Alkyne-Induced Facile C-C Bond Formation of Two eta(2)-Alkynes on Dinuclear Tantalum Bis(alkyne) Complexes To Give Dinuclear Tantalacyclopentadienes

摘要

The dinuclear tantalum-alkyne complexes [(eta(2)-RC=CR)TaCl2](2)(mu-OMe)(2)(mu-thf) (2a, R = Et; 2b, R = Pr-n) were synthesized by treating the mononuclear tantalum alkyne complexes (eta(2)-RC=CR)TaCl3(dme) (1a, R = Et; 1b, R = Pr-n) with 1 equiv of NaOMe in THF. We found that adding a catalytic amount (20 mol %) of 3-hexyne to 2a induced the spontaneous formation of Ta2Cl4(OMe)(2)(mu-C4Et4)(thf) (4a). Similarly, Ta2Cl4(OMe)(2)(mu-(C4Pr4)-Pr-n)(thf) (4b) was obtained by treatment of 2b with a catalytic amount (20 mol %) of 4-octyne. Reaction of 4a,b with 4-dimethylaminopyridine gave 4-dimethylaminopyridine-coordinated complexes 6a,b, whose structures were elucidated by the X-ray structure of 6a. We conducted a control experiment in which 10 equiv of 4-octyne was added to 2a to give Ta2Cl4(OMe)(2)(mu-C-4-2,3-Pr-n(2)-4,5-Et-2)(thf) (7) in 90% yield, indicating that free 4-octyne reacted with the tantalacyclopropene moiety of 2a to form a dissymmetric tantalacyclopentadiene, followed by the release of 3-hexyne. The catalytic activity of 4a-6a for [2 + 2 + 2] cyclotrimerization of 3-hexyne was examined, and we found that their activities were in the order 5a > 4a 6a.