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

摘要

Complex (PNP)TiCH~tBu(CH_2 ~tBu) (PNP - = N[2-P(CHMe_2)_2-4-methylphenyl]_2) eliminates H_3C~tBu to form transient (PNP)TiC ~tBu, which activates the C-F bond of ortho-difluoropyridine and ortho-fluoropyridine to form the alkylidene-fluoride complexes, (PNP)TiC[ ~tBu(NC_5H_3F)](F) (1) and (PNP)TiC[ ~tBu(NC_5H_4)](F) (2), respectively. When (PNP)TiCH~tBu(CH_2 ~tBu) is treated with meta-fluoropyridine, the ring-opened product (PNP)Ti(C(~tBu)CC _4H_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.