46344-B6
Carbanion Intermediates in Proton-Transfer Reactions Between Carbon Acids and Alkoxides
The formation of a carbanion intermediate in methanolic methoxide is investigated using computational chemistry. Working with a freshman during the school year we focused our attention on generating intermediates and searching for transition states in the proton exchange reactions of pentafluorobenzene, C6H5CHClCF3, C6H5CH(CF3)2, and 9-phenylfluorene. All systems where modeled at the B3LYP/6-31+G(d,p) level in the gas phase, the carbon acid reacting with methoxide, with two solvating methanol molecules, and using dielectric wrapping to account for bulk solvent effects. The student working on this project did an excellent job; she had a large learning curve to figure out the Gaussian03W software and without taking Organic Chemistry or Physical Chemistry a lot to learn about molecular orbital theory and thermochemistry. Now she has mastered the software and the basic chemistry of these systems and is working independently. Initial results on these systems indicate that we need a better understanding of the solvent reorganization in the presence of the carbanion. The IEFPCM solvent model as implemented in Gaussian03W was used to account for the dielectric wrapping. This bulk solvent model did not reproduce the experimental reaction kinetics or energetics. The reaction of chloroform with sodium methoxide in methanol produces a carbanion intermediate that can eliminate chloride anion. Elimination of chloride is slow and if the reaction is run in methanol-d1 isotope exchange occurs significantly faster than α-elimination, about 7000 times faster. We used chloroform as due to its small size, compared with the other molecules in this study, which allowed us to investigate an extended solvent shell. Thus we extended the solvent shell from two solvating methanol molecules to eight solvating methanol molecules. To reproduce the experimental reaction kinetics the reaction must proceed through a free carbanion intermediate with no interaction with the solvent. We were only able to identify an encounter complex intermediate and unable to identify a hydrogen-bound complex. We expect that as the carbon-hydrogen bond in chloroform is breaking to form methanol and a carbanion intermediate a new hydrogen bond is formed between a methanol solvent molecule and the carbanion intermediate. Two summer students worked on generating methoxide—methanol clusters using an atmospheric pressure ionization nebulizer coupled to a triple quadrupole mass analyzer. We were able to generate the clusters but due to instrument difficulties we were unable to get reproducible data. Currently, the experimental aspect of this project is stalled due to a burned out power supply. We are now focusing more on calculations and synthesis with a colleague in our department. We are reevaluating our computational methods to find a better solvent model and including solvent motion in our calculations. The student working with me this semester is continuing in this project and making good progress by reading the literature and trying out new methods.
