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 (pK(a)s) 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 alpha-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 VIE 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 pK(a) 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., Delta pK = 12-25 pK units). A further examination of the pK, values also reveals that the thermodynamic stabilities of the Group VB onium ylides are in a decreasing order of P+-C- > As+-C- > N+-C- and of the Group VIB onium ylides in a decreasing order of S+-C- > Se+-C- similar to Te+-C-, if the substituents on the onium atoms are kept similar. The stability order for the ylides of the third-row elements was found to be S+-C- > P+-C-, as implied by the Delta pKs of 1.7-4.2 for the three R3P+-C-/R2S+-C- pairs compared (see text). The pK(a)s of the eight remotely substituted E+-CH2Ar series all correlate well with the sigma(-) constants (see Table 3) with a decreasing order of slopes as (E+ =) Ph2P(O) (-5.86) > Ph3As+ (-5.35) > Bu3P+ (-5.00) > Ph3P+ (-4.46) and Bu2Te+ (-5.50) > Me2Se+ (-5.03) > Me2S+ (-3.40), suggesting a similar trend for the extent of charge localization at the carbanions next to the E+ group. All these observations are consistent with the assumption that at least part of the gained stabilization in the phosphonium and sulfonium ylide cases (especially the latter) has to be attributed to the back-bonding stabilization involving the sigma* and/or 3d orbital participation. Discussion for elucidating this view is presented.