Nucleophilicity and CO2 fixation ability of phosphorus, nitrogen and sulfur ylides: insights on stereoelectronic factors from DFT study

Nucleophilicity and CO2 fixation ability of the ylides of phosphorous, sulfur and nitrogen have been assessed using DFT calculations at M06-2X/6-311++G(d,p)//M06-2X/6-31+G(d) level. Nucleophilicity of 54 ylides and their CO2 adducts have been evaluated using a theoretical nucleophilicity index (N) which shows good linear correlation (R2 = 0.87) with Mayr’s experimental nucleophilicity parameter for the S-ylides. Two key geometrical parameters, condensed Fukui function (fk−) and Gibbs free energy of reaction (ΔGr) for the ylide-CO2 adducts have also been calculated. In absence of steric effect around the ylidic C-atom and any other secondary interaction, binding energy of the CO2-adducts of all the three types of ylides has a good linear correlation with the nucleophilicity. Nucleophilicity of a P-ylide increases when electron-donating groups are introduced as substituents on both P and the ylidic C-atom. An electron-withdrawing group on the same sites reduces nucleophilicity. Free ylides have higher nucleophilicity than their CO2 adducts.