Highly regioselective [2+2+2] cycloaddition of terminal alkynes catalyzed by titanium complexes of p-tert-butylthiacalix[4]arene

Mono- and dinuclear titanium complexes of p-tert-butylthiacalix[4]arene were applied as a catalyst for [2+2+2] cycloaddition of terminal alkynes. They showed high catalytic activity and regioselectivity toward 1,3,5-trisubstituted benzenes over 1,2,4-trisubstituted isomers. The regioselectivity was rationalized in terms of the steric effect of the thiacalixarene skeleton and the coordination of the bridging sulfur atom to the titanium center.