Understanding intermolecular C-F bond activation by a transient titanium neopentylidyne: experimental and theoretical studies on the competition between 1,2-CF bond addition and [2 + 2]-cycloaddition/β-fluoride elimination.

Complex (PNP)Ti=CH(t)Bu(CH(2)(t)Bu) (PNP(-) = N2-P(CHMe(2))(2)-4-methylphenyl) eliminates H(3)C(t)Bu to form transient (PNP)Ti≡C(t)Bu, which activates the C-F bond of ortho-difluoropyridine and ortho-fluoropyridine to form the alkylidene-fluoride complexes, (PNP)Ti=C(t)Bu(NC(5)H(3)F) (1) and (PNP)Ti=C(t)Bu(NC(5)H(4)) (2), respectively. When (PNP)Ti=CH(t)Bu(CH(2)(t)Bu) is treated with meta-fluoropyridine, the ring-opened product (PNP)Ti(C((t)Bu)CC(4)H(3)-3-FNH) (3) is the only recognizable titanium metal complex formed. Theoretical studies reveal that pyridine binding disfavors 1,2-CF bond addition across the alkylidyne ligand in the case of ortho-fluoride pyridines, while sequential [2 + 2]-cycloaddition/β-fluoride elimination is a lower energy pathway. In the case of meta-fluoropyridine, [2 + 2]-cycloaddition and subsequent ring-opening metathesis is favored as opposed to C-H bond addition or sequential [2 + 2]-cycloaddition/β-hydride elimination. In all cases, C-H bond addition of ortho-fluoropyridines or meta-fluoropyridine is discouraged because such substrate must bind to titanium via its C-H bond, which is rather weak compared to the titanium-pyridine binding.