Theoretical Assessment of Carbon Dioxide Reactivity in Methylpiperidines: A Conformational Investigation

In this work, the possible mechanisms for the reactions of CO₂ with various positional isomers of methylpiperidines (MPs) (N-MP, 2-MP, 3-MP, and 4-MP) including the effect of aqueous solvation have been explored using quantum chemical methods. The major pathways investigated for CO₂ capture in aqueous amines are carbamate formation, its hydrolysis, and the bicarbonate formation (CO₂ + H₂O + MP) reaction. The calculations indicate that an axial orientation for the methyl group and an equatorial for the COO^- group could be energetically ideal in the carbamate product of MPs. The proton abstraction step in the carbamate pathway is almost barrierless for the zwitterion-amine route, while a much higher energy barrier is observed for the zwitterion-H₂O route. During carbamate hydrolysis, the addition of even two explicit water molecules does not exhibit any notable effect on the already high energy barrier associated with this reaction. This indicates that bicarbonate formation is less likely to occur via carbamate hydrolysis. The calculations suggest that, although the carbamate pathway is kinetically favored, the MP carbamate could still be a minor product, especially for sterically hindered conformations, and the bicarbonate pathway should be predominant in aqueous MPs.