Organoboron Species Research Articles

Single Heteroatom Fine-Tuning of the Emissive Properties in Organoboron Complexes with 7-(Azaheteroaryl)indole Systems.

The application of organoboron compounds as light-absorbing or light-emitting species in areas as relevant as organic electronics or biomedicine has motivated the search for new materials which contribute to the progress of those applications. This article reports the synthesis of four-coordinate boron complexes based on the unexplored 7-(azaheteroaryl)indole ligands. An easy synthetic approach has enabled the fine-tuning of the electronic structure of the organoboron species by modifying a heteroaromatic component in the conjugated system. Furthermore, a comprehensive characterization by X-ray diffraction, absorption and emission spectroscopy, both in solution and in the solid state, cyclic voltammetry, and computational methods has evidenced the utility of this simple strategy. Large Stokes shifts have been achieved in solid thin-films which show a range of emitted light from blue to orange. The synthesized compounds have been used as biocompatible fluorophores in cell bioimaging.

Homologation Strategy for the Generation of 1-Chloroalkyl Radicals from Organoboranes.

The generation of 1-bromo and 1-chloroalkyl radicals from organoboranes has been investigated. The direct approach involving the hydroboration of halogenated alkenes is impeded by partial dehalogenation taking place during the hydroboration process. An indirect method involving the generation of B-(1-chloroalkyl)catecholborane by homologation of B-alkylcatecholborane with dichloromethyllithium was developed. A reaction sequence involving a hydroboration reaction, a Matteson homologation, and a radical allylation process has been performed as a one-pot process that takes advantage of three different reactivities of organoboron species. Starting from styrene derivatives, it was possible to prepare B-(1-chloro-2-arylpropyl)catecholboranes that are excellent precursors to 1-chloro-2-arylpropyl radicals. A concise approach for the synthesis of an optically active α-methylene-γ-lactone from p-chlorostyrene has been developed on the basis of a two-step sequence involving an enantioselective hydroboration-homologation-cyclization reaction followed by a hydrolysis-lactonization process.

Serendipitous Observation of AlI Insertion into a C-O Bond in L2 PhB (L=Oxazol-2-ylidene).

Oxidative addition of the C-O bond in L2 PhB: (L=oxazol-2-ylidene) to an AlI center induced ring expansion of an oxazol-2-ylidene ring, affording a unique organoboron species formally involving either an Al, N, O mixed heterocyclic carbene or anionic (amino)(boryl)carbene fragment.

Metal free C-H functionalization of diazines and related heteroarenes with organoboron species and its application in the synthesis of a CDK inhibitor, meriolin 1.

Here, we report a metal-free cross-coupling reaction of diazines and related heteroarenes with organoboron species via C-H functionalization. The optimized conditions represent a metal-free method for the activation of aryl/heteroarylboronic acids, which undergo coupling with diazines and related heteroarenes. Optimized conditions also find application in the synthesis of a pyrimidine-based potent CDK inhibitor, meriolin1.

Catalytic Borylation using an Air‐Stable Zinc Boryl Reagent: Systematic Access to Elusive Acylboranes

AbstractThe use of borylzinc reagents in palladium‐catalyzed borylation chemistry is described (i.e. a boron analogue of the Negishi coupling), including a one‐pot bench‐top protocol using an air‐ and moisture‐stable bis(boryl)zinc reagent. The steric/electronic properties of the boryl fragment employed enable a systematic method for accessing acylboranes, a rare class of organoboron species with great potential in chemical synthesis. The reactions proceed under mild conditions, use inexpensive commercial sources of palladium, and demonstrate a remarkable functional‐group tolerance.

Catalytic Borylation using an Air-Stable Zinc Boryl Reagent: Systematic Access to Elusive Acylboranes.

The use of borylzinc reagents in palladium-catalyzed borylation chemistry is described (i.e. a boron analogue of the Negishi coupling), including a one-pot bench-top protocol using an air- and moisture-stable bis(boryl)zinc reagent. The steric/electronic properties of the boryl fragment employed enable a systematic method for accessing acylboranes, a rare class of organoboron species with great potential in chemical synthesis. The reactions proceed under mild conditions, use inexpensive commercial sources of palladium, and demonstrate a remarkable functional-group tolerance.

On the Reaction Mechanism of the Rhodium-Catalyzed Arylation of Fullerene (C60) with Organoboron Compounds in the Presence of Water.

Density functional theory (DFT) calculations were carried out to study the reaction mechanism of the Suzuki–Miyaura rhodium‐catalyzed hydroarylation of fullerene (C60) by phenylboronic acid in the presence of water. As found experimentally, our results confirm that addition of the phenyl group and the hydrogen atom in C60 occurs at the [6,6] bond. The rate‐determining step corresponds to the simultaneous transfer of a hydrogen atom from a water molecule to C60 and the recovery of the active species. The use of 2‐phenyl‐1,3,2‐dioxaborinane and the 4,4,5,5‐tetramethyl‐2‐phenyl‐1,3,2,‐dioxaborolane instead of phenylboronic acid as organoborate agents does not lead to great modifications of the energy profile. The possible higher steric hindrance of 4,4,5,5‐tetramethyl‐2‐phenyl‐1,3,2,‐dioxaborolane should not inhibit its use in the hydroarylation of C60. Overall, we show how organoboron species arylate C60 in rhodium‐based catalysis assisted by water as a source of protons.

Factors affecting reactions of trialkylcyanoborates with imidoyl chlorides/trifluoroacetic anhydride

Methods for generating tert-alkyl organoboron species are in high demand as they are invaluable intermediates for the synthesis of quaternary carbon centres. Herein we report investigations into generation of tert-alkyl organoboron species using imidoyl chlorides as reagents in the organoboron cyanidation reaction. Although alkenyl side-products predominate in particularly hindered cases, tert-alkyl organoboron species can be successfully generated for less hindered examples.

Intercalation of Benzoxaborolate Anions in Layered Double Hydroxides: Toward Hybrid Formulations for Benzoxaborole Drugs

Benzoxaboroles are a family of cyclic derivatives of boronic acids, whose reactivity makes them interesting candidates for the development of novel drugs. In this study, we describe the preparation of the first hybrid organic–inorganic materials involving benzoxaboroles, as a first step toward their use as new formulations for such drugs. The materials were prepared by intercalation of the simplest benzoxaborole (C7H6BO(OH), BBzx) or of its fluorinated analogue (C7H5FBO(OH), AN2690, a recently developed antifungal drug), both taken under their anionic form (benzoxaborolate), into a biodegradable inorganic matrix (a Mg–Al layered double hydroxide). Extensive characterization was carried out on the materials, notably by powder X-ray diffraction and multinuclear (11B, 27Al, 13C, 19F, 25Mg, and 1H) solid state NMR, in order to describe their structure, particularly in the vicinity of the organoboron species. Three crystalline phases involving benzoxaborolate anions in association with Ca2+ or Mg2+ cations wer...

Structures and stabilities of CBHz (z ≤ 8) and CxB3-xHz (x = 1, 2, z ≤ 14): prediction of novel organo-boron radicals

Hydrogenated boron-carbon clusters, i.e. organo-borons, have received considerable attention both theoretically and experimentally. Herein, using a topology searching strategy, we systematically explore the structures and stabilities of small organo-borons with CBHz (z ≤ 8) and CxB3-xHz (x = 1, 2,z≤ 14) stoichiometry, with particular interests in the intrinsic stabilities of the organo-boron radicals. At the CCSD(T)/aug-cc-pVQZ//B3LYP/aug-cc-pVTZ and CCSD(T)/aug-cc-pVQZ//MP2/aug-cc-pVTZ levels, the stabilities of these global minimum organo-boron species were evaluated by considering dissociation pathways and the binding energy per atom. Aside from the five already studied radicals (CBH2, CBH4, C2BH2, C2BH4 and CB2H3), we predict six novel radicals, i.e. CBH6, C2BH6, C2BH8, CB2H, CB2H5 and CB2H7, which could be detected under suitable circumstances. However, observation of the highly hydrogenated CB2H9 radical is much less likely due to its minute stability towards H-extrusion. The computationally determined stable/meta-stable maximum hydrogenation numbers for CB, C2B and CB2 (6, 8 and 8, respectively) are in excellent agreement with a simple electron-counting model for CxBy chains. The newly predicted organo-boron radicals await future laboratory verification.

A Novel Boron Heterocycle Comprised of an Inner Boronium/Borate Adduct

The reaction of [MeSCH2Li·(tmeda)] with 9-Cl-9-BBN led to the formation of the unprecedented boron heterocycle 3, which is an inner salt formally composed of boronium and borate subunits. This product rearranges upon heating to the thermodynamically more stable Ci-symmetric dimer 4, which can also be directly prepared if 9-MeO-9-BBN is chosen as the starting material instead of 9-Cl-9-BBN. The reasons for the different outcomes are discussed, and the solid-state structures of the dimeric products 3 and 4, as well as of the derived α-functionalized organoboron species 6, 9, and 12, are presented.

Role of the Base and Control of Selectivity in the Suzuki–Miyaura Cross‐Coupling Reaction

AbstractThe outcome of the Suzuki–Miyaura cross‐coupling for the direct competition reaction between two boronic acids was evaluated under routine synthesis conditions. The reaction selectivity was found to depend on the amount of the base used, with fewer bases favoring the reactivity of the boronic acid with lower pKa (stronger acid). The dependence of the reaction selectivity on base stoichiometry was found to increase with the increase in the difference in the pKa values of the competing boronic acids. These results confirm a relationship between acid–base chemistry and the Suzuki–Miyaura reaction catalytic cycle. Moreover, the results indicate that under these specific conditions, the most reactive organoboron species toward transmetalation is the borate anion RB(OH)3− instead of the neutral boronic acid RB(OH)2. Hence, the main role of the base in the reaction mechanism is to increase the reactivity of the boronic acid toward the Pd–halide complex by converting it into the respective organoborate. In addition, boric acid, an important reaction byproduct, affects the selectivity in the Suzuki reaction because its gradual formation in the reaction medium disturbs the acid–base equilibrium.

ChemInform Abstract: Iron‐Catalyzed Cross‐Coupling of Electron‐Deficient Heterocycles and Quinone with Organoboron Species via Innate C—H Functionalization: Application in Total Synthesis of Pyrazine Alkaloid Botryllazine A.

AbstractThis reaction works under open flask conditions.

Iron-catalyzed Cross-Coupling of Electron-Deficient Heterocycles and Quinone with Organoboron Species via Innate C–H Functionalization: Application in Total Synthesis of Pyrazine Alkaloid Botryllazine A

Here, we report an iron-catalyzed cross-coupling reaction of electron-deficient heterocycles and quinone with organoboron species via innate C-H functionalization. Iron(II) acetylacetonate along with oxidant (K2S2O8) and phase-transfer catalyst (TBAB) under open flask conditions efficiently catalyzed the cross-coupling of pyrazine with arylboronic acids and gave monoarylated products in good to excellent yields. Optimized conditions also worked for other heterocylces such as quinoxalines, pyridines, quinoline, and isoquinoline as well as quinones. In addition, we demonstrated as a first example its application for the synthesis of anticancer marine pyrazine alkaloid botryllazine A.

Synthesis and Electrochemical Behavior of Mixed Organoboron/Organomercury Compounds

AbstractAs part of our ongoing interest in the synthesis and reduction chemistry of organoboron species, we have investigated the synthesis of mixed organoboron/organomercury complexes by reaction of the Li(THF)4 salt of dimesityl‐1, 8‐naphthalenediylborate with 1, 2‐(HgCl)2C6F4 and 1, 3‐(HgCl)2C6F4, respectively. The resulting tetranuclear B2Hg2 complexes (2 and 3, respectively) were characterized by multinuclear NMR spectroscopy and single‐crystal X‐ray analysis. The cyclic voltammogram of complex 2, which features a B–Hg–Hg‐B core connected by an ortho‐phenylene (Hg–Hg connection) and two peri‐naphthalenediyl linkers (B–Hg connection), shows significant coupling of the two electroactive boryl units, presumably via a direct σ interaction of the vacant p orbitals of the four neighboring Lewis acids. This conclusion is supported by DFT calculations, which show that the LUMO of 2 spans the four Lewis acids, with a major in phase contribution from the boron 2p orbitals and the mercury 6p orbitals.

ChemInform Abstract: The Slow‐Release Strategy in Suzuki—Miyaura Coupling

AbstractReview: 151 refs.

Allylation of Imines and Their Derivatives with Organoboron Reagents: Stereocontrolled Synthesis of Homoallylic Amines

Homoallylic amines serve as important precursors for the synthesis of a variety of natural products and pharmaceutically relevant compounds. One widely used strategy for homoallylic amine synthesis is the addition of allylic metal or metalloid derivatives to imines or related functional groups. The use of allylic boron compounds for these reactions has emerged as an important synthe­tic approach, providing a robust and chemoselective method for efficient and stereocontrolled access to various homoallylic amines. In this review, a comprehensive and critical analysis of imine allyl­boration and crotylboration methods using organoborane, organoboronate and potassium organotrifluoroborate reagents is provided. These approaches include direct methods that accomplish diastereo- or enantiocontrolled addition of the organoboron species. Additionally, organoboron-based transition-metal-catalyzed allylation of imines and their respective surrogates using copper, indium, iridium, palladium or zinc catalysis is discussed. Finally, coverage on the use of allenyl- and propargylboron compounds and their application for the synthesis of homopropargylic and homoallenyl amines is provided.

Distinguishing Between Pathways for Transmetalation in Suzuki−Miyaura Reactions

We report a systematic study of the stoichiometric reactions of isolated arylpalladium hydroxo and halide complexes with arylboronic acids and aryltrihydroxyborates to evaluate the relative rates of the two reaction pathways commonly proposed to account for transmetalation in the Suzuki-Miyaura reaction. On the basis of the relative populations of the palladium and organoboron species generated under conditions common for the catalytic process and the observed rate constants for the stoichiometric reactions between the two classes of reaction components, we conclude that the reaction of a palladium hydroxo complex with boronic acid, not the reaction of a palladium halide complex with trihydroxyborate, accounts for transmetalation in catalytic Suzuki-Miyaura reactions conducted with weak base and aqueous solvent mixtures.

ChemInform Abstract: Highly Effective Alternative Aryl Trihydroxyborate Salts for a Ligand‐Free, On‐Water Suzuki—Miyaura Coupling Reaction.

AbstractReadily accessible and air‐stable sodium aryl trihydroxyborates are utilized as an alternative source of organoboron species in the ligand‐free Pd‐catalyzed title reaction.

The Slow‐Release Strategy in Suzuki–Miyaura Coupling

AbstractDespite great advances in metal‐catalyzed cross‐coupling reactions, their efficacy is often compromised by side reactions, reducing the yield, or requiring a large excess of one component. Suzuki–Miyaura cross‐coupling is no exception, as the boronic acid functionality can be susceptible to a range of undesired processes. A number of methods have been developed to mitigate these side reactions, and herein we focus on the “slow‐release” strategy. These conditions involve deployment of a “masking” reagent that protects the vulnerable boronic acid functionality from degradation, particularly protodeboronation, whilst simultaneously facilitating controlled liberation of the active reagent into the catalytic milieu. Under suitably tailored conditions, this dual‐action approach ensures that the concentration of the free boronic acid is minimized, thus attenuating its degradation but still facilitating transmetallation of the organoboron species with the key organopalladium intermediate.