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: 4.56). 11/1998; 63(20):7072-7077. DOI: 10.1021/jo981129i
Source: PubMed

ABSTRACT 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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