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 Cp2TiMe2 reacts with [Ph3C][B(C6F5)(4)] in CD2Cl2 at 205 K to give, inter alia, [Cp2TiMe(CD2Cl2)][B(C6F5)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 [Cp2TiMe(DMP)](+), which undergoes ready migratory insertion to form the sigma-alkyl complex [Cp2Ti(CH2CMe2CH2CHMe2)](+). The latter, which does not contain a beta-hydrogen atom, rearranges rapidly via an unprecedented 1,5-sigma bond metathesis reaction involving two isomeric epsilon-agostic species to give the sigma-alkyl species [Cp2Ti(CH2CHMeCH2CMe3)](+), this does contain a beta-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 [Cp2Ti{eta(3)-(CH2)(2)CCH2CMe3}](+) (a major product) or the hydride complex [Cp2TiH](+). The latter reacts reversibly with free DMP to give the insertion product [Cp2Ti(CH2CHMeCH2CHMe2)](+) (V, a major product), in which the italicized hydrogen atom engages in a beta-agostic interaction with the metal atom. Compound V is a rare example of both a beta-agostic derivative of a group 4 metallocene and a beta-agostic compound of any metal in which the H-1 resonance of the agostic hydrogen can be identified in the H-1 NMR spectrum (delta -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 [Cp2Ti(H-2){eta(3)-(CH2)(2)CCH2CHMe2}](+), which loses H-2 to form [Cp2Ti{eta(3)-(CH2)(2)CCH2CHMe2}](+) (a minor product). Support for all steps of the proposed reaction scheme comes from product distributions, from labeling studies utilizing [Cp2Ti(CD3)(CD2Cl2)](+), 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 [Cp2ZrMe(DMP)](+) does not undergo migratory insertion and subsequent reactions.