Effects of Atom Pairs O and S on the Structure and Stability of Zwitterionic Tetrahedral Intermediate in the Aminolysis of Esters

Dae Dong Sung 

Dong-A University, 840 Hadan, Saha, Busan 604-714, Korea, South

Abstract

The aminolysis reactions of aryl S-carbamate and O-carbamate with arylamines in acetonitrile have been studied. Rates are much faster than the corresponding reactions of aryl O-carbamates. The rate increase from O-carbamte to S-carbamate is greater than that expected from substitution of thiophenoxide for phenoxide leaving group in the stepwise aminolysis reactions of esters. This large rate increase and the similar change in the aminolysis rates that are reported to occur from aryl ethyl carbamate to aryl ethylthiocarbamate lead us to conclude that the aminolysis of S-carbamate proceeds by a concerted mechanism in contrast to a stepwise process for O-carbamate. The reaction parameter, the negative ρxz values and violation of the reactivity-selectivity principle support the proposed mechanism. The reactivity parameter, the large βx values obtained are considered to indicate a large degree of bond making in the transition state, which is consistent with the relatively large kinetic effects observed. The effects of atom pairs O and S on the structure and stability of putative zwitterionic tetrahedral intermediates in the aminolysis of esters have been calculated using DFT method. The suggested model explicits that the solvation of the zwitterionic tetrahedral intermediates occurs for each of the four types of acetates by n = 0, 1, 3, 4, and 5 water molecules. The stability of the zwitterionic intermediates increases in the order (O,O) < (O,S) < (S,O) <(S,S). This stability order is that expected from a frontier energy gap control of the second order perturbation energy of vicinal charge transfer interaction. In this type of the second order perturbation energy control a thiol S (-S-) has a greater charge transfer to the *C=O (or *C=S) orbital than an alcoholic oxygen (-O-). The result shows there are clear distinction between sp2 and sp3 carbon zwitterions; for the former case, the reorganization energies are much lower than those for the latter case, i.e., the hydrogen bonding energies of the explicit H2Os are not sufficient to rehybridyze the carbonyl carbon center from sp2 to sp3, which requires successively less and less energy in the order, (O,O) > (O,S) > (S,O) >(S,S). In contrast, the increase in the solvation energy is linear with the number of water molecules, n. The calculated values show again that it increases with the same order as that of the stability order: (O,O = -6.7), (O,S=-7.1), (S,O=-7.9) and (S,S=-8.3 kcal/mol).

References:

[1] D. D. Sung, C. K. Kim, K. A. Lee, C. K. Sohn, H. K. Oh, I. Lee, J. Phys. Chem. A. 2005, 109, 2978.

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Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 2, by Dae Dong Sung
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-06-27 12:41
Revised:   2009-06-07 00:44
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