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 sN of the reference nucleophiles
were substituted into the correlation log k2(20 °C) = sN(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.