Kinetic and Electrochemical Studies of the Oxidative Addition of Demanding Organic Halides to Pd(0): the Efficiency of Polyphosphane Ligands in Low Palladium Loading Cross-Couplings Decrypted

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• 作者：Veronika A. Zinovyeva ; Sophal Mom ; Sophie Fournier ; Charles H. Devillers ; H茅l猫ne Cattey ; Henri Doucet ; Jean-Cyrille Hierso ; Dominique Lucas
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：October 21, 2013
• 年：2013
• 卷：52
• 期：20
• 页码：11923-11933
• 全文大小：483K
• 年卷期：v.52,no.20(October 21, 2013)
• ISSN：1520-510X

文摘

Oxidative addition (OA) of organic halides to palladium(0) species is a fundamental reaction step which initiates the C鈥揅 bond formation catalytic processes typical of Pd(0)/Pd(II) chemistry. The use of structurally congested polyphosphane ligands in palladium-catalyzed C鈥揅 bond formation has generated very high turnover numbers (TONs) in topical reactions such as Heck, Suzuki, Sonogashira couplings, and direct sp²C鈥揌 functionalization. Herein, the OA of aryl bromides to Pd(0) complexes stabilized by ferrocenylpolyphosphane ligands L1 (tetraphosphane), L2 (triphosphane), and L3 (diphosphane) is considered. The investigation of kinetic constants for the addition of Ph鈥揃r to Pd(0) intermediates (generated by electrochemical reduction of Pd(II) complexes coordinated by L1鈥?b>L3) is reported. Thus, in the OA of halides to the Pd(0) complex coordinated by L1 the series of rate constants k_app is found (mol^鈥? L s^鈥?): k_app(Ph鈥揃r) = 0.48 > k_app(ClCH₂鈥揅l) = 0.25 k_app(p-MeC₆H₄鈥揃r) = 0.08 鈮?k_app(o-MeC₆H₄鈥揃r) = 0.07 k_app(Ph鈥揅l). Kinetic measurements clarify the influence that the presence of four, three, or two phosphorus atoms in the coordination sphere of Pd has on OA. The presence of supplementary phosphorus atoms in L1 and L2 unambiguously stabilizes Pd(0) species and thus slows down the OA of Ph鈥揃r to Pd(0) of about 2 orders of magnitude compared to the diphosphane L3. The electrosynthesis of the complexes resulting from the OA of organic halides to [Pd(0)/L] is easily performed and show the concurrent OA to Pd(0) of the sp³C鈥揅l bond of dichloromethane solvent. The resulting unstable Pd/alkyl complex is characterized by NMR and single crystal X-ray structure. We additionally observed the perfect stereoselectivity of the OA reactions which is induced by the tetraphosphane ligand L1. Altogether, a clearer picture of the general effects of congested polydentate ligands on the OA of organic halides to Pd(0) is given.