From Carbodiimides to Carbon Dioxide: Quantification of the Electrophilic Reactivities of Heteroallenes

Kinetics of the reactions of isocyanates, isothiocyanates, carbodiimides, carbon disulfide, and carbon dioxide with carbanions or enamines (reference nucleophiles) have been measured photometrically in acetonitrile or DMSO solution at 20 °C. The resulting second-order rate constants and the previously published reactivity parameters N and s_N of the reference nucleophiles were substituted into the correlation log k₂(20 °C) = s_N(N + E) to determine the electrophilicity parameters of the heteroallenes: TsNCO (E = -7.69) ≫ PhNCO (E = -15.38) > CS₂ (E = -17.70) ≈ PhNCS (E = -18.15) > PhNCNPh (E = -20.14) ≫ CyNCNCy (E ≈ -30). An approximate value could be derived for CO₂ (-16 < E < - 11). Quantum chemical calculations were performed at the IEFPCM(DMSO)/B3LYP-D3/6-311+G(d,p) level of theory and compared with experimental Gibbs activation energies. The distortion-interaction model was used to rationalize the different reactivities of O- and S-substituted heteroallenes. Eventually it is demonstrated that the electrophilicity parameters determined in this work can be used as ordering principle for literature-known reactions of heteroallenes.