Elusive metal-free primary amination of arylboronic acids: synthetic studies and mechanism by density functional theory

Stereospecific Enzymatic Conversion of Boronic Acids to Amines

Boronic acids and esters are highly regarded for their safety, unique reactivity, and versatility in synthesizing a wide range of small molecules, bioconjugates, and materials. They are not exploited in biocatalytic synthesis, however, because enzymes that can make, break, or modify carbon-boron bonds are rare. We wish to combine the advantages of boronic acids and esters for molecular assembly with biocatalysis, which offers the potential for unsurpassed selectivity and efficiency. Here, we introduce an engineered protoglobin nitrene transferase that catalyzes the new-to-nature amination of boronic acids using hydroxylamine. Initially targeting aryl boronic acids, we show that the engineered enzyme can produce a wide array of anilines with high yields and total turnover numbers (up to 99% yield and >4000 TTN), with water and boric acid as the only byproducts. We also demonstrate that the enzyme is effective with bench-stable boronic esters, which hydrolyze in situ to their corresponding boronic acids. Exploring the enzyme's capacity for enantioselective catalysis, we found that a racemic alkyl boronic ester affords an enantioenriched alkyl amine, a transformation not achieved with chemocatalysts. The formation of an exclusively unrearranged product during the amination of a boronic ester radical clock and the reaction's stereospecificity support a two-electron process akin to a 1,2-metallate shift mechanism. The developed transformation enables new biocatalytic routes for synthesizing chiral amines.

Amino-λ3 -iodane-Enabled Electrophilic Amination of Arylboronic Acid Derivatives

In this report, we describe the use of amino-λ3 -iodanes in the electrophilic amination of arylboronic acids and boronates. Iodine(III) reagents with transferable amino groups, including one with an NH2 group, were synthesized and used in the amination, allowing the synthesis of a wide range of primary and secondary (hetero)arylamines. Mechanistic studies by DFT calculations indicate that the reaction proceeds through an electrophilic amination process from a tetravalent borate complex with a B-N dative bond.

Metal-Free Oxidative Formation of Aryl Esters by Catalytic Coupling of Acyl and Sulfonyl Chlorides with Arylboronic Acids

A practical and efficient synthesis of aryl esters was developed through metal-free oxidation. This reaction employs stable and readily available acyl or sulfonyl chlorides and arylboronic acids as the starting materials and proceeds under mild reaction conditions without additional precious metal catalysts. This new strategy exhibits broad substrate tolerance and operational simplicity and gives diverse aryl esters in moderate to high yields.

Ammonia surrogates in the synthesis of primary amines

Primary amines are derivatives of ammonia in which one hydrogen atom is replaced by an alkyl or aryl group. Ammonia serves as the primary nitrogen source in amination reactions, and its utilization in solution or as a pure gas has witnessed notable advancements. However, the use of gaseous ammonia remains problematic in academic laboratory settings, while employing aqueous ammonia poses challenges in highly water-sensitive transformations. Consequently, the search for alternative sources of ammonia has garnered considerable attention among the organic chemistry community. This comprehensive literature review focuses on the use of ammonia surrogates in amination reactions, irrespective of the resulting intermediate. The review emphasizes the formation of the C-N bond and underscores the importance of generating intermediate products that can be readily transformed into primary amines through well-established reactions.

Programmable Amine Synthesis via Iterative Boron Homologation

The value of Matteson-type reactions has been increasingly recognized for developing automated organic synthesis. However, the typical Matteson reactions almost exclusively focus on homologation of carbon units. Here, we report the detailed development of sequential insertion of nitrogen and carbon atoms into boronate C-B bonds, which provides a modular and iterative approach to access functionalized tertiary amines. A new class of nitrenoid reagents is uncovered to allow direct formation of aminoboranes from aryl or alkyl boronates via N-insertion. The one-pot N-insertion followed by controlled mono- or double-carbenoid insertion has been realized with widely available aryl boronates. The resulting aminoalkyl boronate products can undergo further homologation and various other transformations. Preliminary success on homologation of N,N-dialkylaminoboranes and sequential N- and C-insertions with alkyl boronates have also been achieved. To broaden the synthetic utility, selective removal of a benzyl or aryl substituent permits access to secondary or primary amine products. The application of this method has been demonstrated in the modular synthesis of bioactive compounds and the programmable construction of diamines and aminoethers. A plausible reaction mechanism, supported by preliminary NMR (nuclear magnetic resonance) and computational studies, is also proposed.

From Cycloalkanols to Heterocycles via Nitrogen Insertion

A variety of cyclic alcohols are found to undergo nitrogen insertion by subjection to O-mesitylsulfonylhydroxylamine. Critical to a successful process is the use of fluorinated alcoholic solvents, which ensures sufficient substrate activation to allow engagement with the ambiphilic aminating agent. This transition-metal-free nitrogen insertion provides access to a variety of medicinally relevant heterocycles such as pyrrolidenes, quinolines, and benzazepines (24 examples). Furthermore, combination with a photochemical Norrish-Yang-type cyclization allows an unprecedented access to indoles from ortho-substituted acetophenones.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.

Chemoselective Primary Amination of Aryl Boronic Acids by PIII/PV═O-Catalysis: Synthetic Capture of the Transient Nef Intermediate HNO

A catalytic approach to intercept the transient HNO for a chemoselective primary amination of arylboronic acids is reported. A phosphetane-based catalyst operating within P^III/P^V═O redox cycling is shown to capture HNO, generated in situ by Nef decomposition of 2-nitropropane, to selectively install the primary amino group at aryl C_sp2 centers. The method furnishes versatile primary arylamines from arylboronic acid substrates with the preservation of otherwise reactive functional groups.

Site-selective amination and/or nitrilation via metal-free C(sp2)-C(sp3) cleavage of benzylic and allylic alcohols

Benzylic/allylic alcohols are converted via site-selective C(sp²)-C(sp³) cleavage to value-added nitrogenous motifs, viz., anilines and/or nitriles as well as N-heterocycles, utilizing commercial hydroxylamine-O-sulfonic acid (HOSA) and Et₃N in an operationally simple, one-pot process. Notably, cyclic benzylic/allylic alcohols undergo bis-functionalization with attendant increases in architectural complexity and step-economy.

Hydroxylamine-mediated C-C amination via an aza-hock rearrangement

Despite the widespread use of anilines, synthetic challenges to these targets still exist. Selectivity is often an issue, when using the traditional nitration-reduction sequence or more modern approaches, including arene C-H aminations catalyzed by transition metals, photosensitizers, or electrodes. Accordingly, there is still a need for general methods to rapidly, directly access specific isomers of substituted anilines. Here, we report a simple route towards the synthesis of such motifs starting from benzyl alcohols, which are converted to anilines by the use of arylsulfonyl hydroxylamines, via an aza-Hock rearrangement. Good to excellent yields are observed. The method is applicable to various benzyl alcohol surrogates (such as ethers, esters, and halides) as well as simple alkylarenes. Functionalizations of pharmaceutically relevant structures are feasible under the reaction conditions. Over ten amination reagents can be used, which facilitates the rapid assembly of a vast set of compounds.

Bicyclohexene-peri-naphthalenes: Scalable Synthesis, Diverse Functionalization, Efficient Polymerization, and Facile Mechanoactivation of Their Polymers

Pursuing polymers that can transform from a nonconjugated to a conjugated state under mechanical stress to significantly change their properties, we developed a new generation of ladder-type mechanophore monomers, bicyclo[2.2.0]hex-5-ene-peri-naphthalene (BCH-Naph), that can be directly and efficiently polymerized by ring-opening metathesis polymerization (ROMP). BCH-Naphs can be synthesized in multigram quantities and functionalized with a wide range of electron-rich and electron-poor substituents, allowing tuning of the optoelectronic and physical properties of mechanically generated conjugated polymers. Efficient ROMP of BCH-Naphs yielded ultrahigh molecular weight polymechanophores with controlled MWs and low dispersity. The resulting poly(BCH-Naph)s can be mechanically activated into conjugated polymers using ultrasonication, grinding, and even simple stirring of the dilute solutions, leading to changes in absorption and fluorescence. Poly(BCH-Naph)s represent an attractive polymechanophore system to explore multifaceted mechanical response in solution and solid states, owing to the synthetic scalability, functional diversity, efficient polymerization, and facile mechanoactivation.

An Improved PIII/PV═O-Catalyzed Reductive C-N Coupling of Nitroaromatics and Boronic Acids by Mechanistic Differentiation of Rate- and Product-Determining Steps

Experimental, spectroscopic, and computational studies are reported that provide an evidence-based mechanistic description of an intermolecular reductive C–N coupling of nitroarenes and arylboronic acids catalyzed by a redox-active main-group catalyst (1,2,2,3,4,4-hexamethylphosphetane P-oxide, i.e., 1·[O]). The central observations include the following: (1) catalytic reduction of 1·[O] to P^III phosphetane 1 is kinetically fast under conditions of catalysis; (2) phosphetane 1 represents the catalytic resting state as observed by ³¹P NMR spectroscopy; (3) there are no long-lived nitroarene partial-reduction intermediates observable by ¹⁵N NMR spectroscopy; (4) the reaction is sensitive to solvent dielectric, performing best in moderately polar solvents (viz. cyclopentylmethyl ether); and (5) the reaction is largely insensitive with respect to common hydrosilane reductants. On the basis of the foregoing studies, new modified catalytic conditions are described that expand the reaction scope and provide for mild temperatures (T ≥ 60 °C), low catalyst loadings (≥2 mol%), and innocuous terminal reductants (polymethylhydrosiloxane). DFT calculations define a two-stage deoxygenation sequence for the reductive C–N coupling. The initial deoxygenation involves a rate-determining step that consists of a (3+1) cheletropic addition between the nitroarene substrate and phosphetane 1; energy decomposition techniques highlight the biphilic character of the phosphetane in this step. Although kinetically invisible, the second deoxygenation stage is implicated as the critical C–N product-forming event, in which a postulated oxazaphosphirane intermediate is diverted from arylnitrene dissociation toward heterolytic ring opening with the arylboronic acid; the resulting dipolar intermediate evolves by antiperiplanar 1,2-migration of the organoboron residue to nitrogen, resulting in displacement of 1·[O] and formation of the target C–N coupling product upon in situ hydrolysis. The method thus described constitutes a mechanistically well-defined and operationally robust main-group complement to the current workhorse transition-metal-based methods for catalytic intermolecular C–N coupling.

Aminoazanium of DABCO: An Amination Reagent for Alkyl and Aryl Pinacol Boronates

The aminoazanium of DABCO (H₂ N-DABCO) has been developed as a general and practical amination reagent for the direct amination of alkyl and aryl pinacol boronates. This compound is stable and practical for use as a reagent. Various primary, secondary. and tertiary alkyl-Bpin and aryl-Bpin substrates were aminated to give the corresponding amine derivatives. The amination is stereospecific. The anti-Markovnikov hydroamination of olefins was easily achieved by catalytic hydroboration with HBpin and in subsequent situ amination using H₂ N-DABCO. Moreover, the combination of 1,2-diboration of olefins, using B₂ pin₂ , with this amination process achieved the unprecedented 1,2-diamination of olefins. The amination protocol was also successfully extended to aryl pinacol boronates.

How to make C–N bonds using boronic acids and their derivatives without transition metals

No need for transition-metal catalysis in amination, amidation, nitration or nitrosation reactions with boron derivatives as reagents.

Redox-neutral ortho-C-H amination of pinacol arylborates via palladium(ii)/norbornene catalysis for aniline synthesis

A palladium(ii)/norbornene cooperative catalysis enabled redox-neutral ortho-C-H amination of pinacol aryl- or heteroarylborates for the synthesis of structurally diverse anilines is reported. This method is scalable, robust (tolerance of air and moisture), phosphine ligand-free, and compatible with a wide range of functionalities. These practical features make this reaction amenable for industry. A plethora of synthetically very useful halogenated anilines, which often cannot be prepared via other transition-metal-catalyzed aminations, are readily produced using this method. Particularly, the orthogonal reactivity between pinacol arylborates and aryl iodides is demonstrated. Preliminary deuterium-labeling studies reveal a redox-neutral ipso-protonation mechanism of this process, which will surely inspire the future development of this field. Overall, the exceptionally broad scope (47 examples) and reliability of this procedure, together with the wide availability of pinacol arylborates, make this chemistry a valuable addition to the existing methods for aniline synthesis.

Cross-Coupling of Heteroatomic Electrophiles

At the advent of cross-coupling chemistry, carbon electrophiles based on halides or pseudohalides were the only suitable electrophilic coupling partners. Almost two decades passed before the first cross-coupling reaction of heteroatom-based electrophiles was reported. Early work by Murai and Tanaka initiated investigations into silicon electrophiles. Narasaka and Johnson pioneered the way in the use of nitrogen electrophiles, while Suginome began the exploration of boron electrophiles. The chemistry reviewed within provides perspective on the use of heteroatomic electrophiles, specifically silicon-, nitrogen-, boron-, oxygen-, and phosphorus-based electrophiles in transition-metal catalyzed cross-coupling. For the purposes of this review, a loose definition of cross-coupling is utilized; all reactions minimally proceed via an oxidative addition event. Although not cross-coupling in a traditional sense, we have also included catalyzed reactions that join a heteroatomic electrophile with an in situ generated nucleophile. However, for brevity, those involving hydroamination or C-H activation as a key step are largely excluded. This work includes primary references published up to and including October 2018.

Direct Observation and Analysis of the Halo-Amino-Nitro Alkane Functional Group

Conventional amide synthesis is a mainstay in discipline-spanning applications, and it is a reaction type that historically developed as a singular paradigm when considering the carbon-nitrogen bond-forming step. Umpolung amide synthesis (UmAS) exploits the unique properties of an α-halo nitroalkane in its reaction with an amine to produce an amide. The "umpolung" moniker reflects its paradigm-breaking C-N bond formation on the basis of evidence that the nucleophilic nitronate carbon and electrophilic nitrogen engage to form a tetrahedral intermediate (TI) that is an unprecedented functional group, a 1,1,1-halo-amino-nitro alkane (HANA). Studies probing HANA transience have failed to capture this (presumably) highly reactive intermediate. We report here the direct observation of a HANA, its conversion thermally to an amide functionality, and quantitative analysis of this process using computational techniques. These findings validate the HANA as a functional group common to UmAS and diverted UmAS, opening the door to its targeted use and creative manipulation.

Deacetylative Amination of Acetyl Arenes and Alkanes with C-C Bond Cleavage

The Brønsted acid-catalyzed synthesis of primary amines from acetyl arenes and alkanes with C-C bond cleavage is described. Although the conversion from an acetyl group to amine has traditionally required multiple steps, the method described herein, which uses an oxime reagent as an amino group source, achieves the transformation directly via domino transoximation/Beckmann rearrangement/Pinner reaction. The method was also applied to the synthesis of γ-aminobutyric acids, such as baclophen and rolipram.

Reaction of Nitrogen-Radicals with Organometallics Under Ni-Catalysis: N-Arylations and Amino-Functionalization Cascades

Herein, we report a strategy for the generation of nitrogen-radicals by ground-state single electron transfer with organyl-NiI species. Depending on the philicity of the N-radical, two types of processes have been developed. In the case of nucleophilic aminyl radicals direct N-arylation with aryl organozinc, organoboron, and organosilicon reagents was achieved. In the case of electrophilic amidyl radicals, cascade processes involving intramolecular cyclization, followed by reaction with both aryl and alkyl organometallics have been developed. The N-cyclization-alkylation cascade introduces a novel retrosynthetic disconnection for the assembly of substituted lactams and pyrrolidines with its potential demonstrated in the short total synthesis of four venom alkaloids.

Theoretical Coupling and Stability of Boronic Acid Adducts with Catecholamines

Background:

Catecholamines combined with boric/boronic acids are attractive chemical agents in drug design because some of their adducts have shown interesting biological activity. Scant information exists about their stability.

Objective:

The aim of the present theoretical study was to explore the role of boron in molecules that combine catecholamines and boric/boronic acids, with a particular interest in examining stability.

Method:

The methodology was based on the US GAMESS program using DFT with the B3LYP exchange-correlation functional and the 6-31G (d,p) split-valence basis set.

Results:

According to the current findings, the boron-containing compounds (BCCs) exhibit weaker bonding to the hydroxyls on the ethylamine moiety than to those in the aromatic ring. The strongest binding site of a hydroxyl group was often found to be in meta-position (relative to ethylamine moiety) for boron-free compounds and in para-position for BCCs. Nonetheless, the methyl substituent in the amino group was able to induce changes in this pattern. We analyzed feasible boronsubstituted structures and assessed the relative strength of the respective C-B bonds, which allowed for the identification of the favorable points for reaction and stability.

Conclusion:

It is feasible to form adducts by bonding on the amine and catechol sides of catecholamines. The presence of boron stabilizes the adducts in para-position. Since some of these BCCs are promising therapeutic agents, understanding the mechanisms of reaction is relevant for drug design.

Copper mediated C-H amination with oximes: en route to primary anilines

Here we report an efficient Cu(i)-mediated C-H amination reaction with oximes as amino donors to introduce NH2 groups directly. Various strongly coordinating heterocycles including quinoline, pyrimidine, pyrazine, pyrazole and triazole were tolerated well. The potential utility was further demonstrated in a late-stage modification of telmisartan (an antagonist for the angiotensin II receptor).

A Protocol for Direct Stereospecific Amination of Primary, Secondary, and Tertiary Alkylboronic Esters

The direct, stereospecific amination of alkylboronic and borinic esters can be conducted by treatment of the organoboron compound with methoxyamine and potassium tert-butoxide. In addition to being stereospecific, this process also enables the direct amination of tertiary boronic esters in an efficient fashion.

Direct Reductive N-Functionalization of Aliphatic Nitro Compounds

The first general protocol for the direct reductive N-functionalization of aliphatic nitro compounds is presented. The nitro group is partially reduced to a nitrenoid, with a mild and readily available combination of B₂ pin₂ and zinc organyls. Thereby, the formation of an unstable nitroso intermediate is avoided, which has so far severely limited reductive transformations of aliphatic nitro compounds. The reaction is concluded by an electrophilic amination of zinc organyls.

Rh-Catalyzed aminative dearomatization of 2-naphthols

β-Naphthalenones bearing a naked α-amino group could be rapidly constructed through a Rh-catalyzed aminative dearomatization reaction of 2-naphthols with O-(2,4-dinitrophenyl)hydroxylamine.

Iridium catalyzed fragmentation/cyclization of N-butynyl 4,4-dimethylisoxazolidine-3,5-diones: a unique access to multiply substituted pyrroles

A novel protocol for the synthesis of multiply substituted pyrroles has been developed through the iridium catalyzed fragmentation/cyclization of N-butynyl 4,4-dimethylisoxazolidine-3,5-diones in hot ethanol.

O-Substituted hydroxyl amine reagents: an overview of recent synthetic advances

Reagents derived from oxygen-substituted hydroxylamine facilitate stereo- and regioselective C–N, N–N, O–N, and S–N bond-formation reactions and intra-molecular cyclizations without any expensive metal catalysts. These remarkable transformations are discussed in this review.

Metal free direct C(sp2)–H arylaminations using nitrosoarenes to 2-hydroxy-di(het)aryl amines as multifunctional Aβ-aggregation modulators

A direct C(sp²)–H arylamination of 2-hydroxyarenes using nitrosoarenes was achieved under metal free conditions without the aid of additional reagents/steps for N–O bond cleavage.

t-BuOK-Mediated Oxidative Dehydrogenative C(sp3)-H Arylation of 2-Alkylazaarenes with Nitroarenes

The first transition-metal free and regioselective C(sp³)-H arylation of 2-alkylazaarenes with nitroarenes has been achieved via t-BuOK-mediated dehydrogenative C(sp³)-C(sp²) coupling. This reaction provides an efficient access to the biologically important and synthetically useful 2-benzyl-substituted azaarenes under mild conditions without the need of prefunctionalization of 2-alkylazaarenes or using the specialized arylating agents.

Practical Singly and Doubly Electrophilic Aminating Agents: A New, More Sustainable Platform for Carbon-Nitrogen Bond Formation

Given the importance of amines in a large number of biologically active natural products, active pharmaceutical ingredients, agrochemicals, and functional materials, the development of efficient C-N bond-forming methods with wide substrate scope continues to be at the frontier of research in synthetic organic chemistry. Here, we present a general and fundamentally new synthetic approach for the direct, transition-metal-free preparation of symmetrical and unsymmetrical diaryl-, arylalkyl-, and dialkylamines that relies on the facile single or double addition of readily available C-nucleophiles to the nitrogen atom of bench-stable electrophilic aminating agents. Practical single and double polarity reversal (i.e., umpolung) of the nitrogen atom is achieved using sterically and electronically tunable ketomalonate-derived imines and oximes. Overall, this novel approach represents an operationally simple, scalable, and environmentally friendly alternative to transition-metal-catalyzed C-N cross-coupling methods that are currently used to access structurally diverse secondary amines.