My group recently discovered a new class of C-C bond-forming reaction: the direct alpha-arylation of ketones catalyzed by palladium complexes. We have now extended this reaction to include the alpha-arylation of carboxylic acid derivatives such as amides and malonates, while finding remarkable catalysts that provide alpha-arylation of ketones at room temperature using aryl bromides and at only 70 C using aryl chlorides. Under NIH support my group would 1) study new alkylphosphine ligands for the catalytic process based on our hypothesis that sterically hindered, chelating alkyl phosphines accelerate reaction rates, 2) expand the scope of this new process to other types of carbonyl compounds, alkyl cyanides, and nitroalkanes and 3) develop a detailed, quantitative mechanistic understanding of the reactions comprising the catalytic cycle. More specifically, we will prepare alkylphosphine ligands containing large and small substituents at phosphorus and with backbones that provide large and small bite angles. These ligands will be used to further improve yields, rates, and substrate scope, while uncovering the features of our current ligands that provide such fast rates. These ligands will also be used in studies toward extending the scope of electrophiles to vinyl and heteroaromatic halides and sulfonates, and to nucleophilic partners such as the anions of alpha-diketones, alpha-siloxy ketones, alpha, beta-unsaturated ketones, beta-dicarbonyl compounds, esters, nitriles, nitroalkanes, and azlactones. A detailed mechanistic description of the catalytic chemistry based on firm quantitative data is an important goal of the proposed research. In general, we will conduct a careful study to determine how ligands steric and electronic properties affect each step of the catalytic cycle, including oxidative addition of aryl halide that is likely to be the rate determining step of the reaction, formation of an arylpalladium enolate complex from the resulting arylpalladium halide complex, and C-C bond-forming reductive elimination that is the crucial coupling step in the catalytic cycle. We have conducted the first direct observation of this type of reductive elimination. Beta-Hydrogen elimination from the palladium enolate complexes, which competes with reductive elimination, will be investigated to determine how this process can be prevented. Finally, we will begin a detailed mechanistic study of the initial asymmetric version of the ketone arylation process in conjunction with Buchwald's synthetic effort as a means for our two groups to create improved enantioselective catalysts.
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