General method for synthesis of 2-heterocyclic N-methyliminodiacetic acid boronates

Transition-Metal-Free Heterocyclic Carbon-Boron Bond Formation

Heteroaryl boronic acids and esters are extremely important and valuable intermediates because of their wide application in the synthesis of marketed drugs and bioactive compounds. Over the last couple of decades, the construction of highly important heteroaryl carbon-boron bonds has created huge attention. The transition-metal-free protocols are more green, less sensitive to air and moisture, and also economically advantageous over the transition-metal-based protocols. The transition-metal-free C-H borylation of heteroarenes and C-X (X=halogen) borylation of heteroaryl halides represents an excellent approach for their synthesis. Also, various cyclization and alkyne activation protocols have been recently established for their synthesis. The goal of this review article is to summarize the existing literature and the current state of the art for transition-metal-free synthesis of heteroaryl boronic acid and esters.

A Modular Synthesis of Teraryl‐based α‐Helix Mimetics, Part 5: A Complete Set of Pyridine Boronic Acid Pinacol Esters Featuring Side Chains of Proteinogenic Amino Acids

Teraryl‐based α‐helix mimetics have proven to be useful compounds for the inhibition of protein‐protein interactions (PPI). We have developed a modular and flexible approach for the synthesis of teraryl‐based α‐helix mimetics using pyridine containing boronic acid building blocks to increase the water solubility. Following our initial publication in which we have introduced the methodology in combination with sequential Pd‐catalyzed cross‐coupling for teraryl assembly, we can now report a complete set of pyridine based boronic acid building blocks decorated with side chains of all proteinogenic amino acids relevant for PPI (Ala, Arg, Asn, Asp, Cys, Gln, Glu, His, Ile, Leu, Lys, Met, Phe, Ser, Thr, Trp, Tyr, Val) to complement the core fragment set. For a representative set of teraryls we have studied the influence of the pyridine rings on the solubility of the assembled oligoarenes.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Heteroaryl-Heteroaryl, Suzuki-Miyaura, Anhydrous Cross-Coupling Reactions Enabled by Trimethyl Borate

Reaction conditions have been developed for refractory heteroaryl-heteroaryl Suzuki-Miyaura cross-couplings. The reported method employs neopentyl heteroarylboronic esters as nucleophiles, heteroaryl bromides and chlorides as the electrophiles, and the soluble base potassium trimethylsilanolate (TMSOK) under anhydrous conditions. The addition of trimethyl borate enhances reaction rates by several mechanisms, including (1) solubilization of in situ-generated boronate complexes, (2) preventing catalyst poisoning by the heteroatomic units, and (3) buffering the inhibitory effect of excess TMSOK. The use of this method enables cross-coupling of diverse reaction partners including a broad range of π-rich and π-deficient heteroaryl boronic esters and heteroaryl bromides. Reactions proceed in good yields and short reaction times (3 h or less).

Light-Mediated Cross-Coupling of Anomeric Trifluoroborates

Stereoselective reactions at the anomeric carbon constitute the cornerstone of preparative carbohydrate chemistry. Here, we report stereoselective C-arylation and etherification reactions of anomeric trifluoroborates derived from BMIDA esters. These reactions are characterized by high anomeric selectivities for 2-deoxysugars and broad substrate scope (24 examples), including disaccharides and trifluoroborates with free hydroxyl groups. Taken together, this new class of carbohydrate reagents adds the palette of anomeric nucleophile reagents suitable for efficient installation of C-C bonds.

Dynamic Ion Speciation during the Hydrolysis of Aryltrifluoroborates*

Organotrifluoroborates serve as coupling partners during transmetalation in the Suzuki-Miyaura reaction but require hydrolysis prior to the coupling reaction. Their anionic nature allows study of their hydrolysis by electrospray ionization mass spectrometry (ESI-MS) through real-time monitoring, complemented by pH analysis. The induction period varied according to the borates employed, and a dynamic series of equilibria for numerous ions was observed during hydrolysis. We found that the induction periods and reaction rates were sensitive to the R group of the borates, the shape of the reaction vessel, and stir rate.

Synthesis of Tridentate [1,2,4] Triazinyl-Pyridin-2-yl Indole Lewis Basic Complexants via Pd-Catalyzed Suzuki-Miyaura Cross-Coupling

Full closure of the nuclear fuel cycle is predicated, in part, on defining efficient separations processes for the effective speciation of the neutron-absorbing lanthanides from the minor actinides post-PUREX. Pursuant to the aforementioned, a class of tridentate, Lewis basic procomplexants have been prepared leveraging a Pd-catalyzed Suzuki-Miyaura cross-coupling between 6-bromo-[1,2,4]-triazinylpyridine derivatives and various protected indole-boronic acids to afford functionalized 2-[6-(5,6-diphenyl-[1,2,4]triazin-3-yl)-pyridin-2-yl]-1H-indoles. A highly active catalyst/ligand system with low loadings was employed rapidly affording 26 examples in yields as high as 85%. Method optimization, substrate and indole scope, comparative analysis between coupling reagents, and a scale-up experiment are reported.

Observation of an Inversion in Photophysical Tuning in a Systematic Study of Luminescent Triazole-Based Osmium(II) Complexes

In a systematic survey of luminescent bis(terdentate) osmium(II) complexes, a tipping point involving a reversal in photophysical tuning is observed whereby increasing stabilization of the ligand-based lowest unoccupied molecular orbital (LUMO) results in a blue shift in the optical absorption and emission bands. The complexes [Os(N^N'^N″)₂]²⁺ [N^N'^N″ = 2,6-bis(1-phenyl-1,2,3-triazol-4-yl)pyridine (Os1), 2,6-bis(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os2), 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2'-bipyridyl (Os3), 2-(pyrid-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os4), 2-(pyrazin-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyridine (Os5), and 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2'-bipyrazinyl (Os6)] have been prepared and characterized, and all complexes display phosphorescence ranging from the orange to near-IR regions of the spectrum. Replacement of the central pyridine in the ligands of Os1 by the more π-accepting pyrazine in Os2 results in a 55 nm red shift in the triplet metal-to-ligand charge-transfer-based emission band, while a larger red shift of 107 nm is observed for the replacement of one of the triazole donors in the ligands of Os1 by a second pyridine ring in Os3 (λ^em_max = 702 nm). Interestingly, replacement of the central pyridine ring in the ligands of Os3 by pyrazine (Os4, λ^em_max = 702 nm) fails to result in a further red shift in the emission band. Reversal of the relative positions of the pyridine and pyrazine donors in Os5 (λ^em_max = 733 nm) compared to Os4 does indeed result in the expected red shift in the emission with respect to that for Os3 based on the increased π-acceptor character of the ligands present. However, an inversion in emission tuning is observed for Os6, in which the incorporation of a second pyrazine donor in the ligand architecture results in a blue shift in the optical absorption and emission maxima (λ^em_max = 710 nm). Electrochemical studies reveal that while incorporating pyrazine in the ligands indeed results in an expected anodic shift in the first reduction potential through stabilization of the ligand-based LUMO, there is also a concomitant anodic shift in the Os^II/Os^III-based oxidation potential. This stabilization of the metal-based highest occupied molecular orbital (HOMO) thus nullifies the effect of stabilization of the LUMO in Os4 compared to Os3, resulting in these complexes having coincident emission maxima. For Os6, stabilization of the HOMO through the incorporation of two pyrazine donors in the ligand structure now exceeds stabilization of the LUMO, resulting in a larger HOMO-LUMO gap and a counterintuitive blue shift in the optical properties in comparison with those of Os5. While it is known that the replacement of ligands (e.g., replacing bipyridyl with bipyrazinyl) can result in a larger HOMO-LUMO energy gap through greater stabilization of the HOMO, these results importantly allow us to capture the tipping point at which this inversion in photophysical tuning occurs. This therefore enables us to explore the limits available in emission tuning with a relatively simple and minimalist ligand structure.

Anthranilamide (aam)-substituted arylboranes in direct carbon–carbon bond-forming reactions

Pd-Catalyzed cross-coupling and Rh-catalyzed 1,4-addition with Ar-B(aam) become feasible without the need for prior acidic deprotection

Designing Cross-Coupling Reactions using Aryl(trialkyl)silanes

Organo(trialkyl)silanes have several advantages, including high stability, low toxicity, good solubility, easy handling, and ready availability compared with heteroatom-substituted silanes. However, methods for the cross-coupling of organo(trialkyl)silanes are limited, most probably because of their exceeding robustness. Thus, a practical method for the cross-coupling of organo(trialkyl)silanes has been a long-standing challenging research target. This article discusses how aryl(trialkyl)silanes can be used in cross-coupling reactions. A pioneering example is Cu^II catalytic conditions with the use of electron-accepting aryl- or heteroaryl(triethyl)silanes and aryl iodides. The reaction forms biaryls or teraryls. This design concept can be extended to Pd/Cu^II -catalyzed cross-coupling polymerization reactions between such silanes and aryl bromides or chlorides and to Cu^I -catalyzed alkylation using alkyl halides.

The Molecular Industrial Revolution: Automated Synthesis of Small Molecules

Today we are poised for a transition from the highly customized crafting of specific molecular targets by hand to the increasingly general and automated assembly of different types of molecules with the push of a button. Creating machines that are capable of making many different types of small molecules on demand, akin to that which has been achieved on the macroscale with 3D printers, is challenging. Yet important progress is being made toward this objective with two complementary approaches: 1) Automation of customized synthesis routes to different targets by machines that enable the use of many reactions and starting materials, and 2) automation of generalized platforms that make many different targets using common coupling chemistry and building blocks. Continued progress in these directions has the potential to shift the bottleneck in molecular innovation from synthesis to imagination, and thereby help drive a new industrial revolution on the molecular scale.

Design, Synthesis, and Application of Polymer-Supported Silicon-Transfer Agents for Cross-Coupling Reactions with Organolithium Reagents

The initial design, synthesis, and validation of polymer-supported siloxane transfer agents have been achieved that permit the direct use of organolithium reagents in the palladium-catalyzed cross-coupling reactions. Through rational design, two generations of polymer support were developed that significantly simplify product purification and the transfer agent recycling.

Use of N-methyliminodiacetic acid boronate esters in suzuki-miyaura cross-coupling polymerizations of triarylamine and fluorene monomers

Polytriarylamine copolymers can be prepared by Suzuki‐Miyaura cross‐coupling reactions of bis N‐methyliminodiacetic acid (MIDA) boronate ester substituted arylamines with dibromo arenes. The roles of solvent composition, temperature, reaction time, and co‐monomer structure were examined and (co)polymers prepared containing 9, 9‐dioctylfluorene (F8), 4‐sec‐butyl or 4‐octylphenyl diphenyl amine (TFB), and N, N′‐bis(4‐octylphenyl)‐N, N′‐diphenyl phenylenediamine (PTB) units, using a Pd(OAc)₂/2‐dicyclohexylphosphino‐2′,6′‐dimethoxybiphenyl (SPhos) catalyst system. The performance of a di‐functionalized MIDA boronate ester monomer was compared with that of an equivalent pinacol boronate ester. Higher molar mass polymers were produced from reactions starting with a difunctionalized pinacol boronate ester monomer than the equivalent difunctionalized MIDA boronate ester monomer in biphase solvent mixtures (toluene/dioxane/water). Matrix‐assisted laser desorption/ionization mass spectroscopic analysis revealed that polymeric structures rich in residues associated with the starting MIDA monomer were present, suggesting that homo‐coupling of the boronate ester must be occurring to the detriment of cross‐coupling in the step‐growth polymerization. However, when comparable reactions of the two boronate monomers with a dibromo fluorene monomer were completed in a single phase solvent mixture (dioxane + water), high molar mass polymers with relatively narrow distribution ranges were obtained after only 4 h of reaction. © 2017 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2798–2806

Synthesis and Characterization of a Series of Bis-homoleptic Cycloruthenates with Terdentate Ligands as a Family of Panchromatic Dyes

A series of six homoleptic bis-cyclometalated ruthenium complexes, Ru(N^N^C)₂, is reported where N^N^C is a 6-(2,4-difluoro-3-R³-phenyl)-4-R²-4'-R¹-2,2'-bipyridine with R³ = -H or -CF₃ and R² and R¹ = -COOEt or -CF₃. An effective synthesis of the ligands and the complexes is described. The UV-visible absorption studies demonstrate that these complexes are panchromatic dyes absorbing up to 900 nm. Importantly, the onset of absorption depends only on the substitution on the metalated phenyl, whereas the intensity of absorption throughout the spectra is a function of substituents on both the phenyl and the bipyridine moieties. The same trend is observed in electrochemistry as the redox gap depends only on the substitution on the metalated phenyl, whereas the oxidation and reduction potentials are a function of substituents on both the phenyl and the bipyridine moieties. Preliminary tests as sensitizer for dye-sensitized solar cells demonstrate that the number of anchoring groups on the dye has a major influence on the device efficiency.

Copper-catalyzed direct borylation of alkyl, alkenyl and aryl halides with B(dan)

Copper-catalyzed reaction of organic halides with an unsymmetrical diboron, (pin)B–B(dan), which leads to the direct formation of masked organoboron compounds, is reported.

Design, Synthesis, and Validation of an Effective, Reusable Silicon-Based Transfer Agent for Room-Temperature Pd-Catalyzed Cross-Coupling Reactions of Aryl and Heteroaryl Chlorides with Readily Available Aryl Lithium Reagents

A reusable silicon-based transfer agent (1) has been designed, synthesized, and validated for effective room-temperature palladium-catalyzed cross-coupling reactions (CCRs) of aryl and heteroaryl chlorides with readily accessible aryl lithium reagents. The crystalline, bench-stable siloxane transfer agent (1) is easily prepared via a one-step protocol. Importantly, this "green" CCR protocol circumvents prefunctionalization, isolation of organometallic cross-coupling partners, and/or stoichiometric waste aside from LiCl. DFT calculations support a σ-bond metathesis mechanism during transmetalation and lead to insights on the importance of the CF3 groups.

Modular Approaches to Diversified Soft Lewis Basic Complexants through Suzuki-Miyaura Cross-Coupling of Bromoheteroarenes with Organotrifluoroborates

Remediation or transmutation of spent nuclear fuel obtained as a function of energy production and legacy waste remains a significant environmental concern. Substantive efforts over the last three decades have focused on the potential of soft-Lewis basic complexants for the chemoselective separation of trivalent actinides from lanthanides in biphasic solvent systems. Recent efforts in this laboratory have focused on the concept of modularity to rapidly prepare complexants and complexant scaffolds not easily accessible via traditional linear methods in a convergent manner to better understand solubility and complexation structure/activity function in process-relevant solvents. The current work describes an efficient method for the construction of diversified complexants through multi-Suzuki-Miyaura cross-coupling of bromoheteroarenes with organotrifluoroborates affording efficient access to 22 novel materials in 43-99% yield over two, three, or four cross-couplings on the same scaffold. Optimization of the catalyst/ligand system, application, and limitations are reported herein.

C2-Alkenylation of N-heteroaromatic compounds via Brønsted acid catalysis

C2-alkenylated heteroaromatics can be accessed by simple Brønsted acid catalysed union of diverse heteroarene N-oxides with alkenes. The scope and limitations of the process are outlined.

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.

Facile synthesis of borofragments and their evaluation in activity-based protein profiling

The discovery of enzyme inhibitors relies on synthetic methods that enable rapid and modular construction of small molecules. Heterocyclic fragments designed to maximize enthalpic interactions with their protein targets represent a particularly desirable class of molecules. Here we describe a reagent that enables straightforward construction of "borofragments", in which a heterocycle is separated from the boron center by two or three rotatable bonds. The stability of these molecules depends on the MIDA group which likely acts as a slow-release element under biological conditions. Borofragments can be used to discover inhibitors of enzymes that use catalytic oxygen nucleophiles. We have employed this method to identify inhibitors of ABHD10 and the predicted carboxypeptidase CPVL. This technique should be applicable to other classes of targets.

Ru-catalysed C-H arylation of indoles and pyrroles with boronic acids: scope and mechanistic studies

The Ru-catalysed C2–H arylation of indoles and pyrroles by using boronic acids under oxidative conditions is reported. This reaction can be applied to tryptophan derivatives and tolerates a wide range of functional groups on both coupling partners, including bromides and iodides, which can be further derivatised selectively. New indole-based ruthenacyclic complexes are described and investigated as possible intermediates in the reaction. Mechanistic studies suggest the on-cycle intermediates do not possess a para-cymene ligand and that the on-cycle metalation occurs through an electrophilic attack by the Ru centre.

A modular synthesis of functionalised phenols enabled by controlled boron speciation

Functionalised biaryl phenols can be rapidly accessed via one-pot Suzuki–Miyaura cross-coupling, chemoselective control of boron solution speciation, and oxidation.

An alkyne hydrosilylation–Hiyama coupling approach to highly functionalised 1,3-dienes

A high yielding and completely stereoselective hydrosilylation–Hiyama protocol has been established for the synthesis of highly functionalised E,E-dienes.

Organometallic chemistry of ethynyl boronic acid MIDA ester, HCCB(O2CCH2)2NMe

The reactions of HCCBMIDA (BMIDA = B(O₂CCH₂)₂NMe) with a range of ruthenium complexes examples of σ-alkynyl, σ-alkenyl and vinylidene complexes bearing 4-coordinate boron substituents.

Boron-containing enamine and enamide linchpins in the synthesis of nitrogen heterocycles

The use of α-boryl enamine and enamide linchpins in the synthesis of nitrogen heterocycles has been demonstrated. Boryl enamines provide ready access to the corresponding α-halo aldehydes, which undergo regioselective annulation to form borylated thiazoles. A condensation/amidation sequence converts α-boryl aldehydes into stable α-boryl enamides without concomitant C → N migration. We also show that palladium-catalyzed cyclization of α-boryl enamides leads to synthetically versatile isoindolones. These molecules can be subsequently used to access polycyclic scaffolds.

Chemoselective boronic ester synthesis by controlled speciation

Control of boronic acid solution speciation is presented as a new strategy for the chemoselective synthesis of boronic esters. Manipulation of the solution equilibria within a cross-coupling milieu enables the formal homologation of aryl and alkenyl boronic acid pinacol esters. The generation of a new, reactive boronic ester in the presence of an active palladium catalyst also facilitates streamlined iterative catalytic C-C bond formation and provides a method for the controlled oligomerization of sp(2) -hybridized boronic esters.