Decarboxylative Amination: Diazirines as Single and Double Electrophilic Nitrogen Transfer Reagents

Decarboxylative stereoretentive C-N coupling by harnessing aminating reagent

In recent decades, strategies involving transition-metal catalyzed carbon-carbon or carbon-heteroatom bond coupling have emerged as potent synthetic tools for constructing intricate molecular architectures. Among these, decarboxylative carbon-nitrogen bond formation using abundant carboxylic acids or their derivatives has garnered notable attention for accessing alkyl- or arylamines, one of key pharmacophores. While several decarboxylative amination methods have been developed, the involvement of a common carboradical intermediate currently poses challenges in achieving stereospecific transformation toward chiral alkylamines. Herein, we present a base-mediated, stereoretentive decarboxylative amidation by harnessing 1,4,2-dioxazol-5-one as a reactive and robust amidating reagent under transition-metal-free ambient conditions, encompassing all types of primary, secondary and tertiary carboxylic acids, thereby providing access to the important pharmacophore, α-chiral amines. This method exhibits high functional group tolerance, convenient scalability, and ease of applicability for 15N-isotope labeling, thus accentuating its synthetic utilities. Experimental and computational mechanistic investigations reveal a sequence of elementary steps: i) nucleophilic addition of carboxylate to dioxazolone, ii) rearrangement to form a dicarbonyl N-hydroxy intermediate, iii) conversion to hydroxamate, followed by a Lossen-type rearrangement, and finally, iv) reaction of the in situ generated isocyanate with carboxylate leading to C-N bond formation in a stereoretentive manner.

Electrochemical Ni-Catalyzed Decarboxylative C(sp3 )-N Cross-Electrophile Coupling

A new electrochemical transformation is presented that enables chemists to couple simple alkyl carboxylic acid derivatives with an electrophilic amine reagent to construct C(sp3 )-N bond. The success of this reaction hinges on the merging of cooperative electrochemical reduction with nickel catalysis. The chemistry exhibits a high degree of practicality, showcasing its wide applicability with 1°, 2°, 3° carboxylic acids and remarkable compatibility with diverse functional groups, even in the realm of late-stage functionalization. Furthermore, extensive mechanistic studies have unveiled the engagement of alkyl radicals and iminyl radicals; and elucidated the multifaceted roles played by i Pr2 O, Ni catalyst, and electricity.

Access to pyrrolines and fused diaziridines by selective radical addition to homoallylic diazirines

Pyrroline derivatives are common in bioactive natural products and therapeutic agents. We report here a synthesis of pyrrolines and fused diaziridines by divergent radical cyclization of homoallylic diazirines, which can serve as an internal radical trap and a nitrogen source. This reaction proceeds by selective radical addition to C[double bond, length as m-dash]C or N[double bond, length as m-dash]N bonds followed by intramolecular cyclization. Frontier molecular orbital analysis provides a deep insight into the origin of the selectivity. The reaction demonstrates a new cyclization mode, broad functional group compatibility and high product diversity, and reveals a much broader chemical space for diazirine studies.

Nickel-Catalyzed Reductive Arylation of Redox Active Esters for the Synthesis of α-Aryl Nitriles: Investigation of a Chlorosilane Additive

A nickel-catalyzed reductive cross-coupling of redox active N-hydroxyphthalimide (NHP) esters and iodoarenes for the synthesis of α-aryl nitriles is described. The NHP ester substrate is derived from cyanoacetic acid, which allows for a modular synthesis of substituted α-aryl nitriles, an important scaffold in the pharmaceutical sciences. The reaction exhibits a broad scope, and many functional groups are compatible under the reaction conditions, including complex highly functionalized medicinal agents. Mechanistic studies reveal that reduction and decarboxylation of the NHP ester to the reactive radical intermediate are accomplished by a combination of a chlorosilane additive and Zn dust. We demonstrate that stoichiometric chlorosilane is essential for product formation and that chlorosilane plays a role beyond activation of the metal reductant.

Synthesis of Indoles Using the Electrophilic Potential of Diazirines

The electrophilic potential of diazirines has been utilized to obtain N-substituted diaziridines that are directly hydrolyzed to produce monosubstituted hydrazines. The hydrazines can undergo the Fisher process with enolizable carbonyls to yield multiple indole derivatives in moderate to high yields. The N-metalated diaziridine intermediates can undergo isomerization prior to electrophilic substitution, to form N,N-disubstituted hydrazones. The latter react with enolizable carbonyls to produce N-protected indole derivatives in a single step. This protocol was used to efficiently synthesize indomethacin, an anti-inflammatory drug.

Decarboxylation of β-boryl NHPI esters enables radical 1,2-boron shift for the assembly of versatile organoborons

In recent years, numerous 1,2-R shift (R = aliphatic or aryl) based on tetracoordinate boron species have been well investigated. In the contrary, the corresponding radical migrations, especially 1,2-boryl radical shift for the construction of organoborons is still in its infancy. Given the paucity and significance of such strategies in boron chemistry, it is urgent to develop other efficient and alternative synthetic protocols to enrich these underdeveloped radical 1,2-boron migrations, before their fundamental potential applications could be fully explored at will. Herein, we have demonstrated a visible-light-induced photoredox neutral decarboxylative radical cross-coupling reaction, which undergoes a radical 1,2-boron shift to give a translocated C-radical for further capture of versatile radical acceptors. The mild reaction conditions, good functional-group tolerance, and broad β-boryl NHPI esters scope as well as versatile radical acceptors make this protocol applicable in modification of bioactive molecules. It can be expected that this methodology will be a very useful tool and an alternative strategy for the construction of primary organoborons via a novel radical 1,2-boron shift mode.

Nickel-Catalyzed Amination of Alkyl Electrophiles

Bimolecular nucleophilic substitution SN2 is the earliest and most important means of amination of alkyl electrophiles; its practical utilization is largely limited to primary or activated substrates. Furthermore, a persistent challenge lies in establishing C(sp3)-N bonds from alkyl substrates in cross-coupling chemistry using palladium and nickel catalysts. Therefore, the methods of constructing C(sp3)-N bonds remain rare from alkyl electrophiles. The existing routes are limited to copper catalysis and photoredox catalysis. Here, we demonstrate an alternative amination strategy for rapid construction of C(sp3)-N bonds from accessible alkyl electrophiles, which were used as radical precursors under nickel catalysis by Ni (III) species reductive eliminations in high efficiency.

Radical Decarboxylative Carbon-Nitrogen Bond Formation

The carbon-nitrogen bond is one of the most prevalent chemical bonds in natural and artificial molecules, as many naturally existing organic molecules, pharmaceuticals, agrochemicals, and functional materials contain at least one nitrogen atom. Radical decarboxylative carbon-nitrogen bond formation from readily available carboxylic acids and their derivatives has emerged as an attractive and valuable tool in modern synthetic chemistry. The promising achievements in this research topic have been demonstrated via utilizing this strategy in the synthesis of complex natural products. In this review, we will cover carbon-nitrogen bond formation via radical decarboxylation of carboxylic acids, Barton esters, MPDOC esters, N-hydroxyphthalimide esters (NHP esters), oxime esters, aryliodine(III) dicarboxylates, and others, respectively. This review aims to bring readers a comprehensive survey of the development in this rapidly expanding field. We hope that this review will emphasize the knowledge, highlight the proposed mechanisms, and further disclose the fascinating features in modern synthetic applications.

Halogen-atom and group transfer reactivity enabled by hydrogen tunneling

The generation of carbon radicals by halogen-atom and group transfer reactions is generally achieved using tin and silicon reagents that maximize the interplay of enthalpic (thermodynamic) and polar (kinetic) effects. In this work, we demonstrate a distinct reactivity mode enabled by quantum mechanical tunneling that uses the cyclohexadiene derivative γ-terpinene as the abstractor under mild photochemical conditions. This protocol activates alkyl and aryl halides as well as several alcohol and thiol derivatives. Experimental and computational studies unveiled a noncanonical pathway whereby a cyclohexadienyl radical undergoes concerted aromatization and halogen-atom or group abstraction through the reactivity of an effective H atom. This activation mechanism is seemingly thermodynamically and kinetically unfavorable but is rendered feasible through quantum tunneling.

Reactions of Frustrated Lewis Pairs with Chloro‐Diazirines: Cleavage of N=N Double Bonds

The reactions of FLPs with diazomethanes leads to the rapid loss of N₂. In contrast, in this work, we reported reactions of phosphine/borane FLPs with chlorodiazirines which led to the reduction of the N=N double bond, affording linked phosphinimide/amidoborate zwitterions of the general form R₃PNC(Ar)NR′BX(C₆F₅)₂. A detailed DFT mechanistic study showed that these reactions proceed via FLP addition to the N=N bond, followed by subsequent group transfer reactions to nitrogen and capture of the halide anion.

Exploration of inhibitors of the bacterial LexA repressor-protease

Resistant and tolerant bacterial infections lead to billions in healthcare costs and cause hundreds of thousands of deaths each year. The bulk of current antibiotic research efforts focus on molecules which, although novel, are not immune from acquired resistance and seldomly affect tolerant populations. The bacterial SOS response has been implicated in several resistance and tolerance mechanisms, making it an attractive antibiotic target. Using small molecule inhibitors targeting a key step in the deployment of the SOS response, our approach focused on preventing the deployment of mechanisms such as biofilm formation, horizontal gene transfer, and error-prone DNA repair. Herein we report the synthesis and testing of analogs of a triazole-containing tricyclic inhibitor of LexA proteolysis, the key event in the SOS response. Our results hint that our inhibitor's may function by adopting a β-hairpin conformation, reminiscent of the native cleavage loop of LexA.

Synthesis of α-Aryl Secondary Amides via Nickel-Catalyzed Reductive Coupling of Redox-Active Esters

The transition-metal-catalyzed α-arylation of secondary amides remains a synthetic challenge due to the presence of a free N-H bond. We report a strategy to synthesize secondary α-aryl amides via a Ni-catalyzed reductive arylation of redox-active N-hydroxyphthalimide (NHP) esters of malonic acid half amides. This transformation proceeds under mild conditions and displays excellent chemoselectivity for amide α-arylation in the presence of other enolizable carbonyls. The NHP ester substrates are readily prepared from Meldrum's acid.

Emerging Applications of Aryl Trifluoromethyl Diazoalkanes and Diazirines in Synthetic Transformations

Aryl trifluoromethyl diazoalkanes and diazirines have become unique as reactants in synthetic methodology. As privileged compounds containing CF₃ groups and ease of synthetic access, aryl trifluoromethyl diazoalkanes and diazirines have been highlighted for their versatility in applications toward a wide range of synthetic transformations. This Perspective highlights the synthetic applications of these reactants as precursors of stabilized metal carbenes, i.e., donor-acceptor-substituted ones.

Photodecarboxylative Amination of Redox-Active Esters with Diazirines

Diazirines have been recently demonstrated to serve as electrophilic amination reagents that afford diaziridines, versatile heterocycles that are readily transformed into amines, hydrazines, and nitrogen-containing heterocycles. Here, we report the photodecarboxylative amination of redox-active esters with diazirines using inexpensive photoactivators under mild conditions with an enhanced scope for primary substrates. The stability of diazirines to blue light is demonstrated, paving the way for further research into other photochemical amination methods with these unique heterocycles.

Carbon Atom Insertion into Pyrroles and Indoles Promoted by Chlorodiazirines

Herein, we report a reaction that selectively generates 3-arylpyridine and quinoline motifs by inserting aryl carbynyl cation equivalents into pyrrole and indole cores, respectively. By employing α-chlorodiazirines as thermal precursors to the corresponding chlorocarbenes, the traditional haloform-based protocol central to the parent Ciamician-Dennstedt rearrangement can be modified to directly afford 3-(hetero)arylpyridines and quinolines. Chlorodiazirines are conveniently prepared in a single step by oxidation of commercially available amidinium salts. Selectivity as a function of pyrrole substitution pattern was examined, and a predictive model based on steric effects is put forward, with DFT calculations supporting a selectivity-determining cyclopropanation step. Computations surprisingly indicate that the stereochemistry of cyclopropanation is of little consequence to the subsequent electrocyclic ring opening that forges the pyridine core, due to a compensatory homoaromatic stabilization that counterbalances orbital-controlled torquoselectivity effects. The utility of this skeletal transform is further demonstrated through the preparation of quinolinophanes and the skeletal editing of pharmaceutically relevant pyrroles.

Reductive cross-coupling to access C–N bonds from aryl halides and diazoesters under dual nickel/photoredox-catalyzed conditions

A reductive cross-coupling reaction to access C–N bonds via dual Ni/photoredox-catalyzed systems has been reported. This method tolerates a broad scope of functional groups and the products can be transformed into several heterocycles.