Mechanistic studies on palladium-catalyzed coupling reactions

Palladium;Cross-Coupling;Coupling;Mechanism;Catalysis

Carrow, Bradley P.

关键词: Palladium;    Cross-Coupling;    Coupling;    Mechanism;    Catalysis;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/24376/Carrow_Bradley.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

A series of mechanistic studies reported here has revealed fundamental information about the individual steps of palladium-catalyzed coupling reactions. In one case, the oxidative addition of PhX (X = I, Br, Cl) to the complexes Pd(PtBu3)2 (1), Pd(1-AdPtBu2)2(2), Pd(CyPtBu2)2 (3), and Pd(PCy3)2 (4) (1-Ad = 1-adamantyl, Cy = cyclohexyl) was studied to determine the effect of steric properties on the coordination number of the species that undergoes oxidative addition and to determine whether 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 process for complexes 1−4 depends more on the halide than on the steric properties of the ligands. The rate-limiting step of the oxidative addition of PhI occurred with L2Pd(0) in all cases, as determined by the lack of dependence of kobs on [PtBu3], [1-AdPtBu2], or [CyPtBu2] and the inverse dependence of the rate constant on [PCy3] when the reaction was initiated with Pd(PCy3)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. An unusual autocatalytic mechanism for oxidative addition of bromobenzene to Pd(PtBu3)2 was also observed when reactions were run in the absence of strong base. Studies on the effect of various additives showed that the degree of rate acceleration followed the trend: (PtBu3)Pd(Ph)(Br) ≈ (HPtBu3)Br < [(PtBu3)Pd(μ-Br)]2 < (PtBu3)2Pd(H)(Br). Studies on the reactions of Pd(PtBu3)2 in the presence of (PtBu3)2Pd(H)(Br) showed that the concentration of (PtBu3)2Pd(H)(Br) decreased only after the Pd(0) complex had been consumed. These data indicated that the catalyst in this process is (PtBu3)2Pd(H)(Br). Thermal decomposition of the three-coordinate oxidative addition product (PtBu3)Pd(Ar)(Br) during the reaction of Pd(PtBu3)2 and bromoarenes ultimately leads to formation of (PtBu3)2Pd(H)(Br). Parallel reactions of bromobenzene with (PtBu3)2Pd(H)(Br) and Pd(PtBu3)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 PtBu3•HBr reacts with the Pd(0) species to form (PtBu3)2Pd(H)(Br), and (PtBu3)2Pd(H)(Br) reacts with the bromoarene, possibly though the anionic species [HPtBu3+][(PtBu3)Pd(Br)−], to form [Pd(PtBu3)(Ar)(Br)].In a separate study, the isolation and reactivity of a series of “ligandless,” anionic arylpalladium complexes of the general structure [Pd(Ar)Br2]22− were conducted. These anionic complexes insert olefins at room temperature, and these fast insertions indicate that the anionic complexes are kinetically competent to be intermediates in Heck−Mizoroki reactions conducted under “ligandless” conditions (lacking added dative ligand). Kinetic studies showed that the anionic complexes insert olefins much faster than the corresponding neutral, P(t-Bu)3-ligated complexes. Addition of halide to the reaction of the neutral complex (tBu3P)Pd(Ar)(Br) and styrene led to a significant rate acceleration, suggesting that the anionic complex forms rapidly and reversibly in situ from the neutral species prior to migratory insertion. These data, along with studies on the regioselectivity for reaction of aryl halides with butyl vinyl ether in the presence of the different starting catalysts, are consistent with the intermediacy of the same anionic, arylpalladium intermediates in Heck reactions catalyzed by palladium complexes containing bulky trialkylphosphine ligands as in reactions conducted under ligandless conditions.Lastly, a systematic study of the stoichiometric reactions of isolated arylpalladium hydroxo and halide complexes with arylboronic acids and aryltrihydroxyborates was conducted to evaluate the relative rates of the two reaction pathways commonly proposed to account for transmetallation in the Suzuki-Miyaura reaction. Based on the relative populations of the palladium and organoboron species generated under conditions common for the catalytic process and the observed rate constants for the stoichiometric reactions between the two classes of reaction components, we conclude that the reaction of a palladium hydroxo complex with boronic acid, not the reaction of a palladium halide complex with trihydroxyborate, accounts for transmetallation in catalytic Suzuki-Miyaura reactions conducted with weak base and aqueous solvent mixtures.