Theoretical Analysis of the Mechanism of Palladium(II) Acetate-Catalyzed Oxidative Heck Coupling of Electron-Deficient Arenes with Alkenes: Effects of the Pyridine-Type Ancillary Ligand and Origins of

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• 作者：Songlin Zhang ; Li Shi ; Yuqiang Ding
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：December 21, 2011
• 年：2011
• 卷：133
• 期：50
• 页码：20218-20229
• 全文大小：1171K
• 年卷期：v.133,no.50(December 21, 2011)
• ISSN：1520-5126

文摘

A systematic theoretical study is carried out on the mechanism for Pd(II)-catalyzed oxidative cross-coupling between electron-deficient arenes and alkenes. Two types of reaction pathways involving either a sequence of initial arene C鈥揌 activation followed by alkene activation, or the reverse sequence of initial alkene C鈥揌 activation followed by arene activation are evaluated. Several types of C鈥揌 activation mechanisms are discussed including oxidative addition, 蟽-bond metathesis, concerted metalation/deprotonation, and Heck-type alkene insertion. It is proposed that the most favored reaction pathway should involve an initial concerted metalation/deprotonation step for arene C鈥揌 activation by (L)Pd(OAc)₂ (L denotes pyridine type ancillary ligand) to generate a (L)(HOAc)Pd(II)鈥揳ryl intermediate, followed by substitution of the ancillary pyridine ligand by alkene substrate and direct insertion of alkene double bond into Pd(II)鈥揳ryl bond. The rate- and regio-determining step of the catalytic cycle is concerted metalation/deprotonation of arene C鈥揌 bond featuring a six-membered ring transition state. Other mechanism alternatives possess much higher activation barriers, and thus are kinetically less competitive. Possible competing homocoupling pathways have also been shown to be kinetically unfavorable. On the basis of the proposed reaction pathway, the regioselectivity predicted for a number of monosubstituted benzenes is in excellent agreement with experimental observations, thus, lending further support for our proposed mechanism. Additionally, the origins of the regioselectivity of C鈥揌 bond activation is elucidated to be caused by a major steric repulsion effect of the ancillary pyridine type ligand with ligands on palladium center and a minor electronic effect of the preinstalled substituent on the benzene ring on the cleaving C鈥揌 bond. This would finally lead to the formation of a mixture of meta and para C鈥揌 activation products with meta products dominating while no ortho products were detected. Finally, the multiple roles of the ancillary pyridine type ligand have been discussed. These insights are valuable for our understanding and further development of more efficient and selective transition metal-catalyzed oxidative C鈥揌/C鈥揌 coupling reactions.