The Slow-Release Strategy in Suzuki-Miyaura Coupling

MOF-253-Pd(OAc)2: a recyclable MOF for transition-metal catalysis in water

We report palladium(ii)-functionalized MOF-253 (MOF-253-Pd(OAc)₂) as a recyclable catalyst to form all-carbon quaternary centers via conjugate additions of arylboronic acids to β,β-disubstituted enones in aqueous media.

Palladium-Catalyzed Suzuki Cross-Coupling of 2-Halo-Deazapurines with Potassium Organotrifluoroborate Salts in the Regioselective Synthesis of Imidazo[4,5-b]pyridine Analogues

In this paper, we report the use of potassium organotrifluoroborate salts as nucleophilic organoboron reagents in the Suzuki cross-coupling reactions of 2-halo deazapurines. Regio-isomeric C-2-substituted imidazo[4,5-b]pyridine analogues were synthesized by employing this protocol in good to excellent yields. Whereas aryl and heteroaryl trifluoroborates reacted readily to give the coupled products in high yields, alkyltrifluoroborates were found to be less reactive. The utilization of tetrabutylammonium acetate was found to play a substantial role in enhancing the reaction rates of the cross-coupling process. Also, a comparative study was performed between boronic acids and potassium organotrifluoroborate salts.

Dosage delivery of sensitive reagents enables glove-box-free synthesis

Contemporary organic chemists employ a broad range of catalytic and stoichiometric methods to construct molecules for applications in many fields, including material sciences¹, pharmaceuticals^2–5, agrochemicals, and sensors⁶. The potential utility of a synthetic method can be greatly reduced if it relies on the use of air- and/or moisture-sensitive reagents or catalysts. Furthermore, many synthetic chemistry laboratories have numerous containers of partially used reagents that have been spoiled by exposure to the ambient atmosphere. This is exceptionally wasteful from both an environmental and a cost perspective. In this manuscript, we report an encapsulation method through which air- and moisture-sensitive sensitive compounds can be rendered stable and stored on a laboratory bench top. We demonstrate this approach in three contexts, by describing single use capsules that contain all of the reagents (i.e., catalysts, ligands, and bases) necessary for palladium-catalyzed carbon–fluorine^7–9, carbon–nitrogen^10,11, and carbon–carbon¹² bond forming reactions. The strategy described in this paper should be broadly applicable to a wide range of reagents and catalysts and should have the power to be transformative in preparative organic chemistry, particularly for inexperienced chemists. In addition, this approach will reduce the amount of tedious and time-consuming weighing procedures for the synthetic chemist performing these techniques on a large number of substrate combinations.

From synthesis to function via iterative assembly of N-methyliminodiacetic acid boronate building blocks

The study and optimization of small molecule function is often impeded by the time-intensive and specialist-dependent process that is typically used to make such compounds. In contrast, general and automated platforms have been developed for making peptides, oligonucleotides, and increasingly oligosaccharides, where synthesis is simplified to iterative applications of the same reactions. Inspired by the way natural products are biosynthesized via the iterative assembly of a defined set of building blocks, we developed a platform for small molecule synthesis involving the iterative coupling of haloboronic acids protected as the corresponding N-methyliminodiacetic acid (MIDA) boronates. Here we summarize our efforts thus far to develop this platform into a generalized and automated approach for small molecule synthesis. We and others have employed this approach to access many polyene-based compounds, including the polyene motifs found in >75% of all polyene natural products. This platform further allowed us to derivatize amphotericin B, the powerful and resistance-evasive but also highly toxic last line of defense in treating systemic fungal infections, and thereby understand its mechanism of action. This synthesis-enabled mechanistic understanding has led us to develop less toxic derivatives currently under evaluation as improved antifungal agents. To access more Csp(3)-containing small molecules, we gained a stereocontrolled entry into chiral, non-racemic α-boryl aldehydes through the discovery of a chiral derivative of MIDA. These α-boryl aldehydes are versatile intermediates for the synthesis of many Csp(3) boronate building blocks that are otherwise difficult to access. In addition, we demonstrated the utility of these types of building blocks in accessing pharmaceutically relevant targets via an iterative Csp(3) cross-coupling cycle. We have further expanded the scope of the platform to include stereochemically complex macrocyclic and polycyclic molecules using a linear-to-cyclized strategy, in which Csp(3) boronate building blocks are iteratively assembled into linear precursors that are then cyclized into the cyclic frameworks found in many natural products and natural product-like structures. Enabled by the serendipitous discovery of a catch-and-release protocol for generally purifying MIDA boronate intermediates, the platform has been automated. The synthesis of 14 distinct classes of small molecules, including pharmaceuticals, materials components, and polycyclic natural products, has been achieved using this new synthesis machine. It is anticipated that the scope of small molecules accessible by this platform will continue to expand via further developments in building block synthesis, Csp(3) cross-coupling methodologies, and cyclization strategies. Achieving these goals will enable the more generalized synthesis of small molecules and thereby help shift the rate-limiting step in small molecule science from synthesis to function.

Organocatalyzed Asymmetric Conjugate Addition of Heteroaryl and Aryl Trifluoroborates: a Synthetic Strategy for Discoipyrrole D

Bis-heteroaryl or bis-aryl stereocenters were formed by an organocatalytic enantioselective conjugate addition using the respective trifluoroborate salts as nucleophiles. Control studies suggested that fluoride dissociation is necessary in the anhydrous conditions. This strategy is applicable to the synthesis of discoipyrrole D, an inhibitor of BR5 fibroblast migration.

Synthesis of Heteroaryl Sulfonamides from Organozinc Reagents and 2,4,6-Trichlorophenyl Chlorosulfate

A method for the preparation of aryl and heteroaryl sulfonamides using 2,4,6-trichlorophenyl chlorosulfate (TCPC) is described. The reaction of 2-pyridylzinc reagents with TCPC resulted in 2,4,6-trichlorophenyl (TCP) pyridine-2-sulfonates, and the parent pyridine-2-sulfonate was shown to react with amines. Less electron-rich aryl- and heteroarylzinc reagents reacted with TCPC to afford sulfonyl chlorides that were converted in situ to sulfonamides.

Dual Role of Acetanilides: Traceless Removal of a Directing Group through Deacetylation/Diazotation and Palladium-Catalyzed C-C-Coupling Reactions

The acetamide group enables regioselective oxidative ortho-C-H activation reactions, such as Pd-catalyzed acylation. The synthetic utility of these transformations can be significantly enhanced by using the acetamide as a quasi-leaving group in a subsequent conventional Pd-catalyzed coupling or cross-coupling reaction. The concept is illustrated herein for the synthesis of o-alkenyl- and o-arylphenones, which have potential for the synthesis of arylated aromatic heterocycles.

A mechanistic proposal for the protodeboronation of neat boronic acids: boronic acid mediated reaction in the solid state

Boronic acids that undergo protodeboronation as solids are stable in solution: the solid state organizes them for reaction.

Base-promoted protodeboronation of 2,6-disubstituted arylboronic acids

Facile based promoted deboronation of electron-deficient arylboronate esters was observed for arylboronates containing two ortho electron-withdrawing group (EWG) substituents. Among 30 representative boronates, only the diortho-substituted species underwent facile C-B fission in aqueous basic conditions (200 mM hydroxide). These results provide fundamental insight into deboronative mechanisms with implications for cross-coupling reactions, regioselective deuteration/tritiation for isotopic labeling, and the design of new (18)F-trifluoroborate radioprosthetics.

Suzuki-Miyaura coupling of halophenols and phenol boronic acids: systematic investigation of positional isomer effects and conclusions for the synthesis of phytoalexins from pyrinae

The Suzuki-Miyaura couplings of o-, m-, and p-halophenols with o-, m-, and p-phenol boronic acids were investigated for all combinations under standardized conditions, using Pd/C as a heterogeneous catalyst and water as a solvent. In the case of iodophenols, conventional heating was used, while for bromophenols significantly better results could be obtained using microwave irradiation. This systematic study revealed that 2,4'-biphenol is particularly difficult to access, irrespective of the starting materials used, but that these difficulties can be overcome by using different additives. The conclusions drawn from this investigation allowed us to identify conditions for the protecting group-free or minimized total synthesis of biaryl-type phytoalexins. These compounds possess antibacterial activity and are produced by fruit trees as a response to microbial infection.

Synthesis and chemoselective ligations of MIDA acylboronates with O-Me hydroxylamines

A chemoselective amide-bond forming ligation of N-methyliminodiacetyl (MIDA) acylboronates and O-Me hydroxylamines, including unprotected peptide substrates, is described.

Enantioselective palladium catalyzed conjugate additions of ortho-substituted arylboronic acids to β,β-disubstituted cyclic enones: total synthesis of herbertenediol, enokipodin A and enokipodin B

Palladium catalyzed asymmetric conjugate addition of ortho-substituted arylboronic acids to cyclic enones and its application in natural product synthesis.

Convenient access to readily soluble symmetrical dialkyl-substituted α-oligofurans

A combination of heteroatom directed lithiation/CuCl₂-induced homocoupling, Wittig olefination/Pd-catalyzed transfer hydrogenation followed by Suzuki–Miyaura or Stille cross-coupling enables convenient access to dialkyl-substituted α-oligofurans of potential interest for organic electronics.

Gold-catalyzed oxidative coupling of arylsilanes and arenes: origin of selectivity and improved precatalyst

The mechanism of gold-catalyzed coupling of arenes with aryltrimethylsilanes has been investigated, employing an improved precatalyst (thtAuBr3) to facilitate kinetic analysis. In combination with linear free-energy relationships, kinetic isotope effects, and stoichiometric experiments, the data support a mechanism involving an Au(I)/Au(III) redox cycle in which sequential electrophilic aromatic substitution of the arylsilane and the arene by Au(III) precedes product-forming reductive elimination and subsequent cycle-closing reoxidation of the metal. Despite the fundamental mechanistic similarities between the two auration events, high selectivity is observed for heterocoupling (C-Si then C-H auration) over homocoupling of either the arylsilane or the arene (C-Si then C-Si, or C-H then C-H auration); this chemoselectivity originates from differences in the product-determining elementary steps of each electrophilic substitution. The turnover-limiting step of the reaction involves associative substitution en route to an arene π-complex. The ramifications of this insight for implementation of the methodology are discussed.

Transition metal-catalyzed functionalization of pyrazines

Transition metal-catalyzed reactions are generally used for carbon-carbon bond formation on pyrazines and include, but are not limited to, classical palladium-catalyzed reactions like Sonogashira, Heck, Suzuki, and Stille reactions. Also a few examples of carbon-heteroatom bond formation in pyrazines are known. This perspective reviews recent progress in the field of transition metal-catalyzed cross-coupling reactions on pyrazine systems. It deals with the most important C-C- and C-X-bond formation methodologies.

Scope and limitations of the Heck-Matsuda-coupling of phenol diazonium salts and styrenes: a protecting-group economic synthesis of phenolic stilbenes

4-Phenol diazonium salts undergo Pd-catalyzed Heck reactions with various styrenes to 4'-hydroxy stilbenes. In almost all cases higher yields and fewer side products were observed, compared to the analogous 4-methoxy benzene diazonium salts. In contrast, the reaction fails completely with 2- and 3-phenol diazonium salts. For these substitution patterns the methoxy-substituted derivatives are superior.

Synthesis of solid 2-pyridylzinc reagents and their application in Negishi reactions

In search of alternatives to unstable or unreliable 2-pyridylboron reagents, we have explored two new varieties of solid, moderately air-stable 2-pyridylzinc reagents. Both reagents can be manipulated in air and are competent nucleophiles in Negishi cross-coupling reactions.

Suzuki-Miyaura cross-coupling of potassium dioxolanylethyltrifluoroborate and aryl/heteroaryl chlorides

A robust and efficient protocol for the introduction of the dioxolanylethyl moiety onto various aryl and heteroaryl halides has been developed, providing cross-coupling yields up to 93%. Copper-catalyzed borylation of 2-(2-bromoethyl)-1,3-dioxolane with bis(pinacolato)diboron followed by treatment with potassium bifluoride provides the key organotrifluoroborate reagent.

Suzuki-Miyaura cross-coupling of potassium alkoxyethyltrifluoroborates: access to aryl/heteroarylethyloxy motifs

The introduction of an alkoxyethyl moiety onto aromatic substructures has remained a long-standing challenge for synthetic organic chemists. The main reasons are the inherent instability of alkoxyethylmetallic species and the lack of general procedures to access them. A new method utilizing a cross-coupling strategy based on the exceptional properties of organotrifluoroborates has been developed, and the method allows an easy and efficient installation of this unit on a broad range of aryl and heteroaryl bromides.

Mechanistic origin of antagonist effects of usual anionic bases (OH-, CO3(2-)) as modulated by their countercations (Na+, Cs+, K+) in palladium-catalyzed Suzuki-Miyaura reactions

The mechanism of the reaction of trans-ArPdBrL(2) (Ar=p-Z-C(6)H(4), Z=CN, H; L=PPh(3)) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4), Z'=H, CN, MeO), which is a key step in the Suzuki-Miyaura process, has been established in N,N-dimethylformamide (DMF) with two bases, acetate (nBu(4)NOAc) or carbonate (Cs(2)CO(3)) and compared with that of hydroxide (nBu(4)NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M(+) (e.g., M(+)OH(-)), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M(+) (Na(+), Cs(+), K(+)) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L(2), partly inhibiting its transmetalation with Ar'B(OH)(2). A decreasing reactivity order is observed when M(+) is associated with OH(-): nBu(4)N(+) > K(+) > Cs(+) > Na(+). Acetates lead to the formation of trans-ArPd(OAc)L(2), which does not undergo transmetalation with Ar'B(OH)(2). This explains why acetates are not used as bases in Suzuki-Miyaura reactions that involve Ar'B(OH)(2). Carbonates (Cs(2)CO(3)) give rise to slower reactions than those performed from nBu(4)NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH(-). The mechanism of the reaction with carbonates is then similar to that established for nBu(4)NOH, involving ArPd(OH)L(2) in the transmetalation with Ar'B(OH)(2). Due to the low concentration of OH(-) generated from CO(3)(2-) in water, both transmetalation and reductive elimination result slower than those performed from nBu(4)NOH at equal concentrations as Cs(2)CO(3). Therefore, the overall reactivity is finely tuned by the concentration of the common base OH(-) and the ratio [OH(-)]/[Ar'B(OH)(2)]. Hence, the anionic base (pure OH(-) or OH(-) generated from CO(3)(2-)) associated with its countercation (Na(+), Cs(+), K(+)) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L(2), acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar'B(OH)(3)(-) and by complexation of ArPd(OH)L(2) by M(+).