A DFT Study Of Mechanism For The Ramberg-b(?)cklund Reaction

The carbon-carbon double bond is one of the most fundamental functional group in organic chemistry. Although there are many methods to form it, chemical workers should be doing more in-depth and detailed research and exploration in this field, because the regioselectivity, stereoselectivity and other questions in the synthesis process, especially, the synthesis process is sensitive to light, heat, acid, basis etc.In the ever-expanding modern organic synthesis methods, the Ramberg–B(a¨)cklund reaction continues to occupy a prominent position among various methods for the construction of carbon-carbon double bond. On treatment with base,α-halo sulfones are converted into alkenes via Ramberg–B(a¨)cklund reaction. Initially, the Ramberg–B(a¨)cklund reaction was completed under the condition of KOH solution. It is now understood that the reaction takes place in the presence of base. The productive rate and the conformation of the alkenes are connected with the reaction conditions, such as the strength of the base, the polarity of the solvent and so on. The advantage of the Ramberg–B(a¨)cklund reaction is that it has good stereoselective and high yield.The Ramberg-B(a¨)cklund reaction has been applied widely in the organic synthesis, and many investigations are to optimize the reaction conditions of composing alkenes. To the best of our knowledge, the mechanism of the Ramberg-B(a¨)cklund reaction has not been studied by using quantum-chemical methods. So studying the mechanism of the reaction by using the method of quantum chemistry calculation is of great significance.Considering the above facets, we start our systemic study decomposition ofα-halo sulfones under different conditions by using the B3LYP method in the density functional theory. The thesis is consisted of five parts as follows:1. A DFT study for the mechanism of theα-halomethyl methyl sulfones decomposition reactionDensity functional theory (DFT) has been used to study the mechanism of theα-halomethyl methyl sulfones (CH₂XSO₂CH₃, X=Cl, Br) decomposition reaction at the B3LYP/6-311+G (d, p) levels. The transition states and the energy barriers of the reactions are obtained. The mechanism can be described by the following reaction sequences:The energy barrier of the first step is 80kcal/mol, which is more higher than the energy barrier of the second step. So the first step is the speed-controlled step, and it is difficult for theα-halomethyl methyl sulfones to decompose.2. The effect of the base strength on the Ramberg–B(a¨)cklund reactionThe mechanisms of decomposition ofα-chloromethyl methyl sulfone (CH₂ClSO₂CH₃) catalyzed by H₂O, NH₃ and OH- are studied by using the density functional theory (DFT) at the B3LYP/6-311+G(d, p) level. The transition states and the energy barriers of the reactions are obtained. The results indicate that the reaction is the easiest catalyzed by hydroxide ion, and the energy barrier of the speed-controlled step is reduced to 22.7kcal/mol.3. The effect of the solvent on the Ramberg–B(a¨)cklund reactionThe mechanisms of the decomposition ofα-chloromethyl methyl sulfone in the solvent H₂O, CH₃OH, and CCl₄ are studied by using the density functional theory (DFT) at the B3LYP/6-311+G(d, p) level. The results indicate that the mechanisms are similar to the reaction catalyzed by hydroxide ion in the gas-phase. The difference is that the energy barriers are lower if H₂O or CH₃OH is used as solvent.4. The effect of the substituent on the the Ramberg–B(a¨)cklund reactionThe effect of the substituent on the reaction is studied by using the density functional theory (DFT) at the B3LYP/6-311+G(d, p) level. The results indicate that the difference is the energy barrier of the second step. The energy barrier is reduced a little if the substituent is methyl. 5. The stereoselective of the Ramberg–B(a¨)cklund reactionThe mechanism of the decompose for theα-chloroethyl ethyl is studied to understand the stereoselective of the Ramberg–B(a¨)cklund reaction. The transition states and the energy barriers of the reactions are obtained. The results indicate that the product E-butylene is corresponding to not only the thermodynamics, but also the kinetics.