Highly enantioselective catalytic cross-dehydrogenative coupling of N-carbamoyl tetrahydroisoquinolines and terminal alkynes

Photomediated One-Pot Three-Component Approach Enables the Formal Direct N-Acylation/Sulfonylation and α-C-H Functionalization of 1,2,3,4-Tetrahydroisoquinoline

N-Acyl/sulfonyl-α-functionalized 1,2,3,4-tetrahydroisoquinolines (THIQs) are significant structural motifs in organic synthesis and drug discovery. However, the one-pot approach enabling direct difunctionalization of THIQs remains challenging. Herein we report a photomediated one-pot three-component strategy to access N-acyl/sulfonyl-α-functionalized THIQs. This method features the use of oxygen (from air) as the green oxidant, high atom and step economy, and decent structural diversity. The synthetic applicability of the method was further demonstrated via the facile construction of valuable bioactive molecules. Mechanistic studies indicated that oxidation with singlet oxygen and the acceptor-less dehydrogenation were involved in the photoredox process.

Catalytic Enantioselective Alkyne Addition to Nitrones Enabled by Tunable Axially Chiral Imidazole-Based P,N-Ligands

Although catalytic enantioselective alkyne addition is an established method for the synthesis of chiral propargylic alcohols and amines, addition to nitrones presents unique challenges, and no general chiral catalyst system has been developed. In this manuscript, we report the first Cu-catalyzed enantioselective alkyne addition to nitrones utilizing tunable axially chiral imidazole-based P,N-ligands. Our approach effectively overcomes difficulties in both reactivity and selectivity, resulting in a simple Cu-catalyzed protocol. The reaction accommodates a wide range of nitrones and alkynes, enabling the streamlined synthesis of chiral propargyl N-hydroxylamines via the enantioselective C-C bond formation. A diverse array of optically active nitrogen-containing compounds, including chiral hydroxylamines, can be accessed directly through facile transformations of the reaction products.

Green Light Promoted Iridium(III)/Copper(I)-Catalyzed Addition of Alkynes to Aziridinoquinoxalines Through the Intermediacy of Azomethine Ylides

This manuscript describes the development of alkyne addition to the aziridine moiety of aziridinoquinoxalines using dual Ir(III)/Cu(I) catalytic system under green light-emitting diode (LED) photolysis (λmax =525 nm). This mild method features high levels of chemo- and regioselectivity and was used to generate 30 highly functionalized substituted dihydroquinoxalines in 36-98 % yield. This transformation was also carried asymmetrically using phthalazinamine-based chiral ligand to provide 9 chiral addition products in 96 : 4 to 86 : 14 e.r. The experimental and quantum chemical explorations of this reaction suggest a mechanism that involves Ir(III)-catalyzed triplet energy transfer followed by a ring-opening reaction ultimately leading to the formation of azomethine ylide intermediates. These azomethine intermediates undergo sequential protonation/copper(I) acetylide addition to provide the products.

Enantioselective Synthesis of C2-Quaternary Indolin-3-ones by Pt-Catalyzed Alkynylation of 2-Aryl-3H-indol-3-one with Alkynylsilanes

An efficient method for the synthesis of five-membered chiral propargylic amines from 2-aryl-3H-indol-3-one and alkynylsilanes has been developed. The reaction proceeded under the catalytic system of PtCl4, oxazoline-based ligand L11, Zn(CF3COO)2, and AcOH in DCE at 95 °C via in situ desilylation of TMS-alkynes. This methodology also highlights a new protocol for the in situ desilylation of alkynylsilanes. The reaction showed a broad substrate scope with good yields and enantioselectivity.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

C(sp3)-H Aerobic Alkenylation of Tetrahydroisoquinolines via Organic Electrosynthesis

A method for the C(sp³)-H alkenylation of N-aryl-tetrahydroisoquinoline (THIQ) has been developed by the combination of electrooxidation and a copper catalyst. The corresponding products were obtained with good to excellent yields under mild conditions. Besides, the addition of TEMPO as an electron mediator is crucial to this transformation, since the oxidative reaction could proceed under a low electrode potential. In addition, the catalytic asymmetric variant has also been demonstrated with good enantioselectivity.

Hydride-Abstraction-Initiated Catalytic Stereoselective Intermolecular Bond-Forming Processes

The stereoselective intermolecular bond-forming reactions through the direct manipulation of ubiquitous yet inert C(sp³)-H bonds represent an important and long-standing goal in chemistry. In particular, developing such a stereoselective bimolecular transformation involving carbocation intermediates generated via site-selective hydride abstraction or formal hydride abstraction by organic oxidants would avoid the preinstallation of directing groups and is therefore attractive. Hydride-abstraction-initiated bimolecular transformations have received considerable attention, but existing examples lack stereoselective studies. Prevalent stereoselective studies typically suffer from the narrow substrate scope of specific and highly reactive N-aryl amines and diarylmethanes together with limited synthetic utility. This Account describes our recent advances in the development and synthetic application of hydride-abstraction-initiated stereoselective intermolecular C-C and C-H bond-forming processes with significantly expanded scopes involving structurally diverse N-acyl amines and ethers together with nitriles, esters, and perfluoroalkyl moieties.We first explored hydride-abstraction-initiated stereoselective intermolecular C-C bond-forming processes. Utilizing triarylmethyl cations or oxoammonium ions as hydride abstractors, we accomplished the diastereoselective oxidative C-H functionalization of structurally diverse N-acyl amines and ethers with a range of organoboranes and C-H components, efficiently installing a series of alkyl, alkenyl, aryl, and alkynyl species into the α-position of heteroatoms with good levels of diastereocontrol. Subsequently, we developed an "acetal pool" strategy as the toolbox to regulate the stability of cationic intermediates and the compatibility of organic oxidants with a delicate asymmetric catalysis system. Utilizing this strategy, we achieved the catalytic enantioselective oxidative C-H alkenylation, arylation, alkynylation, and alkylation of diverse N-acyl heterocycles with a range of boronates and C-H components. Simultaneously, we extended this strategy to the asymmetric oxidative C-H alkylation of ethers. Notably, the method allows solvents that are used daily, such as tetrahydrofuran, tetrahydropyran, and diethyl ether, to be facilely transformed to high-value-added optically pure bioactive molecules. We further expanded the scope of this challenging area from the C(sp³)-H bond adjacent to electron-donating heteroatoms to valuable electron-withdrawing functional groups including nitriles, esters, and perfluoroalkyl moieties for the stereoselective construction of single and vicinal quaternary carbon stereocenters, respectively.We studied hydride-abstraction-initiated catalytic asymmetric intermolecular C-H bond-forming processes, known as redox deracemization. Utilizing the acetal pool strategy, we reported the first redox deracemization of cyclic benzylic ethers. Later, we disclosed an aerobic one-pot deracemization of diverse α-amino acid derivatives with excellent functional group compatibility. We further achieved the deracemization of the tertiary stereogenic center adjacent to electron-withdrawing groups including perfluoroalkyl, cyano, and ester moieties, which are otherwise difficult to construct.

Synthesis of Pyrrolo[2,1-a]isoquinolines through Cu-Catalyzed Condensation/Addition/Oxidation/Cyclization Cascade

We have developed a copper-catalyzed synthesis of pyrrolo[2,1-a]isoquinolines with terminal alkynes, aldehydes, and tetrahydroisoquinolines. A variety of pyrrolo[2,1-a]isoquinolines have been prepared in 17-69% yield via a condensation/Mannich-type addition/oxidation/cyclization cascade sequence. Modifications through simple chemical transformations provided highly functionalized molecules containing a privileged framework.

Stereoselective Synthesis of Benzo[a]quinolizidines via Aerobic DDQ-Catalyzed Allylation and Reductive Cyclization

Stereoselective synthesis of C₄-substituted benzo[a]quinolizidines via redox-controlled catalytic C-C-bond-forming reactions was carried out. Aerobic DDQ-catalyzed allylation of N-Cbz tetrahydroisoquinolines efficiently provided α-allylated products 5, which were transformed to enones 6 via cross-metathesis reactions using the second-generation Hoveyda-Grubbs catalyst. Palladium-catalyzed hydrogenation of 6 prompted alkene reduction, protecting group removal, and intramolecular reductive amination in one step to afford the desired benzo[a]quinolizidines 7 as single diastereomers.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

Electrochemical synthesis of 1,2,4-oxadiazoles from amidoximes through dehydrogenative cyclization

A convenient and efficient method for the generation of the iminoxy radical through anodic oxidation was developed for the synthesis of 3,5-disubstituted 1,2,4-oxadiazoles from N-benzyl amidoximes. The transformation proceeds through 1.5-Hydrogen Atom Transfer (1,5-HAT) and intramolecular cyclization. The process features simple operation, mild conditions, broad substrate scope and high functional group compatibility, and provides a facile and practical way for the preparation of 1,2,4-oxadiazoles.

Copper-Catalyzed Intermolecular Enantioselective Radical Oxidative C(sp3 )-H/C(sp)-H Cross-Coupling with Rationally Designed Oxazoline-Derived N,N,P(O)-Ligands

The intermolecular asymmetric radical oxidative C(sp³ )-C(sp) cross-coupling of C(sp³ )-H bonds with readily available terminal alkynes is a promising method to forge chiral C(sp³ )-C(sp) bonds because of the high atom and step economy, but remains underexplored. Here, we report a copper-catalyzed asymmetric C(sp³ )-C(sp) cross-coupling of (hetero)benzylic and (cyclic)allylic C-H bonds with terminal alkynes that occurs with high to excellent enantioselectivity. Critical to the success is the rational design of chiral oxazoline-derived N,N,P(O)-ligands that not only tolerate the strong oxidative conditions which are requisite for intermolecular hydrogen atom abstraction (HAA) processes but also induce the challenging enantiocontrol. Direct access to a range of synthetically useful chiral benzylic alkynes and 1,4-enynes, high site-selectivity among similar C(sp³ )-H bonds, and facile synthesis of enantioenriched medicinally relevant compounds make this approach very attractive.

Catalytic Asymmetric Alkynylation of 3,4-Dihydro-β-carbolinium Ions Enables Collective Total Syntheses of Indole Alkaloids

Chiral tetrahydro-β-carboline (THβC) is not only a prevailing structural feature of many natural alkaloids but also a versatile synthetic precursor for a vast array of monoterpenoid indole alkaloids. Asymmetric synthesis of C1-alkynyl THβCs remains rarely explored and challenging. Herein, we describe the development of two complementary approaches for the catalytic asymmetric alkynylation of 3,4-dihydro-β-carbolinium ions with up to 96 % yield and 99 % ee. The utility of chiral C1-alkynyl THβCs was demonstrated by the collective total syntheses of seven indole alkaloids: harmicine, eburnamonine, desethyleburnamonine, larutensine, geissoschizol, geissochizine, and akuammicine.

Asymmetric Deoxygenative Alkynylation of Tertiary Amides Enabled by Iridium/Copper Bimetallic Relay Catalysis

A variety of inert tertiary amides have been successfully transformed into synthetically important chiral propargylamines in high yields with good to excellent enantioselectivities via a relayed sequence of Ir catalyzed partial reduction and Cu/GARPHOS catalyzed asymmetric alkynylation with terminal alkynes. The reaction was readily extended to some drug molecules and the transformations of representative products have been demonstrated, thus attesting the practical utilities and the robust nature of the protocol.

Photocatalyzed Csp3-Csp3 cross-dehydrogenative coupling of N-Boc-tetrahydroisoquinolines with α,β-unsaturated ketones

A novel photocatalyzed cross-dehydrogenative coupling reaction of N-Boc-tetrahydroisoquinolines with α,β-unsaturated ketones has been developed. This research provides an easy access to a variety of C1-substituted tetrahydroisoquinolines, which can be further transformed into benzo[a]-quinolizine-2-ones, the skeletons of natural products with a wide range of biological activities. The load of the photocatalyst is low and the oxidant is inexpensive and less toxic.

Direct functionalization of quinoxalin-2(1H)-one with alkanes: C(sp2)-H/C(sp3)-H cross coupling in transition metal-free mode

Considering the significance of pharmaceutically important heterocycles, efficient and highly versatile protocols for the functionalization of diverse heterocycles with easily accessible feedstock are crucial. Here, we have reported selective alkylation of quinoxalin-2(1H)-one with a broad class of hydrocarbons having different C(sp³)-H bonds with varying bond strengths using di-tert-butyl peroxide (DTBP) as an alkoxyl radical mediator for hydrogen atom transfer (HAT). This dehydrogenative coupling approach utilizes feedstock chemicals such as cycloalkanes, cyclic ethers and alkyl arenes as coupling partners. This protocol exhibits good functional group compatibility and selectivity regarding both heterocycles and unactivated alkanes. Moreover, this methodology allows functionalization of relatively strong C-H bonds of adamantane and exclusive selectivity towards 3° C(sp³)-H bonds is observed. We also illustrate the applicability of this C(sp²)-H/C(sp³)-H cross-coupling for practical access to bioactive pharmaceuticals.

Iridium-Catalyzed Asymmetric Cascade Allylation/Pictet-Spengler Cyclization Reaction for the Enantioselective Synthesis of 1,3,4-Trisubstituted Tetrahydroisoquinolines

An iridium-catalyzed trifluoroacetic acid-promoted asymmetric cascade allylation/Pictet-Spengler cyclization reaction of azomethine ylides with aromatic allylic alcohols is reported. This protocol provides a facile and scalable method for the construction of 1,3,4-trisubstituted tetrahydroisoquinolines containing two stereogenic centers in good yields (up to 96%) with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Furthermore, a series of aromatic heterocycle-fused piperidines were also obtained with excellent enantiocontrol by this methodology.

Enantioselective Synthesis of 1-Aryl Tetrahydroisoquinolines by the Rhodium-Catalyzed Reaction of 3,4-Dihydroisoquinolinium Tetraarylborates

The 1-aryl tetrahydroisoquinolines (1-aryl THIQs) are omnipresent in biologically active molecules. Here we report on the direct asymmetric synthesis of these valuable compounds via the reaction of 3,4-dihydroisoquinolinium tetraarylborates. The dual roles of anionic tetraarylborates, which function as both prenucleophiles and stabilizers of 3,4-dihydroisoquinolinium cations, enable this rhodium(I)-catalyzed protocol to convergently provide enantioenriched 1-aryl THIQs in good yields (≤95%) with ≤97% ee, as demonstrated by the formal synthesis of (-)-solifenacin and the facile synthesis of (-)-Cryptostyline I.

Enantioselective Alkynylation of Unstabilized Cyclic Iminium Ions

An enantioselective copper-catalyzed alkynylation of unstabilized cyclic iminium ions has been developed. Whereas such alkynylations typically utilize pyridinium, quinolinium and isoquinolinium intermediates, this method enables use of cyclic iminium ions unstabilized by resonance. With the use of a Lewis acid and copper catalyst, these iminium ions are generated in situ from readily available hemiaminal methyl ethers and transformed into highly enantioenriched α-alkynylated cyclic amines. A variety of terminal alkynes can be incorporated in high yields and enantiomeric excesses.

Direct Conversion of N-Alkylamines to N-Propargylamines through C-H Activation Promoted by Lewis Acid/Organocopper Catalysis: Application to Late-Stage Functionalization of Bioactive Molecules

An efficient catalytic method to convert an α-C-H bond of N-alkylamines into an α-C-alkynyl bond was developed. In the past, such transformations were carried out under oxidative conditions, and the enantioselective variants were confined to tetrahydroisoquinoline derivatives. Here, we disclose a method for the union of N-alkylamines and trimethylsilyl alkynes, without the presence of an external oxidant and promoted through cooperative actions of two Lewis acids, B(C6F5)3 and a Cu-based complex. A variety of propargylamines can be synthesized in high diastereo- and enantioselectivity. The utility of the approach is demonstrated by the late-stage site-selective modification of bioactive amines. Kinetic investigations that shed light on various mechanistic nuances of the catalytic process are presented.

Cross-Dehydrogenative Alkynylation: A Powerful Tool for the Synthesis of Internal Alkynes

Alkynes are among the most fundamentally important organic compounds and are widely used in synthetic chemistry, biochemistry, and materials science. Thus, the development of an efficient and sustainable method for the preparation of alkynes has been a central concern in organic synthesis. Cross-dehydrogenative coupling utilizing E-H and Z-H bonds in two different molecules can avoid the need for prefunctionalization of starting materials and has become one of the most straightforward methods for the construction of E-Z chemical bonds. This Review summarizes recent progress in the preparation of internal alkynes by cross-dehydrogenative coupling with terminal alkynes.

Direct and Efficient C(sp3)-H Functionalization of N-Acyl/Sulfonyl Tetrahydroisoquinolines (THIQs) With Electron-Rich Nucleophiles via 2,3-Dichloro-5,6-Dicyano-1,4-Benzoquinone (DDQ) Oxidation

A highly efficient metal-free oxidative direct C(sp³)–H functionalization of N-acyl/sulfonyl 1,2,3,4-tetrahydroisoquinolines (THIQs) with a wide range of electron-rich nucleophiles was accomplished under mild conditions through oxidation with DDQ and subsequent trapping of the resulting reactive and stable N-acyl/sulfonyl iminium ions. The synthetic utility of this method was illustrated by a concise and efficient total synthesis of (±)-benzo[a]quinolizidine (10) in 3 steps from the known N-Cbz 1,2,3,4-THIQ 4b.

Enantioselective Copper-Catalyzed Alkynylation of Benzylic C-H Bonds via Radical Relay

The first enantioselective alkynylation of benzylic C-H bonds via copper-catalyzed radical relay has been established herein, which provides an easy access to structurally diverse benzylic alkynes in good yields with excellent enantioselectivities. A key step for the asymmetric copper-catalyzed radical relay process is the enantioselective capture of a benzylic radical with chiral (Box)Cu^II-alkynyl species. In addition, the reaction displays good functional group tolerance, broad substrate scope, and mild conditions. The enantioenriched alkynylation products can be readily transformed into highly valuable synthons, such as chiral terminal alkynes, allenes, alkenes, and carboxylic acids. More importantly, our methodology can be applied to the synthesis of bioactive molecule AMG 837.

New Trends in Enantioselective Cross-Dehydrogenative Coupling

The development of cross-dehydrogenative coupling in recent years has simplified the synthesis of many materials, as a result of facile C–H activation, which, together with its greater atom economy and environmental friendliness, has made an impact on modern organic chemistry. Indeed, many C–C and C–X (X = N, O, P, S, B, or Si) coupling reactions can now be performed directly between two C–H bonds or a C–H and an X–H bond, simply by adding catalytic amounts of a metal salt to a mixture of the two and an oxidant to accept the two hydrogen atoms released. Chiral organocatalysts or chiral ligands have been joined to promote enantioselective processes, resulting in the development of efficient reaction cascades that provide products in high yields and high levels of asymmetric induction through cooperative catalysis. In recent years, photochemical oxidation and electrochemistry have widened even more the scope of cross-dehydrogenative coupling (CDC). In this review, we summarized the recent literature in this subject, hoping that it will inspire many new synthetic strategies.

Experimental and Computational Studies Unraveling the Peculiarity of Enolizable Oxoesters in the Organocatalyzed Mannich-Type Addition to Cyclic N-Acyl Iminium Ions

γ− and δ-Oxoesters are easily available starting materials that have been sparingly used in some organocatalyzed reactions proceeding with a high enantioselectivity. In our experimentation we found that the use of these compounds as the enolizable (nucleophilic) component in organocatalyzed Mannich-type reactions using in situ-generated cyclic N-acyl iminium ions gave low diastereoselectivity and low to moderate values of enantioselectivity. This significant drop of facial selectivity with respect to simple aliphatic aldehydes has been rationalized by means of density functional theory (DFT) calculations.

Enantioselective aerobic oxidative cross-dehydrogenative coupling of glycine derivatives with ketones and aldehydes via cooperative photoredox catalysis and organocatalysis

The combination of photoredox catalysis and enamine catalysis has enabled the development of an enantioselective aerobic oxidative cross-dehydrogenative coupling between glycine derivatives and simple ketones or aldehydes, which provides an efficient approach for the rapid synthesis of enantiopure unnatural α-alkyl α-amino acid derivatives in good yield with excellent diastereo- (up to >99 : 1) and enantioselectivities (up to 97% ee). This process includes the direct photoinduced oxidation of glycine derivatives to an imine intermediate, followed by the asymmetric Mannich-type reaction with an enamine intermediate generated in situ from a ketone or aldehyde and a chiral secondary amine organocatalyst. This mild method allows the direct formation of a C–C bond with simultaneous installation of two new stereocenters without wasteful removal of functional groups.

A visible-light-induced enantioselective aerobic oxidative cross-dehydrogenative coupling between glycine derivatives and simple ketones or aldehydes is achieved.

Oxidative Dearomative Cross-Dehydrogenative Coupling of Indoles with Diverse C-H Nucleophiles: Efficient Approach to 2,2-Disubstituted Indolin-3-ones

The oxidative, dearomative cross-dehydrogenative coupling of indoles with various C-H nucleophiles is developed. This process features a broad substrate scope with respect to both indoles and nucleophiles, affording structurally diverse 2,2-disubstituted indolin-3-ones in high yields (up to 99%). The oxidative dimerization and trimerization of indoles has also been demonstrated under the same conditions.