EQUILIBRIUM ACIDITIES AND HOMOLYTIC BOND-DISSOCIATION ENERGIES OF THE ACIDIC C-H BONDS IN P-SUBSTITUTED TRIPHENYLPHOSPHONIUM CATIONS

Equilibrium acidities (pK(HA) values) for the cations in 11 P-substituted triphenylphosphonium salts, [Ph(3)PG](+)X(-), where G is CH3, (CH3)(2)CH, CH(2)Ph, CH(2)CO(2)Et, CH(Me)CO(2)Et, CH2COCH3, CH2CHO, CH2CN, CH(2)COPh, fluorene, and CH(2)SPh, together with the oxidation potentials of their conjugate bases (ylides), have been measured in dimethyl sulfoxide (DMSO) solution. The acidifying effects of the or-triphenylphosphonium groups (alpha-Ph(3)P(+)) on the adjacent C-H bonds in these cations were found to average about 29 kcal/mol, which is about 15 kcal/mol greater than the average for the acidifying effects of alpha-trimethylammonium groups (alpha-Me(3)N(+)) and about 6 kcal/mol greater than the acidifying effects of alpha-pyridinium groups (alpha-PyN(+)) on these G functions. Evidence is presented to show that these large acidifying effects of alpha-Ph(3)P(+) groups are caused by a combination of field/inductive and polarizability effects. The homolytic bond dissociation energies (BDEs) of the acidic C-H bonds in these cations estimated by combining their equilibrium acidities with the oxidation potentials of their conjugate bases (ylides) show that alpha-Ph(3)P(+) groups increase the BDEs in most of these cations by 0-3 kcal/mol, i.e., they slightly destabilize the corresponding radicals. These effects on BDEs are similar to those of alpha-Me(3)N(+) groups but opposite to the effects of alpha-PyN(+) groups, which usually decrease the BDEs by about 4-6 kcal/mol as a consequence of their ability to delocalize odd electrons present on adjacent atoms into the aromatic ring.