Electrophilic α-amination of carbonyl compounds

Photochemical α-amination of carbonyl groups with iodinanes

We report on a photochemical reaction of silyl enol ethers with iminoiodinanes. This aza Rubottom reaction provides a direct access towards α-amino carbonyl compounds under catalyst free reaction conditions with light as the sole source of energy. Control experiments suggest the participation of triplet nitrene intermediates.

Pd-catalyzed three-component [2 + 2 + 1] cycloamination toward carbazoles

Conventional approaches using hydroxylamine derivatives as single nitrogen sources for the preparation of N-heterocyclic molecules rely on two chemical processes involving sequential nucleophilic and electrophilic C-N bond formations. Herein, we report a novel Suzuki reaction/C-H activation/amination sequence for building a myriad of carbazoles in a single transformation using bifunctional secondary hydroxylamines. It is noteworthy that the synthetic utility of this methodology is highlighted by the total synthesis of clausine V and glycoborine by incorporating the title [2 + 2 + 1] cycloamination as the key step. Control experiments were performed to gain a better understanding of the reaction mechanism.

Diphenylphosphinylhydroxylamine (DPPH) Affords Late-Stage S-imination to access free-NH Sulfilimines and Sulfoximines

Sulfilimines, as potential aza-isosteres of sulfoxides, are valued as building blocks, auxiliaries, ligands, bioconjugation handles, and as precursors to versatile S(VI) scaffolds including sulfoximines and sulfondiimines. Here, we report a thioether imination methodology that exploits O-(diphenylphosphinyl)hydroxyl amine (DPPH). Under mild, metal-free, and biomolecule-compatible conditions, DPPH enables late-stage S-imination on peptides, natural products, and a clinically trialled drug, and shows both excellent chemoselectivity and broad functional group tolerance. This methodological report is extended to an efficient and high-yielding one-pot reaction for accessing free-NH sulfoximines with diverse substrates including ones of potential clinical importance. In the presence of a rhodium catalyst, sulfoxides are S-iminated in higher yields to afford free-NH sulfoximines. S-imination was validated on an oxidatively delicate amatoxin to give sulfilimine and sulfoximine congeners. Interestingly, these new sulfilimine and sulfoximine-amatoxins show cytotoxicity. This method is further extended to create sulfilimine and sulfoximine-Fulvestrant and buthionine analogues.

Nickel-catalyzed oxidative thiolation of α-amino carbonyl compounds with thiols

An efficient oxidative thiolation of α-amino carbonyl compounds with thiols by the catalysis of an Earth-abundant nickel salt is disclosed for the first time. A variety of alkyl thiols and (hetero)aryl thiols underwent the reaction well with α-amino ketones and an α-amino ester to produce the desired α,α-aminothiocarbonyl compounds in good to excellent yields under ligand- and base-free conditions.

Enantioselective Amination of 3-Substituted-2-benzofuranones via Non-covalent N-Heterocyclic Carbene Catalysis

Herein, an efficient method for asymmetric α-amination of 2-benzofuranones with N-heterocyclic carbene (NHC) catalysis is reported. The process is based on non-covalent interaction of NHC with substrate, facilitating the formation of a chiral ion-pair that encompasses enolate and azolium salt. The activated enolate adds to an electrophilic amine source with sufficient facial control to furnish an enantioenriched product having an amine substituted quaternary stereocenter. The process displays a broad substrate scope. A preparative scale synthesis has been achieved. Preliminary mechanistic investigations based on experimental and DFT studies suggest a reaction pathway that involves non-covalent substrate/NHC interactions and essentially implicate the role of π-π interaction in diastereomeric transition states for stereo-chemical discrimination.

Catalytic Enantioselective Amination of Enol Silyl Ethers Using a Chiral Paddle-Wheel Diruthenium Complex

A chiral paddle-wheel dinuclear ruthenium catalyst was applied to a catalytic asymmetric nitrene-transfer reaction with enol silyl ethers. The ruthenium catalyst was applicable to aliphatic enol silyl ethers as well as aryl-containing enol silyl ethers. The substrate scope of the ruthenium catalyst was superior to that of analogous chiral paddle-wheel rhodium catalysts. α-Amino ketones derived from aliphatic substrates were obtained in up to 97% ee with the ruthenium catalyst, while analogous rhodium catalysts resulted in only moderate enantioselectivity.

α-Amination of Carbonyl Compounds by Using Hypervalent Iodine-Based Aminating Reagents Containing a Transferable (Diarylmethylene)amino Group

Hypervalent iodine-based aminating reagents containing a transferable (diarylmethylene)amino group can be used for the α-amination of simple carbonyl compounds such as esters, amides, and ketones in the presence of a lithium base. The (diarylmethylene)amino groups of the products can be readily modified, thus providing access to primary amines and diarylmethylamines. The developed method features transition-metal-free conditions and a simple one-pot procedure without the need to prepare enolate equivalents separately, thus offering a general and practical approach to the synthesis of a wide variety of α-amino carbonyl compounds. Experimental mechanistic investigations indicate that this amination proceeds through a unique radical coupling of an α-carbonyl radical with an iminyl radical; they are generated through a single-electron transfer between a lithium enolate and the hypervalent iodine reagent.

Palladium(II)-Catalyzed [2+2+1] Annulation of Alkynes and Hydroxylamines: A Rodox-Neutral Approach to Fully Substituted Pyrroles

A palladium-catalyzed [2+2+1] annulation of alkynes and hydroxylamines has been developed for the rapid construction of fully substituted pyrroles. This transformation involves sequential nucleophilic-addition of hydroxylamine to alkyne, alkyne migratory insertion, and synergistic demetallization cyclization, which provides a redox-neutral annulation approach to pyrrole derivatives. Moreover, the strategy enabled alteration of the photophysical properties of pyrrole products by varying the aryl substituents, thus leading to the development of N-functionalized tetraarylpyrroles as new fluorophores.

Aza-Quasi-Favorskii Reaction: Construction of Highly Substituted Aziridines through a Concerted Multibond Rearrangement Process

A new molecular rearrangement, the aza-Quasi-Favorskii rearrangement, has been developed for the construction of highly substituted aziridines. Electron-deficient O-sulfonyl oximes react readily with α,α-disubstituted acetophenone-derived enolates to furnish highly substituted aziridines via this unprecedented domino process. In-depth computational studies reveal an asynchronous yet concerted nitrenoid-type rearrangement pathway.

Chain Walking as a Strategy for Iridium-Catalyzed Migratory Amidation of Alkenyl Alcohols to Access α-Amino Ketones

Catalytic carbon-nitrogen bond formation in hydrocarbons is an appealing synthetic tool to access valuable nitrogen-containing compounds. Although a number of synthetic approaches have been developed to construct a bifunctional α-amino carbonyl scaffold in this realm, installation of an amino functionality at the remote and unfunctionalized aliphatic sites remains underdeveloped. Here we present a tandem iridium catalysis that enables the redox-relay amidation of alkenyl alcohols via chain walking and metal-nitrenoid transfer, which eventually offers a new route to various α-amino ketones with excellent regioselectivity. The virtue of this transformation is that an unrefined isomeric mixture of alkenyl alcohols can be utilized as the readily available starting materials to lead to the regioconvergent amidation. Mechanistic investigations revealed that the reaction proceeds via a tandem process involving two key components of redox-relay chain walking and intermolecular nitrenoid transfer with the assistance of hydrogen bonding, thus representing the competence of Ir catalysis for the olefin migratory C-N coupling with high efficiency and exquisite selectivity.

Cinchona Alkaloid Derived Iodide Catalyzed Enantioselective Oxidative α-Amination of Carbonyl Compounds toward the Construction of Spiroindolyloxindole

Novel cinchona alkaloid derived iodide catalysts were developed for the enantioselective oxidative α-amination of 2-oxindoles, providing various functionalized spiropyrrolidine oxindoles in high yields and with good enantioselectivities. This iodide/ROOH catalytic system features a one-step synthesis of a catalyst with multiple functionalities, ease of operation, and good scalability, thereby enriching the repertoire of iodide catalysis for enantioselective oxidative coupling reactions.

A multifaceted approach towards understanding the peculiar behavior of (α)-hydroxyiminophosphonates

A unique multifaceted approach involving the interplay of NMR, XRD, LC, DFT and MS/IM-MS techniques toward the understanding of the peculiar isomer-selective reduction of (α)-hydroxyiminophosphonates into (α)-hydroxyaminophosphonate derivatives.

Palladium-catalyzed benzylic C(sp3)-H carbonylative arylation of azaarylmethyl amines with aryl bromides

A highly selective palladium-catalyzed carbonylative arylation of weakly acidic benzylic C(sp3)-H bonds of azaarylmethylamines with aryl bromides under 1 atm of CO gas has been achieved. This work represents the first examples of use of such weakly acidic pronucleophiles in this class of transformations. In the presence of a NIXANTPHOS-based palladium catalyst, this one-pot cascade process allows a range of azaarylmethylamines containing pyridyl, quinolinyl and pyrimidyl moieties and acyclic and cyclic amines to undergo efficient reactions with aryl bromides and CO to provide α-amino aryl-azaarylmethyl ketones in moderate to high yields with a broad substrate scope and good tolerance of functional groups. This reaction proceeds via in situ reversible deprotonation of the benzylic C-H bonds to give the active carbanions, thereby avoiding prefunctionalized organometallic reagents and generation of additional waste. Importantly, the operational simplicity, scalability and diversity of the products highlight the potential applicability of this protocol.

Electrochemical Oxidative Coupling Between Benzylic C(sp3)-H and N-H of Secondary Amines: Rapid Synthesis of α-Amino α-Aryl Esters

An intermolecular electrochemical coupling between the benzylic C(sp³)-H bond and various secondary amines is reported. The electronic behavior of two electronically rich units viz the α-position of α-aryl acetates and amines was engineered electrochemically, thus facilitating their reactivity for the direct access of α-amino esters. A series of acyclic/cyclic secondary amines and α-aryl acetates were tested to furnish the corresponding α-amino esters with high yields (up to 92%) under mild conditions.

Electrophilic Aminating Agents in Total Synthesis

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C−N bond‐forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

Recent advances in the synthesis of α-amino ketones

Due to the importance of the amino ketone motif in synthetic and medicinal chemistry, the number of protocols developed in recent years has considerably increased. This review serves to collate and critically evaluate novel methodologies published since 2011 towards this high value synthon. The chapters are divided by the requisite functionality in the starting material, and an emphasis is placed on discussing functional group compatibility and resultant product substitution patterns. Throughout, applications to medicinal targets are highlighted and mechanistic details are presented, and we further provide a short outlook for future development and emerging potential within this area.

Oxidative Deconstruction of Azetidinols to α-Amino Ketones

A silver-mediated synthesis of α-amino ketones via the oxidative deconstruction of azetidinols has been developed using a readily scalable protocol with isolated yields up to 80%. The azetidinols are easily synthesized in one step and can act as protecting groups for these pharmaceutically relevant synthons. Furthermore, mechanistic insights are presented and these data have revealed that the transformation is likely to proceed through the β-scission of an alkoxy radical, followed by oxidation and C-N cleavage of the resulting α-amido radical.

Hydroxylamines As Bifunctional Single-Nitrogen Sources for the Rapid Assembly of Diverse Tricyclic Indole Scaffolds

Conventional approaches on using hydroxylamine derivatives as single nitrogen sources, for the construction of n-membered (n > 3) N-heterocycles, rely upon two chemical operations by involving sequential nucleophilic and electrophilic C-N bond formations. Here, we report a highly efficient cascade of alkyne insertion/C-H activation/amination for the rapid preparation of a myriad of tricyclic indoles, in a single-step transformation, by using bifunctional secondary hydroxylamines. It is noteworthy that judicious selection of applicable amino agents, for enabling the prior oxidative addition of aryl iodide to initial Pd(0) species and subsequent two C-N bonds formation, was the key to the success of this reaction. Control experiments indicated that a five-membered palladacyclic intermediate played a crucial role in promoting the final aminative ring closure.

Synthesis of α-heterosubstituted ketones through sulfur mediated difunctionalization of internal alkynes

Synthesis of α-heterosubstituted ketones was achieved through sulfur mediated difunctionalization of internal alkynes in one pot. The reaction design involves: phenyl substituted internal alkyne attacking triflic anhydride activated diphenyl sulfoxide to give a sulfonium vinyl triflate intermediate, hydrolysis to give an α-sulfonium ketone, and then substitution with various nucleophiles. This method provides a unified route to access α-amino ketones, α-acyloxy ketones, α-thio ketones, α-halo ketones, α-hydroxy ketones, and related heterocyclic structures, in a rapid fashion.

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.

Aza-Rubottom Oxidation: Synthetic Access to Primary α-Aminoketones

An aza analogue of the Rubottom oxidation is reported. This facile transformation takes place at ambient temperature and directly converts silyl enol ethers to the corresponding primary α-aminoketones. The use of hexafluoroisopropanol (HFIP) as the solvent is essential for the success of this reaction. Overall this process is well-suited for the aza-functionalization and derivatization of complex organic molecules.

Synthesis of α-(4-Oxazolyl)amino Esters via Brønsted Acid Catalyzed Tandem Reaction

A one-step, Brønsted acid catalyzed tandem reaction for the synthesis of α-(4-oxazolyl)amino esters was developed. 4-Nitrobenzenesulfonic acid was found to be an efficient catalyst for the coupling of ethyl 2-oxobut-3-ynoates with amides to provide various α-(4-oxazolyl)amino esters. The experimental and X-ray crystallographic data suggest that a series of bond-forming reactions including imine formation, intermolecular Michael addition, and intramolecular Michael addition are involved to generate both the oxazole and amino acid functionalities.

Multifunctionalization of Unactivated Cyclic Ketones via Synergistic Catalysis of Copper and Diarylamine: Access to Cyclic α-Enaminone

A multifunctionalization of unactivated cyclic ketones via synergistic catalysis of copper and diarylamine for the direct synthesis of cyclic α-enaminone is reported for the first time. This reaction goes through oxidative α-amination, followed by a desaturation, and features mild reaction conditions, a broad substrate scope, and great functional group tolerance.

Organocatalytic Asymmetric Decarboxylative Amination of β-Keto Acids: Access to Optically Active α-Amino Ketones and 1,2-Amino Alcohols

An organocatalytic asymmetric decarboxylative amination reaction of β-keto acids is described. Under mild reaction conditions, a series of chiral α-amino ketones were obtained in good to high yields (up to 99%) and enantioselectivities (up to 95% ee). A chiral 1,2-amino alcohol was synthesized from the corresponding decarboxylative amination product in several steps without loss of enantioselectivity.

Enantioselective magnesium-catalyzed transformations

This review updates the major progress in the field of enantioselective transformations promoted by chiral magnesium catalysts, covering the literature since 2007, illustrating the power of these green catalysts to promote many types of reactions.

Enantioselective Silver-Catalyzed Transformations

This review collects the major progress in the field of enantioselective transformations promoted by chiral silver catalysts, covering the literature since 2008, well illustrating the power of these especially mild Lewis acid catalysts to provide novel asymmetric reactions.

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.