Highly regioselective [2+2+2] cycloaddition of terminal alkynes catalyzed by eta(6)-arene complexes of titanium supported by dimethylsilyl-bridged p-tert-butyl calix[4]arene ligand
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abstract
Two new Ti-6-arene complexes [(DMSC)Ti{6-1,2,4- C6H3(SiMe3)3}] (6) and [(DMSC)Ti{6-1,3,5-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 Me3SiCCH revealed first-order dependence on [6] and [Me3SiCCH]; and activation parameters, H(+) = 14 kcal/mol, and S(+) = -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 6-arene compound. The high selectivity for 1,2,4-substituted benzene may be understood in terms of the directing influence of the DMSC ligand.
