Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C−H Bonds in As -Substituted Triphenylarsonium and Related Cations 1
Department of Chemistry, Nankai University, Tianjin, 300071, People's Republic of China and Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459.The Journal of Organic Chemistry (Impact Factor: 4.72). 11/1998; 63(20):7072-7077. DOI: 10.1021/jo981129i
Equilibrium acidities (pK(HA)s) of As-fluorenyltriphenylarsonium, As-phenacyltriphenylarsonium, six As-(para-substituted benzyl)triphenylarsonium [p-GC(6)H(4)CH(2)(+)AsPh(3)] (G = H, Me, CF(3), CO(2)Me, CN, and NO(2)), and six P-(para-substituted benzyl)tri(n-butyl)phosphonium [p-GC(6)H(4)CH(2)(+)P(n-Bu)(3)] (G = H, Me, CF(3), CO(2)Me, CN, and NO(2)) bromide salts, together with the oxidation potentials [E(ox)(A(-))] of their conjugate bases (ylides) have been determined in dimethyl sulfoxide (DMSO) solution. Introduction of an alpha-triphenylarsonium (alpha-Ph(3)As(+)) group was found to increase the adjacent C-H bond acidities by 13-20 pK units (18-27 kcal/mol). The equilibrium acidities for the two series p-GC(6)H(4)CH(2)(+)AsPh(3) and p-GC(6)H(4)CH(2)(+)P(n-Bu)(3) cations were found to be nicely correlated with the Hammett sigma(-) constants of the corresponding para-substituents (G) (Figures 1 and 2). The homolytic bond dissociation enthalpies (BDEs) of the acidic C-H bonds determined by using eq 1 reveal that an alpha-Ph(3)As(+) group increases the BDE value of the adjacent acidic C-H bond by 2-5 kcal/mol, whereas the substituent effects for an alpha-Ph(3)P(+) or alpha-(n-Bu)(3)P(+) group was found to be dependent on the nature of the substituents attached to the alpha-carbon atom. Good linear correlations were obtained for the equilibrium acidities of As-(para-substituted benzyl)triphenylarsonium and P-(para-substituted benzyl)tri(n-butyl)phosphonium cations with the oxidation potentials of their conjugate bases (ylides) as shown in Figures 3 and 4, respectively.
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ABSTRACT: 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). A further examination of the pKa 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 ΔpKs of 1.7−4.2 for the three R3P+−C-/R2S+−C- pairs compared (see text). The pKas of the eight remotely substituted E+-CH2Ar series all correlate well with the σ- 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 σ* and/or 3d orbital participation. Discussion for elucidating this view is presented.
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