The mechanism of catalyst formation from tungsten hexachloride and cocatalyst in olefin metathesis

Summary The mechanism of the catalyst formation in hept-2-ene metathesis was investigated by gas chromatographic analysis of the products from reactions of tungsten hexachloride with R₄Sn(R = Ph, n-Bu), RLi, RMgl (R = n-Bu) or R₃Al(R = Et) as cocatalyst. The products are chiefly RH, RCl and RR species, and the highest catalytic activity is observed for an equimolar ratio of products to tungsten hexachloride. The facts suggested that the active catalyst, a WCl₄ species, is formed by the elimination of two R groups from R₂WCl₄ and/or one R group and chlorine from RWCl₅. The total quantity and composition of the products varies with the cocatalyst:WCl₆ ratio, and the difference between the behavior of tetra-n-butylstannane and n-butyllithium in the catalyst formation is explainable on the basis of their relative nucleophilicities. Gas chromatographic analysis of the organotin compounds resulting from the WCl₆:: Bu₄Sn system shows that tetra-n-butylstannane provides only one butyl group per mole of WCl₆.