Unraveling strong bonds: Small molecule activation via metal-ligand multiple bonds.

摘要

Metal-ligand multiple bonds have had a significant impact in organometallic chemistry and have been the platform for a variety of reaction chemistry as well as participants in many industrial processes. In particular, transition metal alkylidynes have been applied in catalytic processes such as alkyne metathesis and polymerization reactions. Although transition metal alkylidynes are known for groups 4 and 5, they are far more common for group 6. Terminal alkylidyne versions of group 4 or any 3d transition metal are unknown. The complexes (PNP)Ti=CHtBu(CH2tBu) and (PNP)Ti=CHSiMe3(CH2SiMe3) can undergo intramolecular alpha-hydrogen abstractions to generate transient titanium alkylidynes of the type (PNP)Ti=CR (R = tBu or SiMe3). One major goal of this research is to trap or isolate the titanium alkylidyne functionality as well as explore the reactive nature of this transient species.;The cleavage of the strong C--N bond in pyridines and picolines by the transient (PNP)Ti=CtBu results in ring-opening of the N-heterocycle and formation of a new titanium complex. Addition of chlorosilanes to the ring-opened product facilitates the complete denitrogenation of the former N-heterocycle. A detailed mechanistic study of this unique transformation will be presented and discussed. The reactive alkylidyne linkage has also been the basis for the isolation of stable azatitanacyclobutadienes, which upon addition of an electrophile extrudes alkyne and forms a titanium imido complex. The synthesis of an alkylidyne analogue of Tebbe's reagent has shown to be a masked form of the reactive alkylidyne complex.;The complexes (PNP)Ti=CHtBu(CH2tBu) and (PNP)Ti=CHSiMe3(CH2SiMe3) have proven to be particularly reactive toward hydrofluoroarenes and fluoroalkanes to promote a series of sequential C-H/C-F activation reactions. A detailed understanding of the dehydrofluorination of saturated and unsaturated hydrofluorocarbons has been investigated through a variety of kinetic and isotopic labeling studies. Likewise, the (PNP)Ti=CtBu can participate in the C-F bond cleavage reactions of perfluorinated arenes and fluoropyridines via either 1,2-CF bond additions or [2+2] cycloaddition reactions.