Mechanistic Studies into Amine-Mediated Electrophilic Arene Borylation and Its Application in MIDA Boronate Synthesis

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• 作者：Viktor Bagutski ; Alessandro Del Grosso ; Josue Ayuso Carrillo ; Ian A. Cade ; Matthew D. Helm ; James R. Lawson ; Paul J. Singleton ; Sophia A. Solomon ; Tommaso Marcelli ; Michael J. Ingleson
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：January 9, 2013
• 年：2013
• 卷：135
• 期：1
• 页码：474-487
• 全文大小：817K
• 年卷期：v.135,no.1(January 9, 2013)
• ISSN：1520-5126

文摘

Direct electrophilic borylation using Y₂BCl (Y₂ = Cl₂ or o-catecholato) with equimolar AlCl₃ and a tertiary amine has been applied to a wide range of arenes and heteroarenes. In situ functionalization of the ArBCl₂ products is possible with TMS₂MIDA, to afford bench-stable and easily isolable MIDA-boronates in moderate to good yields. According to a combined experimental and computational study, the borylation of activated arenes at 20 掳C proceeds through an S_EAr mechanism with borenium cations, [Y₂B(amine)]⁺, the key electrophiles. For catecholato-borocations, two amine dependent reaction pathways were identified: (i) With [CatB(NEt₃)]⁺, an additional base is necessary to accomplish rapid borylation by deprotonation of the borylated arenium cation (蟽 complex), which otherwise would rather decompose to the starting materials than liberate the free amine to effect deprotonation. Apart from amines, the additional base may also be the arene itself when it is sufficiently basic (e.g., N-Me-indole). (ii) When the amine component of the borocation is less nucleophilic (e.g., 2,6-lutidine), no additional base is required due to more facile amine dissociation from the boron center in the borylated arenium cation intermediate. Borenium cations do not borylate poorly activated arenes (e.g., toluene) even at high temperatures; instead, the key electrophile in this case involves the product from interaction of AlCl₃ with Y₂BCl. When an extremely bulky amine is used, borylation again does not proceed via a borenium cation; instead, a number of mechanisms are feasible including via a boron electrophile generated by coordination of AlCl₃ to Y₂BCl, or by initial (heteroarene)AlCl₃ adduct formation followed by deprotonation and transmetalation.