A general solution for the 2-pyridyl problem

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Late-Stage Functionalization of Peptides and Cyclopeptides Using Organozinc Reagents

We report a new late-stage functionalization of small peptides and cyclopeptides relying on the Negishi cross-coupling of readily prepared iodotyrosine- or iodophenylalanine-containing peptides with aryl-, heteroaryl-, and alkylzinc pivalates or halides. In silico and in vitro determinations of membrane permeability parameters of the modified cyclopeptides showed that in most cases, the solubility was improved by the introduction of polar pyridyl units while the cell-membrane permeability was maintained.

Nickel-Catalyzed Stille Cross Coupling of C-O Electrophiles

Aryl sulfamates, tosylates, and mesylates undergo efficient Ni-catalyzed cross coupling with diverse organostannanes in the presence of relatively unhindered alkylphosphine ligands and KF. The coupling is valuable for difficult bond constructions, such as aryl- heteroaryl, aryl-alkenyl, and aryl-alkynyl, using non-triflate phenol derivatives. A combination of experimental and computational studies implicate an unusual mechanism for transmetalation involving an 8-centered cyclic transition state. This reaction is inhibited by chloride sources due to slow transmetalation of organostannanes at a Ni(II)-chloride intermediate. These studies help to explain why prior efforts to achieve Ni-catalyzed Stille coupling of phenol derivatives were unsuccessful.

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

Radical Hydroarylation of Functionalized Olefins and Mechanistic Investigation of Photocatalytic Pyridyl Radical Reactions

We report the photoredox alkylation of halopyridines using functionalized alkene and alkyne building blocks. Selective single-electron reduction of the halogenated pyridines provides the corresponding heteroaryl radicals, which undergo anti-Markovnikov addition to the alkene substrates. The system is shown to be mild and tolerant of a variety of alkene and alkyne subtypes. A combination of computational and experimental studies support a mechanism involving proton-coupled electron transfer followed by medium-dependent alkene addition and rapid hydrogen atom transfer mediated by a polarity-reversal catalyst.

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.

Borylated reagents for multicomponent reactions

Over the past two decades, there has been an increasing interest in the therapeutic potential of organoboron reagents due to the discovery of bortezomib (Velcade). This has motivated synthetic chemists to develop novel routes for the preparation of heteroatom-rich boron-containing molecules (BCMs). In particular, the development of borylated building blocks has provided facile access to difficult-to-access heteroatom-rich BCMs. In this review, we will discuss the methods used to prepare boron-containing molecules of biological relevance from multicomponent reactions with borylated building blocks.

Suzuki Cross-Coupling of [2.2]Paracyclophane Trifluoroborates with Pyridyl and Pyrimidyl Building Blocks

We report a new Suzuki cross-coupling protocol for high yielding derivatization of [2.2]paracyclophane with pyridyl and pyrimidyl substituents. The [2.2]paracyclophane trifluoroborate salt presented herein is a bench stable, easily accessible, and convenient substitute to former cross-coupling substrates. This will be of very high interest for future paracyclophane derivatization endeavors.

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.

Toward Generalization of Iterative Small Molecule Synthesis

Small molecules have extensive untapped potential to benefit society, but access to this potential is too often restricted by limitations inherent to the customized approach currently used to synthesize this class of chemical matter. In contrast, the "building block approach", i.e., generalized iterative assembly of interchangeable parts, has now proven to be a highly efficient and flexible way to construct things ranging all the way from skyscrapers to macromolecules to artificial intelligence algorithms. The structural redundancy found in many small molecules suggests that they possess a similar capacity for generalized building block-based construction. It is also encouraging that many customized iterative synthesis methods have been developed that improve access to specific classes of small molecules. There has also been substantial recent progress toward the iterative assembly of many different types of small molecules, including complex natural products, pharmaceuticals, biological probes, and materials, using common building blocks and coupling chemistry. Collectively, these advances suggest that a generalized building block approach for small molecule synthesis may be within reach.

Mechanistic study of the ligand controlled regioselectivity in iridium catalyzed C–H borylation of aromatic imines

DFT calculation indicates that in iridium catalyzed C–H borylation of aromatics, the ortho selectivity is proposed to be attributed to the electron donating effect of AQ ligand, while the meta selectivity is due to steric hindrance of TMP ligand.

Catalyst Selection Facilitates the Use of Heterocyclic Sulfinates as General Nucleophilic Coupling Partners in Palladium-Catalyzed Coupling Reactions

A range of 5- and 6-membered heterocycle-derived sulfinates are shown to be effective nucleophilic coupling partners with aryl chlorides and bromides using Pd(0) catalysis. The use of optimal reaction conditions, specifically incorporating a P(t-Bu)₂Me-derived Pd catalyst, allowed reactions to be performed at moderate temperatures and enabled the inclusion of a variety of sensitive functional groups. Challenging heterocyclic sulfinates, including pyrazine, pyridazine, pyrimidine, pyrazole, and imidazole, were all shown to perform well.

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.

Base-Catalyzed Aryl-B(OH)2 Protodeboronation Revisited: From Concerted Proton Transfer to Liberation of a Transient Aryl Anion

Pioneering studies by Kuivila, published more than 50 years ago, suggested ipso protonation of the boronate as the mechanism for base-catalyzed protodeboronation of arylboronic acids. However, the study was limited to UV spectrophotometric analysis under acidic conditions, and the aqueous association constants (K_a) were estimated. By means of NMR, stopped-flow IR, and quenched-flow techniques, the kinetics of base-catalyzed protodeboronation of 30 different arylboronic acids has now been determined at pH > 13 in aqueous dioxane at 70 °C. Included in the study are all 20 isomers of C₆H_nF_(5-n)B(OH)₂ with half-lives spanning 9 orders of magnitude: <3 ms to 6.5 months. In combination with pH-rate profiles, pK_a and ΔS^⧧ values, kinetic isotope effects (²H, ¹⁰B, ¹³C), linear free-energy relationships, and density functional theory calculations, we have identified a mechanistic regime involving unimolecular heterolysis of the boronate competing with concerted ipso protonation/C-B cleavage. The relative Lewis acidities of arylboronic acids do not correlate with their protodeboronation rates, especially when ortho substituents are present. Notably, 3,5-dinitrophenylboronic acid is orders of magnitude more stable than tetra- and pentafluorophenylboronic acids but has a similar pK_a.

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.

2,6-Bis(diazaboryl)pyridine: A Superbasic Sterically Demanding Pyridine Ligand

Herein a convenient one-pot route to a sterically demanding superbasic pyridine is presented. Functionalization of the 2- and 6-positions with the strongly σ-donating boryl-groups shifts the calculated gas phase basicity of the pyridine nitrogen atom to 1012 kJ mol^-1 , which outperforms the "proton sponge" 1,8-bis(dimethylamino)naphthalene (996 kJ mol^-1 ). The diazaboryl groups are oriented orthogonally to the pyridine ring and do not block the N-position, which resembles the geometry of commonly used N-heterocyclic carbenes. This allows the substituted pyridine to be used as a neutral N-donor ligand in coordination chemistry that is demonstrated herein with the Lewis adducts of haloboranes. Contrary to NHCs, which can form extraordinarily stable adducts, the pyridine ligand is intended to act as a weaker-coordinating alternative and could allow for alternative ligand chemistry.

New Inhibitor Targeting Signal Transducer and Activator of Transcription 5 (STAT5) Signaling in Myeloid Leukemias

Signal transducers and activators of transcription 5 (STAT5s) are crucial effectors of tyrosine kinase oncogenes in myeloid leukemias. Inhibition of STAT5 would contribute to reducing the survival of leukemic cells and also tackling their chemoresistance. In a first screening experiment, we identified hit 13 as able to inhibit STAT5 phosphorylation and leukemic cell growth. The synthesis of 18 analogues of 13 allowed us to identify one compound, 17f, as having the most potent antileukemic effect. 17f inhibited the growth of acute and chronic myeloid leukemia cells and the phosphorylation and transcriptional activity of STAT5. Importantly, 17f had minimal effects on bone marrow stromal cells that play vital functions in the microenvironment of hematopoietic and leukemic cells. We also demonstrated that 17f inhibits STAT5 but not STAT3, AKT, or Erk1/2 phosphorylation. These results suggest that 17f might be a new lead molecule targeting STAT5 signaling in myeloid leukemias.

Exhaustive Suzuki-Miyaura reactions of polyhalogenated heteroarenes with alkyl boronic pinacol esters

A novel Suzuki-Miyaura protocol is described that enables the exhaustive alkylation of polychlorinated pyridines. This method facilitates a formal synthesis of normuscopyridine and the rapid assembly of a dumbbell shaped portion of a [2]rotaxane.

Engineering High-Potential Photo-oxidants with Panchromatic Absorption

Challenging photochemistry demands high-potential visible-light-absorbing photo-oxidants. We report (i) a highly electron-deficient Ru(II) complex (eDef-Rutpy) bearing an E_1/2^0/+ potential more than 300 mV more positive than that of any established Ru(II) bis(terpyridyl) derivative, and (ii) an ethyne-bridged eDef-Rutpy-(porphinato)Zn(II) (eDef-RuPZn) supermolecule that affords both panchromatic UV-vis spectral domain absorptivity and a high E_1/2^0/+ potential, comparable to that of Ce(NH₄)₂(NO₃)₆ [E_1/2(Ce^3+/4+) = 1.61 V vs NHE], a strong and versatile ground-state oxidant commonly used in organic functional group transformations. eDef-RuPZn exhibits ∼8-fold greater absorptive oscillator strength over the 380-700 nm range relative to conventional Ru(II) polypyridyl complexes, and impressive excited-state reduction potentials (¹E^-/* = 1.59 V; ³E^-/* = 1.26 V). eDef-RuPZn manifests electronically excited singlet and triplet charge-transfer state lifetimes more than 2 orders of magnitude longer than those typical of conventional Ru(II) bis(terpyridyl) chromophores, suggesting new opportunities in light-driven oxidation reactions for energy conversion and photocatalysis.

3-Cyanoallyl boronates are versatile building blocks in the synthesis of polysubstituted thiophenes

We report the preparation of hitherto unprecedented 3-cyanoallyl boronates using condensation of the parent α-boryl aldehyde and nitriles. The resulting allyl boronates have been used to generate a wide range of borylated thiophenes, which represent a valuable class of heterocycles in modern drug discovery. Subsequent Suzuki-Miyaura cross-coupling enabled the synthesis of pharmaceutically important 3,5-disubstituted aminothiophenes. Moreover, late stage functionalization gave access to borylated bromothiophene and thieno[2,3-b]pyridines.

Pyridine sulfinates as general nucleophilic coupling partners in palladium-catalyzed cross-coupling reactions with aryl halides

Pyridine rings are ubiquitous in drug molecules; however, the pre-eminent reaction used to form carbon-carbon bonds in the pharmaceutical industry, the Suzuki-Miyaura cross-coupling reaction, often fails when applied to these structures. This phenomenon is most pronounced in 2-substituted pyridines, and results from the difficulty in preparing, the poor stability of, and low efficiency in reactions of pyridine-2-boronates. We demonstrate that by replacing these boronates with pyridine-2-sulfinates, a cross-coupling process of unrivalled scope and utility is realized. The corresponding 3- and 4-substituted pyridine variants are also efficient coupling partners. In addition, we apply these sulfinates in a library format to the preparation of medicinally relevant derivatives of the drugs varenicline (Chantix) and mepyramine (Anthisan).