Oxidative Addition of Aryl Sulfonates to Palladium(0) Complexes of Mono- and Bidentate Phosphines. Mild Addition of Aryl Tosylates and the Effects of Anions on Rate and Mechanism

Oxidative additions of aryl halides to palladium complexes have been studied extensively
in recent years, but studies on the oxidative addition of aryl sulfonates to palladium
complexes are less common. Here, oxidative additions of aryl triflates to palladium(0)
complexes of hindered mono- and bis(phosphines) and the rare oxidative addition of aryl
tosylates to palladium(0) complexes of hindered bis(phosphines) are reported. Pd[P(o-tolyl)3]2
reacted with PhOTf in the presence of added halide salts to produce the corresponding
{Pd[P(o-tolyl)3](Ar)(μ-X)}2 complexes (X = halide). The rate of oxidative addition was
accelerated by the addition of coordinating anions but was not affected by the addition of
weakly coordinating anions. This result suggests that the oxidative addition of aryl triflates
occurs to an anionic palladium complex ligated by halide. The addition of phenyl triflate to
Pd(PPF-t-Bu)[P(o-tolyl)3] in the presence of added bromide produced [(PPF-t-Bu)Pd(Ph)(Br)]. The rate of this oxidative addition was accelerated by added bromide but was more
greatly accelerated by the addition of PF6-, a typically noncoordinating anion. This result
indicates that a medium effect, not coordination of bromide prior to oxidative addition,
accounts for the acceleration of the rate of addition of aryl triflates to Pd(PPF-t-Bu)[P(o-tolyl)3]. The oxidative addition of phenyl tosylate to Pd(PPF-t-Bu)[P(o-tolyl)3] and to Pd(CyPF-t-Bu)[P(o-tolyl)3] also occurred at room temperature to produce the corresponding
phenylpalladium tosylate complexes. In the presence of added bromide, arylpalladium
bromide complexes were formed. Like the addition of phenyl triflate to Pd(PPF-t-Bu)[P(o-tolyl)3], the rate of oxidative addition was accelerated by addition of either strongly or weakly
coordinating anions. The additions of aryl tosylates were also faster in more polar solvents.
Thus, the effect of halide on the rate of addition of aryl tosylates to Pd(PPF-t-Bu)[P(o-tolyl)3]
is also more likely to result from a medium effect, rather than from coordination of anion to
palladium prior to oxidative addition.