Dinuclear titanium(IV) complex of p-tert-butylthiacalix[4]arene as a novel bidentate Lewis acid catalyst

Treatment of p-tert-butylthiacalix[4]arene (H4L, 2) with TiCl4 in dichloromethane gave two novel dinuclear titanium(IV) complexes formulated as [Ti2LCl4]. One of the complexes was subjected to X-ray crystallographic analysis to show that the calixarene ligand (L4−) adopted a cone conformation, forcing the two metal centers to reside in close vicinity with syn arrangement with respect to the mean plane defined by the macrocycle (3). The other one was assigned to be an anti titanium(IV) complex (4) based on 1H NMR spectroscopy. The syn complex showed high catalytic activity in the Mukaiyama–aldol reaction of aromatic aldehydes with silyl enol ethers, indicating the double-activation ability of the bidentate Lewis acid toward the aldehydes.