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/beta-fluoride elimination

Abstract

Complex (PNP)Ti=(CHBu)-Bu-t((CH2Bu)-Bu-t) (PNP- = N[2-P(CHMe2)(2)-4-methylphenyl](2)) eliminates (H3CBu)-Bu-t to form transient (PNP)Ti (CBu)-Bu-t, which activates the C-F bond of ortho-difluoropyridine and ortho-fluoropyridine to form the alkylidene-fluoride complexes, (PNP)Ti=C[Bu-t(NC5H3F)](F) (1) and (PNP)Ti=C[Bu-t(NC5H4)](F) (2), respectively. When (PNP)Ti=(CHBu)-Bu-t((CH2Bu)-Bu-t) is treated with meta-fluoropyridine, the ring-opened product (PNP)Ti(C(Bu-t)CC4H3-3-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/beta-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/beta-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.