 4 and 3-methyl-1,4-pentadiene. The reaction of cyclopentanone and propylene with 1 yields a small amount of 2-cyclopentylpropene. Intramolecular deoxygenative coupling occurs with 6- and 7-en-2-ones to form 1-methyl-2-methylene-substituted cyclopentyl and cyclohexyl ring systems, respectively. The net result of these reactions is transfer of the ketone oxygen atom to tungsten, accompanied with its replacement by a hydrogen and a vinyl group. The suggested mechanism for this deoxygenative coupling (Scheme I) is coordination of both the ketone and ethylene to tungsten, coupling to form a 2-oxametallacyclopentane, β-hydrogen elimination to an allyloxy hydride species, C-O bond cleavage to an oxo allyl hydride complex, and reductive elimination of alkene. Consistent with the suggestion of an oxametallacycle, hydrolysis of the reaction mixture of 1 and 6-hepten-2-one provides stereospecifically trans-1,2-dimethylcyclopentanol. The enones methyl vinyl ketone and 5-hexen-2-one react with 1 to form stable complexes in which the enone is bound in an η⁴ fashion, similar to the proposed mixed alkene ketone intermediates in the coupling reactions. A related tungsten(II) butadiene complex, WCl₂(CH₂= CHCH=CH₂) (PMePh₂)₂, has also been isolated.