700846-84-0Relevant academic research and scientific papers
Competitive ArC-H and ArC-X (X = Cl, Br) activation in halobenzenes at cationic titanium centers
Ma, Kuangbiao,Piers, Warren E.,Parvez, Masood
, p. 3303 - 3312 (2007/10/03)
The titanium methyl cation [Cp*(tBu3P=N) TiCH3]+[B(C6F5)4] - reacts rapidly with H2 to give the analogous cationic hydride [Cp*(tBu3P=N)TiH(THF)n] +[B(C6F5)4]- (n = 0, 1), which can be trapped and isolated as its THF adduct 1·THF (n = 1). When generated in the presence of chloro or bromobenzene, 1 undergoes C-X activation or ortho-C-H activation, depending on the amount of dihydrogen present in the reaction medium. At ～4 atm of H2, C-X activation is preferred, giving the halocations [Cp*(tBu3P= N)TiX] +[B(C6F5)4]- (2X) and C6H6/biphenyl mixtures. At lower pressures of H 2 (> 1 atm), the β-halophenyl cations [Cp*(Bu 3P=N)Ti(2-X-C6H4)]+[B(C 6F5)4]- (3X) are the products isolated. In the absence of H2, these compounds are quite thermally stable, but undergo β-halogen elimination upon moderate heating, to give 2X (～20%) and compounds 4X which are the result of reaction between 2X and benzyne via addition of the benzyne C-C triple bond across the Ti-N bond of the phosphinimide ligand. Thus, three separate bond activation processes are operative in this system: direct C-X activation, ortho-C-H activation, and indirect C-X activation via β-halogen elimination. Mechanistic studies on all three processes have been done and support a radical pathway for direct C-X cleavage, σ-bond metathesis of the ortho-C-H bond of η1- coordinated C6H5X, and β-halogen elimination from base-free compound 3X.
Isolation and Characterization of a Monomeric Cationic Titanium Hydride
Ma, Kuangbiao,Piers, Warren E.,Gao, Yuan,Parvez, Masood
, p. 5668 - 5669 (2007/10/03)
Methyl cations 1-Cp and 1-Cp*, stabilized by the tri-tert-butylphophinimine ligand and either C5H5 or C5Me5, were generated from the neutral dimethyl precursors and [Ph3C]+[B(C6F5)4]-. Reaction of these compounds with H2 resulted in contrasting reactions. For 1-Cp, hydrogenolysis of the Ti-CH3 group led to rapid reduction to Ti(III) and production of a cationic Ti(III) dimer, 2, presumably formed upon loss of H2 from a transiently generated Ti(IV) hydride. Compound 2 was characterized crystallographically and via its cleavage with donor solvents such as THF to form the monomeric [Cp(L)Ti(THF)2]+[B(C6F5)4]-, 3. In contrast, 1-Cp* reacted rapidly with H2 to form a cationic Ti(IV) hydride species, 4, which was resistant to reduction. While only moderately stable in solution under H2, a stable, isolable THF adduct preciptitated upon addition of THF, giving 4·THF, which was fully characterized, including via X-ray crystallography. Naked hydride 4 was very reactive toward haloarene solvents such as bromobenzene, giving the cationic bromide [Cp·(L)TiBr]+[B(C6F5)4]-, 5, which was fully characterized as its THF adduct 5·THF. The contrasting behavior of 1-Cp and 1-Cp* is a result of the greater steric protection and electron donation provided by the Cp* ligand relative to the Cp donor. Copyright
