Surprising Differences in the Cycloaddition Reactions of Cumulenes and Heterocumulenes to Vinylidenetitanocene: Formation of Four‐ or Five‐Membered Titanacycles

Liberation of ethylene from titaniumcyclobutane 1 generates the corresponding metallaallene, Cp 2*TiCCH₂, which surprisingly behaves differently in trapping reactions with various compounds. In the presence of carbon dioxide, ketenes, and isocyanates the four‐membered titanacycles 2 (X  O, CR₂, and NR, respectively) form, whereas with metal carbonyls [L_nMCO] the products are the Fischer carbene complexes 3, which have a planar metallacyclopentene unit. Cp^*  η⁵‐C₅Me₅. (Figure Presented.)