Stannyl Research Articles

Chemo-selective Stille-type coupling of acyl-chlorides upon phosphine-borane Au(i) catalysis.

Phosphine-boranes do not promote oxidative addition of acyl chlorides to gold, but the phosphine-borane gold triflimide complex [iPr2P(o-C6H4)BCy2]AuNTf2 was found to catalyze the coupling of acyl chlorides and aryl stannanes. The reaction involves aryl/chloride-bridged dinuclear gold(i) complexes as key intermediates, as substantiated by spectroscopic and crystallographic analyses. Similar to Pd(0)/Pd(ii)-catalyzed Stille coupling with phosphine-borane ligands, the gold-catalyzed variant shows complete chemoselectivity for acyl chlorides over aryl iodides and bromides, enabling straightforward access to halogenated aryl ketones.

Effective Preparation of [18F]Flumazenil Using Copper-Mediated Late-Stage Radiofluorination of a Stannyl Precursor.

(1) Background: [18F]Flumazenil 1 ([18F]FMZ) is an established positron emission tomography (PET) radiotracer for the imaging of the gamma-aminobutyric acid (GABA) receptor subtype, GABAA in the brain. The production of [18F]FMZ 1 for its clinical use has proven to be challenging, requiring harsh radiochemical conditions, while affording low radiochemical yields. Fully characterized, new methods for the improved production of [18F]FMZ 1 are needed. (2) Methods: We investigate the use of late-stage copper-mediated radiofluorination of aryl stannanes to improve the production of [18F]FMZ 1 that is suitable for clinical use. Mass spectrometry was used to identify the chemical by-products that were produced under the reaction conditions. (3) Results: The radiosynthesis of [18F]FMZ 1 was fully automated using the iPhase FlexLab radiochemistry module, affording a 22.2 ± 2.7% (n = 5) decay-corrected yield after 80 min. [18F]FMZ 1 was obtained with a high radiochemical purity (>98%) and molar activity (247.9 ± 25.9 GBq/µmol). (4) Conclusions: The copper-mediated radiofluorination of the stannyl precursor is an effective strategy for the production of clinically suitable [18F]FMZ 1.

Organogermanes as Orthogonal Coupling Partners in Synthesis and Catalysis.

Since the advent of metal-catalyzed cross-coupling technology more than 40 years ago, the field has grown to be ever-increasingly enabling, yet the employed coupling partners are largely still those that were originally employed in the context of Pd-catalyzed cross-coupling, namely, arylboronic esters/acids, aryl silanes, aryl stannanes, or organometallic reagents (RMgX, RZnX). Aryl germanes have little precedent in the literature; they were historically explored in the context of Pd0/PdII-catalyzed cross-coupling reactions but were found to be much less reactive than the already established reagents. Consequently, few efforts were made by the community on their further mechanistic or synthetic exploration.In 2019, our group described trialkyl aryl germanes as robust, convenient, and nontoxic reagents. Although structurally similar to trialkyl aryl stannanes or silanes, the GeEt3 site does not engage in the traditional transmetalation mode of PdII complexes. Our studies instead provided strong support for an unprecedented and orthogonal reactivity of organogermanes that follows electrophilic aromatic substitution (SEAr)-type reactivity. This mode of bond activation allowed us to devise a number of synthetic strategies in which the Ge functionality was for the first time more reactive and exclusively functionalized in preference over several of the established coupling partners (e.g., silanes, boronic acids/esters, halogens).Within the past year we have showcased the unique reactivity of organogermanes in C-C and C-X bond-forming transformations. Because of the exquisite mode of bond activation, the new strategies offer access to complementary chemical transformations, tolerating other cross-coupling enabling functionalities, and allow for their further downstream diversification. We have for instance demonstrated that organogermanes can be coupled efficiently with aryl halides under Pd nanoparticle conditions with tolerance of all other established cross-coupling partners, while under homogeneous Pd0/PdII catalysis all of the other established groups can be functionalized preferentially over the Ge functionality. We similarly were able to harness this orthogonal reactivity mode in oxidative gold catalysis, where organogermanes proved to be more reactive than the established silanes or boronic esters. We have also developed an orthogonal approach for metal-free halogenation of organogermanes with convenient halogenation agents, offering access to the chemo- and regioselective installation of valuable halide motifs in the presence of alternative groups that can also engage in electrophilic halogenations.In this Account, we wish to provide an overview of (i) the historic versus current reactivity findings and synthetic utility of organogermanes, (ii) the current state of mechanistic understanding of their reactivity, and (iii) the synthetic repertoire and ease of installing the germanium functionality in organic molecules.

Palladium-catalyzed cross-coupling of aroyl chlorides with aryl stannanes in the presence of triethylsilane: Efficient access to aromatic ketones

Herein, we report the development of a palladium-catalyzed cross-coupling reaction that focuses on the preparation of aromatic ketones. Aroyl chlorides react quickly at 120 °C with aryl stannanes in the presence of Pd(PPh3)4 and Et3SiH to efficiently give the corresponding ketones without the formation of significant decarbonylated byproducts. In other words, the decarbonylative side reaction is practically suppressed by simply adding Et3SiH to the reaction mixture, which reduces the amount of biaryl impurities in the products.

Modular Synthesis of Diarylalkanes by Nickel-Catalyzed 1,1-Diarylation of Unactivated Terminal Alkenes

A nickel-catalyzed 1,1-diarylation of electronically unbiased alkenes has been developed, providing straightforward access to diarylalkanes from readily available materials. Importantly, both the e...

Synthesis of Arylstannanes via Palladium‐Catalyzed Decarbonylative Coupling of Aroyl Fluorides

AbstractAryl stannanes are valuable precursors in organic transformations, but their synthetic methods are limited. Here we present a Pd‐catalyzed decarbonylative stannylation of acid fluorides in the absence of exogenous base. Various aryl stannanes were efficiently prepared from bench‐stable transition metal catalyst and ligand with broad functional group compatibility and substrate scope including natural products and pharmaceuticals. This protocol was also successfully used to a late‐stage diversification of an existing uricosuric drug probenecid.magnified image

Visible-Light-Driven Synthesis of Arylstannanes from Arylazo Sulfones.

The visible-light-driven preparation of (hetero)aryl stannanes was carried out under both photocatalyst- and metal-free conditions via irradiation of arylazo sulfones in the presence of hexaalkyldistannanes. The reaction shows a high efficiency and a wide substrates scope. The resulting crude organotin derivatives can be directly employed in a Stille protocol.

Synthesis of Aryl Trimethylstannane via BF3·OEt2-Mediated Cross-Coupling of Hexaalkyl Distannane Reagent with Aryl Triazene at Room Temperature.

BF3·OEt2-mediated cross-coupling of (SnMe3)2 with aryl triazene offers a new strategy for the synthesis of aryl stannane. A variety of synthetically useful aryl trimethylstannanes were produced in moderate to good yields with this metal-free approach. One-pot sequential Stille cross-coupling with different aryl bromides provides a short entry to both symmetrical and unsymmetrical biaryl compounds.

Trifluoromethyl Benzoate: A Versatile Trifluoromethoxylation Reagent.

Trifluoromethyl benzoate (TFBz) is developed as a new shelf-stable trifluoromethoxylation reagent, which can be easily prepared from inexpensive starting materials using KF as the only fluorine source. The synthetic potency of TFBz is demonstrated by trifluoromethoxylation-halogenation of arynes, nucleophilic substitution of alkyl (pseudo)halides, cross-coupling with aryl stannanes, and asymmetric difunctionalization of alkenes. The unprecedented trifluoromethoxylation-halogenation of arynes proceeds smoothly at room temperature with the aid of a crown ether-complexed potassium cation, which significantly stabilizes the trifluoromethoxide anion derived from TFBz.

Stannylation of Aryl Halides, Stille Cross‐Coupling, and One‐Pot, Two‐Step Stannylation/Stille Cross‐Coupling Reactions under Solvent‐Free Conditions

Solvent‐free protocols for palladium‐catalyzed stannylation of aryl halides, Stille cross‐coupling, and one‐pot, two‐step stannylation/Stille cross‐coupling (SSC) are reported for the first time. (Het)aryl halides bearing acceptor, donor, as well as sterically demanding substituents are stannylated and/or coupled in high yields. The reactions are catalyzed by conventional palladium(II) acetate/PCy3 [Pd(OAc)2/PCy3] under air, using available base CsF, and without the use of high purity reagents. The developed synthetic procedures are versatile, robust, and easily scalable. The absence of solvent, and the elimination of isolation procedures of aryl stannanes makes the SSC protocol simple, step economical, and highly efficient for the synthesis of biaryls in a one‐pot two‐step procedure.

Controllable Stereoselective Synthesis of (Z)- and (E)-Homoallylic Alcohols Using a Palladium-Catalyzed Three-Component Reaction.

Diastereoselective synthesis of (Z)- and (E)-homoallylic alcohols using a Pd-catalyzed three-component reaction of 3-(pinacolatoboryl)allyl benzoates, aldehydes, and aryl stannanes was developed, which provides an alternative method for the allylboration of aldehydes using α,γ-diaryl-substituted allylboronates. Both sets of reaction conditions enable access to either (Z)- or (E)-homoallylic alcohols with good to high alkene stereocontrol. The present method showed good functional group compatibility and generality. Efficient chirality transfer reactions to afford enantioenriched (Z)- and (E)-homoallylic alcohols were also achieved.

A Conjugated Microporous Polymer for Palladium‐Free, Visible Light‐Promoted Photocatalytic Stille‐Type Coupling Reactions

The Stille coupling reaction is a versatile method to mainly form aromatic C—C bonds. However, up to now, the use of palladium catalysts is necessary. Here, a palladium‐free and photocatalytic Stille‐type coupling reaction of aryl iodides and aryl stannanes catalyzing a conjugated microporous polymer‐based phototcatalyst under visible light irradiation at room temperature is reported. The novel coupling reaction mechanism occurs between the photogenerated aryl radical under oxidative destannylation of the aryl stannane, and the electron‐activated aryl iodide, resulting into the aromatic C—C bond formation reaction. The visible light‐promoted Stille‐type coupling reaction using the polymer‐based pure organic photocatalyst offers a simple, sustainable, and more economic synthetic pathway toward palladium‐free aromatic C—C bond formation.

Copper-Mediated Oxidative Fluorination of Aryl Stannanes with Fluoride.

A regiospecific method for the oxidative fluorination of aryl stannanes using tetrabutylammonium triphenyldifluorosilicate (TBAT) and copper(II) triflate is described. This reaction is robust, uses readily available reagents, and proceeds via a stepwise protocol under mild conditions (60 °C, 3.2 h). Broad functional group tolerance, including arenes containing protic and nucleophilic groups, is demonstrated.

Intramolecular trans-Dicarbofunctionalization of Alkynes by a Formal anti-Carbopalladation/Stille Cascade.

An intramolecular Pd-catalyzed trans-dicarbofunctionalization of internal alkynes using aryl bromides and aryl stannanes is presented. Tetrasubstituted double bonds embedded in an oligocyclic ring system are obtained in a regio- and diastereoselective fashion. The transformation features a broad substrate scope and functional-group tolerance.

ChemInform Abstract: Total Syntheses of Secalonic Acids A and D.

Total syntheses of the dimeric tetrahydroxanthone natural products secalonic acids A and D are described. Key steps involve kinetic resolution of the tetrahydroxanthone core structure using homobenzotetramisole catalysis and late-stage copper(I)-mediated homodimerization of complex aryl stannane monomers.

Total syntheses of secalonic acids A and D.

Total syntheses of the dimeric tetrahydroxanthone natural products secalonic acids A and D are described. Key steps involve kinetic resolution of the tetrahydroxanthone core structure using homobenzotetramisole catalysis and late-stage copper(I)-mediated homodimerization of complex aryl stannane monomers.

Total Syntheses of Secalonic Acids A and D

AbstractTotal syntheses of the dimeric tetrahydroxanthone natural products secalonic acids A and D are described. Key steps involve kinetic resolution of the tetrahydroxanthone core structure using homobenzotetramisole catalysis and late‐stage copper(I)‐mediated homodimerization of complex aryl stannane monomers.

Synthesis of Aryl Trimethylstannanes from Aryl Amines: A Sandmeyer‐Type Stannylation Reaction

Sandmeyer-type stannylation: Stille coupling is one of the most powerful coupling reactions for CC bond formation, whereas there are only limited methods to access aryl stannane compounds. A mild stannylation process based on a Sandmeyer-type transformation using aromatic amines as the starting materials is described. DCE: 1,2-dichloroethane.

ChemInform Abstract: Mild Copper‐Mediated Fluorination of Aryl Stannanes and Aryl Trifluoroborates.

This communication describes a mild copper-mediated fluorination of aryl stannanes and aryl trifluoroborates with N-fluoro-2,4,6-trimethylpyridinium triflate. This protocol demonstrates broad substrate scope and functional group tolerance, and does not require the use of any noble metal additives. The reaction is proposed to proceed via an arylcopper(III) fluoride intermediate.

ChemInform Abstract: Preparation of Imides via the Palladium‐Catalyzed Coupling Reaction of Organostannanes with Methyl N‐[Methoxy(methylthio)methylene]carbamate.

AbstractThe palladium‐catalyzed coupling of aryl(tributyl)stannanes (II) with carbamate (I) in the presence of cuprous 2‐thiophenecarboxylate leads to imino ethers (III), which are subjected to acid hydrolysis to yield imides (IV).