Theoretical study on the aminolysis of ester catalyzed by TBD: Hydrogen bonding or covalent bonding of the catalyst?

In this work, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-catalyzed aminolysis reaction of methyl acetate with methylamine has been investigated at the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory. In our calculations, we take into account three possible pathways for the reaction. Path 1 is a 'full concerted' mechanism via a SN2-like O → N acyl-transfer transition state through the hydrogen bonding of TBD; Path 2 is a stepwise mechanism involving the tetrahedral intermediates through the hydrogen bonding of TBD; and Path 3 is also a stepwise mechanism including the nucleophilic attack of TBD upon ester with covalently bonding of TBD. Results indicate that Path 2 is the likely mechanism and that the hydrogen bonding of TBD has an important role in the aminolysis of ester. The role of the TBD catalyst is to facilitate the proton transfer and accelerate the reaction.