Mechanism of metal-catalyzed CC-coupling reactions with titanocene vinylidene. A theoretical study

The activation barriers and reaction energies for the [2+2]-cycloaddition of titanocene vinylidene with different reagents with double and triple bonds have been investigated at the B3LYP level of theory, using an effective core potential for Ti with a large valence basis set. For nonpolar reagents like ethylene or acetylene the reaction proceeds via a facile [2+2]-cycloaddition. In contrast to that polar reagents like formaldehyde or HCN react via primary formation of a donor–acceptor complex with the electrophilic titanium atom. This adduct rearranges to the transition state of the [2+2]-cycloaddition yielding the four membered titanacyclus. The analysis of the molecular orbitals of the 2-methylenetitanacyclobutene Cp2H, the 2-methyleneazatitanacyclobutene Cp2 and the 2-methyleneoxatitanacyclobutane Cp2 with the extended Hückel method makes the different reactivity of these compounds understandable. Subsequent reactions of the titanacyclobutanes and -butenes have been investigated as well: Cycloreversion occurs for titanacyclobutane, and with a substantial higher activation barrier for titanacyclobutene. Electrocyclic ring opening is proposed for azatitanacyclobutene. Metathesis reactions are possible for titanaoxetanes. A mechanism for the rearrangement of titanaoxetanes with the exocyclic methylene group in α-position to Ti into titanaoxetanes with the exocyclic methylene group in β-position has been proposed.