α- and β-Agostic Alkyl–Titanocene Complexes

Reaction of [Cp2Ti(Me)(CD2Cl2)]+ (I) with 3,3-dimethyl-1-butene in CD2Cl2 at 205 K produces the α-agostic insertion product [Cp2TiCH2CHMetBu]+ (II), in which the chirality at C(2) induces preferential agostic binding of one of the diastereotopic α-H atoms; subsequent coordination of ethyl ether to II leaves the α-agostic interaction weakened but intact. On warming, II undergoes β-hydrogen elimination and the resulting hydride reacts further with excess 3,3-dimethyl-1-butene to form the β-agostic species [Cp2TiCH2CH2tBu]+. NMR data and calculated (DFT) energies support the assignments

[Cp2TiCH2CHMe(SiMe3)]+, an alkyl-titanium complex which (a) exists in equilibrium between a β-agostic and a lower energy γ-agostic isomer and (b) undergoes hydrogen atom exchange between α-, β-, and γ-sites via a combination of conventional β-hydrogen elimination-reinsertion and a nonconventional CH bond activation process which involves proton tunnelling

The compound [Cp2Ti(Me)(CD2Cl2)][B(C6F5)4] reacts with trimethylvinylsilane (TMVS) to form the 1,2-insertion product [Cp2TiCH2CHMe(SiMe3)](+) (III), which exists in solution as equilibrating β- and γ-agostic isomers. In addition, while free rotation of the β-methyl group results in a single, averaged γ-H atom resonance at higher temperatures, decoalescence occurs below ~200 K, and the resonance of the γ-agostic hydrogen atom at δ ~ -7.4 is observed. Reaction of [Cp2Ti(CD3)(CD2Cl2)](+) with TMVS results in the formation of [Cp2TiCH2CH(CD3)(SiMe3)](+), which converts, via reversible β-elimination, to an equilibrium mixture of specifically [Cp2TiCH2CH(CD3)(SiMe3)](+) and [Cp2TiCD2CD(CH3)(SiMe3)](+). Complementing this conventional process, exchange spectroscopy experiments show that the β-H atom of [Cp2TiCH2CHMe(SiMe3)](+) undergoes exchange with the three hydrogen atoms of the β-methyl group (β-H/γ-H exchange) but not with the two α-H atoms. This exchange process is completely shut down when [Cp2TiCH2CH(CD3)(SiMe3)](+) is used, suggesting an H/D kinetic isotope effect much larger (apparently >16,000) than the maximum possible for an over-the-barrier process. It is proposed that β-H/γ-H exchange is facilitated by quantum mechanical proton tunnelling in which a hydrogen atom of the 2-methyl group of the alkene-hydride deinsertion product [Cp2TiHCH2═CMe(SiMe3)](+) undergoes reversible exchange with the hydride ligand via the allyl dihydrogen species [Cp2TiH2(η(3)-CH2C(SiMe3)CH2](+). Complementing these findings, DFT calculations were carried out to obtain energies and NMR parameters for all relevant species and thence to obtain better insight into the agostic preference(s) of complex III and the observed exchange processes. In all cases where comparisons between experimental and calculated data were possible, agreement was excellent

Mechanisms of α-, β-, and γ-H(D) Exchange Processes in the α-Agostic Alkyltitanocene(IV) Complexes [Cp2TiCH2CH(CH3)(CMe3)]+and [Cp2TiCH2CH(CD3)(CMe3)]+: Stark Contrasts with Their γ-SiMe3Analogues

The α-agostic alkyltitanocene(IV) complex [Cp2TiCH2CH(CH3)(CMe3)]+ (II) and its D3 isotopologue [Cp2TiCH2CH(CD3)(CMe3)]+ (II-CD3) undergo reversible, intramolecular H−H(D) exchange among the α-, β-, and γ- (β-Me) positions of the alkyl ligands in addition to concomitant intermolecular H−H(D) exchange with the vinylic and 2-methyl sites of the product of β-hydrogen elimination, CH2=CMeCMe3, results which are in strong contrast to the exchange behavior of the γ-agostic silyl analogues [Cp2TiCH2CH(CH3)(SiMe3)]+ (III) and [Cp2TiCH2CH-(CD3)(SiMe3)]+ (III-CD3). As has been previously shown, III undergoes tunnelling-expedited exchange of the β-H (but not α-H) with the hydrogen atoms of the β-methyl group (β-H/γ-H exchange) while III-CD3 isomerizes reversibly but specifically to the isotopomer [Cp2TiCD2CD(CH3)(SiMe3)]+, forgoing β- H/γ-H exchange completely. In this paper we show that all of the exchange processes of II/II-CD3 and III/III-CD3 initially involve conventional β-H elimination processes and thus, at some stage, intermediacy of the corresponding hydrido alkene complexes [Cp2TiH(CH2=CMeCMe3)]+ (VI) or [Cp2TiH(CH2=CMeSiMe3)]+ (or their D3 isotopologues). The major reason for the vastly different exchange behaviors of the two structurally very similar alkyltitanocene systems II and III is simply that, for steric and electronic reasons, the alkene in VI dissociates reversibly to the hydride, [Cp2TiH]+, and free alkene while that in [Cp2TiH(CH2=CMeSiMe3)]+ does not; CH2=CMeSiMe3 remains coordinated at the temperature and on the time scale of the experiments. DFT calculations generally support our mechanistic conclusions and furthermore point to a subtle ion pairing effect which hinders intramolecular η2-alkene rotation in VI and [Cp2TiH(CH2=CMeSiMe3)]+, thereby exerting a surprisingly important, heretofore unanticipated, influence on the nature of the chemistry involved