Fixation of heterocumulenes. 1. A theoretical study on the irreversible reaction of CO2 with a 2-lithio-1,3-dithiane

The mechanism of electrophilic addition of CO2 to 2-lithio-2-phenyl-1,3-dit hiane-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 includ ed in the study. The overall reaction is exothermic by 18.8 kcal/mol and pr oceeds via precomplexation of the CO2 (necessary for activation). The react ion barrier is calculated to be 11.9 kcal/mol (relative to the precomplex 2 a) and represents a lower boundary for the activation energy. The reaction barrier is shown to originate from specific solvent effects. A detailed sur vey of the electronic effects governing the reaction is given. The ab initi o results were then compared with semiempirical PM3 calculations, which wer e extended to include the heterocumulenes COS and CS2. As expected, the red uced electrophilicity of these molecules leads to a higher activation barri er by retention of the same reaction mechanism.