Abstract:

We report that oxidative addition of bromobenzene to Pd(P^tBu₃)₂ occurs by an unusual autocatalytic mechanism. Studies on the effect of various additives showed that the degree of rate acceleration followed the trend: (P^tBu₃)Pd(Ph)(Br) ≈ (HP^tBu₃)Br < [(P^tBu₃)Pd(μ-Br)]₂ < (P^tBu₃)₂Pd(H)(Br). Studies on the reactions of Pd(P^tBu₃)₂ in the presence of (P^tBu₃)₂Pd(H)(Br) showed that the concentration of (P^tBu₃)₂Pd(H)(Br) decreased only after the Pd(0) complex had been consumed. These data indicated that the catalyst in this process is (P^tBu₃)₂Pd(H)(Br). Thermal decomposition of the three-coordinate oxidative addition product (P^tBu₃)Pd(Ar)(Br) during the reaction of Pd(P^tBu₃)₂ and bromoarenes ultimately leads to formation of (P^tBu₃)₂Pd(H)(Br). Parallel reactions of bromobenzene with (P^tBu₃)₂Pd(H)(Br) and Pd(P^tBu₃)₂ showed that the bromoarenes reacted considerably faster with the Pd(II) species than with the Pd(0) species. We therefore propose a catalytic cycle for oxidative addition in which PBu^t₃·HBr reacts with the Pd(0) species to form (P^tBu₃)₂Pd(H)(Br), and (P^tBu₃)₂Pd(H)(Br) reacts with the bromoarene, possibly though the anionic species [HP^tBu₃⁺][(P^tBu₃)Pd(Br)⁻], to form [Pd(P^tBu₃)(Ar)(Br)].