| Description |
Ni(COD)2 and Xantphos are known to form a bis-chelated (Xantphos)2Ni species, which was believed to be a thermodynamic sink and detrimental to catalysis. This species was found to react with nitriles to form a transient (Xantphos)Ni(nitrile) complex, which can be trapped by alkenes and alkynes for productive (Xantphos)Ni(alkene/alkyne) complexes. Nitrile-induced dissociation of Xantphos from (Xantphos)2Ni indicates that this species is not a thermodynamic sink and that its activation is a kinetics problem. Aryl alkyl ketenes were synthesized in three steps: addition of an alkyl chain to the ?-position of aryl acetic acids, conversion to the acyl chloride, and amine mediated dehydrohalogenation of the acyl chloride. Ketenes with primary alkyl chains form smoothly at room temperature while ketenes with secondary chains require a reaction temperature of 60 ºC. Formation of ketenes with electron rich aromatic rings requires longer reaction times than for electron poor aromatic rings. A series of (dppf)Ni(aryl alkyl ketene) complexes were synthesized and fully characterized. The ketene is coordinated to the metal through the C=O bond in the solid state and this mode is usually the only one observed in solution. However, C=C coordination of the ketene is observed in solution for some ketenes. The mechanism of thermal decomposition of these complexes was investigated through a combination of experimental and theoretical techniques. Synergy between these techniques gave deep insight into this mechanism, which involves isomerization of the ketene from C=O coordinated to C=C, followed by decarbonylation, giving an intermediate carbonyl carbene complex. Rearrangement of the carbene ligand is irreversible and results in a carbonyl alkene complex. While decarbonylation is usually rate limiting, carbene rearrangement is rate limiting for electron poor or sterically large ketenes. Initial attempts toward a Ni-catalyzed Wittig reaction on CO2 led to formation of symmetrical allenes, which raises the question: Does the trapping reagent react with free or coordinated ketene? The mechanism of ketene self-exchange was investigated. Interestingly, this ligand exchange proceeds through an autoinductive process that can be accelerated through addition of water or inhibited by a number of additives. |
