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 Interacti

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• 作者：Francoise Sauriol ; Jessica F. Sonnenberg ; Sarah J. Chadder ; Alexandre F. Dunlop-Brire ; Michael C. Baird ; Peter H. M. Budzelaar
• 刊名：Journal of the American Chemical Society
• 出版年：2010
• 出版时间：September 29, 2010
• 年：2010
• 卷：132
• 期：38
• 页码：13357-13370
• 全文大小：485K
• 年卷期：v.132,no.38(September 29, 2010)
• ISSN：1520-5126

文摘

The compound Cp₂TiMe₂ reacts with [Ph₃C][B(C₆F₅)₄] in CD₂Cl₂ at 205 K to give, <i>inter aliai>, [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 σ-alkyl complex [Cp₂Ti(CH₂CMe₂CH₂CHMe₂)]⁺. 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 [Cp₂Ti(CH₂CHMeCH₂CMe₃)]⁺; this does contain a β-hydrogen atom and, in concurrent processes, eliminates H₂ or 2,4,4-trimethyl-1-pentene (a major product) to form respectively the allylic complex [Cp₂Ti{η³-(CH₂)₂CCH₂CMe₃}]⁺ (a major product) or the hydride complex [Cp₂TiH]⁺. The latter reacts reversibly with free DMP to give the insertion product [Cp₂Ti(CH₂C<i>Hi>MeCH₂CHMe₂)]⁺ (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 ¹H resonance of the agostic hydrogen can be identified in the ¹H 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 [Cp₂Ti(H₂){η³-(CH₂)₂CCH₂CHMe₂}]⁺, which loses H₂ to form [Cp₂Ti{η³-(CH₂)₂CCH₂CHMe₂}]⁺ (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.