Borinic Acid Research Articles

Stereoselective Synthesis of 1,1′‐Disaccharides by Organoboron Catalysis

AbstractThe highly stereoselective synthesis of 1,1′‐disaccharides was achieved by using 1,2‐dihydroxyglycosyl acceptors and glycosyl donors in the presence of a tricyclic borinic acid catalyst. In this reaction, the complexation of the diols and the catalyst is crucial for the activation of glycosyl donors, as well as for the 1,2‐cis‐configuration of the products. The anomeric stereochemistry of the glycosyl donor depends on the employed glycosyl donor. Applications of the produced 1,1′‐disaccharides are also described.

Stereoselective Synthesis of 1,1′‐Disaccharides by Organoboron Catalysis

The highly stereoselective synthesis of 1,1'-disaccharides was achieved by using 1,2-dihydroxyglycosyl acceptors and glycosyl donors in the presence of a tricyclic borinic acid catalyst. In this reaction, the complexation of the diols and the catalyst is crucial for the activation of glycosyl donors, as well as for the 1,2-cis-configuration of the products. The anomeric stereochemistry of the glycosyl donor depends on the employed glycosyl donor. Applications of the produced 1,1'-disaccharides are also described.

A Bond-Weakening Borinate Catalyst that Improves the Scope of the Photoredox α-C–H Alkylation of Alcohols

The development of catalyst-controlled, site-selective C(sp3)–H functionalization reactions is currently a major challenge in organic synthesis. In this paper, a novel bond-weakening catalyst that recognizes the hydroxy group of alcohols through formation of a borate is described. An electron-deficient borinic acid–ethanolamine complex enhances the chemical yield of the α-C–H alkylation of alcohols when used in conjunction with a photoredox catalyst and a hydrogen atom transfer catalyst under irradiation with visible light. This ternary hybrid catalyst system can, for example, be applied to functional-group-enriched­ peptides.

Absolute stereochemical determination of 1,2-diols via complexation with dinaphthyl borinic acid.

Rapid derivatization of chiral 1,2-diols with dinaphthyl borinic acid (DBA) leads to a cyclic boronate, enabling the absolute stereochemical prediction via exciton-coupled circular dichroic (ECCD) of the naphthyl groups. Aryl- and alkyl-substituted 1,2-diols derivatized with DBA yield a predictable ECCD, which is also in agreement with theoretical predictions derived from computationally minimized structures.

The Introduction of the Radical Cascade Reaction into Polymer Chemistry: A One-Step Strategy for Synchronized Polymerization and Modification.

SummaryPolymerization and modification play central roles in polymer chemistry and are generally implemented in two steps, which suffer from the time-consuming two-step strategy and present considerable challenge for complete modification. By introducing the radical cascade reaction (RCR) into polymer chemistry, a one-step strategy is demonstrated to achieve synchronized polymerization and complete modification in situ. Attributed to the cascade feature of iron-catalyzed three-component alkene carboazidation RCR exhibiting carbon-carbon bond formation and carbon-azide bond formation with extremely high efficiency and selectivity in one step, radical cascade polymerization therefore enables the in situ synchronized polymerization through continuous carbon-carbon bond formation and complete modification through carbon-azide bond formation simultaneously. This results in a series of α, β, and γ poly(amino acid) precursors. This result not only expands the methodology library of polymerization, but also the possibility for polymer science to achieve functional polymers with tailored chemical functionality from in situ polymerization.

Intramolecular Borylation via Sequential B-Mes Bond Cleavage for the Divergent Synthesis of B,N,B-Doped Benzo[4]helicenes.

New symmetric and unsymmetric B,N,B-doped benzo[4]helicenes 3-6 a/b have been achieved in good yields, using a three-step process, starting from N(tolyl)3 in a highly divergent manner (7 examples). A borinic acid functionalized 1,4-B,N-anthracene 1 was found to display unprecedented reactivity, acting as a convenient and highly effective precursor for selective formation of bromo-substituted B,N,B-benzo[4]helicenes 2 a/2 b via intramolecular borylation and sequential B-Mes bond cleavage in the presence of BBr3 . Subsequent reaction of 2 a/2 b with Ar-Li provided a highly effective toolbox for the preparation of symmetrically/unsymmetrically functionalized B,N,B-helicenes. Their high photoluminescence quantum yields along with the small ΔEST suggest their potential as thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs).

Electron-Deficient Borinic Acid Polymers: Synthesis, Supramolecular Assembly, and Examination as Catalysts in Amide Bond Formation.

The Lewis acidic character of borinic-acid-functionalized polymers suggests broad potential applications in supramolecular materials, chemo- and biosensors, as well as supported catalysts. Two highly electron-deficient borinic acid copolymers (3 a and 3 b) with variable steric hindrance at the boron center were prepared by reaction of aryldibromoboranes ArBBr2 (2, Ar=2,4-Cl2 Ph, 3,5-Cl2 Ph) with a 10 % stannylated polystyrene random copolymer, followed by conversion to the desired PS-B(Ar)OH functionalities. The supramolecular assembly of these polymers through Lewis acid-Lewis base interactions and reversible covalent B-O-B bond formation was investigated. Exposure of a polymer solution of 3 a to pyridine triggered spontaneous gelation, whereas 3 b only gelled upon addition of molecular sieves to favor formation of boroxane crosslinks. The crosslinking process was readily reversed by addition of small amounts of water or wet solvent. The dynamic processes were studied in detail by variable-temperature (VT) NMR by using molecular model compounds. The polymers and their corresponding model compounds were also examined as catalysts in the amide bond formation reaction between phenylacetic acid and benzylamine. The 3,5-dichlorophenyl borinic acid derivatives proved to be the more effective catalysts. Mechanistic studies suggested that the borane Lewis acid-catalyzed coupling involves initial acid-induced protodeboronation to release the dichlorophenyl boronic acid as the active catalyst.

A Highly Warped Heptagon‐Containing sp2 Carbon Scaffold via Vinylnaphthyl π‐Extension

AbstractA new strategy is demonstrated for the synthesis of warped, negatively curved, all‐sp2‐carbon π‐scaffolds. Multifold C−C coupling reactions are used to transform a polyaromatic borinic acid into a saddle‐shaped polyaromatic hydrocarbon (2) bearing two heptagonal rings. Notably, this Schwarzite substructure is synthesized in only two steps from an unfunctionalized alkene. A highly warped structure of 2 was revealed by X‐ray crystallographic studies and pronounced flexibility of this π‐scaffold was ascertained by experimental and computational studies. Compound 2 exhibits excellent solubility, visible range absorption and fluorescence, and readily undergoes two reversible one‐electron oxidations at mild potentials.

A Highly Warped Heptagon‐Containing sp2 Carbon Scaffold via Vinylnaphthyl π‐Extension

A new strategy is demonstrated for the synthesis of warped, negatively curved, all‐sp2‐carbon π‐scaffolds. Multifold C−C coupling reactions are used to transform a polyaromatic borinic acid into a saddle‐shaped polyaromatic hydrocarbon (2) bearing two heptagonal rings. Notably, this Schwarzite substructure is synthesized in only two steps from an unfunctionalized alkene. A highly warped structure of 2 was revealed by X‐ray crystallographic studies and pronounced flexibility of this π‐scaffold was ascertained by experimental and computational studies. Compound 2 exhibits excellent solubility, visible range absorption and fluorescence, and readily undergoes two reversible one‐electron oxidations at mild potentials.

Introducing Perylene as a New Member to the Azaborine Family

AbstractSubstitutional doping of perylene with two boron atoms at the 6b/12b positions and two oxygen or nitrogen atoms at the 1/7 positions has been achieved. The modular synthesis route developed for these bis‐BO‐ (3) and bis‐BN‐perylenes (5) starts from the readily accessible borinic acid derivative of the doubly brominated 9,10‐dihydro‐9,10‐diboraanthracene (DBA), 1,5‐Br2DBA(OH)2. A Stille‐type reaction first furnishes the alkynyl‐substituted species 1,5‐(RCC)2DBA(OH)2 (2), which undergo double ring closure to afford 3 via the gold‐catalyzed addition of the O−H bonds to the C≡C bonds. Treatment of 2 with MeN(SiMe3)2 leads to aminoborane intermediates 1,5‐(RCC)2DBA(N(H)Me)2, which can be ring‐closed to give 5 in a similar manner as in the case of 3. Different substituents R (such as Me, tBuPh) can be introduced at the 2/8 positions of the perylene core. The products obtained undergo reversible reduction and are efficient blue/turquoise emitters.

Introducing Perylene as a New Member to the Azaborine Family.

Substitutional doping of perylene with two boron atoms at the 6b/12b positions and two oxygen or nitrogen atoms at the 1/7 positions has been achieved. The modular synthesis route developed for these bis-BO- (3) and bis-BN-perylenes (5) starts from the readily accessible borinic acid derivative of the doubly brominated 9,10-dihydro-9,10-diboraanthracene (DBA), 1,5-Br2 DBA(OH)2 . A Stille-type reaction first furnishes the alkynyl-substituted species 1,5-(RCC)2 DBA(OH)2 (2), which undergo double ring closure to afford 3 via the gold-catalyzed addition of the O-H bonds to the C≡C bonds. Treatment of 2 with MeN(SiMe3 )2 leads to aminoborane intermediates 1,5-(RCC)2 DBA(N(H)Me)2 , which can be ring-closed to give 5 in a similar manner as in the case of 3. Different substituents R (such as Me, tBuPh) can be introduced at the 2/8 positions of the perylene core. The products obtained undergo reversible reduction and are efficient blue/turquoise emitters.

An Acid-Free Anionic Oxoborane Isoelectronic with Carbonyl: Facile Access and Transfer of a Terminal B═O Double Bond.

We disclose the synthesis and structural characterization of the first acid-free anionic oxoborane, [K(2.2.2-crypt)][(HCDippN)2BO] (1) (Dipp = 2,6- iPr2C6H3), which is isoelectronic with classical carbonyl compounds. 1 can readily be accessed from its borinic acid by a simple deprotonation/sequestration sequence. Crystallographic and density functional theory (DFT) analyses support the presence of a polarized terminal B═O double bond. Subsequent π bond metathesis converts the B═O bond to a heavier B═S containing system, affording the first anionic thioxoborane [K(2.2.2-crypt)][(HCDippN)2BS] (2), isoelectronic with thiocarbonyls. Facile B═O bond cleavage can also be achieved to access B-H and B-Cl bonds and, via a remarkable oxide (O2-) ion abstraction, to generate a borenium cation [(HCDippN)2B(NC5H5)][OTf] (4). By extension, 1 can act as an oxide transfer agent to organic substrates, a synthetic role traditionally associated with transition-metal compounds. Hence we show that B-O linkages, which are often considered to be thermodynamic sinks, can be activated under mild conditions toward bond cleavage and transfer, by exploiting the higher reactivity inherent in the B═O double bond.

Sequential C−H Borylation and N‐Demethylation of 1,1′‐Biphenylamines: Alternative Route to Polycyclic BN‐Heteroarenes

AbstractDescribed herein is an unprecedented access to BN‐polyaromatic compounds from 1,1′‐biphenylamines by sequential borane‐mediated C(sp2)−H borylation and intramolecular N‐demethylation. The conveniently in situ generated Piers’ borane from a borinic acid reacts with a series of N,N‐dimethyl‐1,1′‐biphenyl‐2‐amines in the presence of PhSiH3 to afford six‐membered amine‐borane adducts bearing a C(sp2)−B bond at the C2′‐position. These species undergo an intramolecular N‐demethylation with a B(C6F5)3 catalyst to provide BN‐isosteres of polyaromatics. According to computational studies, a stepwise ionic pathway is suggested. Photophysical characters of the resultant BN‐heteroarenes shown them to be distinctive from those of all‐carbon analogues.

Sequential C-H Borylation and N-Demethylation of 1,1'-Biphenylamines: Alternative Route to Polycyclic BN-Heteroarenes.

Described herein is an unprecedented access to BN-polyaromatic compounds from 1,1'-biphenylamines by sequential borane-mediated C(sp2 )-H borylation and intramolecular N-demethylation. The conveniently in situ generated Piers' borane from a borinic acid reacts with a series of N,N-dimethyl-1,1'-biphenyl-2-amines in the presence of PhSiH3 to afford six-membered amine-borane adducts bearing a C(sp2 )-B bond at the C2'-position. These species undergo an intramolecular N-demethylation with a B(C6 F5 )3 catalyst to provide BN-isosteres of polyaromatics. According to computational studies, a stepwise ionic pathway is suggested. Photophysical characters of the resultant BN-heteroarenes shown them to be distinctive from those of all-carbon analogues.

Tuning the Electrochemical Redox Potentials of Catechol with Boronic Acid Derivatives.

A strategy to control the oxidation potential of catechol using borinic acids is presented. Borinic acids reversibly bind catechol to form boron "ate" complexes (BACs) that alter the electron density on the oxygen atoms of catechol and, in turn, the propensity of the catechol toward electrochemical oxidation. The effect of different substituents on the borinic acid are investigated to determine their efficacy in tuning the electron density within the BAC and the resulting oxidation potential.

Promoting water dissociation performance by borinic acid for the strong-acid/base-free hydrogen evolution reaction.

A new type of organic proton donor for the strong-acid/base-free hydrogen evolution reaction is reported, by taking advantage of the electron deficiency of the boron center of borinic acid and the strong affinity and hydrolysis reaction between borinic acid and water with hydrogen ion formation. The borinic acid proton donor exhibits even higher current density than H2SO4 to promote hydrogen evolution performance in DMF.

Aminofluoroalkoxide amido and boryloxo lead(ii) complexes.

We report here on the utilisation of a readily available bidentate aminofluoroalkoxide in lead(ii) chemistry. Stable heteroleptic three-coordinate complexes can be produced in high yields, including a convenient amido synthetic precursor and a rare case of PbII-boryloxide.

Borinic Acid Mediated Hydrosilylations: Reductions of Carbonyl Derivatives

4‐Fluoro‐2‐chlorophenylborinic acid acts as a precatalyst in the presence of phenylsilane for the facile reduction of ketones, aldehydes and imines. Notably, synergistic mediation of a tertiary amine was found essential to trigger silicon to boron hydride transfer to generate a key amine–diarylhydroborane Lewis complex.

A comparison of the structure and bonding in the donor–acceptor complexes H3N → BR(OH)2 and H3N → BRH(OH) (R = H; NH2, OH, and F): a computational investigation

Boronic acids, R–B(OH)2, play an important role in synthetic, biological, medicinal, and materials chemistry. Borinic acids, R–BH(OH), find relevance in similar fields, although their properties, e.g., binding affinity to diols, can vary significantly. Dative boron–nitrogen bonds, N → B, are critical for protecting the boron atom in these acids from nucleophilic attack. In this article, we study the structure, bonding, and formation thermodynamics of the model donor–acceptor complexes H3N → BR(OH)2 and H3N → BRH(OH) (R = H; NH2, OH, and F). Geometry optimizations were performed using second-order Moller–Plesset perturbation theory (MP2) with the Dunning–Woon aug-cc-pVTZ basis set; single-point CCSD(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ level calculations were used to generate a QCI density for analyses of the bonding. Extensive comparisons are made with results from density functional theory (DFT) optimizations with/without empirical dispersion corrections. The addition of an ammonia molecule dative bonded to the boron atom for these boronic and borinic acids results in the elongation of bonds to the boron atom, e.g., the boron–oxygen bond lengths increase in the range from ~ 4.5 to ~ 6.5%. The calculated values of ∆ \( {H}_{298}^0 \) for the ammoniation reactions, R–B(OH)2 + NH3 → H3N → BR(OH)2 are − 0.6, + 4.6, + 0.1, and − 6.0 kcal/mol for R = H, NH2, OH, and F, respectively, at the CCSD(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ level; the corresponding values of ∆\( {H}_{298}^0 \) for the borinic acid reactions are − 8.9, + 2.1, − 1.5, and − 7.9 kcal/mol.

Borinic Acid-Catalyzed, Regioselective Ring Opening of 3,4-Epoxy Alcohols.

Diarylborinic acids (Ar2BOH) catalyze the C3-selective ring opening of 3,4-epoxy alcohols with aniline, dialkylamine and arenethiol nucleophiles. The regiochemical outcome is consistent with a catalytic tethering mechanism in which the borinic acid interacts with both the electrophile and the nucleophile. The rate acceleration resulting from this induced intramolecularity effect is sufficient to overcome steric biases that would otherwise favor C4-selective opening of the substituted epoxy alcohols.