Introduction

Aryl C–N bonds are widely found in pharmaceuticals, agrochemicals, and functional molecules. Transition-metal-catalyzed amination reactions such as Buchwald–Hartwig amination typically employ aryl halides or pseudohalides as aryl electrophiles, whereas classical nucleophilic aromatic substitution (S_NAr) relies on an appropriate leaving group and electronic activation of the aromatic ring.[2,3] In contrast to these substrates, ordinary phenols do not provide an aryl electrophilic system suitable for direct nucleophilic aromatic substitution by amines.

In 2024, Fier and Kim reported in the Journal of the American Chemical Society a C–N cross-coupling method between phenols and primary amines using 5,6-dichloropyrazine-2,3-dicarbonitrile (DCPDC) as a multifunctional reagent. The reaction does not require a transition metal for C–N bond formation, nor does it require prior conversion of the phenol into a conventional aryl halide or pseudohalide. The authors described the design principles as induced proximity and electronic activation of both the nucleophile and electrophile, ultimately enabling net dehydrative C–N coupling between phenols and primary amines.[1]

Through sequential S_NAr reactions, DCPDC installs the amine and phenol onto the same electron-deficient pyrazine scaffold, establishing a temporary covalent linkage. An intramolecular Smiles aryl migration then forms the target aryl C–N bond, followed by reductive cleavage to remove the pyrazine scaffold.[1] A bond-forming process that would otherwise require direct reaction between two free molecules is thereby converted into a preorganized intramolecular aryl migration.

1. Reactivity Barriers to Direct Deoxygenative C–N Bond Formation between Phenols and Amines

Primary amines are effective nucleophiles, but the aryl C–O(H) unit of an ordinary phenol is not a typical substrate for nucleophilic aromatic substitution. Classical S_NAr generally requires an aromatic ring with sufficient electron deficiency and a substituent capable of acting as a leaving group during the nucleophilic addition–elimination sequence. Ordinary phenols lack such a typical aryl leaving group, and the aromatic ring itself generally does not provide sufficient electronic activation for direct nucleophilic substitution by an amine.

Transition-metal-catalyzed aryl amination employs a different bond-forming mode. In 1994, the Hartwig and Buchwald groups independently reported early Pd-catalyzed C–N coupling reactions of aryl halides with tin amides or in situ generated aminostannanes, providing important methodological foundations for the subsequent development of Buchwald–Hartwig amination.[2,3] These reactions do not require an amine to directly displace the hydroxyl group of a phenol; instead, they employ aryl electrophiles with appropriate reactivity.

The Fier and Kim method uses phenols and primary amines as coupling partners. The initial challenge is therefore not to further increase the nucleophilicity of the free amine, but to establish a reaction system capable of supporting aryl migration.[1]

DCPDC provides a different pathway: its electron-deficient heteroaromatic ring, which readily undergoes S_NAr, is first used to connect the two reaction partners independently, after which the target C–N bond is formed within this temporary linked system.

2. Sequential S_NAr: Establishing a Temporary Covalent Linkage on DCPDC

DCPDC contains an electron-deficient pyrazine ring, two cyano groups, and two adjacent chloro substituents. The two C–Cl sites can undergo sequential nucleophilic aromatic substitution, allowing the reagent to first accept a nitrogen nucleophile and subsequently an oxygen nucleophile.

The reaction begins with the first S_NAr between a primary amine and DCPDC, generating a monoaminated chloropyrazine intermediate. Phenol is then added, and the phenoxide nucleophile generated under basic conditions displaces the remaining chloride to afford a pyrazine intermediate bearing both the amino unit and the aryloxy unit.[1] For the representative benzylamine–phenol system, the reported procedure first forms the initial S_NAr intermediate in K₃PO₄/1,4-dioxane at 50 °C, followed by addition of phenol and DMSO and heating to 100 °C for the second substitution and rearrangement stage.

These two S_NAr reactions have not yet formed the final Ar–N bond, but they alter the connectivity between the two reaction partners:

Free primary amine + free phenol

→ Aminopyrazine intermediate

→ Covalent assembly containing both the N unit and the Ar–O unit.

The amine and the aryl group to be transferred are therefore no longer part of two independent molecules, but are organized within the same pyrazine-derived intermediate.

The induced proximity described in the original study arises from this covalent assembly.[1] DCPDC therefore not only provides chemical activation but also changes the spatial organization of the reaction: the subsequent aryl migration is converted from an intermolecular bond-forming event into a reaction occurring within a single intermediate.

3. Formation of the Target C–N Bond: Smiles Aryl Migration

After completion of the two S_NAr reactions, the aryl group remains connected to the pyrazine scaffold through oxygen, and the target Ar–N bond has not yet formed.

According to the mechanism proposed by Fier and Kim, the N–H-containing pyrazine intermediate forms the corresponding nitrogen nucleophilic center under basic conditions, followed by an intramolecular Smiles rearrangement. Nitrogen attacks the ipso position of the aromatic ring attached through the aryloxy group, reorganizing the original Ar–O linkage. The aryl group migrates from oxygen to nitrogen, forming a new Ar–N bond and the rearranged pyrazine-derived intermediate.[1]

The change in connectivity can be summarized as follows:

Ar–O–pyrazine–NH–R

→ Smiles aryl migration

→ Ar–N(R)–pyrazine-derived intermediate.

The target C–N bond in this method is therefore not formed when the amine attacks DCPDC, nor when the phenol is incorporated into DCPDC, but instead arises from the subsequent intramolecular O→N aryl migration.

The Smiles rearrangement is intrinsically an intramolecular nucleophilic aromatic substitution. Previous studies have used Smiles rearrangements of aminopyrazine and aminoquinoxaline derivatives to construct N-arylated products.[4] The distinctive feature of the 2024 work is the use of DCPDC to convert this aryl-migration mode into a cross-coupling strategy between phenols and externally added primary amines.

4. Multifunctionality of DCPDC: Proximity, Electronic Activation, and Scaffold Removal

4.1 Electron-Deficient Pyrazine Supports Sequential S_NAr

The pyrazine ring of DCPDC is intrinsically electron deficient, and the two cyano groups further enhance the electron-withdrawing character of the ring system, while the two chloro groups provide the reactive sites required for sequential nucleophilic substitution.

This structural combination allows the remaining C–Cl site to react further with a phenoxide nucleophile after the first amine substitution. DCPDC can therefore sequentially incorporate two different nucleophilic components, nitrogen and oxygen, to construct the disubstituted intermediate required for the subsequent rearrangement.[1]

Sequential S_NAr reactions constitute the entry point into the entire reaction pathway. Without this ordered assembly process, the amine and aryl group would remain independent reaction partners, and the intramolecular connectivity required for the subsequent Smiles aryl migration could not be established.

4.2 Covalent Assembly Generates Induced Proximity

After the second S_NAr reaction, the nitrogen nucleophilic unit and the aryloxy group reside within the same pyrazine-derived intermediate.

This covalent preorganization reduces the dependence of intermolecular bond formation on random molecular collisions and mutual orientation, allowing nitrogen attack at the aryl ipso position to proceed through an intramolecular process.

Thus, the role of DCPDC is not limited to increasing the intrinsic reactivity of an individual reaction center; it also changes the relative positions and connectivity of the atoms involved in bond formation.

4.3 Connectivity Alters the Electronic Environments of the Nucleophile and Electrophile

The original paper also emphasizes the electronic activation of both the nucleophile and electrophile.[1]

After the amine is incorporated into the electron-deficient pyrazine, the resulting N–H unit occupies an electronic environment distinct from that of the free amine. In the mechanism proposed by the authors, this N–H group is converted under basic conditions into the nitrogen nucleophilic center that participates in the Smiles rearrangement.

Meanwhile, the phenol no longer undergoes direct attack by an amine in its free Ar–OH form. Instead, it is first converted into an aryloxy intermediate linked to the electron-deficient pyrazine. This connectivity provides an appropriate intramolecular nucleophilic substitution architecture for the subsequent aryl migration.

The reaction therefore does not activate the phenol or amine independently, but instead uses the same pyrazine scaffold to establish both spatial proximity and electronic matching.[1]

4.4 Reductive Removal of the Pyrazine Scaffold after Rearrangement

After the Smiles rearrangement forms the target Ar–N bond, the pyrazine moiety remains attached to the nitrogen-containing intermediate. The reaction subsequently employs Zn/AcOH for reductive cleavage, removing the pyrazine fragment that served in assembly and aryl migration and releasing the final arylamine product.[1]

Under the representative conditions, Zn and AcOH are added after completion of the rearrangement stage, followed by treatment at 80 °C to complete reductive cleavage.

Thus, DCPDC sequentially serves as:

a substrate for sequential nucleophilic substitution → a temporary linking scaffold → a proximity- and electronic-modulation unit → a cleavable auxiliary structure.

5. Substrate Scope and Reaction Boundaries

5.1 Phenols Do Not Require Prior Strong Electronic Activation Typical of Classical S_NAr

The method can employ phenols bearing a variety of substitution patterns as aryl sources. Unlike classical S_NAr, the aryl electrophilic component does not need to be prefunctionalized with typical strongly electron-withdrawing groups.[1] The method therefore reduces the requirement for prior electronic activation of the phenolic aromatic ring.

This does not mean that Smiles aryl migration is completely insensitive to the electronic properties of the aromatic ring. Through induced proximity and activation of the reacting units, DCPDC allows some phenols that are not strongly preactivated in the manner required for classical S_NAr to enter this reaction pathway.

5.2 Nitrogen Coupling Partners Are Primarily Primary Amines

Fier and Kim define the method as a cross-coupling reaction between phenols and primary amines.[1] The substrate scope includes aliphatic primary amines as well as aromatic primary amines such as anilines, enabling formation of the corresponding N-alkyl arylamines and diarylamines.

Following the first S_NAr reaction, a primary amine forms an N–H-containing aminopyrazine intermediate, which subsequently participates in the deprotonation and Smiles rearrangement proposed by the authors. The primary-amine substrate scope is therefore directly related to the bond-forming pathway employed in this reaction.

5.3 Aryl Halides and Coordinating Functional Groups Show Good Compatibility

The system exhibits broad functional-group tolerance and can retain aryl halides and functional groups capable of coordination.[1] In some transition-metal-catalyzed coupling reactions, these structural features may compete through oxidative addition, metal coordination, or catalyst deactivation. Because C–N bond formation in the DCPDC system does not rely on a transition-metal catalytic cycle, it provides a distinct chemoselectivity profile.

The original study also applied the reaction to late-stage functionalization of complex substrates, demonstrating that the sequential S_NAr–Smiles rearrangement can operate in multifunctional molecules.[1]

5.4 Direct Conversion of Phenols into Primary Arylamines

In addition to cross-coupling phenols with externally added primary amines, the authors extended the design to deoxyamination of phenols, in which the phenolic hydroxyl group is directly replaced with NH₂ to afford primary arylamines.

This variant uses an aminopyrazine intermediate as the nitrogen source and employs slow addition of this intermediate during the reaction. Compared with conventional phenol–primary amine cross-coupling, this deoxyamination reaction has a greater requirement for electronic activation of the aryl group, and more electron-deficient phenols are more favorable for formation of the desired products.

Thus, the same DCPDC–Smiles design can be used to form both Ar–NHR and Ar–NH₂ structures, although the applicable substrate windows of the two reactions are not identical.

6. From Enhancing Reactivity to Reconstructing the Bond-Forming Pathway

6.1 Converting Intermolecular Bond Formation into Intramolecular Migration

The design value of the DCPDC system lies in changing the pathway through which the target bond is formed, rather than simply increasing the intrinsic reactivity of free phenol or free amine. Sequential S_NAr reactions temporarily install the two reaction partners onto the same pyrazine scaffold, after which the Ar–N bond is formed through an intramolecular Smiles aryl migration.

This reaction illustrates a design strategy in which a removable temporary covalent linkage is used to reconstruct the spatial and electronic relationships between reaction partners, converting an initially intermolecular bond-forming process into an intramolecular aryl migration.

6.2 Complementarity with Conventional C–N Coupling

The method does not require a transition metal for C–N bond formation, does not require prior conversion of the phenol into a conventional aryl halide or pseudohalide, and, as explicitly noted in the original study, does not require rigorous exclusion of air or moisture.[1]

At the same time, DCPDC is used stoichiometrically, and Zn/AcOH is subsequently required to cleave the pyrazine scaffold. The method can therefore be regarded as a complementary pathway to conventional transition-metal coupling and classical S_NAr, with its practical application depending on substrate structure, functional-group compatibility, sequence of operations, and the specific synthetic objective.

From a methodological perspective, the principal significance of this work is that it demonstrates an alternative approach to challenging bond formation: when direct intermolecular coupling is limited, temporary covalent linkage can be used to reconstruct the spatial and electronic relationships between reaction partners, followed by intramolecular migration to complete formation of the target bond.

7. Representative Chemicals and Research Applications Related to DCPDC-Mediated Phenol–Amine C–N Bond Formation

Table 1. Core Pyrazine Reagents and Chemicals Related to the Reaction Conditions

Table 2. Phenols and Nitrogen-Containing Heteroaryl Oxygen-Containing Substrates

Table 3. Aliphatic and Benzyl Primary Amine Coupling Partners

Table 4. Aromatic Primary Amine Coupling Partners

Note: The products listed above are representative Aladdin products relevant to scientific research. Some are representative chemicals related to this reaction type and are not intended to indicate that all were individually evaluated as substrates in the original study by Fier and Kim. Specific applications should be determined according to product specifications, batch COAs, and the intended reaction or evaluation system. Additional information on product specifications, grades, and COAs can be retrieved from the Aladdin website using the product name, CAS number, or catalog number.

References

[1] Fier, P. S.; Kim, S. Transition-Metal-Free C–N Cross-Coupling Enabled by a Multifunctional Reagent. J. Am. Chem. Soc. 2024, 146, 6476–6480. DOI: 10.1021/jacs.4c00871.

[2] Paul, F.; Patt, J.; Hartwig, J. F. Palladium-Catalyzed Formation of Carbon-Nitrogen Bonds. Reaction Intermediates and Catalyst Improvements in the Hetero Cross-Coupling of Aryl Halides and Tin Amides. J. Am. Chem. Soc. 1994, 116, 5969–5970. DOI: 10.1021/ja00092a058.

[3] Guram, A. S.; Buchwald, S. L. Palladium-Catalyzed Aromatic Aminations with in situ Generated Aminostannanes. J. Am. Chem. Soc. 1994, 116, 7901–7902. DOI: 10.1021/ja00096a059.

[4] Bi, F. C.; Aspnes, G. E.; Guzman-Perez, A.; Walker, D. P. Novel Syntheses of 3-Anilino-pyrazin-2(1H)-ones and 3-Anilino-quinoxalin-2-(1H)-ones via Microwave-Mediated Smiles Rearrangement. Tetrahedron Lett. 2008, 49, 1832–1835. DOI: 10.1016/j.tetlet.2008.01.056.

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