Reactivity of Permethylzirconocene and Permethyltitanocene toward Disubstituted 1,3-Butadiynes:  η⁴- vs η²-Complexation or C−C Coupling with the Permethyltitanocene

This paper describes the reactivity of permethylzirconocene and permethyltitanocene toward different 1,3-butadiynes. A pointed dependence on the metals and the diyne substituents was found. Unusual, but still stable, five-membered zirconacyclocumulenes (η⁴-diyne complexes, zirconacyclopenta-2,3,4-trienes) Cp*₂Zr(η⁴-1,2,3,4-RC₄R), R = Ph and SiMe₃, were prepared using two new and effective synthetic routes. One starts with the permethylzirconocene bisacetylides Cp*₂Zr(C⋮CR)₂, R = Ph (1a), SiMe₃ (1b), which rearrange in sunlight to form the stable five-membered zirconacyclocumulenes Cp*₂Zr(η⁴-1,2,3,4-RC₄R), R = Ph (2a), SiMe₃ (2b). The alternative route to 2a and 2b is the reduction of Cp*₂ZrCl₂ with Mg in the presence of the adequate disubstituted butadiynes RC⋮C−C⋮CR. Both methods failed to produce the analogous titanacyclocumulenes, which seemed extremely unstable. Nevertheless, we were able to obtain distinct products employing the reduction pathway with permethyltitanocene. For R = SiMe₃, the novel titanacyclopropene (η²-complex) Cp*₂Ti(η²-1,2-Me₃SiC₂C⋮CSiMe₃) (3) was isolated. For R = Ph, an activation of both pentamethylcyclopentadienyl ligands was observed resulting in the complex [η⁵-C₅Me₄−(CH₂)−]Ti[−C(CHPh)C(CHPh)CH₂-η⁵-C₅Me₄] (4). The reaction of 4 with carbon dioxide led to the Cp*-substituted titanafuranone Cp*Ti[−OC(O)C(Ph)C(−)C(CHPh)CH₂-η⁵-C₅Me₄] (5). The zirconacyclocumulene 2b surprisingly inserted two molecules of CO₂ to give the unprecedented cumulenic dicarboxylate Cp*₂Zr [−OC(O)C(SiMe₃)CCC(SiMe₃)C(O)O−] (6). The η²-complex 3 (titanacyclopropene) took up one molecule of carbon dioxide to afford the titanafuranone Cp*₂Ti[OC(O)C(SiMe₃)C(C⋮CSiMe₃)−] (7).