The oxidative addition of PhX (X = I, Br, Cl) to the complexes Pd(P t Bu 3 ) 2 (1), Pd(1-AdP t Bu 2 ) 2 (2), Pd(CyP t Bu 2 ) 2 (3), and Pd(PCy 3 ) 2 (4) were studied to determine the effect of steric properties on the coordination number of the species that undergoes oxidative addition and to determine if the type of halide affects the identity of this species. The kinetic data imply that the number of phosphines coordinated to the complex that reacts in the irreversible step of the oxidative addition processes for complexes 1-4 depends on the halide more than the steric properties of the ligands. The rate-limiting step of the oxidative addition of PhI occurred with L 2 Pd (0) 3 . The irreversible step of the oxidative addition of PhCl occurred with a monophosphine species in each case, as signaled by an inverse dependence of the rate constant on the concentration of ligand. The irreversible step of the oxidative addition of PhBr occurred with a bisphosphine species, as signaled by the zeroth-order or small dependence of the rate constant on the concentration of phosphine. Thus, the additions of the less reactive chloroarenes occur through lower-coordinate intermediates than additions of the more reactive haloarenes.

Direct arylations of pyridine N-oxide (PyO), a convenient method to prepare 2-arylpyridines, catalyzed by Pd(OAc)2 and PtBu3 have been proposed to occur by the generation of a PtBu3-ligated arylpalladium acetate complex (PtBu3)Pd(Ar)(OAc) and the reaction of this complex with PyO. We provide strong evidence that (PtBu3)Pd(Ar)(OAc) does not react directly with pyridine N-oxide. Instead, our data imply that the cyclometallated complex [Pd(OAc)(tBu2PCMe2CH2)]2, which is generated from the decomposition of (PtBu3)Pd(Ar)(OAc), reacts with PyO and serves as a catalyst for the reaction of PyO with (PtBu3)Pd(Ar)(OAc). The reaction of PyO with (PtBu3)Pd(Ar)(OAc) occurs with an induction period, and the reaction of (PtBu3)Pd(Ar)(OAc) with excess PyO in the presence of [Pd(OAc)(tBu2PCMe2CH2)]2 is zero-order in (PtBu3)Pd(Ar)(OAc). Moreover, the rates of reactions of PyO with bromobenzene catalyzed by [Pd(OAc)(tBu2PCMe2CH2)]2 and [Pd(PtBu3)2] depend on the concentration of [Pd(OAc)(tBu2PCMe2CH2)]2, but not on the concentration of [Pd(PtBu3)2]. Finally, the reaction of (PtBu3)Pd(Ar)(OAc) with the model heteroarylpalladium complex containing a cyclometallated phosphine [(PEt3)Pd(2-benzothienyl)(tBu2PCMe2CH2)] rapidly formed the arylated heterocycle. Together, these data imply that the rate-determining C-H bond cleavage occurs between PyO and the cyclometallated [Pd(OAc)(tBu2PCMe2CH2)]2, rather than between PyO and (PtBu3)Pd(Ar)(OAc). In this case, the resulting heteroarylpalladium complex transfers the heteroaryl group to (PtBu3)Pd(Ar)(OAc), and C-C bondformation occurs from (PtBu3)Pd(Ar)(2-pyridyl oxide). This mechanism proposed for the direct arylation of PyO constitutes an example of C-H bond functionalization in which C-H activation occurs at one metal center, and the activated moiety undergoes functionalization after transfer to a second metal center.

We report an example of a bisphosphine palladium(0) complex with hindered ligands that undergoes oxidative addition of chloro-, bromo-, and iodoarenes in high yield. Addition of PhX (X = I, Br, Cl) to [Pd(Q-phos-tol)2] produced [Pd(Q-phos-tol)(Ph)(I)], [Pd(Q-phos-tol)(Ph)(Br)], and [Pd(Q-phos-tol)(Ph)(Cl)]2. To study the mechanisms of the oxidative addition of the three haloarenes to [Pd(Q-phos-tol)2], we determined the order of the reaction on the concentration of ligand and haloarene. The different haloarenes reacted through different mechanistic pathways. Addition of iodobenzene occurred by irreversible associative displacement of a phosphine. Addition of bromobenzene occurred by rate-limiting dissociation of phosphine. Addition of chlorobenzene occurred by reversible dissociation of phosphine, followed by rate-limiting oxidative addition. The mechanism of exchange of ligands from the Pd(0)L2 was also studied. The rate constant value for dissociation of ligand calculated from ligand exchange experiments is in agreement with the value calculated through experiments on oxidative addition.