Remarkable reactions and intermediates in titanocene(IV) chemistry: migratory insertion reactions of 2,2-disubstituted-1-alkenes, intramolecular 1,5-σ bond metathesis via ε-agostic interactions, and a rare example of a β-agostic alkyltitanocene complex.

The compound Cp(2)TiMe(2) reacts with [Ph(3)C][B(C(6)F(5))(4)] in CD(2)Cl(2) at 205 K to give, inter alia, [Cp(2)TiMe(CD(2)Cl(2))][B(C(6)F(5))(4)]. This solvent-separated ion pair reacts in turn with 2,4-dimethyl-1-pentene (DMP) to give a series of cationic species, the first being the alkene complex Cp(2)TiMe(DMP), which undergoes ready migratory insertion to form the σ-alkyl complex Cp(2)Ti(CH(2)CMe(2)CH(2)CHMe(2)). The latter, which does not contain a β-hydrogen atom, rearranges rapidly via an unprecedented 1,5-σ bond metathesis reaction involving two isomeric ε-agostic species to give the σ-alkyl species Cp(2)Ti(CH(2)CHMeCH(2)CMe(3)); this does contain a β-hydrogen atom and, in concurrent processes, eliminates H(2) or 2,4,4-trimethyl-1-pentene (a major product) to form respectively the allylic complex Cp(2)Ti{η(3)-(CH(2))(2)CCH(2)CMe(3)} (a major product) or the hydride complex Cp(2)TiH. The latter reacts reversibly with free DMP to give the insertion product Cp(2)Ti(CH(2)CHMeCH(2)CHMe(2)) (V, a major product), in which the italicized hydrogen atom engages in a β-agostic interaction with the metal atom. Compound V is a rare example of both a β-agostic derivative of a group 4 metallocene and a β-agostic compound of any metal in which the (1)H resonance of the agostic hydrogen can be identified in the (1)H NMR spectrum (δ -3.43). Interestingly, a NOESY experiment on V indicates slow mutual exchange between the agostic hydrogen atom, the hydrogen atoms on C(1), and those of Me(2). These observations are consistent with the intermediacy of the allylic dihydrogen species Cp(2)Ti(H(2)){η(3)-(CH(2))(2)CCH(2)CHMe(2)}, which loses H(2) to form Cp(2)Ti{η(3)-(CH(2))(2)CCH(2)CHMe(2)} (a minor product). Support for all steps of the proposed reaction scheme comes from product distributions, from labeling studies utilizing Cp(2)Ti(CD(3))(CD(2)Cl(2)), and from extensive DFT calculations. The observed titanocene-based chemistry stands in stark contrast to that of the analogous zirconium system, in which the unusual but well-characterized cationic methyl alkene complex Cp(2)ZrMe(DMP) does not undergo migratory insertion and subsequent reactions.