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

Group 4 metallocene mono- and bis-σ-alkynyl complexes of the type L2M(σ-CCR) and L2M(σ-CCR)2 with M=titanium and zirconium in the oxidation states +3 and +4 and L=Cp (η5-cyclopentadienyl) and Cp* (η5-pentamethylcyclopentadienyl) are important compounds for stoichiometric and catalytic CC single bond coupling and cleavage reactions. Detailed investigations show five-membered metallacyclocumulenes L2M(η4-1,2,3,4-RC4R) as the key intermediates in both reactions of a CC single bond cleavage of different 1,4-substituted 1,3-butadiynes RCCCCR 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 σ-to-π 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.