EQUILIBRIUM ACIDITIES AND HOMOLYTIC BOND-DISSOCIATION ENTHALPIES OF THE ACIDIC C-H BONDS IN P-(PARA-SUBSTITUTED BENZYL)TRIPHENYLPHOSPHONIUMCATIONS AND BELATED CATIONS

Equilibrium acidities (pK(HA)) of six P-(para-substituted benzyl)triph enylphosphonium (p-GC(6)H(4)-CH(2)PPh(3)(+)) cations, P-allyltriphenyl phosphonium cation, P-cinnamyltriphenylphosphonium cation, and As-(p-c yanobenzyl)triphenylarsonium cation, together with the oxidation poten tials [E(ox)(A(-))] of their conjugate anions (ylides) have been measu red in dimethyl sulfoxide (DMSO) solution. The acidifying effects of t he a-triphenylphosphonium groups on the acidic C-H bonds in toluene an d propene were found to be ca 25 pK(HA) units (34 kcal/mol). Introduct ion of an electron-withdrawing group such as 4-NO2, 4-CN, or 4-Br into the para position of the benzyl ring in p-GC(6)H(4)CH(2)PPh(3)(+) cat ions resulted in an additional acidity increase, but introduction of t he 4-OEt electron-donating group decreases the acidity. The equilibriu m acidities of p-GC(6)H(4)CH(2)PPh(3)(+) cations were nicely linearly correlated with the Hammett sigma(-) constants of the substituents (G) with a slope of 4.78 pK(HA) units (R(2) = 0.992) (Figure 1). Reversib le oxidation potentials of the P-(para-substituted benzyl)triphenylpho sphonium ylides were obtained by fast scan cyclic voltammetry. The hom olytic bond dissociation enthalpies (BDEs) of the acidic C-H bonds in these cations, estimated by combining their equilibrium acidities with the oxidation potentials of their corresponding conjugate anions, sho wed that the alpha-Ph(3)P(+) groups have negligible stabilizing or des tabilizing effects on the adjacent radicals. The equilibrium acidity o f As-(p-cyanobenzyl)triphenylarsonium cation is 4 pK(HA) units weaker than that of P-(p-cyanobenzyl)triphenylphosphonium cation, but the BDE of the acidic C-H bond in As-(p-cyanobenzyl)triphenylarsonium cation is ca 2 kcal/mol higher than that in P-(p-cyanobenzyl)triphenylphospho nium cation.