Synthesis, characterization, and alkyne cyclotrimerization chemistry of titanium complexes supported by calixarene-derived bis(aryloxide) ligation

Proximally bridged calix[4]arene compounds (DESC)H₂ (3), (DMSHC)H₂ (4), (DMSMC)H₂ (5), and (DPSC)H₂ (6), in which one R₂Si group (R=alkyl or aryl) bridges adjacent oxygens, were synthesized via reaction between dialkyl- or diaryldichlorosilane and the corresponding calix[4]arene. Treatment of p- tert -butylcalix[4]arene with Ph₂SiCl₂ at room temperature or (o-MeC₆ H₄)₂SiCl₂ at 80 °C gave (ClPh₂SiCl)₂Calix-H₂ (7) and (o-Tol₂SiCl)₂Calix-H₂ (8), respectively. Titanium dichloride complexes 9-12 (L₂ TiCl₂, where L₂=DESC, DMSHC, DMSMC, or DPSC) were prepared in high yield from reaction of 3-6 with TiCl₄. The molecular structures of 7 and 12 were established by single-crystal X-ray diffraction studies. Reduction of 9, 11, and 12 with activated magnesium (Mg*) in the presence of an excess of Me₃SiC≡CH produced titananorbornadiene complexes L₂Ti{η⁶-1,2,4-C₆H₃ (SiMe₃)₃} (13-15, L₂=DESC, DMSMC, or DPSC), which were characterized in solution. Catalytic cyclotrimerization of both terminal and internal alkynes was achieved using catalyst systems derived from L₂TiCl₂ complexes 9-12 and Mg*. For unsymmetrically substituted internal alkynes, preference for 1,2,4-substitution decreased as the size difference of the substituent groups decreased. The cyclotrimerization of PhC≡CMe was more facile when the calixarene-derived bis(aryloxide) ligand was DPSC versus DMSMC, suggesting that the DPSC ligand may provide a less crowded titanium center and exert greater kinetic control over the course of the cyclotrimerization.