Theoretical and experimental investigation of the formation of E- and Z-aldimines from the reaction of methylamine with acetaldehyde

The reaction of methylamine with acetaldehyde in pentane at 230 K leads to formation of both E- and Z- aldimines, the Z-isomer being the main product and generated upon kinetic control. The E- form is more stable than the Z- form by 3.3 kcal/mol (Delta G*:). Ab initio calculations have indicated tha t a tetrahedral zwitterion intermediate is not formed, and the bimolecular process, forming the aminoalcohol intermediate, occurs through a four cente r transition state, with concerted formation of C-N bond and proton transfe r. However, the predicted Delta G*(not equal) is very high, indicating that the true mechanism for this step is not bimolecular. A catalyzed pathway m ust be actuating. The following step, unimolecular elimination of water, al so presents a very high activation free energy. Similarly, a catalyzed mech anism is needed. The isomerization from the Z- form to the E- form through inversion of the nitrogen atom has a predicted Delta G*(not equal) of 27.0 kcal/mol at 298.15 K, resulting in a lifetime of four months for the Z-isom er.