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 (p K a s) of six families of remotely substituted benzyl onium salts (i.e., 4-G-C 6 H 4 CH 2 -E + ·Br -, where E + = Ph 3 P +, Ph 2 PO, Et 3 N +, Me 2 S +, Me 2 Se +, and Bu 2 Te + and G = H, Me, CF 3, CO 2 Me, CN, and NO 2 ), one family of α-E + substituted acetophenones (i.e., PhCOCH 2 -E + ·Br -, where E + = Me 2 S +, Bu 2 Te +, Bu 3 P +, and Ph 3 As + ), and one family of 9-E + -substituted fluorenes (i.e., 9-E + -FlH·Br -, where E + = Bu 2 S +, Me 2 Se +, Bu 2 Te +, Bu 3 P +, and Ph 3 As + ) 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 p K 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., Δp K = 12−25 p K units). A further examination of the p K a 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 - ∼ 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 Δp K s of 1.7−4.2 for the three R 3 P + −C - /R 2 S + −C - pairs compared (see text). The p K a s of the eight remotely substituted E + -CH 2 Ar series all correlate well with the σ - constants (see Table 3) with a decreasing order of slopes as (E + =) Ph 2 P(O) (−5.86) > Ph 3 As + (−5.35) > Bu 3 P + (−5.00) > Ph 3 P + (−4.46) and Bu 2 Te + (−5.50) > Me 2 Se + (−5.03) > Me 2 S + (−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 σ* and/or 3d orbital participation. Discussion for elucidating this view is presented.