Highly regioselective [2 + 2 + 2] cycloaddition of terminal alkynes catalyzed by η⁶-arene complexes of titanium supported by dimethylsilyl- bridged p-tert-butyl calix[4]arene ligand

Two new Ti-η⁶-arene complexes [(DMSC)Ti{η⁶-1,2,4- C₆H₃(SiMe₃)₃}] (6) and [(DMSC)Ti{η⁶-1,3,5-C₆H₃Bu(t)₃}] (7) containing 1,2-alternate, Me₂Si-bridged p-tert-butylcalix[4]arene (DMSC) ancillary ligand have been synthesized. The solid-state structure of 6 revealed a highly folded arene ligand [with a dihedral angle of 29.7(7)°] and suggests that 6 is better described as a 7-titananorbornadiene species. Both 6 and 7 are efficient catalysts for highly regioselective [2 + 2 + 2] cycloaddition of terminal alkynes to yield 1,2,4-substituted benzenes. Kinetic studies of the catalytic [2 + 2 + 2] cycloaddition of Me₃SiC≡CH revealed first-order dependence on [6] and [Me₃SiC≡CH]; and activation parameters, ΔH(+) = 14 kcal/mol, and ΔS(+) = -11 cal/mol K, that are consistent with an associative mechanism. The reaction rate is influenced by the steric requirements of both the alkyne and the η⁶-arene compound. The high selectivity for 1,2,4-substituted benzene may be understood in terms of the directing influence of the DMSC ligand.