CO2 fixation : catalytic synthesis of β-hydroxycarboxylic acids

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Flowers, Brendan John Scott
Carbon dioxide fixation , Asymmetric catalysis , Carboxylation , Hydrogenation , Ketones
Although carbon dioxide as a greenhouse gas is a serious environmental concern, it remains a valuable C1 source if viable methods are available for its conversion into useful products. Herein, we present recent progress in the synthesis of aliphatic, aromatic, cyclic, and bicyclic beta-ketocarboxylic acids and the promising results from subsequent asymmetric hydrogenation to give beta-hydroxycarboxylic acids. For the synthesis of the beta-ketocarboxylic acids, we investigated the effects of temperature, reaction time, and amount of 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU), which is a promoter for carbon-carbon bond formation with CO2. The highest-yielding conditions for this DBU-promoted carboxylation reaction were used to carboxylate a number of aliphatic and aromatic substrates. In order to determine whether the hydrogenation reaction will effectively compete with the in situ decarboxylation of the beta-ketocarboxylic acids, 1H NMR spectroscopy was used to monitor the rate of decarboxylation. The solvent, electronic, and steric effect on the rate of decarboxylation was investigated by testing a variety of beta-ketocarboxylic acids. Using œRuCl2{(S)-BINAP} catalyst precursor, we determined the effect that solvent, H2 pressure, base, and substrate substitution had on the enantioselectivity of the asymmetric hydrogenation. CH2Cl2 and MeOH were determined to be the best solvents because of the high hydrogenation selectivity, high enantioselectivity, and decreased reaction times. These standard conditions were used to hydrogenate the variety of aliphatic and aromatic beta-ketocarboxylic acids previously synthesized. Additional experiments, including deuterium labelling, were performed in an attempt to elucidate the hydrogenation mechanism and the actively hydrogenated tautomer. These results lead us to believe that different reaction pathways occur in protic versus aprotic solvents. The results discussed herein represent the first in depth investigation of transition metal catalyzed hydrogenation of beta-ketocarboxylic acids. These results are very encouraging because enantioselectivities greater than 99 % were achieved for multiple beta-keto acids. This synthesis is industrially advantageous due to the limited number of reactants required, their low-cost, and the potential for recycling unused materials.
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