Claisen Rearrangement And Similar Reaction And Aromatic Transition State Theory Research

Pericyclic reaction is a kind of improtant chemical reactions in organic synthesis.It is characterized by interactional molecular orbital which forms a ring in the transitionstate. Many researchers proposed a great number of theories about the reactionmechanism, one of which is aromatic transition state theory which is proposed byZimmerman. The theory doesn’t consider the symmetry of the track, but it presents thepericyclic reaction experiences an aromatic transition state. The reaction depends onwhether the cyclic transition state has aromaticity or not.Aromatic transition state theory emphasizes that the ring transition has aromaticity,which makes system stable, so it is useful for the reaction. The aromaticity of thetransition state is the key to the problem. But people can not measure aromaticity of thetransition state, it inevitably doubts: Is the transition state of the pericyclic reactionreally aromatic? How is it? In the pericyclic reaction process, from reactants to thetransition states to the products, how does aromaticity change? These issues are veryimportant for aromatic transition state theory, but as far as we know, there is no answer.In this paper, we calculate an important pericyclic reaction-Claisen rearrangementreaction. On the reaction channel, from the reactant to the transition state to the product,we calculate nucleus independent chemical shifts(NICS) to judge and verify aromatictransition state theory. So we can find the change regular of aromaticity with thereaction processing.We research Claisen rearrangement and similar reaction mechanisms at the level ofB3LYP/6-311++G(d,p) by reading reviews. We calculate NICS to examine aromatictransition state theory on the IRC reaction channel. Specific researches include:1. A DFT study of Claisen rearrangement reactionThe reaction mechanism of Claisen rearrangement can be showed as: DFT has been used to optimize reactants, transition states and products at the levelof B3LYP/6-311++G(d,p). On the basis of the optimizing transition state, we conductIRC calculation to look for reaction channel. Then calculating NICS of some pointswhich are on reaction channel can help us to study change regular of aromaticity.In the figure above, we research Claisen rearrangement for the group of-F and-OH on the position of1,4and5respectively and discuss the change of aromaticity withreaction coordinate. Then we analyze activation energy is affected by the group of-Fand-OH. The results show that:(1) With the reaction proceeding, aromaticity gradually becomes larger. When itgets the transition state, aromaticity is the largest. But the reaction goes on processing,aromaticity becomes smaller. It shows the transition state has aromaticity andaromaticity is the largest. That indicates the aromatic transition state theory isreasonable.(2) In different positions, substituents have little effect on aromaticity. It shows thatthe transition state which is a six-membered ring plays an important part in aromaticity.(3) In the same position,–OH could reduce Eaand1-substituent plays a greaterrole.2. A DFT study of Claisen rearrangement of1-N-4-O-1,5-hexadieneThe reaction mechanism can be showed as:At the same level, key configurations are majorizated. Calculating NICS of somepoints are uesd to research the variation of aromaticity.In the figure above, we discuss Claisen rearrangement of1-N-4-O-1,5-hexadienefor the group of-F and–OCH3on the position of2,3and6respectively.The studycontains the change of aromaticity with reaction coordinate and analysis about theactivation energy. The results show that:(1) On the IRC passage, with the reaction processing, aromaticity becomes larger.The aromaticity of the transition state is the largest. With the reaction go on proceeding,aromaticity becomes smaller.(2) In different positions, the substituents have little effect on aromaticity.(3) In the same position,-OCH3could reduce Eaand1-substituent plays a more important part.3. A DFT study of Claisen rearrangement of1-S-4-O-1,5-hexadieneThe reaction mechanism can be showed as:In the figure above, we discuss Claisen rearrangement of1-S-4-O-1,5-hexadienefor the group of-F and–OCH3on the position of2,3and6respectively.Then thechanges of aromaticity and activation energy are discussed.The results show that:(1) On the IRC passage, with the reaction proceeding, aromaticity becomes larger.The aromaticity of the transition state is the largest. With the reaction go on proceeding,aromaticity becomes smaller.(2) In different positions, the substituents have little effect on aromaticity.(3)1-substituent plays a more important part in aromaticity.