Theoretical study of the gas-phase decomposition of neutral α-amino acid ethyl esters. Part 2 - Elimination of ethyl picolinate and ethyl 1-methylpipecolinate

Theoretical studies of the thermolysis in the gas phase of two α-amino acid ethyl esters, ethyl picolinate and ethyl 1-methylpipecolinate, were carried out using ab initio theoretical methods, at the MP2/6-31G(d) and MP2/6-311+G(2d,p) levels of theory. The reactions studied have two steps: the first corresponds to the formation of a neutral amino acid intermediate via a six-membered cyclic transition state, and the second is the rapid decarboxylation of this intermediate via a five-membered cyclic transition state. The progress of the first step of the reactions was followed by means of the Wiberg bond indices. The results indicate that the transition states have an intermediate character between reactants and products, and the calculated synchronicities show that the reactions are concerted and slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming processes, indicating a bond deficiency in the transition states. The kinetic parameters calculated for both reactions agree very well with the available experimental results.