Annulation reaction of (triphenylmethyl)allene on a cationic metal complex and the reaction mechanism

Article abstract of DOI:10.1021/om00034a032

Treatment of CpW(CO)₃CH₂C≡CH, 1a (Cp = η⁵-C₅H₅), with Ph₃CPF₆ (Ph = C₆H₅) generates a cationic η²-(triphenylmethyl)allene (tritylallene) complex [CpW(CO)₃(η²-CH₂=C=CHCPh ₃)]PF₆, 2a. When reacted with excess Me₃NO, complex 2a liberates free tritylallene CH₂=C=CHCPh₃, 3. In acetonitrile, the tritylallene ligand of 2a undergoes an annulation reaction followed by decomplexation to give a mixture of 1,1-diphenyl-1,4-dihydronaphthalene, 4a, and 1,1-diphenyl-1,2-dihydronaphthalene, 4b. The first step of the cyclization is the intramolecular nucleophilic attack of the phenyl groups on the metal-coordinated allene moiety generating an arenium intermediate. Deprotonation of this intermediate by PPh₃ affords a neutral metal-vinyl complex Cp(CO)₃W[σ-(C₁₀H₇)(C₆H ₅)₂], 7a, which has been isolated. Protonation of complex 7a with HBF₄ gives 4a and 4b quantitatively. In the initial stage of this protonation, a π-complex Cp(CO)₃W[η²-(C₁₀H₈)(C ₆H₅)₂]BF₄, 9b, is observed and a fast workup enables isolation of 9b for spectroscopic characterization. Complex 9b gives 4b exclusively. The mechanism for the cyclization reaction is established by a spectroscopic characterization of 7a especially through 2D NMR techniques. The isomerization pathway that leads to 4b is established by protonation reaction of 7a using CF₃COOD. Reactivity of such a cyclization reaction in a number of metal systems is rationalized by the electron donor/acceptor ability of the metal fragment.