Copper(I) Chloride Initiated Decomposition of 7-Phosphanorbornadiene. Evidence for a Solvent-Assisted Catalytic Mechanism

Density functional theory is used to explore the mechanism of the copper(I)-chloride-catalyzed decomposition of W(CO)₅-complexed 7-phosphanorbornadiene and the subsequent olefin trapping of the terminal phosphinidene complex, CuCl lowers the activation barrier by interacting directly with the breaking P-C bond. Contrary to the prevailing notion that a free terminal phosphinidene complex (W(CO)₅=PR) is generated in the CuCl-catalyzed cheletropic elimination of the 7-phosphanorbornadiene-W(CO) ₅ complex, the present mechanism suggests that CuCl is attached to the terminal phosphinidene. Furthermore, a "chloride shuttle" takes place where the chloride first migrates to the phosphorus center and then is returned back to the copper center by the incoming olefin in an S_N2 reaction step. When the substituent on phosphorus is a phenyl group (R = Ph), the uncatalyzed reaction has an activation barrier of 17.9 kcal/mol, which is reduced by 10.9 kcal/mol on including the CuCl catalyst. The CuCl-catalyzed decomposition of 7-phosphanorbornadiene followed by olefin trapping of the terminal phosphinidene complex has a close parallel with the Cu(I)-catalyzed cyclopropanation reaction of diazoalkane. In both catalyzed reactions, copper(I) is coordinated to the phosphinidene/carbene as a Lewis acid, while a Lewis base is displaced from the phosphorus/carbon center as the olefin is added.