Ab initio study on pericyclic reactions involving heteroatoms
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This thesis consists of work involving use of computational chemistry for studying the reactions of heteroatomic systems. It is interesting to observe how the reaction mechanism for classical organic reactions are affected by the introduction of sulfur or phosphorous as heteroatoms. The ab initio molecular orbital theory was used to investigate the sigmatropic rearrangement involving sulfur atoms. The energetics of the [1,3]- hydrogen shift in thioacetaldehyde and 1,3-propanedithial are calculated at MP4SDTQ/6-31 **// MP2/6-31G** + ZPE(HF/6-31G**). The results are compared to the [l,3]-hydrogen shift in acetaldehyde and malonaldehyde. The thiocarbonyl form is compared with the thiol form in terms of stability. The activation energy for the [l,5]-hydrogen shift in 1,3-propanedithial is 2.70 kcal/mol at MP4SDTQ/6- 31**+ ZPE(HF/6-31G**), which is 0.4 kcal/mol lower than in malonaldehyde. The activation barrier for the antarafacial TS of 1,3-propanedithial is approximately 15 kcal/mol lower than that of malonaldehyde. The second part of the thesis includes the study of phosphorous Bergmantype cyclization based on computational chemistry. These results will be compared with the Bergman-type cyclization which has been studied extensively, both experimentally and theoretically. The theoretical calculations have been carried at CASSCF level. The activation energy obtained in the electrocyclization of (Z)- hexa-l,5-diphospha-l,5-diyne-3-ene was approximately 27.8 kcal/mol at CASSCF/6-31G* level.