700846-92-0Relevant 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.
