Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C-H Bonds in As-Substituted Triphenylarsonium and Related Cations(1).
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.
The Journal of Organic Chemistry 10/1998; 63(22). DOI:10.1021/jo981078p · 4.64 Impact Factor
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ABSTRACT: A systematic investigation of the methylene C−H bond dissociation energies (BDEs) of the onium-substituted toluene series (1−6) and of the adjacent electron-pulling group-substituted acetophenone and fluorene series (7 and 8) (Chart 1) was carried out using a modified eletrochemical method (eq 1) incorporating the pKa's measured in DMSO solution. This provided the first opportunity to examine the stabilization effect of substituents on carbon radicals (or C−H bonds) of varying degrees of electron deficiency. The relative BDE (ΔBDE) values estimated for these substrates within a family showed that for most systems, the adjacent or remote electron-withdrawing groups (EWGs) were found to strengthen the benzylic (or methylene) C−H bonds (i.e., the O-type), which is in sharp contrast to the universally observed C−H bond-weakening effect of EWGs (i.e., the S-type) in the literature. This general phenomenon reveals that it is the apparent electronegativity of the methylene carbon, rather than the nonbonded electron pair as suggested in the literature, that governs the direction of radical substituent effects.Journal of the American Chemical Society 09/2000; 122(41). DOI:10.1021/ja994120m · 11.44 Impact Factor
The Journal of Organic Chemistry 05/1999; 64(11). DOI:10.1021/jo982037w · 4.64 Impact Factor