以理論計算方法探討一些簡單有機分子的化學反應

theoretical study of reaction mechanisms of some simple organic molecules

楊仲瑜

指導教授 ： 何嘉仁

國立臺灣師範大學/理學院/化學系/碩士(2003年)

摘要

無資料

關鍵字

甲基化 ； 熱分解

並列摘要

We carried out theoretical studies for two important reactions involving HNCO peptide bond: 1. the methylation of Hydroxamic Acid (RC(=X)NHOH, X=O、R=H) and its derivatives (X=S、R=CH3,Cl,CHCH2) with diazomethane , and 2. the thermal deamination and denitrosation of N-Nitrosoamides . The descriptions are following:
Part 1： There are seven possible pathways of methylation of (Z)HAs to generate O-alkylation and N-alkylation products. Path Z7 generating (Z)O,O-dimethylhydroxamates 9 has the lowest energy barrier 17.24 kcal/mol. The reaction between HAs and CH2N2 results in predominant formation of product 9 strictly high-site selective. When the substitutents of CH3, Cl and CHCH2 are added, the energy barrier of pathways and the site selection of product will be affected. The probability for the formation of (Z)N,O-dimethylhydroxamates 6 will be increase.
There are six possible pathways in the E-form Has methylation. Path E5、E6 generate (E)O,O-dimethylhydroxamate 24 , with the energy barriers, 22.85 kcal/mol, and 23.06 kcal/mol respectively. The substituents CH3, Cl and CHCH2 have little effect to the site-selectivity of the products except the Cl-substituent changes the rate-determine step. The production of (E)N,O-dimethylhydroxamates 27 has higher energy barrier.
Part 2： The similar methylation reaction of thiohydroxamic acid is investigated and the result compared with that of hydroxamic counterparts. We found that the relative energies、structures of transition state and the pathways of reaction were affected by two major factors: the size and the electronegativity of O and S atom. However products of Z and E isomers of SHAs also have high site-selectivity , with N-methyl product obtained at higher barrier, similar to Has counterparts.
Part 3： We studied theoretical the thermal decomposition of N-Nitrosoamides and designed two isodesmic reaction to understand the substitutions effects on the stability of the reactant and transition state . We found that the substitution at the different sites (N and C) of N-Nitrosoamides led to very different results. The silyl substituent at C-site even followed different pathway of decomposition.
The thermal decomposition of N-Nitrosoamides in acidic condition may undergo deaminative (loss of N2) and denitrosative (loss of NO+) two competitive pathways. According to our result, we may understand how the acidity, temperature and added nucleophilicity affect reaction barriers.

並列關鍵字

hydroxamic acid ； thiohydroxamic acid ； nitrosoamides