Generation and Cross-Coupling of Organozinc Reagents in Flow

Low-cost transition metal catalysed Negishi coupling: an update

The Negishi coupling is a significant C-C bond-forming reaction to access synthetically valuable organic compounds. In recent years, researchers have developed sustainable first-row transition metal (Fe, Co, Ni and Cu) based complexes in place of the conventional Pd catalyst for this reaction. Several such low-cost metal-based catalysts showed high efficiency and potential application in natural product synthesis. This review focuses on the recent achievements in low-cost transition metal-based Negishi coupling reactions, covering reports from 2016.

Heterogeneous Single-Atom Zinc on Nitrogen-Doped Carbon Catalyzed Electrochemical Allylation of Imines

Organometallic reagents are effective for carbon-carbon bond formation; however, consumption of stoichiometric amounts of metals is problematic. We developed electrochemical allylation reactions of imines catalyzed by nitrogen-doped carbon-supported single-atom zinc, which were fixed on a cathode to afford a range of homoallylic amines efficiently. The system could suppress generation of metallic waste, and the catalyst electrode showed advantages over bulk zinc in terms of activity and robustness. An electrochemical flow reaction was also successfully performed to produce the homoallylic amine continuously with minimum amounts of waste.

End-to-End Automated Synthesis of C(sp3)-Enriched Drug-like Molecules via Negishi Coupling and Novel, Automated Liquid-Liquid Extraction

Herein, we report an end-to-end process including synthesis, work-up, purification, and post-purification with minimal human intervention using Negishi coupling as a key transformation to increase Fsp3 in bioactive molecules. The main advantages of this protocol are twofold. First, the automated sequential generation of organozinc reagents from readily available alkyl halides offers a large diversity of alkyl groups to functionalize (hetero)aryl halide scaffolds via Pd-catalyzed Negishi coupling in continuous flow. Second, a fully automated liquid-liquid extraction has been developed and successfully applied for unattended operations. The workflow was completed with mass-triggered preparative high-performance liquid chromatography HPLC, providing an efficient production line of compounds with enriched sp3 character and better drug-like properties. The modular nature allows a smooth adaptation to a wide variety of synthetic methods and protocols and makes it applicable to any medchem laboratory.

(2-Ethylhexyl)sodium: A Hexane-Soluble Reagent for Br/Na-Exchanges and Directed Metalations in Continuous Flow

We report the on-demand generation of hexane-soluble (2-ethylhexyl)sodium (1) from 3-(chloromethyl)heptane (2) using a sodium-packed-bed reactor under continuous flow conditions. Thus, the resulting solution of 1 is free of elemental sodium and therefore suited for a range of synthetic applications. This new procedure avoids the storage of an alkylsodium and limits the handling of metallic sodium to a minimum. (2-Ethylhexyl)sodium (1) proved to be a very useful reagent and undergoes in-line Br/Na-exchanges as well as directed sodiations. The resulting arylsodium intermediates are subsequently trapped in batch with various electrophiles such as ketones, aldehydes, Weinreb-amides, imines, allyl bromides, disulfides and alkyl iodides. A reaction scale-up of the Br/Na-exchange using an in-line electrophile quench was also reported.

Disposable cartridge concept for the on-demand synthesis of turbo Grignards, Knochel-Hauser amides, and magnesium alkoxides

Magnesium organometallic reagents occupy a central position in organic synthesis. The freshness of these compounds is the key for achieving a high conversion and reproducible results. Common methods for the synthesis of Grignard reagents from metallic magnesium present safety issues and exhibit a batch-to-batch variability. Tubular reactors of solid reagents combined with solution-phase reagents enable the continuous-flow preparation of organomagnesium reagents. The use of stratified packed-bed columns of magnesium metal and lithium chloride for the synthesis of highly concentrated turbo Grignards is reported. A low-cost pod-style synthesizer prototype, which incorporates single-use prepacked perfluorinated cartridges and bags of reagents for the automated on-demand lab-scale synthesis of carbon, nitrogen, and oxygen turbo magnesium bases is presented. This concept will provide access to fresh organomagnesium reagents on a discovery scale and will do so independent from the operator’s experience in flow and/or organometallic chemistry.

Selective ortho-Functionalization of Adamantylarenes Enabled by Dispersion and an Air-Stable Palladium(I) Dimer

Contrary to the general belief that Pd-catalyzed cross-coupling at sites of severe steric hindrance are disfavored, we herein show that the oxidative addition to C-Br ortho to an adamantyl group is as favored as the corresponding adamantyl-free system due to attractive dispersion forces. This enabled the development of a fully selective arylation and alkylation of C-Br ortho to an adamantyl group, even if challenged with competing non-hindered C-OTf or C-Cl sites. The method makes use of an air-stable PdI dimer and enables straightforward access to diversely substituted therapeutically important adamantylarenes in 5-30 min.

A Practical Method for Continuous Production of sp3-Rich Compounds from (Hetero)Aryl Halides and Redox-Active Esters

A practically useful coupling reaction between aromatic halides and redox-active esters was realized by nickel catalysis through the use of a packed zinc bed column in continuous flow. Multiple reuse of the column showed a negligible decrease in efficiency, affording high space/time yields. A wide range of substrates, including a number of heteroaryl halides and polyfunctional materials were coupled in generally good yields. Longer-time and larger-scale experiments further demonstrates the robustness of the system.

Flow Technology for the Genesis and Use of (Highly) Reactive Organometallic Reagents

In the field of organic synthesis, the advent of flow chemistry and flow microreactor technology represented a tremendous novelty in the way of thinking and performing chemical reactions, opening the doors to poorly explored or even impossible transformations using batch methods. In this Concept article, we would like to highlight the impact of flow chemistry for exploiting highly reactive organometallic reagents, and how, alongside the well-known advantages concerning safety, scalability, and productivity, flow chemistry makes possible processes that are impossible to control by using the traditional batch approach.

A Ball-Milling-Enabled Reformatsky Reaction

An operationally simple one-jar one-step mechanochemical Reformatsky reaction using in situ generated organozinc intermediates under neat grinding conditions has been developed. Notable features of this reaction protocol are that it requires no solvent, no inert gases, and no pre-activation of the bulk zinc source. The developed process is demonstrated to have good substrate scope (39-82 % yield) and is effective irrespective of the initial morphology of the zinc source.