Titanocene and zirconocene σ-alkynyl complexes in C-C single bond coupling and cleavage reactions

Group 4 metallocene mono- and bis-sigma-alkynyl complexes of the type L2M(sigma-CdropCR) and L2M(sigma-CdropCR)(2) with M = titanium and zirconium in the oxidation states +3 and +4 and L = Cp (eta(5)-cyclopentadienyl) and Cp* (eta(5)-pentamethylcyclopentadienyl) are important compounds for stoichiometric and catalytic C-C single bond coupling and cleavage reactions. Detailed investigations show five-membered metallacyclocumulenes L2M(eta(4)-1,2,3,4-RC4R) as the key intermediates in both reactions of a C-C single bond cleavage of different 1,4-substituted 1,3-butadiynes RCdropC-CdropCR to alkynyl groups and the opposite reaction of C-C single bond formation starting from alkynyl groups under the formation of 1,4-substituted 1,3-butadiynes. Depending on different metals M and ligands L, coupling or cleavage is favoured. Combination of both reactions offered the first C-C single bond metathesis in homogeneous solution, which is photocatalyzed and titanocene-mediated. It proceeds via titanocene-mono-alkynyl complexes, which are interesting species also for other stoichiometric and catalytic C-C coupling reactions. Some similarities regarding the (sigma-to-pi conversion exist between the coupling of the alkynyl groups at titano- and zirconocenes to complexed 1,3-butadiynes on one side and the coupling of phenyl groups at chromium to complexed diphenyl on the other side. (C) 2002 Elsevier Science B.V. All rights reserved.