Borylation Of Arenes Research Articles

Borylation of Arenes with Bis(hexylene glycolato)diboron

A series of diolate-substituted diboron compounds were investigated as reagents for iridium-catalyzed C–H borylations of arenes. These studies showed that commercially available bis(hex­ylene glycolato)diboron reacts with arenes in the presence of the catalyst generated from [Ir(COD)OMe]2 and di-tert-butylbipyridine. This reagent is prepared from a less expensive diol than the more commonly used bis(pinacolato)diboron reagent.

Mechanistic Studies into Amine-Mediated Electrophilic Arene Borylation and Its Application in MIDA Boronate Synthesis

Direct electrophilic borylation using Y(2)BCl (Y(2) = Cl(2) or o-catecholato) with equimolar AlCl(3) and a tertiary amine has been applied to a wide range of arenes and heteroarenes. In situ functionalization of the ArBCl(2) products is possible with TMS(2)MIDA, to afford bench-stable and easily isolable MIDA-boronates in moderate to good yields. According to a combined experimental and computational study, the borylation of activated arenes at 20 °C proceeds through an S(E)Ar mechanism with borenium cations, [Y(2)B(amine)](+), the key electrophiles. For catecholato-borocations, two amine dependent reaction pathways were identified: (i) With [CatB(NEt(3))](+), an additional base is necessary to accomplish rapid borylation by deprotonation of the borylated arenium cation (σ complex), which otherwise would rather decompose to the starting materials than liberate the free amine to effect deprotonation. Apart from amines, the additional base may also be the arene itself when it is sufficiently basic (e.g., N-Me-indole). (ii) When the amine component of the borocation is less nucleophilic (e.g., 2,6-lutidine), no additional base is required due to more facile amine dissociation from the boron center in the borylated arenium cation intermediate. Borenium cations do not borylate poorly activated arenes (e.g., toluene) even at high temperatures; instead, the key electrophile in this case involves the product from interaction of AlCl(3) with Y(2)BCl. When an extremely bulky amine is used, borylation again does not proceed via a borenium cation; instead, a number of mechanisms are feasible including via a boron electrophile generated by coordination of AlCl(3) to Y(2)BCl, or by initial (heteroarene)AlCl(3) adduct formation followed by deprotonation and transmetalation.

Pushing the σ‐Donor Strength in Iridium Pincer Complexes: Bis(silylene) and Bis(germylene) Ligands Are Stronger Donors than Bis(phosphorus(III)) Ligands

Breaking the donor limits: The remarkable coordination chemistry of ECHE (E=Si, Ge) ligands with Group 9 metals (Ir, Rh) and their application in catalytic CH borylation of arenes was investigated. The spectroscopic and structural features of the first [ECE] iridium complexes show the stronger σ-donating properties of the divalent Si and Ge pincer ligands compared to analogous PIII-based ligands.

Spontaneous reduction and C–H borylation of arenes mediated by uranium(III) disproportionation

Transition-metal-arene complexes such as bis(benzene)chromium Cr(η(6)-C(6)H(6))(2) are historically important to d-orbital bonding theory and have modern importance in organic synthesis, catalysis and organic spintronics. In investigations of f-block chemistry, however, arenes are invariably used as solvents rather than ligands. Here, we show that simple uranium complexes UX(3) (X=aryloxide, amide) spontaneously disproportionate, transferring an electron and X-ligand, allowing the resulting UX(2) to bind and reduce arenes, forming inverse sandwich molecules [X(2)U(µ-η(6):η(6)-arene)UX(2)] and a UX(4) by-product. Calculations and kinetic studies suggest a 'cooperative small-molecule activation' mechanism involving spontaneous arene reduction as an X-ligand is transferred. These mild reaction conditions allow functionalized arenes such as arylsilanes to be incorporated. The bulky UX(3) are also inert to reagents such as boranes that would react with the traditional harsh reaction conditions, allowing the development of a new in situ arene C-H bond functionalization methodology converting C-H to C-B bonds.

Intermolecular C–H Bond Activation of Alkanes and Arenes by NCN Pincer Iridium(III) Acetate Complexes Containing Bis(oxazolinyl)phenyl Ligands

Carbon–hydrogen bond activation of arenes by the pincer Ir(III) acetate complex (phebox)Ir(OAc)2(H2O) (1; phebox = bis(oxazolinyl)phenyl) proceeded at 100 °C to give the corresponding aryl Ir(III) complexes (phebox)Ir(Ar)(OAc) in high yields. Reactions of the monosubstituted arenes anisole, toluene, chlorobenzene, acetophenone, and nitrobenzene resulted in the formation of meta- and para-substituted aryl complexes in high yields. Competitive experiments between two monosubstituted arenes exhibited relative reaction rates in the order C6H5OMe > C6H5NO2 > C6H5COMe > C6H5Cl > C6H5Me. The kinetic isotope effects of C–H bond activation of benzene and nitrobenzene were determined to be 2.9 ± 0.1 and 2.0 ± 0.4, respectively. The Ir complex 1 underwent catalytic borylation of arenes with bis(pinacolato)diboron (B2(pin)2) or pinacolborane (HB(pin)), giving the corresponding borylated products. The Ir complex 1 was also reactive toward alkane C–H bond activation, demonstrated by the reactions with n-heptane and n-octane at 160 °C that cleanly afforded the corresponding alkyl complexes (phebox)Ir(n-CnH2n+1)(OAc) (n = 7, 8, respectively).

A Perspective on Direct Electrophilic Arene Borylation

Direct electrophilic arene borylation is introduced as a new synthetic approach to aryl boronate esters. This perspective covers both the key historic breakthroughs and the more recent developments in this area.

ChemInform Abstract: Highly Selective Trifluoromethylation of 1,3‐Disubstituted Arenes Through Iridium‐Catalyzed Arene Borylation.

AbstractThe sequential iridium‐catalyzed borylation and copper‐catalyzed trifluoromethylation of arenes is performed under mild conditions and tolerates a variety of functional groups.

Sequential Ir-Catalyzed Borylation and Cu-Catalyzed Trifluoromethylation of Arenes

Sequential Ir-Catalyzed Borylation and Cu-Catalyzed Trifluoromethylation of Arenes

ChemInform Abstract: Simple Inexpensive Boron Electrophiles for Direct Arene Borylation.

AbstractThe regioselective title synthesis can be used for large‐scale production.

Correction to Chelate Restrained Boron Cations for Intermolecular Electrophilic Arene Borylation

Correction to Chelate Restrained Boron Cations for Intermolecular Electrophilic Arene Borylation

Reactivity of Lewis Acid Activated Diaza- and Dithiaboroles in Electrophilic Arene Borylation

Hydride abstraction from N,N′-bis(adamantyl)-1-hydrido-1,3,2-benzodiazaborole with catalytic [Ph3C][closo-CB11H6Br6] resulted in a low yield of arene borylation and a major product derived from migration of both adamantyl groups to the arene backbone. In contrast, the related aryl-substituted diazaborole N,N′-(2,6-diisopropylphenyl)-1-bromo-1,3,2-diazaborole did not borylate benzene or toluene, being resistant to halide abstraction even with strong halide acceptors: e.g., [Et3Si][closo-CB11H6Br6]. The reactivity disparity arises from greater steric shielding of the boron pz orbital in the 2,6-diisopropylphenyl-substituted diazaboroles. Boron electrophiles derived from 1-chloro-1,3,2-benzodithiaborole ((CatS2)BCl) are active for arene borylation, displaying reactivity between that of catecholato- and dichloro-boron electrophiles. [(CatS2)B(NEt3)][AlCl4] is significantly less prone to nucleophile-induced transfer of halide from [AlCl4]¯ to boron compared to catecholato and dichloro borocations, enabling it to borylate arenes containing nucleophilic −NMe2 moieties in high conversion (e.g., N,N,4-trimethylaniline and 1,8-bis(dimethylamino)naphthalene). Calculations indicate that the magnitude of positive charge at boron is a key factor in determining the propensity of chloride transfer from [AlCl4]¯ to boron on addition of a nucleophile.

Ortho-C–H Borylation of Arenes Using Silica-Supported Rhodium Complexes

ortho-C–H Borylation of Arenes Using Silica-Supported Rhodium Complexes

Regioselective ortho-C–H Borylation of Arenes via Iridium Catalysis

Regioselective ortho-C–H Borylation of Arenes via Iridium Catalysis

Highly Selective Trifluoromethylation of 1,3‐Disubstituted Arenes through Iridium‐Catalyzed Arene Borylation

The old one two: A sequential iridium-catalyzed borylation and copper-catalyzed trifluoromethylation of arenes is described (see scheme; Pin = pinacol). The reaction is conducted under mild reaction conditions and tolerates a variety of functional groups. The advantages of this tandem procedure are demonstrated by the late-stage trifluoromethylation of a number of biologically active molecules.

Highly Selective Trifluoromethylation of 1,3‐Disubstituted Arenes through Iridium‐Catalyzed Arene Borylation

Katalyse im Doppelpack: Eine sequenzielle Iridium-katalysierte Borylierung und Kupfer-katalysierte Trifluormethylierung von Arenen (siehe Schema; Pin=Pinacolato) gelingt unter milden Reaktionsbedingungen und toleriert verschiedenste funktionelle Gruppen. Die Vorzüge dieses Tandemverfahrens werden mit der späten Trifluormethylierung mehrerer biologisch aktiver Moleküle belegt.

Borylation and Silylation of C–H Bonds: A Platform for Diverse C–H Bond Functionalizations

Methods that functionalize C-H bonds can lead to new approaches for the synthesis of organic molecules, but to achieve this goal, researchers must develop site-selective reactions that override the inherent reactivity of the substrates. Moreover, reactions are needed that occur with high turnover numbers and with high tolerance for functional groups if the C-H bond functionalization is to be applied to the synthesis of medicines or materials. This Account describes the discovery and development of the C-H bond functionalization of aliphatic and aromatic C-H bonds with borane and silane reagents. The fundamental principles that govern the reactivity of intermediates containing metal-boron bonds are emphasized and how an understanding of the effects of the ligands on this reactivity led us to broaden the scope of main group reagents that react under mild conditions to generate synthetically useful organosilanes is described. Complexes containing a covalent bond between a transition metal and a three-coordinate boron atom (boryl complexes) are unusually reactive toward the cleavage of typically unreactive C-H bonds. Moreover, this C-H bond cleavage leads to the formation of free, functionalized product by rapid coupling of the hydrocarbyl and boryl ligands. The initial observation of the borylation of arenes and alkanes in stoichiometric processes led to catalytic systems for the borylation of arenes and alkanes with diboron compounds (diborane(4) reagents) and boranes. In particular, complexes based on the Cp*Rh (in which Cp is the cyclopentadienyl anion) fragment catalyze the borylation of alkanes, arenes, amines, ethers, ketals, and haloalkanes. Although less reactive toward alkyl C-H bonds than the Cp*Rh systems, catalysts generated from the combination of bipyridines and iridium(I)-olefin complexes have proven to be the most reactive catalysts for the borylation of arenes. The reactions catalyzed by these complexes form arylboronates from arenes with site-selectivity for C-H bond cleavage that depends on the steric accessibility of the C-H bonds. These complexes also catalyze the borylation of heteroarenes, and the selectivity for these substrates is more dependent on electronic effects than the borylation of arenes. The products from the borylation of arenes and heteroarenes are suitable for a wide range of subsequent conversions to phenols, arylamines, aryl ethers, aryl nitriles, aryl halides, arylboronic acids, and aryl trifluoroborates. Studies of the electronic properties of the ancillary ligand on the rate of the reaction show that the flat structure and the strong electron-donating property of the bipyridine ligands, along with the strong electron-donating property of the boryl group and the presence of a p-orbital on the metal-bound atom, lead to the increased reactivity of the iridium catalysts. Based on this hypothesis, we studied catalysts containing substituted phenanthroline ligands for a series of additional transformations, including the silylation of C-H bonds. A sequence involving the silylation of benzylic alcohols, followed by the dehydrogenative silylation of aromatic C-H bonds, leads to an overall directed silylation of the C-H bond ortho to hydroxyl functionality.

Synthesis of BN-Fused Polycyclic Aromatics via Tandem Intramolecular Electrophilic Arene Borylation

A tandem intramolecular electrophilic arene borylation reaction has been developed for the synthesis of BN-fused polycyclic aromatic compounds such as 4b-aza-12b-boradibenzo[g,p]chrysene (A) and 8b,11b-diaza-19b,22b-diborahexabenzo[a,c,fg,j,l,op]tetracene. These compounds adopt a twisted conformation, which results in a tight and offset face-to-face stacking array in the solid state. Time-resolved microwave conductivity measurements prove that the intrinsic hole mobility of A is comparable to that of rubrene, one of the most commonly used organic semiconductors, indicating that BN-substituted PAHs are potential candidates for organic electronic materials.

ChemInform Abstract: Regioselectivity of the Borylation of Alkanes and Arenes

AbstractReview: 95 refs.

ChemInform Abstract: Pinacol Boronates by Direct Arene Borylation with Borenium Cations.

AbstractIt is demonstrated, that catecholato ligated borenium cations, generated in solution from CCB, react as electrophiles with activated arenes and hetarenes to give a variety of pinacol boronates after quenching with pinacol.

Direct Arene Borylation with Borenium Cations to Yield Pinacol Boronates

Direct Arene Borylation with Borenium Cations to Yield Pinacol Boronates