Chlorine-38 for chlorine substitution in geometric and diastereomeric compounds: mechanism and stereochemistry

Abstract

In order to determine how solvents control the stereochemical course of the hot substitution reactions, the stereochemistry of the energetic ³⁸Cl for Cl was studied in diastereomeric 1,2-dichloro-1,2-difluoroethane and the geometric isomers of 1,2-dichlorohexa-fluorocyclobutane. In each system, the conformer population was determined by NMR and dipole moment techniques. A comparison was made between the changes of the stereochemical course of the substitution reaction and the concentration and nature of the solvent.

The results presented in this investigation indicate that the stereochemical course of a hot atom substitution reaction is directly controlled by solvents participating in the de-excitation stabilization of excited compounds or in the relaxation process of intermediate radicals. This work also indicates that the nature of the solvent cage wall can control the progress of the hot reaction. Abstraction reactions of the Cl can occur in the cage with the compounds composing the cage wall. These reactions can effectively change the stereochemical course of the hot substitution reaction.

It appears that the liquid phase hot atom substitution reactions observed in this investigation are best explained by the caged radical-radical combination model. Strong solute-solvent interactions reduce the time for relaxation of the radicals in the solvent cage thus preventing the radicals from obtaining planarity and consequently retention products. On the other hand, weak solute-solvent interactions increase the time for relaxation of the intermediate which leads to a racemization of reaction products.