Copper(I) complexes bearing pyrrole-bridged S,N and N-donor ligands as catalysts for tandem hydroamination-alkynylation: effect of anions on product formation

In this study, the reaction between 2-(3,5-dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)pyrrole and thiophenol under heating conditions afforded the new ligand 2-(3,5-dimethylpyrazolylmethyl)-5-(phenylthiomethyl)pyrrole 2. The reaction of 2 with meta-chloroperbenzoic acid provided sulfoxide 3 and sulfone 4 group-containing ligands. The reaction of 2 with copper(I) halides provided the binuclear complexes [Cu(μ-X){μ-C4H3N-2-(CH2Me2pz)-5-(CH2SPh)-κ2-S,N}]2 (X = Cl, Br and I, 5-7) in high yields. Conversely, the analogous reaction of 4 with copper(I) halides yielded two types of complexes, three coordinate bi- and mononuclear of the type [Cu(μ-Cl){C4H3N-2-(CH2Me2pz)-5-(CH2SO2Ph)-κ1-N}]28 and [CuX{C4H3N-2-(CH2Me2pz)-5-(CH2SO2Ph)-κ1-N}2], X = Br, 9 and I, 10. When the reaction was carried out in the presence of KPF6, the two-coordinate complex [Cu{C4H3N-2-(CH2Me2pz)-5-(CH2SO2Ph)-κ1-N}2]PF6-11a was isolated, whereas its BF4- analogue 11b was synthesized by the reaction of 8 with AgBF4. The structures of these complexes were determined using single-crystal X-ray crystallography. These copper complexes catalyzed the hydroamination-alkynylation reaction between several secondary amines and alkyl and aryl terminal alkynes. Using 1 mol% of complexes 5-10 as catalysts, both tri- and tetra-substituted propargylamines were isolated. Alternatively, phenylacetylene and different secondary amines afforded the corresponding trisubstituted propargylamines as the major products and alkyl terminal alkynes gave the tetrasubstituted products in excellent yields. In addition, the role of counter anions such as TfO-, PF6-, BF4-, PO43- and Ph4B- on the product selectivity was studied. When fluorinated anions such as TfO-, PF6-, and BF4- were present with the copper complexes, the hydroamination-hydrovinylation product 1-aminodiene 23 was observed, which was not formed with PO43-, Ph4B- or halide ions. Specifically, TfO- and PF6- favored the formation of 23, while BF4- favored the tetrasubstituted product as the major product. This was further supported by the isolated copper(I) complexes containing PF6- and BF4- and by other specific reactions. The peaks for enamines and [LCu]+ species in the HRMS spectra of the reaction mixtures and the isolation of the morpholinium copper(I) salt support the proposed mechanism.

Catalytic Asymmetric Cyclizative Rearrangement of Anilines and Vicinal Diketones to Access 2,2-Disubstituted Indolin-3-ones

The efficient synthesis of chiral 2,2-disubstituted indolin-3-ones is of great importance due to its significant synthetic and biological applications. However, catalytic enantioselective methods for de novo synthesis of such heterocycles remain scarce. Herein, a novel cyclizative rearrangement of readily available anilines and vicinal diketones for the one-step construction of enantioenriched 2,2-disubstituted indolin-3-ones is presented. The reaction proceeds through a self-sorted [3+2] heteroannulation/regioselective dehydration/1,2-ester shift process. Only chiral phosphoric acid is employed to promote the entire sequence and simplify the manipulation of this protocol. Various common aniline derivatives are successfully applied to asymmetric synthesis as 1,3-binuclephiles for the first time. Remarkably, the observed stereoselectivity is proposed to originate from an amine-directed regio- and enantioselective ortho-Csp2-H addition of the anilines to the ketones. A range of synthetic transformations of the resulting products are demonstrated as well.

Sequential Dearomatization/Rearrangement of Quinazoline-Derived Azomethine Imines for the Synthesis of Nitrogen-Rich Three-Dimensional Cage-Like Molecules

A sequential dearomatization/rearrangement reaction between quinazoline-derived azomethine imines and crotonate sulfonium salts has been developed to provide a series of three-dimensional cage-like molecules. The reaction involves two dearomatizations, two cyclizations, and two C-C bond and three C-N bond formations in one step. The new transformation has a broad substrate scope, does not require any added reagents, and proceeds under room temperature in a short time. A mechanistic rationale for the sequential dearomatization/rearrangement is also presented. Furthermore, the synthetic compounds are evaluated for their glucose control effect. Compounds 3aa and 3aj were found to be hyperglycemic, which might be lead compounds for treating hypoglycemia.

Multicomponent Synthesis of α-Branched Amines via a Zinc-Mediated Carbonyl Alkylative Amination Reaction

Methods for the synthesis of α-branched alkylamines are important due to their ubiquity in biologically active molecules. Despite the development of many methods for amine preparation, C(sp3)-rich nitrogen-containing compounds continue to pose challenges for synthesis. While carbonyl reductive amination (CRA) between ketones and alkylamines is the cornerstone method for α-branched alkylamine synthesis, it is sometimes limited by the sterically demanding condensation step between dialkyl ketones and amines and the more restricted availability of ketones compared to aldehydes. We recently reported a "higher-order" variant of this transformation, carbonyl alkylative amination (CAA), which utilized a halogen atom transfer (XAT)-mediated radical mechanism, enabling the streamlined synthesis of complex α-branched alkylamines. Despite the efficacy of this visible-light-driven approach, it displayed scalability issues, and competitive reductive amination was a problem for certain substrate classes, limiting applicability. Here, we report a change in the reaction regime that expands the CAA platform through the realization of an extremely broad zinc-mediated CAA reaction. This new strategy enabled elimination of competitive CRA, simplified purification, and improved reaction scope. Furthermore, this new reaction harnessed carboxylic acid derivatives as alkyl donors and facilitated the synthesis of α-trialkyl tertiary amines, which cannot be accessed via CRA. This Zn-mediated CAA reaction can be carried out at a variety of scales, from a 10 μmol setup in microtiter plates enabling high-throughput experimentation, to the gram-scale synthesis of medicinally-relevant compounds. We believe that this transformation enables robust, efficient, and economical access to α-branched alkylamines and provides a viable alternative to the current benchmark methods.

Recent asymmetric synthesis of natural products bearing an α-tertiary amine moiety via temporary chirality induction strategies

Covering: 2013 to 2023The α-tertiary amine moiety is a common structural motif in natural alkaloids and is frequently associated with intriguing biological activities and inherent synthetic challenges. A major hurdle in the total synthesis of these alkaloids is the asymmetric construction of the α-tertiary amine moiety. Temporary chirality inductions have been effective strategies employed to address this issue, particularly in natural product synthesis. The temporary chirality induction strategies in α-tertiary amine synthesis can be broadly classified into three categories based on the types of temporary chirality involved: Seebach's self-regeneration of stereocenters (SRS), C-to-N-to-C chirality transfer, and memory of chirality (MOC). This review highlights the recent advancements in temporary chirality induction strategies for the total synthesis of α-tertiary amine-containing natural products between 2013 and 2023.

Multicomponent Cyclizative 1,2-Rearrangement Enabled Enantioselective Construction of 2,2-Disubstituted Pyrrolinones

The 1,2-rearrangement reaction is one of the most important approaches to construct carbon-carbon bonds in organic synthesis. However, the development of catalytic asymmetric 1,2-rearrangements is still far from mature and often suffers from problems such as complex substrates, single product structure, and lack of synthetic application. Multicomponent reaction has been recognized as a robust tool for the synthesis of diverse and tunable products from readily available starting material. Conceptionally and practically, the development of multicomponent asymmetric 1,2-rearrangements is highly desirable. In this regard, we report herein a three-component benzilic acid-type rearrangement of 2,3-diketoesters, aromatic amines and aldehydes for the asymmetric construction of synthetically challenging pyrrolinones bearing aza-quaternary stereocenters. To the best of our knowledge, this reaction represents the first example of organocatalyzed multicomponent asymmetric 1,2-rearrangements.

Selective and Functional-Group-Tolerant Photoalkylation of Imines by Energy-Transfer Photocatalysis

Basic amines show broad bioactivity and remain a promising source of new medicines. The direct photoalkylation of imines offers a promising strategy for complex amines. However, the lack of efficient imine photoreactivity hinders this reaction and remains a fundamental limitation in organic photochemistry. We report an efficient photoalkylation of imines that provides primary amines directly without protecting or leaving groups. The transformation effects C-H addition across N-H imines under energy-transfer photocatalysis by a ketone. Our method is distinguished from organometallic, metal-catalyzed, and photoredox approaches to imine alkylation by its lack of protecting groups and its broad scope, which includes unactivated alkanes, protic substrates, basic amines, heterocycles, and ketone imines. We highlight this scope through the condensation and alkylation of two pharmaceutical ketones, providing complex amines succinctly. Our mechanistic analysis supports a three-step process, involving hydrogen-atom transfer to an imine triplet excited state, intersystem crossing, and radical recombination, with photocatalytic enhancement through energy transfer. We further show that N-H imines are more photoreactive than N-substituted imines, a distinction partially explained by sterics and side reactions. To fully explain this distinction, we introduce the thermodynamic parameter excited-state hydrogen-atom affinity, which is highly effective at predicting the photoreactivity of imines.

Iridium(III)-Catalyzed Intermolecular C(sp3 )-H Amidation for the Synthesis of Chiral 1,2-Diamines

Chiral 1,2-diamines are privileged scaffolds among bioactive natural products, active pharmaceutical ingredients, ligands for transition-metal-based asymmetric catalysis and organocatalysts. Despite this interest, the construction of chiral 1,2-diamine motifs still remains a challenge. To address this, an iridium(III)-catalyzed intermolecular C(sp3 )-H amidation reaction was developed. This method relies on the design of a new, cheap and cleavable exo-protecting/directing group derived from camphorsulfonic acid, which is directly installed from easily accessible precursors, and furnishes scalemic free 1,2-diamines upon cleavage of both nitrogen substituents. It was found applicable to both α-secondary and α-tertiary-1,2-diamines, for which a two-step protocol involving intermolecular olefin hydroamination and C(sp3 )-H amidation was developed. Kinetic and computational studies provided insights into the observed reactivity difference between pairs of diastereoisomeric substrates.

Scope and Mechanistic Probe into Asymmetric Synthesis of α-Trisubstituted-α-Tertiary Amines by Rhodium Catalysis

Asymmetric reactions that convert racemic mixtures into enantioenriched amines are of significant importance due to the prevalence of amines in pharmaceuticals, with about 60% of drug candidates containing tertiary amines. Although transition-metal catalyzed allylic substitution processes have been developed to provide access to enantioenriched α-disubstituted allylic amines, enantioselective synthesis of sterically demanding α-tertiary amines with a tetrasubstituted carbon stereocenter remains a major challenge. Herein, we report a chiral diene-ligated rhodium-catalyzed asymmetric substitution of racemic tertiary allylic trichloroacetimidates with aliphatic secondary amines to afford α-trisubstituted-α-tertiary amines. Mechanistic investigation is conducted using synergistic experimental and computational studies. Density functional theory calculations show that the chiral diene-ligated rhodium promotes the ionization of tertiary allylic substrates to form both anti and syn π-allyl intermediates. The anti π-allyl pathway proceeds through a higher energy than the syn π-allyl pathway. The rate of conversion of the less reactive π-allyl intermediate to the more reactive isomer via π-σ-π interconversion was faster than the rate of nucleophilic attack onto the more reactive intermediate. These data imply that the Curtin-Hammett conditions are met in the amination reaction, leading to dynamic kinetic asymmetric transformation. Computational studies also show that hydrogen bonding interactions between β-oxygen of allylic substrate and amine-NH greatly assist the delivery of amine nucleophile onto more hindered internal carbon of the π-allyl intermediate. The synthetic utility of the current methodology is showcased by efficient preparation of α-trisubstituted-α-tertiary amines featuring pharmaceutically relevant secondary amine cores with good yields and excellent selectivities (branched-linear >99:1, up to 99% enantiomeric excess).

Rapid Access to 2,2-Disubstituted Indolines via Dearomative Indolic-Claisen Rearrangement: Concise, Enantioselective Total Synthesis of (+)-Hinckdentine A

The construction of 2,2-disubstituted indolines has long presented a synthetic challenge without any general solutions. Herein, we report a robust protocol for the dearomative Meerwein-Eschenmoser-Claisen rearrangement of 3-indolyl alcohols that provides efficient access to 2-substituted and 2,2-disubstituted indolines. These versatile subunits are useful for natural product synthesis and medicinal chemistry. The title [3,3] sigmatropic rearrangement proceeds in generally excellent yield and transfers the C3-indolic alcohol chirality to the C2 position with high fidelity, thus providing a reliable method for the construction of enantioenriched 2,2-disubstituted indolines. The power of this methodology is demonstrated through the concise and strategically unique total synthesis of the marine natural product hinckdentine A, which features a dearomative Claisen rearrangement, a diastereocontrolled hydrogenation of the alkene product, a one-pot amide-to-oxime conversion using Vaska's complex, and a regioselective late-stage tribromination.

Stereoselective Synthesis of Less Accessible α-Tertiary Amino Ketimines via Electrophilic Amination of α-Branched N-tert-Butanesulfinyl Ketimines

A stereocontrolled electrophilic amination of α-branched N-tert-butanesulfinyl ketimines was developed to construct α-aminoketone derivatives containing less accessible α-tetrasubstituted stereocenters. Stereospecific α-deprotonation of ketimines with potassium tert-butoxide gave stereodefined metalloenamine intermediates that could act as nucleophiles to attack azodicarboxylic derivatives, affording α-aminated products in high yields with excellent stereoselectivities.

A highly efficient method to access unprotected C-3 bifunctional quaternary 3-allyl-3-(amino)oxindoles

A highly efficient Rh(II) catalyzed non-radical protocol to access NH-free C-3 bifunctional oxindoles, which possess 3-allyl and 3-amino simultaneously, was first achieved by employing an intermolecular [2,3]-sigmatropic rearrangement reaction between diazooxindoles and tertiary allylic amines. Utilizing readily available allylamines as the nitrogen and allyl source concurrently, a wide range of bio-active 3-allyl-3-(amino)oxindoles were obtained in excellent yields under very mild reaction conditions; meanwhile, the TON can be up to 90 000. Our study addresses a gap in the literature by investigating intermolecular rearrangements of ammonium ylides with diazoamides, which have been relatively understudied.

Recent advances in the synthesis of chiral α-tertiary amines via transition-metal catalysis

The significance of chiral α-tertiary amines in medicinal chemistry and drug development has been unquestionably established in the last few decades. α-Tertiary amines are attractive structural motifs for natural products, bioactive molecules and pharmaceuticals and are preclinical candidates. Their syntheses have been the focus of intensive research, and the development of new methods has continued to attract more and more attention. In this review, we present the progress in the last decade in the development of synthetic methods for the assembly of chiral ATAs via transition-metal catalysis. To date, the effective approaches in this area could be categorized into three strategies: enantioselective direct and indirect Mannich addition to ketimines; umpolung asymmetric alkylation of imine derivatives; and asymmetric C-N cross-coupling of tertiary alkyl electrophiles. Several related developing strategies for the synthesis of ATAs, such as hydroamination of alkenes, HAT amination approaches and the C-C coupling of α-aminoalkyl fragments, are also described in this article. These strategies have emerged as attractive C-C and C-N bond-forming protocols for enantioselective construction of chiral α-tertiary amines, and to some extent are complementary to each other, showing the prospect of application in medicinal chemistry and chemical biology.

Photoredox-Nickel Dual-Catalyzed C-Alkylation of Secondary Nitroalkanes: Access to Sterically Hindered α-Tertiary Amines

The preparation of tertiary nitroalkanes via the nickel-catalyzed alkylation of secondary nitroalkanes using aliphatic iodides is reported. Previously, catalytic access to this important class of nitroalkanes via alkylation has not been possible due to the inability of catalysts to overcome the steric demands of the products. However, we have now found that the use of a nickel catalyst in combination with a photoredox catalyst and light leads to much more active alkylation catalysts. These can now access tertiary nitroalkanes. The conditions are scalable as well as air and moisture tolerant. Importantly, reduction of the tertiary nitroalkane products allows rapid access to α-tertiary amines.

A Chiral Amine Transfer Approach to the Photocatalytic Asymmetric Synthesis of α-Trialkyl-α-tertiary Amines

A long-standing challenge within radical chemistry is that of controlling the absolute stereochemistry of the products. Here, we report the stereocontrolled addition of α-amino radicals reductively generated from imines via visible-light-mediated photoredox-catalysis to alkenes, giving rise to enantioenriched α-trialkyl-α-tertiary amines. This process exploits a commercially available phenylglycinol derivative as a source of both nitrogen and chiral information. DFT studies support a stereochemical model whereby an intramolecular H-bond rigidifies the transition state of the enantiodetermining step.

Asymmetric C-Alkylation of Nitroalkanes via Enzymatic Photoredox Catalysis

Tertiary nitroalkanes and the corresponding α-tertiary amines represent important motifs in bioactive molecules and natural products. The C-alkylation of secondary nitroalkanes with electrophiles is a straightforward strategy for constructing tertiary nitroalkanes; however, controlling the stereoselectivity of this type of reaction remains challenging. Here, we report a highly chemo- and stereoselective C-alkylation of nitroalkanes with alkyl halides catalyzed by an engineered flavin-dependent "ene"-reductase (ERED). Directed evolution of the old yellow enzyme from Geobacillus kaustophilus provided a triple mutant, GkOYE-G7, capable of synthesizing tertiary nitroalkanes in high yield and enantioselectivity. Mechanistic studies indicate that the excitation of an enzyme-templated charge-transfer complex formed between the substrates and cofactor is responsible for radical initiation. Moreover, a single-enzyme two-mechanism cascade reaction was developed to prepare tertiary nitroalkanes from simple nitroalkenes, highlighting the potential to use one enzyme for two mechanistically distinct reactions.

Design of Photoredox-Catalyzed Giese-Type Reaction for the Synthesis of Chiral Quaternary α-Aryl Amino Acid Derivatives via Clayden Rearrangement

Chiral quaternary α-aryl amino acids are biologically valued but synthetically challenging building blocks. Herein, we report a strategy for the synthesis of molecular architectures by unifying a photoredox catalytic asymmetric Giese-type reaction and Clayden rearrangement. A new class of chiral Karady-Beckwith dehydroalanines is designed and serves as a versatile handle for the photoredox-mediated highly stereoselective Giese-type reaction with feedstock carboxylic acids and tertiary amines. Subsequent Clayden rearrangement delivers chiral quaternary α-aryl amino acid derivatives with high stereoselectivity. The versatile approach offers a reliable source for the assembly of highly demanding chiral building blocks.

Crystallization-Enabled Henry Reactions: Stereoconvergent Construction of Fully Substituted [N]-Asymmetric Centers

Tetrasubstituted stereogenic carbon centers bearing a nitrogen substituent represent important motifs in medicinal chemistry and natural products; therefore, the development of efficient methods for the stereoselective synthesis of this class of compounds continues to be an important problem. This article describes stereoconvergent Henry reactions of γ,γ-disubstituted nitroalkanes to deliver highly functionalized building blocks containing up to five contiguous stereogenic centers including a fully substituted [N]-asymmetric center. Henry reactions of higher order nitroalkanes are often characterized by their reversibility and minimal accompanying thermodynamic stereocontrol. In contrast, mechanistic studies for the present case suggest a scenario in which reversibility is productively leveraged through crystallization-based stereocontrol, thereby enabling the efficient sequential π-additions of readily accessible starting materials to assemble complex acyclic stereoarrays.

Diverse synthesis of α-tertiary amines and tertiary alcohols via desymmetric reduction of malonic esters

Amines and alcohols with a fully substituted α-carbon are structures of great value in organic synthesis and drug discovery. While conventional methods towards these motifs often rely on enantioselective carbon-carbon or carbon-heteroatom bond formation reactions, a desymmetric method is developed here by selectively hydrosilylating one of the esters of easily accessible α-substituted α-amino- and -oxymalonic esters. The desymmetrization is enabled by a suite of dinuclear zinc catalysts with pipecolinol-derived tetradentate ligands and can accommodate a diverse panel of heteroatom substituents, including secondary amides, tertiary amines, and ethers of different sizes. The polyfunctionalized reduction products, in return, have provided expeditious approaches to enantioenriched nitrogen- and oxygen-containing molecules, including dipeptides, vitamin analogs, and natural metabolites.

Chiral α-tertiary amines and tertiary alcohols are prevalent in bioactive molecules yet challenging targets to access. Here, the authors provide a dinuclear zinc-catalyzed desymmetric approach based on readily available malonic esters.

Sigmatropic [1,5] Carbon Shift of Transient C3 Ammonium Enolates

The stereospecific sigmatropic [1,5] carbon shift of C3 ammonium enolates is discovered. According to mechanistic, kinetic and computational experiments, this new rearrangement proceeds via the catalytic generation of a transient C3 ammonium enolate by intramolecular aza-Michael addition. This intermediate rapidly undergoes [1,5] sigmatropic carbon migration to furnish the respective tetrahydroquinoline-4-ones with excellent diastereoselectivities of d.r. >99 : 1 and in 61-98 % yield.

Stereoselective Access to Iminosugar C,C-Glycosides from 6-Azidoketopyranoses

We report the synthesis of iminosugar C,C-glycosides starting from 6-azidoketopyranoses. Their Staudinger-azaWittig-mediated cyclization provided bicyclic N,O-acetals, which were stereoselectively opened with AllMgBr to afford β-hydroxyazepanes with a quaternary carbon α to the nitrogen. Their ring contraction via a β-aminoalcohol rearrangement produced the six-membered l-iminosugars with two functional handles at the pseudoanomeric position. Inversion of the free OH at the azepane level furnished the d-iminosugars.

Synthesis of Unnatural α-Amino Acid Derivatives via Photoredox Activation of Inert C(sp3)-H Bonds

The synthesis of unnatural, tertiary amino acids is a challenging task. While decarboxylation-radical addition has been an important strategy for their formation, the use of alkyl radicals from C(sp³)-H bonds has not been fully explored. Herein, we report a photocatalytic protocol for the synthesis of unnatural α-amino esters employing abundant alkanes and imines retaining full atom economy. When this method is applied, several amino acid derivatives are synthesized in moderate to good yields.

Efficient access to general α-tertiary amines via water-accelerated organocatalytic multicomponent allylation

A tetrasubstituted carbon atom connected by three sp³ or sp²-carbons with single nitrogen, i.e., the α-tertiary amine (ATA) functional group, is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. The synthetic approach toward ATA structures is intricate, therefore, a straightforward catalytic method has remained a substantial challenge. Here we show an efficient water-accelerated organocatalytic method to directly access ATA incorporating homoallylic amine structures by exploiting readily accessible general ketones as useful starting material. The synergistic action of a hydrophobic Brønsted acid in combination with a squaramide hydrogen-bonding donor under aqueous condition enabled the facile formation of the desired moiety. The developed exceptionally mild but powerful system facilitated a broad substrate scope, and enabled efficient multi-gram scalability.

The α-tertiary amine functional group is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. Here the authors show an efficient water-accelerated organocatalytic method to access α-tertiary amines incorporating homoallylic amine structures by exploiting ketones as useful starting material.

Iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides for synthesis of α-tertiary amines

A protocol for the synthesis of α-tertiary amines was developed by iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides. Nucleophilic 1,2-addition of organolithium reagents to carboxamides forms anionic tetrahedral carbinolamine (hemiaminal) intermediates, which are subsequently treated with bromotrimethylsilane (Me₃SiBr) followed by organomagnesium (Grignard) reagents, organolithium reagents or tetrabutylammonium cyanide, affording α-tertiary amines. Employment of (trimethylsilyl)methylmagnesium bromide as the 2^nd nucleophile allowed for aza-Peterson olefination of the resulting α-tertiary (trimethylsilyl)methylamines with acidic work-up, resulting in the formation of 1,1-diarylethylenes.

α-Tertiary Propargylamine Synthesis via KA2-Type Coupling Reactions under Solvent-Free CuI-Zeolite Catalysis

The potential of copper(I)-zeolite catalysis was evaluated in the three-component KA²-coupling mediated synthesis of α-tertiary propargylamines. Our archetypal copper(I)-doped zeolite Cu^I-USY proved to be efficient under ligand- and solvent-free conditions at 80 °C. Usable up to four times, this catalytic material enables the coupling of diverse ketones, alkynes, and amines with a broad functional group tolerance. A decarboxylative and a desilylative version, respectively, involving an alkynoic acid and trimethylsilylacetylene as alkyne surrogates, was also set up to bypass selectivity issues and/or to access α-tertiary propargylamines that are unattainable under standard KA² conditions. Interestingly, the KA²-type coupling reactions were successfully linked to other Cu^I-catalyzed reactions, thus resulting in sequential one-pot processes under full Cu^I-USY catalysis.

Modular Photocatalytic Synthesis of α-Trialkyl-α-Tertiary Amines

Molecules displaying an α-trialkyl-α-tertiary amine motif provide access to an important and versatile area of biologically relevant chemical space but are challenging to access through existing synthetic methods. Here, we report an operationally straightforward, multicomponent protocol for the synthesis of a range of functionally and structurally diverse α-trialkyl-α-tertiary amines, which makes use of three readily available components: dialkyl ketones, benzylamines, and alkenes. The strategy relies on the of use visible-light-mediated photocatalysis with readily available Ir(III) complexes to bring about single-electron reduction of an all-alkyl ketimine species to an α-amino radical intermediate; the α-amino radical undergoes Giese-type addition with a variety of alkenes to forge the α-trialkyl-α-tertiary amine center. The mechanism of this process is believed to proceed through an overall redox neutral pathway that involves photocatalytic redox-relay of the imine, generated from the starting amine-ketone condensation, through to an imine-derived product. This is possible because the presence of a benzylic amine component in the intermediate scaffold drives a 1,5-hydrogen atom transfer step after the Giese addition to form a stable benzylic α-amino radical, which is able to close the photocatalytic cycle. These studies detail the evolution of the reaction platform, an extensive investigation of the substrate scope, and preliminary investigation of some of the mechanistic features of this distinct photocatalytic process. We believe this transformation will provide convenient access to previously unexplored α-trialkyl-α-tertiary amine scaffolds that should be of considerable interest to practitioners of synthetic and medicinal chemistry in academic and industrial institutions.

Multicomponent alkene azidoarylation by anion-mediated dual catalysis

Molecules that contain the β-arylethylamine motif have applications in the modulation of pain, treatment of neurological disorders and management of opioid addiction, among others, making it a privileged scaffold in drug discovery^1,2. De novo methods for their assembly are reliant on transformations that convert a small class of feedstocks into the target compounds via time-consuming multistep syntheses^3-5. Synthetic invention can drive the investigation of the chemical space around this scaffold to further expand its capabilities in biology^6-9. Here we report the development of a dual catalysis platform that enables a multicomponent coupling of alkenes, aryl electrophiles and a simple nitrogen nucleophile, providing single-step access to synthetically versatile and functionally diverse β-arylethylamines. Driven by visible light, two discrete copper catalysts orchestrate aryl-radical formation and azido-group transfer, which underpin an alkene azidoarylation process. The process shows broad scope in alkene and aryl components and an azide anion performs a multifaceted role both as a nitrogen source and in mediating the redox-neutral dual catalysis via inner-sphere electron transfer^10,11. The synthetic capabilities of this anion-mediated alkene functionalization process are likely to be of use in a variety of pharmaceutically relevant and wider synthetic applications.

Highly chemoselective synthesis of hindered amides via cobalt-catalyzed intermolecular oxidative hydroamidation

α-Tertiary amides are of great importance for medicinal chemistry. However, they are often challenging to access through conventional methods due to reactivity and chemoselectivity issues. Here, we report a single-step approach towards such amides via cobalt-catalyzed intermolecular oxidative hydroamidation of unactivated alkenes, using nitriles of either solvent- or reagent-quantities. This protocol is selective for terminal alkenes over groups that rapidly react under known carbocation amidation conditions such as tertiary alcohols, electron-rich alkenes, ketals, weak C−H bonds, and carboxylic acids. Straightforward access to a diverse array of hindered amides is demonstrated, including a rapid synthesis of an aminoadamantane-derived pharmaceutical intermediate.

α-Tertiary amides are common in bioactive natural products and pharmaceuticals, but challenging to access by conventional methods. Here, the authors report a single-step approach toward α-tertiary amides via cobalt-catalyzed intermolecular oxidative hydroamidation of unactivated alkenes.

Crystal structure of tert-butyl 4-[4-(4-fluoro-phen-yl)-2-methyl-but-3-yn-2-yl]piperazine-1-carboxyl-ate

The title sterically congested piperazine derivative, C20H27FN2O2, was prepared using a modified Bruylants approach. A search of the Cambridge Structural Database identified 51 compounds possessing an N-tert-butyl piperazine substructure. Of these only 14 were asymmetrically substituted on the piperazine ring and none with a synthetically useful second nitro-gen. Given the novel chemistry generating a pharmacologically useful core, determination of the crystal structure for this compound was necessary. The piperazine ring is present in a chair conformation with di-equatorial substitution. Of the two N atoms, one is sp 3 hybridized while the other is sp 2 hybridized. Inter-molecular inter-actions resulting from the crystal packing patterns were investigated using Hirshfeld surface analysis and fingerprint analysis. Directional weak hydrogen-bond-like inter-actions (C-H⋯O) and C-H⋯π inter-actions with the dispersion inter-actions as the major source of attraction are present in the crystal packing.

Shono-Type Oxidation for Functionalization of N-Heterocycles

The development of facile synthetic methods for stereodefined aliphatic cyclic amines is an important research field in synthetic organic chemistry since such scaffolds constitute a variety of natural products and biologically active compounds. N-Acyl cyclic N,O-acetals which prepared by electrochemical oxidation of the corresponding cyclic amines have proven to be useful and versatile precursors for the synthesis of such skeletons. In this Personal Account, we introduce our efforts toward the development of synthetic strategies for the diastereo- and/or enantioselective synthesis of cyclic amines by using electrochemically prepared cyclic N,O-acetals. In addition, the investigation of the "memory of chirality" in the electrooxidative methoxylation of N-acyl amino acid derivatives, the strategy for the synthesis of chiral azabicyclic compounds by utilizing electrochemical oxidation, and halogen cation-mediated synthesis of nitrogen-containing heterocycles are also described.

Visible-Light-Mediated Carbonyl Alkylative Amination to All-Alkyl α-Tertiary Amino Acid Derivatives

The all-alkyl α-tertiary amino acid scaffold represents an important structural feature in many biologically and pharmaceutically relevant molecules. Syntheses of this class of molecule, however, often involve multiple steps and require activating auxiliary groups on the nitrogen atom or tailored building blocks. Here, we report a straightforward, single-step, and modular methodology for the synthesis of all-alkyl α-tertiary amino esters. This new strategy uses visible light and a silane reductant to bring about a carbonyl alkylative amination reaction that combines a wide range of primary amines, α-ketoesters, and alkyl iodides to form functionally diverse all-alkyl α-tertiary amino esters. Brønsted acid-mediated in situ condensation of primary amine and α-ketoester delivers the corresponding ketiminium species, which undergoes rapid 1,2-addition of an alkyl radical (generated from an alkyl iodide by the action of visible light and silane reductant) to form an aminium radical cation. Upon a polarity-matched and irreversible hydrogen atom transfer from electron rich silane, the electrophilic aminium radical cation is converted to an all-alkyl α-tertiary amino ester product. The benign nature of this process allows for broad scope in all three components and generates structurally and functionally diverse suite of α-tertiary amino esters that will likely have widespread use in academic and industrial settings.

Regio- and enantioselective amination of acyclic branched α-alkynyl ketones: asymmetric construction of N-containing quaternary stereocenters

Direct regio- and enantioselective amination of acyclic α-branched ketones enabled by the α-alkynyl group.

Lewis acid-catalyzed regioselective C–H carboxamidation of indolizines with dioxazolones via an acyl nitrene type rearrangement

An efficient, direct, and novel Lewis acid-catalyzed regioselective C–H carboxamidation of indolizines with dioxazolones via an acyl nitrene type rearrangement under metal-free conditions has been documented.

Enantioconvergent Amination of Racemic Tertiary C-H Bonds

Racemization is considered to be an intrinsic stereochemical feature of free radical chemistry as can be seen in traditional radical halogenation reactions of optically active tertiary C-H bonds. If the facile process of radical racemization could be effectively combined with an ensuing step of bond formation in an enantioselective fashion, then it would give rise to deracemizative functionalization of racemic tertiary C-H bonds for stereoselective construction of chiral molecules bearing quaternary stereocenters. As a demonstration of this unique potential in radical chemistry, we herein report that metalloradical catalysis can be successfully applied to devise Co(II)-based catalytic system for enantioconvergent radical amination of racemic tertiary C(sp³)-H bonds. The key to the success of the radical process is the development of Co(II)-based metalloradical catalyst with fitting steric, electronic, and chiral environments of the D₂-symmetric chiral amidoporphyrin as the supporting ligand. The existence of optimal reaction temperature is recognized as an important factor in the realization of the enantioconvergent radical process. Supported by an optimized chiral ligand, the Co(II)-based metalloradical system can effectively catalyze the enantioconvergent 1,6-amination of racemic tertiary C(sp³)-H bonds at the optimal temperature, affording chiral α-tertiary amines in excellent yields with high enantiocontrol of the newly created quaternary stereocenters. Systematic studies, including experiments utilizing optically active deuterium-labeled C-H substrates as a model system, shed light on the underlying mechanistic details of this new catalytic process for enantioconvergent radical C-H amination. The remarkable power to create quaternary stereocenters bearing multiple functionalities from ubiquitous C-H bonds, as showcased with stereoselective construction of bicyclic N-heterocycles, opens the door for future synthetic applications of this new radical technology.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

1,3-Dipolar cycloaddition of isatin N,N'-cyclic azomethine imines with α,β-unsaturated aldehydes catalyzed by DBU in water

A simple and green procedure was established by [3 + 3] cycloaddition reaction of isatin derived cyclic imine 1,3-dipoles with α,β-unsaturated aldehydes, giving the desired spiro heterocyclic oxindoles with aza-quaternary centers in good yields and diastereoselectivities. It should be noted that water can be employed as a suitable solvent for the improvement of diastereoselectivity.

A simple and green procedure was established by [3 + 3] cycloaddition reaction of isatin derived cyclic imine 1,3-dipoles with α,β-unsaturated aldehydes, giving spirooxindoles with aza-quaternary center in good yields and diastereoselectivities.

Regio- and Enantioselective Synthesis of Trifluoromethyl-Substituted Homoallylic α-Tertiary NH2 -Amines by Reactions Facilitated by a Threonine-Based Boron-Containing Catalyst

A method for catalytic regio- and enantioselective synthesis of trifluoromethyl-substituted and aryl-, heteroaryl-, alkenyl-, and alkynyl-substituted homoallylic α-tertiary NH₂ -amines is introduced. Easy-to-synthesize and robust N-silyl ketimines are converted to NH-ketimines in situ, which then react with a Z-allyl boronate. Transformations are promoted by a readily accessible l-threonine-derived aminophenol-based boryl catalyst, affording the desired products in up to 91 % yield, >98:2 α:γ selectivity, >98:2 Z:E selectivity, and >99:1 enantiomeric ratio. A commercially available aminophenol may be used, and allyl boronates, which may contain an alkyl-, a chloro-, or a bromo-substituted Z-alkene, can either be purchased or prepared by catalytic stereoretentive cross-metathesis. What is more, Z-trisubstituted allyl boronates may be used. Various chemo-, regio-, and diastereoselective transformations of the α-tertiary homoallylic NH₂ -amine products highlight the utility of the approach; this includes diastereo- and regioselective epoxide formation/trichloroacetic acid cleavage to generate differentiated diol derivatives.