Transient Imine as a Directing Group for the Metal-Free o-C-H Borylation of Benzaldehydes

Sterically Controlled Lewis Acid-Base Interaction Toward para-Selective Borylation of Aromatic Aldimines and Benzylamines

Site-selective C-H bond functionalization of arenes at the para position remains extremely challenging primarily due to its relative inaccessibility from the catalytic site. As a consequence, it is significantly restricted to limited molecular scaffolds. Herein, we report a method for the para-C-H borylation of aromatic aldimines and benzylamines using commercially available ligands under iridium catalysis. The established method displays excellent para selectivity for variously substituted aromatic aldimines, benzylamines and bioactive molecules. Based on several control experiments, it is proposed that a Lewis acid-base interaction between the nitrogen and boron functionality guides the para selectivity via a steric shield for the aromatic aldimines, where Bpin acts as a transient directing group. However, the steric shield of the in situ generated N-Bpin moiety controlled the overall selectivity for the para borylation of benzylamines.

Metal-free Borylation of α-Naphthamides and Phenylacetic Acid Drug

Site-selective C-H borylation is an important strategy for constructing molecular diversity in arenes and heteroarenes. Although transition-metal-catalyzed borylation is well explored, developing metal-free strategies remains scarce. Herein, we developed a straightforward approach for BBr3-mediated selective C-H borylation of naphthamide and phenyl acetamide derivatives under metal-free conditions. This methodology appears to be economical and cost-effective. Successful borylation of drug molecules such as ibuprofen and indoprofen demonstrates the versatility and utility of this metal-free borylation. An exclusive monoselectivity was observed without a trace of diboration. Despite the possibility of forming a 5-membered boronated intermediate at the ortho-position, the selectively 6-membered intermediate paved the way for the formation of the peri-product, which was further supported by detailed computational investigation.

Transforming cyclopropanes to enamides via σ-C-C bond eliminative borylation

Recent strides in C-H borylation have significantly expanded our toolkit for the preparation of organoboronates. Nevertheless, avenues alternative to obtain these compounds via σ-C-C cleavage, thereby facilitating molecular scaffold editing, remain scarce. Several methodologies have been proposed for hydroboration of cyclopropanes by activating C-C bonds, conventionally relying on noble and hazardous metal catalysts to control reaction outcomes. Here, we present a strategy for crafting stereochemically precise γ-borylenamides through ring-opening of cyclopropanes avoiding any metallic entities. Boryl species, generated through a ternary reaction with BCl3, cyclopropanes, and a tertiary amine, selectively undergo C-C bond eliminative borylation under the directing of N-acyl group, thereby ensuring enhanced selectivity and efficiency along the reaction pathway. Such inherently stereoconvergent approach accommodates precursors of diverse geometries, including cis/trans isomeric blends.

Spectroscopic Characterization of the 1-Boratricyclo-[4.1.0.02,7]-heptane Radical with a Delocalized Four-Center-One-Electron Bond

The boron atom is a highly electrophilic reagent due to the presence of its empty p orbital, making it prone to undergo electrophilic addition reactions with the carbon-carbon double bonds of olefins. In this study, the classical C=C reaction pathway occurs when a boron atom attacks the C=C bond of cyclohexene, resulting in the formation of the η2 (1,2)-BC6H10 complex (A) that contains a borirane radical subunit. This complex can further undergo photoisomerization, leading to the formation of a 3,4,5,6-tetrahydroborepine radical (C) through the cleavage of C-C bonds. In addition, two 1-boratricyclo[4.1.0.02,7]heptane radicals with chair (B) and boat (B') conformations were observed through α C-H cleavage reactions. Bonding analysis indicates that these radicals involve a four-center-one-electron (4c-1e) bond. Under UV light irradiation, these two radicals undergo ring-opening and rearrangement reactions, resulting in the formation of a 1-cyclohexen-1-yl-borane radical (D), which is a sp2 C-H activation product. These findings delineate a potential pathway for the synthesis of organoboron radicals through boron-mediated C-H and C-C bond cleavage reactions in cycloolefins.

Regulating iminophosphorane PN bond reactivity through geometric constraints with cage-shaped triarylphosphines

Structure-reactivity investigations and quantum-chemical parametrization of steric and electronic properties of geometrically constrained iminophosphoranes enabled the design of new frustrated Lewis pairs and revealed unusual properties at the phosphonium center embedded in the cage-shaped triptycene tricyclic scaffold.

Study on Pyridine-Boryl Radical-Promoted, Ketyl Radical-Mediated Carbon-Carbon Bond-Forming Reactions

Ketyl radicals are synthetically versatile reactive species, but their applications have been hampered by harsh generation conditions employing highly reducing metals. Recently, the pyridine-boryl radical received wide attention as a promising organic reductant because of its mildness as well as convenience in handling. While probing the utility of the pyridine-boryl radical, our group observed facile pinacol coupling reactivity that had not been known at that time. This serendipitous finding was successfully rendered into a practical synthesis of tetraaryl-1,2-diols in up to 99% yield within 1 h. Subsequently, upon examinations of various reaction manifolds, a diastereoselective ketyl-olefin cyclization was accomplished to produce cycloalkanols such as trans-2-alkyl-1-indanols. Compared to the previous methods, the stereocontrolling ability was considerably enhanced by taking advantage of the structurally modifiable boryl group that would be present near the bond-forming site. In this full account, our synthetic efforts with the O-boryl ketyl radicals are disclosed in detail, covering the discovery, optimization, scope expansion, and mechanistic analysis, including density functional theory (DFT) calculations.

Boron-mediated one-pot access to salicylaldehydes via ortho-C-H hydroxylation of benzaldehydes

A novel protocol has been devised for the ortho-C-H hydroxylation of benzaldehydes. Directed by a transient imine group, the borylation of benzaldehydes, sequentially followed by the hydroxylation, furnishes diverse salicylaldehydes in a one-pot manner. The resultant salicylaldehydes could be readily applied in the downstream synthesis to produce bioactive molecules.

Synthetic progress of organic thermally activated delayed fluorescence emitters via C-H activation and functionalization

Thermally activated delayed fluorescence (TADF) emitters have become increasingly prominent due to their promising applications across various fields, prompting a continuous demand for developing reliable synthetic methods to access them. This review aims to highlight the progress made in the last decade in synthesizing organic TADF compounds through C-H bond activation and functionalization. The review begins with a brief introduction to the basic features and design principles of TADF emitters. It then provides an overview of the advantages and concise development of C-H bond transformations in constructing TADF emitters. Subsequently, it summarizes both transition-metal-catalyzed and non-transition-metal-promoted C-H bond transformations used for the synthesis of TADF emitters. Finally, the review gives an outlook on further challenges and potential directions in this field.

Chemoselective C-H Hydroxylation and Borylation of N-Phenylbenzamides using BBr3

A novel metal-free chemoselective C-H hydroxylation and borylation of N-phenylbenzamides using BBr3 is described. The protocol generates the corresponding phenols and arylboronic esters in moderate to excellent yields under mild conditions with brilliant chemoselectivity. Additionally, this strategy can be realized in one pot, and several potential bioactive derivatives can be synthesized efficiently. Density functional theory calculations certify that the preferred pathway for this metal-free C-H hydroxylation process is the formation of a five-membered boracycle.

Metal-Free ortho C-H Borylation of Thiobenzamides

A BBr3-mediated S-directed ortho C-H borylation of thiobenzamides was developed. A variety of ortho-borylated thiobenzamides were obtained in moderate to good yields with a wide functional group tolerance under simple and metal-free conditions. This transformation provided a convenient and practical route to important functionalized thiobenzamides.

When is an Imine Directing Group a Transient Imine Directing Group in C-H Functionalization?

'Transient' C-H functionalization has emerged in recent years to describe the use of a dynamic linkage, often an imine, to direct cyclometallation and subsequent functionalization. As the field continues to grow in popularity, we consider the features that make an imine directing group transient. A transient imine should be i) formed dynamically in situ, ii) avoid discrete introduction or cleavage steps, and iii) offer the potential for catalysis in both the directing group and metal. This concept article contrasts transient imines with pioneering early studies of imines as directing groups for the formation of metallacycles and the use of preformed imines in C-H functionalization. Leading developments in the use of catalytic additives to form transient directing groups (as aldehyde or amine) are covered including selected highlights of the most recent examples of catalytic imine directed C-H functionalization with transition metals.

Carbonyl group directed synthesis of 3-boryl-3-substituted alkenyl oxindoles and tetrasubstituted β-borylenones

Transition metal-free carbonyl directed boron-Wittig reaction of α-bis(boryl)carbanions with the corresponding isatins or with the α-keto esters/amides was achieved to access alkenyl oxindoles in good yield and high stereoselectivity.

Regioselective ortho halogenation of N-aryl amides and ureas via oxidative halodeboronation: harnessing boron reactivity for efficient C-halogen bond installation

The installation of the C-halogen bond at the ortho position of N-aryl amides and ureas represents a tool to prepare motifs that are ubiquitous in biologically active compounds. To construct such prevalent bonds, most methods require the use of precious metals and a multistep process. Here we report a novel protocol for the long-standing challenge of regioselective ortho halogenation of N-aryl amides and ureas using an oxidative halodeboronation. By harnessing the reactivity of boron over nitrogen, we merge carbonyl-directed borylation with consecutive halodeboronation, enabling the precise introduction of the C-X bond at the desired ortho position of N-aryl amides and ureas. This method offers an efficient, practical, and scalable solution for synthesizing halogenated N-heteroarenes under mild conditions, highlighting the superiority of boron reactivity in directing the regioselectivity of the reaction.

Borylation directed borylation of N-alkyl anilines using iodine activated pyrazaboles

Doubly electrophilic pyrazabole derivatives (pyrazabole = [H2B(μ-C3N2H3)]2) combined with one equiv. of base effect the ortho-borylation of N-alkyl anilines. Initial studies found that the bis(trifluoromethane)sulfonimide ([NTf2]-) pyrazabole derivative, [H(NTf2)B(μ-C3N2H3)]2, is highly effective for ortho-borylation, with this process proceeding through N-H borylation and then ortho C-H borylation. The activation of pyrazabole by I2 was developed as a cheaper and simpler alternative to using HNTf2 as the activator. The addition of I2 forms mono or ditopic pyrazabole electrophiles dependent on stoichiometry. The ditopic electrophile [H(I)B(μ-C3N2H3)]2 was also effective for the ortho-borylation of N-alkyl-anilines, with the primary C-H borylation products readily transformed into pinacol boronate esters (BPin) derivatives. Comparison of borylation reactions using the di-NTf2-and the diiodo-pyrazabole congeners revealed that more forcing conditions are required with the latter. Furthermore, the presence of iodide leads to competitive formation of side products, including [HB(μ-C3N2H3)3BH]+, which are not active for C-H borylation. Using [H(I)B(μ-C3N2H3)]2 and 0.2 equiv. of [Et3NH][NTf2] combines the higher yields of the NTf2 system with the ease of handling and lower cost of the iodide system generating an attractive process applicable to a range of N-alkyl-anilines. This methodology represents a metal free and transiently directed C-H borylation approach to form N-alkyl-2-BPin-aniline derivatives.

Chiral Transient Ligand Enabled Enantioselective Synthesis of Atropisomers Decorated with Unactivated Olefins via a Palladium-Catalyzed C-H Olefination

Herein, atroposelective synthesis of axially chiral biaryls with unactivated olefins by a palladium-catalyzed C-H olefination using a chiral transient directing group strategy has been disclosed. This protocol is well compatible with a variety of biaryl-2-aldehydes as well as various olefins such as allyl sulfonamides and allyl sulfones to provide the atroposelective olefinated products in synthetically useful yields with excellent enantioselectivities up to >99% ee. In addition, a wide number of axially chiral biaryl alcohols were synthesized by the simple diversification of the products in excellent enantioselectivity.

Metal-Free Stereoconvergent C-H Borylation of Enamides

Enamides, functional derivatives of enamines, play a significant role as synthetic targets. However, the stereoselective synthesis of these molecules has posed a longstanding challenge in organic chemistry, particularly for acyclic enamides that are less thermodynamically stable. In this study, we present a general strategy for constructing β-borylenamides by C-H borylation, which provides a versatile platform for generating the stereodefined enamides. Our approach involves the utilization of metalloid borenium cation, generated through the reaction of BBr3 and enamides in the presence of two different additives, avoiding any exogenous catalyst. Importantly, the stereoconvergent nature of this methodology allows for the use of starting materials with mixed E/Z configurations, thus highlighting the unique advantage of this chemistry. Mechanistic investigations have shed light on the pivotal roles played by the two additives, the reactive boron species, and the phenomenon of stereoconvergence.

Metal-Free Directed Site-Selective Csp3 -H Borylation of Saturated Cyclic Amines

The borylation of Csp3 -H bonds is a challenging transformation that is typically restricted to transition metal catalysis. Herein, we report the site-selective metal-free Csp3 -H borylation of saturated cyclic amines. It is possible to selectively borylate piperidine derivatives at the α or β positions according to the reaction conditions. The mechanism was supported by NMR spectroscopy, calorimetry experiments and density functional theory (DFT) computations. It suggests that the piperidine is dehydrogenated by complexation with BBr3 to produce an enamine intermediate, which is in turn borylated at either the α or β position according to the reaction conditions.

C-H Borylation of Benzophenones Using Hydrazone as the Traceless Directing Group

C-H borylation is one of the powerful C-H bond functionalization reactions. In this context, a metal-free C-H borylation of benzophenones using hydrazone as the traceless directing group has been reported. The dibromoboron intermediates can be obtained in excellent yields, and the corresponding arylboronic esters are generated in moderate to excellent yields. Furthermore, the borylated compounds can be transformed in a one-pot method, avoiding the loss of overall yield caused by the separation of the arylboronic esters.

Synthesis of Naphthalimides through Tandem Pd(II)-Catalyzed C(sp3)-H Oxidation and Diels-Alder Reaction Using a Transient Directing Group Strategy

Naphthalimides have found extensive applications in materials science and pharmaceuticals. It is still highly desirable to develop efficient methods for the synthesis of naphthalimides with structural diversity. In this work, we developed a new approach for the synthesis of naphthalimides via a tandem reaction of o-methylbenzaldehydes and maleimides. The tandem reaction involves Pd(II)-catalyzed benzylic C(sp³)-H oxidation using an amino acid as the transient directing group and Diels-Alder reaction. The subsequent dehydration forms naphthalimides. The reaction introduces the imide moiety and constructs a benzene ring simultaneously, allowing for easy access to a range of naphthalimides with a variety of substituents.

Amides as modifiable directing groups in electrophilic borylation

Amide directed C-H borylation using ≥two equiv. of BBr3 forms borenium cations containing a R2N(R')C[double bond, length as m-dash]O→B(Ar)Br unit which has significant Lewis acidity at the carbonyl carbon. This enables reduction of the amide unit to an amine using hydrosilanes. This approach can be applied sequentially in a one-pot electrophilic borylation-reduction process, which for phenyl-acetylamides generates ortho borylated compounds that can be directly oxidised to the 2-(2-aminoethyl)-phenol. Other substrates amenable to the C-H borylation-reduction sequence include mono and diamino-arenes and carbazoles. This represents a simple method to make borylated molecules that would be convoluted to access otherwise (e.g. N-octyl-1-BPin-carbazole). Substituent variation is tolerated at boron as well as in the amide unit, with diarylborenium cations also amenable to reduction. This enables a double C-H borylation-reduction-hydrolysis sequence to access B,N-polycyclic aromatic hydrocarbons (PAHs), including an example where both the boron and nitrogen centres contain functionalisable handles (N-H and B-OH). This method is therefore a useful addition to the metal-free borylation toolbox for accessing useful intermediates (ArylBPin) and novel B,N-PAHs.

Transition Metal-Free Aromatic C-H, C-N, C-S and C-O Borylation

Aromatic organoboron compounds are highly valuable building blocks in organic chemistry. They were mainly synthesized through aromatic C-H and C-Het borylation, in which transition metal-catalysis dominate. In the past decade, with increasing attention to sustainable chemistry, numerous transition metal-free C-H and C-Het borylation transformations have been developed and emerged as efficient methods towards the synthesis of aromatic organoboron compounds. This account mainly focuses on recent advances in transition metal-free aromatic C-H, C-N, C-S, and C-O borylation transformations and provides insights to where further developments are required.

Boron-Mediated Regioselective Aromatic C-H Functionalization via an Aryl BF2 Complex

An efficient regioselective functionalization of 2-aryl-heteroarenes and aryl aldehydes via an azaaryl BF₂ complex has been developed. Mechanistically the reaction comprises fluoride to bromide ligand exchange on an aryl boron species and consecutive C-B bond cleavage to deliver a broad range of functionalized products. The reaction is high yielding, has a broad substrate scope where several different heteroarenes can be functionalized with chloro, bromo, iodo, hydroxyl, amine and BF₂ in a highly regioselective fashion. The method can be applied for late-stage functionalization or for rapid skeleton remodeling with for instance cross-couplings.

Two Directing Groups Used for Metal Catalysed Meta-C-H Functionalisation Only Effect Ortho Electrophilic C-H Borylation

Two templates used in meta-directed C-H functionalisation under metal catalysis do not direct meta-C-H borylation under electrophilic borylation conditions. Using BCl3 only Lewis adduct formation with Lewis basic sites in the template is observed. While combining BBr3 and the template containing an amide linker only led to amide directed ortho C-H borylation, with no pyridyl directed meta borylation. The amide directed borylation is selective for the ortho borylation of the aniline derived unit in the template, with no ortho borylation of the phenylacetyl ring - which would also form a six membered boracycle - observed. In the absence of other aromatics amide directed ortho borylation on to phenylacetyl rings can be achieved. The absence of meta-borylation using two templates indicates a higher barrier to pyridyl directed meta borylation relative to amide directed ortho borylation and suggests that bespoke templates for enabling meta-directed electrophilic borylation may be required.

Adamantane-1-Carbonyl-Directed C-H Borylation and Hydroxylation of Benzenethiols

A novel route has been described for C-H borylation and hydroxylation of benzenethiols directed by adamantane-1-carbonyl using BBr₃. The protocol generates corresponding arylboronic esters and phenols in moderate to excellent yields under metal-free conditions. In addition, the borylated product can be transformed and the directing group can be removed in good yields, which will facilitate the synthesis of structurally diverse benzenethiols.

Borylation Directed Borylation of Indoles Using Pyrazabole Electrophiles: A One-Pot Route to C7-Borylated-Indolines

Pyrazabole (1) is a readily accessible diboron compound that can be transformed into ditopic electrophiles. In 1 (and derivatives), the B⋅⋅⋅B separation is ca. 3 Å, appropriate for one boron centre bonding to N and one to the C7 of indoles and indolines. This suitable B⋅⋅⋅B separation enables double E-H (E=N/C) functionalisation of indoles and indolines. Specifically, the activation of 1 with HNTf₂ generates an electrophile that transforms N-H indoles and indolines into N/C7-diborylated indolines, with N-H borylation directing subsequent C7-H borylation. Indole reduction to indoline occurs before C-H borylation and our studies indicate this proceeds via hydroboration-C3-protodeboronation to produce an intermediate that then undergoes C7 borylation. The borylated products can be converted in situ into C7-BPin-N-H-indolines. Overall, this represents a transient directed C-H borylation to form useful C7-BPin-indolines.

Metal-catalysed C-H bond activation and borylation

Transition metal-catalysed direct borylation of hydrocarbons via C-H bond activation has received a remarkable level of attention as a popular reaction in the synthesis of organoboron compounds owing to their synthetic versatility. While controlling the site-selectivity was one of the most challenging issues in these C-H borylation reactions, enormous efforts of several research groups proved instrumental in dealing with selectivity issues that presently reached an impressive level for both proximal and distal C-H bond borylation reactions. For example, in the case of ortho C-H bond borylation reactions, innovative methodologies have been developed either by the modification of the directing groups attached with the substrates or by creating new catalytic systems via the design of new ligand frameworks. Whereas meta and para selective C-H borylations remained a formidable challenge, numerous innovative concepts have been developed within a very short period of time by the development of new catalytic systems with the employment of various noncovalent interactions. Moreover, significant advancements have occurred for aliphatic C(sp³)-H borylations as well as enantioselective borylations. In this review article, we aim to discuss and summarize the different approaches and findings related to the development of directed proximal ortho, distal meta/para, aliphatic (racemic and enantioselective) borylation reactions since 2014. Additionally, considering the C-H borylation reaction as one of the most important mainstream reactions, various applications of this C-H borylation reaction toward the synthesis of natural products, therapeutics, and applications in materials chemistry will be summarized in the last part of this review article.

Metal-Free Boron-Mediated ortho-C-H Hydroxylation of N-Benzyl-3,4,5-tribromopyrazoles

A novel route has been reported for C-H hydroxylation of benzyl compounds directed by a 3,4,5-tribromopyrazole auxiliary via boronation/oxidation using BBr₃ and NaBO₃·4H₂O. The strategy exhibits outstanding site selectivity and affords the corresponding phenols in moderate to excellent yields under metal-free conditions. Besides, this protocol can be achieved in one pot, which is highly promising as a practical method for use in a multistep organic synthetic process.

A Desaturative Approach for Aromatic Aldehyde Synthesis via Synergistic Enamine, Photoredox and Cobalt Triple Catalysis

Aromatic aldehydes are fundamental intermediates that are widely utilised for the synthesis of important materials across the broad spectrum of chemical industries. Accessing highly substituted derivatives can often be difficult as their functionalizations are generally performed via electrophilic aromatic substitution, S_EAr. Here we provide an alternative and mechanistically distinct approach whereby aromatic aldehydes are assembled from saturated precursors via a desaturative process. This novel strategy harnesses the high‐fidelity of Diels–Alder cycloadditions to quickly construct multi‐substituted cyclohexenecarbaldehyde cores which undergo desaturation via the synergistic interplay of enamine, photoredox and cobalt triple catalysis.

Origin of the Enhanced Reactivity in the ortho C-H Borylation of Benzaldehydes with BBr3

The metal-free ortho C-H borylation of benzaldehyde derivatives using a transient imine directing group was recently developed by our group, providing an efficient strategy for the synthesis of organoboron reagents. Herein, we report on an extensive investigation of the reaction mechanism using density functional theory (DFT) calculations. Computations for the reaction pathway with various imine substrates, as well as the effect of an added base were examined, and the experimentally observed reactivity enhancement is proposed to originate from the tunability of the destabilizing strain energies that results in a reversible complexation process with BBr₃.

BBr3 -Mediated P(III)-Directed C-H Borylation of Phosphines

Transition-metal-catalyzed C-H borylation has been widely used in the preparation of organoboron compounds. Here, we developed a general protocol on metal-free P(III)-directed C-H borylation of phosphines mediated by BBr₃ , resulting in the formation of products bearing both phosphorus and boron. The development of the metal-free strategy to mimic previous metallic processes has shown low cost, superior practicality, and environmental friendliness. Density functional theory (DFT) calculations demonstrate the preferred pathway for this metal-free directed C-H borylation process.

Construction of 1,2,3-Benzodiazaborole by Electrophilic Borylation of Azobenzene and Nucleophilic Dialkylative Cyclization

1,2,3-Benzodiazaboroles can be conveniently prepared from azobenzenes by a two-step protocol involving electrophilic ortho-borylation with BBr₃ and dialkylative cyclization with the Grignard reagent. The methodology provides a diverse range of products equipped with functionalities from azobenzenes containing substituents (Me, t-Bu, F, Cl, Br, I, and OCF₃) and a series of Grignard reagents (alkyl- and arylmagnesium reagents). Moreover, this study displays the moderate aromaticity of the B-N-N-containing five-membered ring and mechanistic investigations of the cyclization reaction.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Pyrimidine-directed metal-free C-H borylation of 2-pyrimidylanilines: a useful process for tetra-coordinated triarylborane synthesis

Convenient, easily handled, laboratory friendly, robust approaches to afford synthetically important organoboron compounds are currently of great interest to researchers. Among the various available strategies, a metal-free approach would be overwhelmingly accepted, since the target boron compounds can be prepared in a metal-free state. We herein present a detailed study of the metal-free directed ortho-C–H borylation of 2-pyrimidylaniline derivatives. The approach allowed us to synthesize various boronates, which are synthetically important compounds and various four-coordinated triarylborane derivatives, which could be useful in materials science as well as Lewis-acid catalysts. This metal-free directed C–H borylation reaction proceeds smoothly without any interference by external impurities, such as inorganic salts, reactive functionalities, heterocycles and even transition metal precursors, which further enhance its importance.

We present the metal-free ortho-C–H borylation of 2-pyrimidylanilines to afford synthetically important boronic esters and tetra-coordinated triarylboranes, which could be useful in materials science as well as Lewis-acid catalysts.