Coupling and decarboxylation mechanism of oxaloacetic acid and ethylenediamine: A theoretical investigation

The decarboxylation mechanism of deprotonated oxaloacetate at pH = 8.0 in aid of protonated ethylenediamine was investigated systematically by full optimization at M06‐2X/6‐311++G(d,p) level combined with the CPCM solvation model to consider the effect of bulk water, where the roles of the carbinolamine and imine intermediates were elucidated. In the minimum energy path, the NH3+ group binds to the β‐carboxyl group of oxaloacetate via a hydrogen bond, and the amino group as both a nucleophile and an electrophile connects to the CO group by a hydrogen transfer process with a free‐energy barrier of 131.9 kJ/mol. Then the carbinolamine intermediate is dehydrated to form an imine with a total barrier of 164.2 kJ/mol, which is the rate‐limiting step in this energetically most favorable channel. After a proton transfer process, the β‐decarboxylation barrier is only 49.6 kJ/mol. The detailed coupling and dehydration‐decarboxylation mechanism of oxaloacetate and ethylenediamine, and the role and reactivity of key intermediates were elucidated.