Titanium-catalyzed head-to-tail dimerization of tert-butylacetylene. Crystal structures of [(C5HMe4)(2)Ti(mu-H)(2)Mg(THF)(mu-Cl)](2) (THF-tetrahydrofuran) and (C5HMe4)(2)TiOCMe3

tert-Butylacetylene (TBUA) is readily dimerized exclusively to 2,4-di-tert- butyl-1-buten-3-yne, head-to-tail dimer (HTTD), in the presence of the (eta (5)-C5HMe4)(2)TiCl2/Mg/THF system. The ESR investigation revealed the forma tion of Ti-Mg hydride complexes [(eta(5)-C5HMe4)(2)Ti(mu-H)(2)Mg(THF)(mu-Cl )](2) (2) and [(eta(5)-C5HMe4)(2)Ti(mu-H)(2)](2)Mg (3) in the absence of TB UA and a tweezer-type complex, probably [eta(5)C(5)HMe(4))(2)Ti(eta(1)-C dr op CCMe3)(2)](-) [Mg(THF)Cl](+) (4) in its presence. The catalytic system a s well as complexes 2-4 were deactivated by presumably tert-butanol contain ed in TBUA to give (eta(5)-C5HMe4)(2)TiOCMe3 (5). Purification of TBUA impr oved the turnover by up to 8.8 x 10(3) mol TBUA per 1 mol Ti using complex 3 as a catalyst, however, complex 5 remained the only observable product of deactivation. The crystal structures of 2 and 5 were determined by X-ray d iffraction analysis. (C) 1999 Elsevier Science S.A. All rights reserved.