Mechanism of Pd-Catalyzed Ar-Ar Bond Formation Involving Ligand-Directed C-H Arylation and Diaryliodonium Oxidants: Computational Studies of Orthopalladation at Binuclear Pd(II) Centers, Oxidation To Form Binuclear Palladium(III) Species, and Ar•••Ar Reductive Coupling
Mechanism of Pd-Catalyzed Ar-Ar Bond Formation Involving Ligand-Directed C-H Arylation and Diaryliodonium Oxidants: Computational Studies of Orthopalladation at Binuclear Pd(II) Centers, Oxidation To Form Binuclear Palladium(III) Species, and Ar•••Ar Reductive Coupling
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DOI：
10.1021/om301013w
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发表时间：
2013-01-28
期刊：
影响因子：
2.8
通讯作者：
Yates, Brian F.
中科院分区：
文献类型：
--
作者：
Canty, Allan J.;Ariafard, Alireza;Yates, Brian F.
对 Pd 催化的 3-甲基 1-2-苯基吡啶 (mppH) 与 [Ph2I]BF4 偶联形成 mppPh 的计算分析支持了先前的合成和动力学研究，该研究表明双核钯物质参与限速氧化步骤。 Pd(OAc)(2) 预催化剂形成“蛤壳”正钯配合物 [Pd(mpp)(mu-OAc)32 (8) 作为活性催化剂，在催化循环中所有步骤中能量需求最高的反应中被 [Ph2I] 氧化。在涉及 Ph+ 正式转移的氧化反应中，亲电碘中心最初与 [Pd(mpp)(mu-OAc)12 (8)，“Pd-O 中心点 IPh2”的桥联乙酸氧原子相互作用，转变为过渡结构，保留 O 中心点中心点中心点 I 相互作用，并在 a 中形成“Pd(mu-Ph-eta(1))I”桥。四元环，“Pd 中心点中心点中心点 Ph 中心点中心点中心点 I(Ph)中心点中心点中心点 O-Pd”，然后消除 PhI，形成含有 Pd Pd 键的双核 Pd(III) 阳离子，[Ph(mpp)Pd(mu-OAc)(2)Pd(mpp)] (14)。阳离子 14 在一个 Pd 中心经历 mpp 中心点中心点 Ph 偶联，形成双核 Pd(II) 阳离子 [(mppPh-N)Pd(mu-OAc)(2)Pd(mpp)](+) (Da)。阳离子 Da 可能会碎裂以释放 mppPh 和单核钯物质，然后在单核中心进行正钯化。然而，在乙酸浓度非常低和氮供体浓度很高的环境中，Da 更有可能发生配体交换，释放 mppPh 并形成 [(mppH-N)Pd(mu-OAc)(2)Pd(mpp)(+) (I)。计算显示了阳离子 1 正钯化的低能途径，其中涉及氮供体试剂 mppH 和 mppPh 作为碱，以除去作为 [HN 供体] 的质子。这种正钯化将完成循环并再生催化剂，[Pd(mpp)(mu-OAc)](2) (8)。对 [(p-X-C6H4)(Mes)I]BF4 (X = H, Me, OMe, F, Cl, COMe, CF3) 反应计算分析得到的哈米特图，反应常数 (rho) 为 1.8，与实验结果 (rho = 1.7 +/- 0.2) 吻合良好。与此一致，分析揭示了过渡结构中含钯和含碘片段的相互作用能的主导作用。
A computational analysis of the Pd-catalyzed coupling of 3-methy1-2-phenylpyridine (mppH) with [Ph2I]BF4 to form mppPh is supportive of a prior synthetic and kinetic study implicating binuclear palladium species in a rate-limiting oxidation step. The Pd(OAc)(2) precatalyst forms the "clamshell" orthopalladated complex [Pd(mpp)(mu-OAc)32 (8) as the active catalyst, which is oxidized by [Ph2I] in a reaction having the highest energy requirement of all steps in the catalytic cycle. In the oxidation reaction, involving formal transfer of Ph+, the electrophilic iodine center interacts initially with a bridging acetate oxygen atom of [Pd(mpp)(mu-OAc)12 (8), "Pd-O center dot center dot center dot IPh2", which transforms to a transition structure with retention of the O center dot center dot center dot I interaction and formation of a "Pd(mu-Ph-eta(1))I" bridge in a four-membered ring, "Pd center dot center dot center dot Ph center dot center dot center dot I(Ph)center dot center dot center dot O-Pd", followed by elimination of PhI with formation of a binuclear Pd(III) cation containing a Pd Pd bond, [Ph(mpp)Pd(mu-OAc)(2)Pd(mpp)] (14). Cation 14 undergoes mpp center dot center dot center dot Ph coupling at one Pd center to form the binuclear Pd(II) cation [(mppPh-N)Pd(mu-OAc)(2)Pd(mpp)](+) (Da). Cation Da may fragment to release mppPh and mononuclear palladium species, followed by orthopalladation at a mononuclear center. However, in an environment of very low acetate concentration and high nitrogen-donor concentration, it is considered far more likely that Da undergoes ligand exchange with release of mppPh and formation of [(mppH-N)Pd(mu-OAc)(2)Pd(mpp)(+) (I). Computation shows a low-energy pathway for orthopalladation at cation 1 that involves nitrogen-donor reagents mppH and mppPh acting as bases to remove a proton as [HN-donor]. This orthopalladation would complete the cycle and regenerate the catalyst, [Pd(mpp)(mu-OAc)](2) (8). A Hammett plot obtained from a computational analysis of the reaction of [(p-X-C6H4)(Mes)I]BF4 (X = H, Me, OMe, F, Cl, COMe, CF3) has a reaction constant (rho) of 1.8, which compares well with the experimental result (rho = 1.7 +/- 0.2). Consistent with this, the analysis reveals the dominant role of the interaction energy for palladium- and iodine-containing fragments in the transition structure.