Free energy profile and microkinetic modeling of base-catalyzed conjugate addition reaction of nitroalkanes to α,β-unsaturated ketones in polar and apolar solvents

Abstract

Michael reactions involving nitroalkanes and enones are important carbon-carbon bond formation reactions. These reactions are base-catalyzed, and during the past 15years, the asymmetric version using bifunctional amino-thiourea organocatalyst has been developed. In this work, the reaction of nitromethane and 4-phenyl-3-buten-2-one, catalyzed by the methoxide ion and piperidine as bases, was investigated by theoretical calculations. We obtained the theoretical free energy profile and did a microkinetic analysis of the catalytic cycle. The direct reaction of the CH2NO2- ion and the enone is very favorable, with a free energy of activation of 21.1kcalmol-1 in methanol solvent. However, the generated MS2 product works like an inhibitor of the catalysis, and the effective barrier in the catalytic cycle becomes 25.5kcalmol-1, leading to slow kinetics at room temperature. In the case of the reaction in apolar solvent (toluene), we found a pathway involving isomerization from the CH3NO2 reactant to the CH2NO2H species, and the latter makes a nucleophilic attack on the enone. Piperidine works like a bifunctional catalyst. In this case, the barrier is very high (32.5kcalmol-1), indicating the importance of the polar environment to accelerate the reaction in the catalytic cycle. Graphical abstract Base-catalyzed conjugate addition reaction of nitroalkanes to α,β-unsaturated ketones.