Theoretical Studies For Reaction Of Allylic Tin With Formaldehyde Promoted By SnCl₄ And Cyclization Of 1, 6-Enyne Catalyzed By GaCl₃

It is of great interest in computational and theoretical chemistry to investigate the chemical reactions and determine the reaction mechanism by using ab initio quantum chemistry calculations recently. Such studies are very useful in understanding mechanism at the molecular level, designing the special synthetic routes, and inventing new higher functional catalysts. In this dissertation, we have investigated the mechanism of reaction of allylic tin with formaldehyde promoted by SnCl4 and cyclization of 1,6-enyne catalyzed by GaCl3 by using B3LYP hybrid density functional method. Electronic structure calculations were performed to identify the stationary structures and energies on the potential energy surfaces. The reaction pathways were characterized and used to explain the selectivity observed by experiments successfully. In the series studies on reaction of allylic tin with formaldehyde promoted by SnCl4, we studied transmetalation of 2-trimethylstannylbuta-1,3-diene with SnCl4 at the B3LYP/6-31G** level at first. Our calculations showed that the reaction between the reactant and SnCl4, which generates 1-trichlorostannylbuta-2,3-diene via transmetalation, has the lowest energy barrier of 78.1kJ.mol-1. The interaction between reactants and product was also involved to study the following isomerization process. It turned out that the isomerization process from 1-trichlorostannylbuta-2,3-diene to 2-trichloro- stannylbuta-1,3-diene via transmetalation with SnCl4 is more energetically favorable than other possible isomerization processes.We have also performed theoretical calculation for two reactions of 1-trichlorostannylbuta-2,3-diene and 2-trichlorostannylbuta-1,3-diene with formaldehyde. The structures and energies of the stationary points were calculated to produce the activation barriers, free energy changes and equilibrium constants. Both reactions in CH2Cl2 solvent at 1atm, 298.2K have also been studied by the SCRF(PCM) method. The results showed that both reactions have a six-membered chair like cyclic transition state and low energy barrier, whereas the free energies are significantly different. It is showed from the calculated the equilibrium constants that the global reaction takes place with high regioselectivity and mainly produce the product of the reaction of the 1-trichlorostannylbuta-2,3-diene with formaldehyde, which is in good agreement with the recent experimental results (Hatakeyama, S. et al. Synlett,1999, 1109).In order to access more closely the experimental conditions, we have studied the mechanism of the reaction of 1-trichlorostannylbuta-2,3-diene with formaldehyde involved both SnCl4 and DMF. Two reaction pathways were obtained and discussed. Our results showed that the global reaction take place with high regioselectivity and mainly produce the product of the reaction of the 1-trichlorostannylbuta-2,3-diene with formaldehyde involved DMF. DMF also reduces the energy barrier and promotes the reaction. Since the difference between energy barriers of two reaction pathways is up to 60.7kJ.mol-1, the regioselectivity of reaction becomes higher when DMF is involved.In addition, we have carried out ab inito calculations for the cyclization of (Z)-8-(trimethylstannyl)oct-6-enal. Three reaction pathways were indentified. Our results showed that the reaction takes place with high stereoselectivity. Among the three reactions, the product in which both -OSn(CH3)3 and –C2H3 moieties are in the equatorial position is the most stable one. However, the mainly product is obtained through the double-six-membered chair like cyclic transition state with the lowest barrier height, in which -OSn(CH3)3 moiety is in the axial position. Stereoselectivity agrees well with experiment, which demonstrates the rationality of the mechanism including the six-membered cyclic transition state. On the other hand, the mechanism of cyclization of 1,n-enynes catalyzed by metals is not very clear till now. Chatani and co-workers(J. Am. Chem. Soc. 2002, 12, 10294)studied the skeletal reorganization...