Effect of oxygen on metathesis of cis-2-pentene by a binary catalyst system of W(CO)₅P(C₆H₅)₃ and (C₂H₅)AlCl₂

Metathesis of cis-2-pentene has been studied with the catalytic system W(CO)₅Pφ₃, EtAlCl₂. Oxygen has a drastic promoting effect on the catalytic system which exhibits, after O₂ introduction, a very high efficiency for metathesis of internal as well as α-olefins. Whereas no significant interaction occurs between W(CO)₅Pφ₃ and EtAlCl₂, O₂ activation induces slow departure of two carbonyl groups. It is at the beginning of the release of CO that the system is the most active. Spectroscopic studies (uv, ir) indicate at this stage a complexing of a Lewis acid to the terminal oxygen of the trans-carbonyl group of W(CO)₅Pφ₃, with a net decrease of the elecronic density of W, so that the strength of the WCO bonds of the square plane is considerably lowered. It is therefore suggested that the main effect of O₂ is to enhance the Lewis acidity of EtAlCl₂, so as to favor an electrophilic attack of the Lewis acid to the tungsten through the carbonyl ligand.