Fixation of heterocumulenes. 1. A theoretical study on the irreversible reaction of CO(2) with a 2-lithio-1,3-dithiane

The mechanism of electrophilic addition of CO(2) to 2-lithio-2-phenyl-1, 3-dithiane-tetrahydrofuran-tetramethylethylendiamine, a Corey-Seebach "umpolungs" reagent for nucleophilic acylation, was investigated at the B3YLP/6-311+G//HF/6-31+G level of theory with specific solvation effects being included in the study. The overall reaction is exothermic by 18.8 kcal/mol and proceeds via precomplexation of the CO(2) (necessary for activation). The reaction barrier is calculated to be 11.9 kcal/mol (relative to the precomplex 2a) and represents a lower boundary for the activation energy. The reaction barrier is shown to originate from specific solvent effects. A detailed survey of the electronic effects governing the reaction is given. The ab initio results were then compared with semiempirical PM3 calculations, which were extended to include the heterocumulenes COS and CS(2). As expected, the reduced electrophilicity of these molecules leads to a higher activation barrier by retention of the same reaction mechanism.