Competitive ArC−H and ArC−X (X = Cl, Br) Activation in Halobenzenes at Cationic Titanium Centers

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 H2 (>1 atm), the β-halophenyl cations [Cp*(tBu3PN)Ti(2-X−C6H4)]+[B(C6F5)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.