Catalytic Borylation Research Articles

Catalyst Development in the Dehydrogenative Borylation of Alkenes for the Synthesis Vinylboronate Esters

AbstractCatalytic dehydrogenative borylation of alkenes provides an efficient and straightforward method for the preparation of synthetically useful vinylboronate esters. Here, we present a summary of developments and recent advances in this area, classified according to the various reactants and catalyst systems.1 Introduction2 Catalytic Dehydrogenative Borylation of Alkenes by Using Boranes3 Catalytic Dehydrogenative Borylation of Alkenes by Using Diboranes4 Zirconium-Catalyzed H 2 –Acceptorless Dehydrogenative Borylation of Alkenes with Boranes5 Conclusion and Outlook

C(sp2)-H Borylation of Heterocycles by Well-Defined Bis(silylene)pyridine Cobalt(III) Precatalysts: Pincer Modification, C(sp2)-H Activation and Catalytically Relevant Intermediates.

Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp2)-H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and pyridines. Specifically, [( Ar SiNSi)CoH3]·NaHBEt3 ( Ar SiNSi = 2,6-[EtNSi(NtBu)2CAr]2C5H3N, Ar = C6H5 (1-H 3 ·NaHBEt 3 ), 4-MeC6H4 (2-H 3 ·NaHBEt 3 )) and trans-[( Ar SiNSi)Co(H)2BPin] (Ar = C6H5 (1-(H) 2 BPin), 4-MeC6H4 (2-(H) 2 BPin), Pin = pinacolato) were prepared and employed as single component precatalysts for the C(sp2)-H borylation of 2-methylfuran, benzofuran and 2,6-lutidine. The cobalt(III) precursors, 2-H 3 ·NaHBEt 3 and 2-(H) 2 BPin also promoted C(sp2)-H activation of benzofuran, yielding [(ArSiNSi)CoH(Bf)2] (Ar = 4-MeC6H4, 2-H(Bf) 2 , Bf = 2-benzofuranyl). Monitoring the catalytic borylation of 2-methylfuran and 2,6-lutidine by 1H NMR spectroscopy established the trans-dihydride cobalt(III) boryl as the catalyst resting state at low substrate conversion. At higher conversion two distinct pincer modification pathways were identified, depending on the substrate and the boron source.

Facile Synthesis of NH-Free 5-(Hetero)Aryl-Pyrrole-2-Carboxylates by Catalytic C-H Borylation and Suzuki Coupling.

A convenient two-step preparation of NH-free 5-aryl-pyrrole-2-carboxylates is described. The synthetic route consists of catalytic borylation of commercially available pyrrole-2-carboxylate ester followed by Suzuki coupling without going through pyrrole N–H protection and deprotection steps. The resulting 5-aryl substituted pyrrole-2-carboxylates were synthesized in good- to excellent yields. This synthetic route can tolerate a variety of functional groups including those with acidic protons on the aryl bromide coupling partner. This methodology is also applicable for cross-coupling with heteroaryl bromides to yield pyrrole-thiophene, pyrrole-pyridine, and 2,3’-bi-pyrrole based bi-heteroaryls.

Mechanism and Scope of Nickel-Catalyzed Decarbonylative Borylation of Carboxylic Acid Fluorides.

This Article describes the development of a base-free, nickel-catalyzed decarbonylative coupling of carboxylic acid fluorides with diboron reagents to selectively afford aryl boronate ester products. Detailed studies were conducted to assess the relative rates of direct transmetalation between aryl boronate esters and diboron reagents and a bisphosphine nickel(aryl)(fluoride) intermediate. These investigations revealed that diboron reagents undergo transmetalation with this Ni(aryl)(fluoride) intermediate at rates significantly faster than their aryl boronate ester congeners. Furthermore, the reactivity of both boron reagents toward transmetalation is enhanced with increasing electrophilicity of the boron center. These mechanistic insights were leveraged to develop a catalytic decarbonylative borylation of acid fluorides that proved applicable to a variety of (hetero)aryl carboxylic acid fluorides as well as diverse diboron reagents. The acid fluorides can be generated in situ directly from carboxylic acids. Furthermore, themechanistic studies directed the identification of various air-stable Ni pre-catalysts for this transformation.

Chiral Bicyclic NHC/Cu Complexes for Catalytic Asymmetric Borylation of α,β-Unsaturated Esters.

The potential of using chiral bicyclic NHC ligands that exhibit modularity was investigated in the Cu-catalyzed asymmetric borylation reaction of α,β-unsaturated esters. After screening for ligands and optimization of the reaction conditions, the corresponding products were afforded with good enantioselectivities (up to 85% ee).

Cobalt-Catalyzed Borylation of Fluorinated Arenes: Thermodynamic Control of C(sp2)-H Oxidative Addition Results in ortho-to-Fluorine Selectivity.

The mechanism of C(sp2)-H borylation of fluorinated arenes with B2Pin2 (Pin = pinacolato) catalyzed by bis(phosphino)pyridine (iPrPNP) cobalt complexes was studied to understand the origins of the uniquely high ortho-to-fluorine regioselectivity observed in these reactions. Variable time normalization analysis (VTNA) of reaction time courses and deuterium kinetic isotope effect measurements established a kinetic regime wherein C(sp2)-H oxidative addition is fast and reversible. Monitoring the reaction by in situ NMR spectroscopy revealed the intermediacy of a cobalt(I)-aryl complex that was generated with the same high ortho-to-fluorine regioselectivity associated with the overall catalytic transformation. Deuterium labeling experiments and stoichiometric studies established C(sp2)-H oxidative addition of the fluorinated arene as the selectivity-determining step of the reaction. This step favors the formation of ortho-fluoroaryl cobalt intermediates due to the ortho fluorine effect, a phenomenon whereby ortho fluorine substituents stabilize transition metal-carbon bonds. Computational studies provided evidence that the cobalt-carbon bonds of the relevant intermediates in (iPrPNP)Co-catalyzed borylation are strengthened with increasing ortho fluorine substitution. The atypical kinetic regime involving fast and reversible C(sp2)-H oxidative addition in combination with the thermodynamic preference for forming cobalt-aryl bonds adjacent to fluorinated sites are the origin of the high regioselectivity in the catalytic borylation reaction.

Methane functionalization by an Ir(III) catalyst supported on a metal–organic framework: an alternative explanation of steric confinement effects

A highly selective Ir catalyst supported on the metal–organic framework (MOF) UiO-67 for the catalytic borylation of methane has recently been synthesized. The high chemoselectivity of the catalyst toward monoborylated methane (CH3Bpin, Bpin = pinacolborane) instead of diborylated methane (CH2Bpin2) was speculated to be caused by the steric confinement of MOF UiO-67. In this study, we applied quantum mechanical methods to determine: (1) the steric effect of the UiO-67 framework in promoting the chemoselectivity of the Ir catalyst toward CH3Bpin and (2) the borylation mechanisms over the Ir catalyst supported on UiO-67. Our results show that UiO-67 framework sterically obstructs the diffusion of the larger CH2Bpin2 molecule within the MOF while allowing the smaller CH3Bpin molecule to pass through with little energy penalty. The diffusion of CH2Bpin2 from the tetrahedral pore to the tetragonal pyramidal pore within modified UiO-67 with coordinated Ir(Bpin)3 complex has an estimated barrier of 24.7 kcal/mol and is 14.2 kcal/mol higher than the diffusion of CH3Bpin. The electronic and steric effects of the support at the Ir catalytic center are much smaller than this confinement effect on diffusion, and the catalytic center behaves similarly to the homogeneous Ir catalyst. We determined an overall free energy of activation of 34.6 kcal/mol for the CH4 borylation reaction using the Ir(III) catalyst. We also determined that the turnover-determining step for the catalytic methane borylation is the isomerization of seven-coordinated Ir(V) complex instead of the commonly assumed C–H bond activation by oxidative addition.

Metal-Organic Framework Stabilizes a Low-Coordinate Iridium Complex for Catalytic Methane Borylation.

Catalytic borylation has recently been suggested as a potential strategy to convert abundant methane to fine chemicals. However, synthetic utility of methane borylation necessitates significant improvement of catalytic activities over original phenanthroline- and diphosphine-Ir complexes. Herein, we report the use of metal-organic frameworks (MOFs) to stabilize low-coordinate Ir complexes for highly active methane borylation to afford the monoborylated product. The mono(phosphine)-Ir based MOF, Zr-P1-Ir, significantly outperformed other Ir catalysts in methane borylation to afford CH3Bpin with a turnover number of 127 at 110 °C. Density functional theory calculations indicated a significant reduction of activation barrier for the rate limiting oxidative addition of methane to the four-coordinate (P1)IrIII(Bpin)3 catalyst to form the six-coordinate (P1)IrV(Bpin)3(CH3)(H) intermediate, thus avoiding the formation of sterically encumbered seven-coordinate IrV intermediates as found in other Ir catalysts based on chelating phenanthroline, bipyridine, and diphosphine ligands. MOF thus stabilizes the homogeneously inaccessible, low-coordinate (P1)Ir(boryl)3 catalyst to provide a unique strategy to significantly lower the activation barrier for methane borylation. This MOF-based catalyst design holds promise in addressing challenging catalytic reactions involving highly inert substrates.

Recent Synthesis Developments of Organoboron Compounds via Metal-Free Catalytic Borylation of Alkynes and Alkenes.

Diboron reagents have been traditionally regarded as “Lewis acids”, which can react with simple Lewis base to create a significant nucleophilic character in one of boryl moieties. In particular, bis(pinacolato)diboron (B2pin2) reacts with simple Lewis bases, such as N-heterocyclic carbenes (NHCs), phosphines and alkoxides. This review focuses on the application of trivalent nucleophilic boryl synthon in the selective preparation of organoboron compounds, mainly through metal-free catalytic diboration and the β-boration reactions of alkynes and alkenes.

Cobalt Pincer Complexes in Catalytic C-H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes.

The mechanism for the borylation of an aromatic substrate by a cobalt pincer complex was investigated by density functional theory calculations. Experimental observations identified trans-(iPrPNP)CoH2(BPin) as the resting state in the borylation of five-membered heteroarenes, and 4-BPin-(iPrPNP)Co(N2)BPin as the resting state in the catalytic borylation of arene substrates. The active species, 4-R-(iPrPNP)CoBPin (R=H, BPin), were generated by reductive elimination of H2 in the former, through Berry pseudorotation to the cis isomer, and N2 loss in the latter. The catalytic mechanism of the resulting Co(I) complex was computed to involve three main steps: C-H oxidative addition of the aromatic substrate (C6H6), reductive elimination of PhBPin, and regeneration of the active complex. The oxidative addition product formed through the most favorable pathway, where the breaking C-H bond of C6H6 is parallel to a line between the two phosphine atoms, leaves the complex with a distorted PNP ligand, which rearranges to a more stable complex via dissociation and re-association of HBPin. Alternative pathways, σ-bond metathesis and the oxidative addition in which the breaking C-H bond is parallel to the Co-B bond, are predicted to be unlikely for this Co(I) complex. The thermodynamically favorable formation of the product PhBPin via reductive elimination drives the reaction forward. The active species regenerates through the oxidative addition of B2Pin2 and reductive elimination of HBPin. In the overall reaction, the flipping (refolding) of the five-membered phosphine rings, which connects the species with two phosphine rings folded in the same direction and that with them folded in different directions, is found to play an important role in the catalytic process, as it relieves steric crowding within the PNP ligand and opens Co coordination space. Metal-ligand cooperation based on the ligand's aromatization/dearomatization, a common mechanism for heavy-metal pincer complexes, and the dissociation of one phosphine ligand, do not apply in this system. This study provides guidance for understanding important features of pincer ligands with first-transition-row metals that differ from those in heavier metal complexes.

Ferroceneboronic Acid and Derivatives: Synthesis, Structure, Electronic Properties, and Reactivity in Directed C–H Bond Activation

Known since 1959, ferroceneboronic acid (1) and its derivatives have been mainly used as polyol sensors and in cross-coupling reactions. However, a literature survey revealed that there is not a paper describing the full characterization of ferroceneboronic acid derivatives and that useful boron protecting groups have not been studied in the ferrocene series. Here, we present an optimized multigram-scale synthesis of the known ferroceneboronic acid (1) using a phase-switch purification process. It was furthermore functionalized to reach the diaminonaphthalene (FcB(dan), 15), anthranilamides (FcB(aam), 16; FcB(Me-aam), 17 and 18), potassium trifluoroborate (FcBF3K, 19), triolborate (FcB(triolborate), 20) and N-methyliminodiacetic acid (FcB(MIDA), 21) derivatives. Their structures were unambiguously assigned by NMR and X-ray analysis, and the data collected provided a general overview of the electronic and structural features of these compounds. From the data obtained, B(dan) and B(amm) were classified as electron-withdrawing groups, whereas trifluoroborate and triolborate behave as electron-donating groups. We report the first catalytic silylation of ferrocene C–H bonds to access di- and trisubstituted derivatives. Catalytic borylation was also attempted, highlighting a switch in regioselectivity that was unambiguously assigned by X-ray analysis.

Direct Borylation of Tertiary Anilines via C-N Bond Activation.

The first successful catalytic borylation of unactivated aromatic C-N bonds of tertiary anilines without the preactivation or any directing groups is demonstrated. The reactivity of both N,N-dialkylarylamines and N-arylpyrroles were investigated systematically, and the targeted products were furnished in moderate to good yields. The DFT calculation results indicated that the catalytic cycle is furnished via a five-membered cyclic transition-state due to the steric hindrance of the Ni/NHC catalytic system.

Copper(I)-Catalyzed Enantioselective Nucleophilic Borylation of Aliphatic Ketones: Synthesis of Enantioenriched Chiral Tertiary α-Hydroxyboronates.

A new method was developed for the first catalytic enantioselective borylation of aliphatic ketones. A variety of substrates reacted efficiently with bis(pinacolato)diboron in the presence of a copper(I)/chiral N-heterocyclic carbene complex catalyst to furnish optically active tertiary α-hydroxyboronates with moderate to high enantioselectivities (up to 94 % ee). Notably, the product could be converted into the chiral tertiary alcohol derivative using a stereospecific boron functionalization process. The theoretical study of the mechanism for the enantioselectivity is also described.

Copper(I)‐Catalyzed Enantioselective Nucleophilic Borylation of Aliphatic Ketones: Synthesis of Enantioenriched Chiral Tertiary α‐Hydroxyboronates

AbstractA new method was developed for the first catalytic enantioselective borylation of aliphatic ketones. A variety of substrates reacted efficiently with bis(pinacolato)diboron in the presence of a copper(I)/chiral N‐heterocyclic carbene complex catalyst to furnish optically active tertiary α‐hydroxyboronates with moderate to high enantioselectivities (up to 94 % ee). Notably, the product could be converted into the chiral tertiary alcohol derivative using a stereospecific boron functionalization process. The theoretical study of the mechanism for the enantioselectivity is also described.

Catalytic Aromatic Borylation via in situ-Generated Borenium Species

Catalytic Aromatic Borylation via in situ-Generated Borenium Species

ChemInform Abstract: Asymmetric Catalytic Borylation of α,β‐Unsaturated Acceptors

AbstractReview: 49 refs.

Cobalt-Catalyzed C(sp2)–H Borylation with an Air-Stable, Readily Prepared Terpyridine Cobalt(II) Bis(acetate) Precatalyst

A bench-stable, 4-aryl-substituted terpyridine supported, high-spin cobalt(II) bis(acetate) complex, (ArTpy)Co(OAc)2 (ArTpy = 4′-(4-N,N′-dimethylaminophenyl)-2,2′:6′,2″-terpyridine), is active for the C(sp2)–H borylation of arenes and heteroarenes with B2Pin2 (Pin = pinacolato). Optimization of the catalytic borylation reaction revealed improved performance in the presence of LiOMe and turnover numbers of up to 100 have been observed using all air-stable components. EPR specstroscopy identified formation of inactive cobalt species, promoted by excess HBPin. A high-spin cobalt(II) bis[(diacetoxy)pinacolatoborate−κ3O,O,O] compound has been isolated and characterized by X-ray diffraction and is the result of catalyst deactivation.

ChemInform Abstract: Facile Synthesis of Chiral Benzimidazolium Salts and the Application in Asymmetric Catalytic Borylation.

AbstractVarious chiral benzimidazolium salts are synthesized by a multistep procedure.

Catalytic borylation of methane.

Despite steady progress in catalytic methods for the borylation of hydrocarbons, methane has not yet been subject to this transformation. Here we report the iridium-catalyzed borylation of methane using bis(pinacolborane) in cyclohexane solvent. Initially, trace amounts of borylated products were detected with phenanthroline-coordinated Ir complexes. A combination of experimental high-pressure and high-throughput screening, and computational mechanism discovery techniques helped to rationalize the foundation of the catalysis and identify improved phosphine-coordinated catalytic complexes. Optimized conditions of 150°C and 3500-kilopascal pressure led to yields as high as ~52%, turnover numbers of 100, and improved chemoselectivity for monoborylated versus diborylated methane.

Direct synthesis of dicarbonyl PCP-iron hydride complexes and catalytic dehydrogenative borylation of styrene.

A new and efficient method based on the simple metalating reagent Fe(CO)5 has been developed for the straightforward synthesis of well defined cyclometalled PCP iron carbonyl pincer complexes. The reaction proceeds cleanly under mild conditions at 30 °C and UV irradiation. Four hydride pincer complexes are synthesized and fully characterized as well as an intermediate dinuclear species. The new iron complexes are active and selective catalytic precursors for the dehydrogenative borylation of styrene with HBpin.