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

Abstract

Two new Ti−η6-arene complexes [(DMSC)Ti{η6-1,2,4-C6H3(SiMe3)3}] (6) and [(DMSC)Ti{η6-1,3,5-C6H3But 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.

Regioselective cyclotrimerization of phenylacetylenes to 1,2,4-triarylbenzenes catalyzed by iridium–diphosphine complexes

“…To the best of our knowledge there are only two examples in the literature of comparable regioselectivity in phenylacetylene cyclotrimerization, i.e., the reaction catalyzed by supported titanium-arene complexes described by Ladipo and coworkers [17] and the palladium-catalyzed cyclotrimerization of p-MeC 6 H 4 CBCH reported by Jiang and Chen [18].…”

Titanium redox catalysis: insights and applications of an earth-abundant base metal

“…Ladipo 17 and Rothwell 18 explicitly studied the mechanism and selectivity of this reaction using calixerene and aryloxide ligands, respectively, although this reaction can be catalyzed across a very broad spectrum of Ti complexes. 19 The overall catalytic cycle for the trimerization of alkynes using the (DMSC)TiCl 2 catalyst developed by Ladipo is shown in Figure 3b.…”

Cyclic versus acyclic structures of six-carbon ligands in binuclear cobalt carbonyl derivatives: Some thermochemical observations

“…The [2+2+2] Reppe cyclotrimerization of alkynes leading to benzene derivatives using a variety of transition metal catalysts [1][2][3][4] has been an effective method for the synthesis of substituted arenes [5][6][7][8]. Cobalt carbonyls such as Co 2 (CO) 8 have been among the most frequently used catalyst precursors for such reactions [9][10][11].…”

“…Such complexes have been isolated and characterized structurally in systems with large R substituents, e.g. [1,3,6-tBu 3 C 6 H 3 ]Co 2 (CO) 4 .…”