Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C-H Bonds in As-Substituted Triphenylarsonium and Related Cations(1).

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.