Recyclable NHC Catalyst for the Development of a Generalized Approach to Continuous Buchwald–Hartwig Reaction and Workup

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• 作者：Anthony Chartoire ; Carmen Claver ; Martin Corpet ; Jamin Krinsky ; Julie Mayen ; David Nelson ; Steven P. Nolan ; Itziar Peñafiel ; Robert Woodward ; Rebecca E. Meadows
• 刊名：Organic Process Research & Development
• 出版年：2016
• 出版时间：February 19, 2016
• 年：2016
• 卷：20
• 期：2
• 页码：551-557
• 全文大小：386K
• ISSN：1520-586X

文摘

A generalized approach to the optimization and implementation of Buchwald–Hartwig reactions in flow is reported, through the combination of three key factors: a highly active palladium catalyst, a universal approach for continuous workup and purification, and a methodology for catalyst recycling and reuse. The palladium N-heterocyclic carbene (NHC) precatalyst [Pd(IPr*)(cin)Cl] 4 (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene; cin = η³-cinnamyl) is an excellent choice for continuous Buchwald–Hartwig reactions, due to its inherent high activity and stability. In preparation for running this reaction in flow (published concurrently), a detailed study has been carried out into its water stability, ultimately allowing the recycling of the catalyst in the organic phase up to three times in batch mode. A “right-first-time” workup methodology has also been developed, resulting in a universal protocol that allows the selective extraction of the Buchwald–Hartwig product into the aqueous stream as a salt, while retaining the aryl bromide starting material in the organic stream with the catalyst, thus negating the requirement for further purification. It is therefore envisaged that this approach will be particularly amenable to exploitation in the pharmaceutical industry. An optimized, scalable synthesis of [Pd(IPr*)(cin)Cl] is also reported on the multihundred gram scale.