Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C−H Bonds in P-(Para-substituted benzyl)triphenylphosphonium Cations and Related Cations

Equilibrium acidities (pKHA) of six P-(para-substituted benzyl)triphenylphosphonium (p-GC6H4CH2PPh3+) cations, P-allyltriphenylphosphonium cation, P-cinnamyltriphenylphosphonium cation, and As-(p-cyanobenzyl)triphenylarsonium cation, together with the oxidation potentials [Eox(A-)] of their conjugate anions (ylides) have been measured in dimethyl sulfoxide (DMSO) solution. The acidifying effects of the α-triphenylphosphonium groups on the acidic C−H bonds in toluene and propene were found to be ca 25 pKHA units (34 kcal/mol). Introduction of an electron-withdrawing group such as 4-NO2, 4-CN, or 4-Br into the para position of the benzyl ring in p-GC6H4CH2PPh3+ cations resulted in an additional acidity increase, but introduction of the 4-OEt electron-donating group decreases the acidity. The equilibrium acidities of p-GC6H4CH2PPh3+ cations were nicely linearly correlated with the Hammett σ- constants of the substituents (G) with a slope of 4.78 pKHA units (R2 = 0.992) (Figure 1). Reversible oxidation potent...