An Extensive Ylide Thermodynamic Stability Scale Predicted by First-Principle Calculations

Thermodynamic stabilities of ylides are measured by the ease of the carbanion formation through the removal of a proton from their precursors. A full-spectrum scale of ylide thermodynamic stability is important to understand the reactivities and selectivities in ylide chemistry. In the present study is reported the first theoretical protocol for predicting the acidities of structurally unrelated ylide precursors in DMSO whose reliability has been tested against almost all the available experimental data. The ONIOM/G3B3// HF//CPCM/Bondi method is found to be the optimal protocol to handle the N-, P-, and S-containing ylides, whereas the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d)//HF//CPCM/Bondi method can be used to deal with those systems for which the ONIOM/G3B3 method is not feasible. Extensive calculations on about 80 experimentally characterized ylide precursors show that this theoretical protocol can reliably predict the pK(a) values of diverse structurally unrelated ylide precursors in DMSO with an error bar of ca. 1.6-1.9 pK(a) units. With the authorized theoretical protocol in hand, we have developed an extensive scale of ylide thermodynamic stability that may find applications in synthetic organic chemistry.