Water as dual functional cocatalyst: A theoretical study on the mechanism of direct aldol reaction on water catalyzed by a leucine derivative

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The role of water and stereoselectivity in the direct syn-aldol reaction involving 3-pentanone and 4-nitrobenzaldehyde catalyzed by amino acid derivatives on water has been investigated by density functional theory. Calculations indicate that the formation of intermediate enamine is the rate determining step via a three-step process with activation enthalpies of 50 kcal/mol in the gas phase and 21 kcal/mol in the presence of water. The subsequent nucleophilic addition of enamine to aldehyde is relatively easier with activation enthalpies below 10 kcal/mol both in the gas phase and in the presence of water. The diastereoselective formation of syn- and anti-aldol products results from the preferential formation of Z-enamine to E-enamine, kinetically and thermodynamically. The enantioselectivity of both syn- and anti-products is controlled by the steric repulsive interactions between the amino alcohol moiety of catalyst and the phenyl ring of aldehyde. Calculations show that water molecule can act as a proton shuttle in the proton-transport catalytic processes. The water-assisted proton-transfer is very efficient to reduce the activation barriers via protonation and deprotonation in the formation of C-N and C-C bonds, dehydration, and β-elimination processes by inhibiting the generation of zwitterionic transition states. The theoretical discoveries indicate that in the present proton-transport assistance, the amino alcohol moiety of the catalyst plays a critical role as hydrogen bond donor to anchor substrates with carbonyl group close to the amine or enamine moiety so that the water molecule can bridge the NH of amine and the oxygen of carbonyl by hydrogen bonding interaction around the reactive site to activate the reactants and promote the reaction effectively.