Effects of Adjacent Onium Cations and Remote Substituents on the H−A+ Bond Equilibrium Acidities in Dimethyl Sulfoxide Solution. An Extensive Ylide Thermodynamic Stability Scale and Implication for the Importance of Resonance Effect on Ylide Stabilities

The equilibrium acidities (pKas) of six families of remotely substituted benzyl onium salts (i.e., 4-G-C6H4CH2-E+·Br-, where E+ = Ph3P+, Ph2PO, Et3N+, Me2S+, Me2Se+, and Bu2Te+ and G = H, Me, CF3, CO2Me, CN, and NO2), one family of α-E+ substituted acetophenones (i.e., PhCOCH2-E+·Br-, where E+ = Me2S+, Bu2Te+, Bu3P+, and Ph3As+), and one family of 9-E+-substituted fluorenes (i.e., 9-E+-FlH·Br-, where E+ = Bu2S+, Me2Se+, Bu2Te+, Bu3P+, and Ph3As+) have been determined in a single solvent, dimethyl sulfoxide (DMSO). This allowed meaningful comparisons of the thermodynamic stabilities for an extensive range of Group VB and VIB ylides covering up to six elements (N, P, As; S, Se, Te) to be made for the first time on the basis of a unified standard. A comparison of the pKa values of onium salts with those of their parents shows that all the onium substituents studied in the present work are strongly ylide-stabilizing, covering an anion stabilization energy range of 17−35 kcal/mol (i.e., ΔpK = 12−25 pK units). ...