Ferrier Glycosylation Mediated by the TEMPO Oxoammonium Cation

The TEMPO oxoammonium cation has been proven to be both an efficient oxidizing reagent and an electrophilic substrate frequently found in organic reactions. Here, we report that this versatile chemical reagent can also be used as an efficient promoter for C- and N-glycosylation reactions through a Ferrier rearrangement with moderate to high yields. This unprecedented reactivity is explained in terms of a Lewis acid activation of glycal by TEMPO+ forming a type of glycal-TEMPO+ mesomeric structure, which occurs through an extended vinylogous hyperconjugation toward the π*(O═N+) orbital [LP(O1) → π*(C1═C2), π*(C1═C2) → σ*(C3-O3), and LP(O6) → π*(O═N+)]. This enables the formation of the respective Ferrier glycosyl cation, which is trapped by various nucleophiles. The extended hyperconjugation (or double hyperconjugation) toward the π*(O═N+) orbital, which confers the Lewis acid character of the TEMPO cation, was supported by natural bond orbital analysis at the M06-2X/6-311+G** level of theory.

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.

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.

Photoinduced α-C-H Amination of Cyclic Amine Scaffolds Enabled by Polar-Radical Relay

Herein, we report a polar-radical relay strategy for α-C-H amination of cyclic amines with N-chloro-N-sodio-carbamates. The relay is initiated by in situ generation of cyclic iminium intermediate using N-iodosuccinimide (NIS) oxidant as an initiator, which then operates through a series of polar (addition and elimination) and radical (homolysis, hydrogen- and halogen atom transfer) reactions to enable the challenging C-N bond formation in a controlled manner. A broad range of α-amino cyclic amines were readily accessed with excellent regioselectivity, and the superb applicability was further demonstrated by functionalization of biologically relevant compounds.

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.

Contrasteric coupling of allenes and tetrahydroisoquinolines by iron-catalysed allenic C(sp2)-H functionalisation

An iron-catalysed C-H functionalisation of simple monosubstituted allenes for the synthesis of 1-tetrahydroisoquinolinyl 1,1-disubstituted allenes is reported. This transformation represents the first example of a direct conversion of allenic C-H bonds to C-C bonds through cross dehydrogenative coupling. The optimized protocol features broad scope and employs mild, functional group tolerant conditions.

Mechanism-Based Approach to Reagent Selection for Oxidative Carbon-Hydrogen Bond Cleavage Reactions

Numerous hydride-abstracting agents generate the same cationic intermediate, but substrate features such as intermediate cation stability, oxidation potential, and steric environment can influence reaction rates in an oxidant-dependent manner. This manuscript provides experimental data to illustrate the role that structural features play in the kinetics of hydride abstraction reactions with commonly used quinone-, oxoammonium ion-, and carbocation- based oxidants. Computational studies of the transition state structures and energies explain these results and energy decomposition analysis calculations reveal unique sensitivities to electrostatic attraction and steric repulsions. Rigorous rate studies of select reactions validated the capacity of the calculations to predict reactivity trends. Additionally, kinetics studies demonstrate the potential for product inhibition in DDQ-mediated reactions. These studies provide a clear guide to select the optimal oxidant for structurally disparate substrates and lead to predictions of reactivity that were validated experimentally.

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.

Transition-Metal-Free Strategies for the Synthesis of C-1 Aryl-Substituted Tetrahydroisoquinolines

1-Aryl-1,2,3,4-tetrahydroisoquinolines are important structural motifs and are widely found in bioactive molecules, pharmaceuticals and synthetic drugs. In view of increasing environmental awareness, the development of transition-metal-free strategies for the synthesis of these compounds is highly desirable. Metal-free oxidative coupling and lithiation methodologies have emerged as effective tools in this area as they exclude the use of transition-metal catalysts and help in reducing unwanted and toxic-metal-based chemical waste in the environment. This review highlights recent advances on the direct arylation of tetrahydroisoquinolines for the synthesis of the title compounds in the absence of a metal salt. Also, the emphasis has been placed on mechanistic considerations of these reactions.

1 Introduction

2 Arylation of Tetrahydroisoquinolines via Oxidative Coupling

2.1 Arylation Using Grignard Reagents

2.2 Arylation Using Other Organometallic Reagents

2.3 Arylation Using Aryl Organoboranes or Arenes

3 Arylation of Tetrahydroisoquinolines via Lithiation

3.1 Intermolecular Arylation

3.2 Intramolecular Arylation

4 Conclusion and Outlook

C(sp3)–H functionalization with isocyanides

This review highlights the state-of-the-art advances in C(sp³)–H functionalization involving isocyanides through the synergistic combination of isocyanide insertion and C(sp³)–H bond activation.

CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization to access 5,6-dihydro-pyrrolo[2,1-a]isoquinolines

An efficient and enviromentally friendly CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization cascade was realized with N-substituted tetrahydroisoquinolines and electron-deficient olefins. Under the mild conditions, the reaction proceeded smoothly and displayed excellent functional group tolerance, affording 5,6-dihydro-pyrrolo[2,1-a]isoquinolines in good to high yields. This protocol exhibits a broad substrate scope to both N-alkyl tetrahydroisoquinolines and dipolarophile substrates.

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.

Toward Rational Understandings of α-C-H Functionalization: Energetic Studies of Representative Tertiary Amines

Functionalization of α-C–H bonds of tertiary amines to build various α-C–X bonds has become a mainstream in synthetic chemistry nowadays. However, due to lack of fundamental knowledge on α-C–H bond strength as an energetic guideline, rational exploration of new synthetic methodologies remains a far-reaching anticipation. Herein, we report a unique hydricity-based approach to establish the first integrated energetic scale covering both the homolytic and heterolytic energies of α-C–H bonds for 45 representative tertiary amines and their radical cations. As showcased from the studies on tetrahydroisoquinolines (THIQs) by virtue of their thermodynamic criteria, the feasibility and mechanisms of THIQ oxidation were deduced, which, indeed, were found to correspond well with experimental observations. This integrated scale provides a good example to relate bond energetics with mechanisms and thermodynamic reactivity of amine α-C–H functionalization and hence, may be referenced for analyzing similar structure-property problems for various substrates.

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A unique hydricity-based methodology for bond energy determination

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The first integrated α-C–H bond energy scale of tertiary amines

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Thermodynamics-based diagnosis of the feasibility/mechanism of amine oxidation

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.

Copper-Catalyzed Aerobic Oxidative Cyclization Cascade to Construct Bridged Skeletons: Total Synthesis of (-)-Suaveoline

Based on the discovery of copper-catalyzed cyclopropanol ring-opening addition to iminium ions, an unprecedented catalytic aerobic C-H oxidation/cyclopropanol cyclization cascade using CuCl₂ as the multifunctional catalyst and air as the oxidant was developed to construct the azabicyclo[3.3.1]nonane skeleton, which is widespread in natural products and medicines. Using this method, concise asymmetric total synthesis of the indole alkaloid (-)-suaveoline was achieved. This study not only provides an efficient, low-cost, and environmentally benign method for constructing such bridged frameworks, but also enriches the realm of cyclopropanol chemistry and C-H functionalization.

Direct (het)arylation of tetrahydroisoquinolines via a metal and oxidant free C(sp3)-H functionalization enabled three component reaction

An unprecedented method for the direct arylation and heteroarylation of tetrahydroisoquinolines under metal and oxidant free conditions is reported. The arylation reactions occurred via a C(sp3)-H functionalization enabled three component condensation of tetrahydroisoquinolines, 9-fluorenone imine, and arenes without involving a pre-functionalization/pre-derivatization step. A wide range of arenes and heteroarenes participated in the reaction to provide structurally diverse arylated tetrahydroisoquinolines with good to excellent yields.

Visible-light-mediated de-aminative alkylation of N-arylamines with alkyl Katritzky salts

A visible-light-mediated de-aminative alkylation of N-arylamines has been developed, providing direct access to α-amino C–H functionalization from readily available Katritzky salts under mild conditions.

Bimolecular oxidative C–H alkynylation of α-substituted isochromans

The first bimolecular oxidative C–H functionalization of secondary benzylic ethers for tertiary ether synthesis has been established in high efficiency.

Iridium-Catalyzed Highly Regioselective and Diastereoselective Allylic Etherification To Access cis-2,6-Disubstituted Dihydropyridinones

A highly regio- and diastereoselective method to access cis-2,6-disubstituted dihydropyridinones under mild conditions by an iridium-catalyzed allylic etherification is reported. cis-2,6-Disubstituted dihydropyridinones are important precursors for the de novo synthesis of the corresponding piperidine alkaloids and iminosugars. This strategy features a broad substrate scope, high yields, and excellent regio- and diastereoselectivities. A π-allyl-Ir intermediate is involved in the mechanism. The strong A^1,3-strain from the tosyl group may also favor the formation of cis-products in this transformation.

Copper-catalyzed oxidative cross-dehydrogenative coupling of 2H-chromenes and terminal alkynes

An efficient copper-catalyzed cross-dehydrogenative coupling of 2H-chromenes and terminal alkynes mediated by DDQ has been established. A protic additive, EtOH, proved to be crucial for harmonizing the oxidation with a subsequent alkynylation step by retaining the oxidation state of an oxocarbenium ion in the form of acetal. The CDC reaction exhibits a good substrate scope, with a range of terminal aryl- and alkyl alkynes being well tolerated. The copper-catalyzed alkynylation of 2H-chromene acetals with terminal alkynes was also explored.

Multicomponent Synthesis of Tetrahydroisoquinolines

A multicomponent synthesis of tetrahydroisoquinolines from carboxylic acids, alkynyl ethers, and dihydroisoquinolines is described. This process features readily available starting materials, simple experimental procedures for achievement of molecule complexity, and structural diversity. The preliminary control experiment and crossover reaction provide important insight into the reaction mechanism. The formed tetrahydroisoquinolines could be transformed to an array of compounds.

Direct oxidative C-H alkynylation of N-carbamoyl tetrahydroisoquinolines and dihydroisoquinolines

An efficient oxidative C-H alkynylation of N-carbamoyl tetrahydroisoquinolines mediated by a TEMPO oxoammonium salt has been established. A variety of electronically varied N-carbamoyl tetrahydroisoquinolines reacted with a range of alkynyl potassium trifluoroborates smoothly under mild metal-free conditions. Dihydroisoquinolines were also suitable components for the reaction. The synthetic applicability of the method for facile access to structurally diverse bioactive molecules was further demonstrated.

A novel tetrahydroisoquinoline (THIQ) analogue induces mitochondria-dependent apoptosis

Lung cancer continues to be a leading cause of cancer-related death worldwide, with non-small cell lung cancer (NSCLC) accounting for more than 80% of lung cancer cases. Current therapies for NSCLC have only limited effect and treatment resistance develops rapidly. In a previous study, we have shown that C₁-phenylethynyl tetrahydroisoquinoline (THIQ) analogue 4 has anti-proliferative activity against PC3 human prostate cancer cells. However, this anticancer effect was achieved with relatively high IC₅₀ in A549 lung cancer cells. To improve the potency of the drug, in the present study, a series of novel THIQ analogues (analogues 5a-d) were prepared by using an oxidative C-H functionalization strategy, and their potential anticancer activities on A549 lung cancer cells were investigated. Among these analogues, analogue 5c can markedly inhibit A549 cell proliferation in a dose-dependent manner with a reasonable IC₅₀ of 14.61 ± 1.03 μM. This effect was mediated by analogue 5c-induced G0/G1 phase arrest and cell apoptosis. Treatment with analogue 5c was shown to induce reactive oxygen species (ROS) accumulation, disruption of mitochondrial membrane potential, reduction of glutathione, elevation of intracellular calcium ion (Ca²⁺), and activation of Caspase-3. Furthermore, analogue 5c can lead to DNA double-strand break and the activation of p53 pathway in A549 cells. In conclusion, the oxidative C-H functionalization strategy to generate analogue 5c could improve the drug anticancer efficacy by inducing mitochondria-dependent apoptosis in A549 cells.

Oxidative Difluoromethylation of Tetrahydroisoquinolines Using TMSCF2SPh: Synthesis of Fluorinated Pyrrolo[2,1-a]isoquinolines and Benzo[a]quinolizidines

An efficient C1-difluoromethylation of tetrahydroisoquinolenes was achieved using TMSCF₂SPh as a difluoromethylating agent and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^-) as an oxidant. The process provides an access to a variety of C1-difluoro(phenylsulfanyl)methylated tetrahydroisoquinoline adducts in good yields. These adducts were employed as key precursors for preparing fluorinated pyrrolo[2,1-a]isoquinoline and benzo[a]quinolizidines.

Substituted tetrahydroisoquinolines: synthesis, characterization, antitumor activity and other biological properties

This work deals with the molecular design, synthesis and biological activity of a series of tetrahydro[1,4]dioxanisoquinolines and dimethoxyisoquinoline analogues. This study describes the synthesis strategy of these potential antitumor compounds, their multi-step synthesis and their optimization. A series of tetrahydroisoquinolines was synthesized and their cytotoxicity evaluated. Some of these tetrahydroisoquinolines showed promising KRas inhibition, antiangiogenesis activity and antiosteoporosis properties. Molecular modeling studies showed that compound 12 bind in the p1 pocket of the KRas protein making interactions with the hydrophobic residues Leu56, Tyr64, Tyr71 and Thr74 and hydrogen bonds with residues Glu37 and Asp38.

Auto-oxidation promoted sp3 C–H arylation of glycine derivatives

An auto-oxidation promoted sp³ C–H arylation reaction between N-aryl glycine derivatives and electron-rich arenes, leading to the formation of N-aryl α-aryl α-amino acid derivatives, is described. This atom-economical and environmentally benign reaction proceeds smoothly under mild reaction conditions and requires only Brønsted acid and oxygen (balloon). A plausible radical involved mechanism is proposed.

t-BuOK-Mediated Oxidative Dehydrogenative C(sp3)-H Arylation of 2-Alkylazaarenes with Nitroarenes

The first transition-metal free and regioselective C(sp³)-H arylation of 2-alkylazaarenes with nitroarenes has been achieved via t-BuOK-mediated dehydrogenative C(sp³)-C(sp²) coupling. This reaction provides an efficient access to the biologically important and synthetically useful 2-benzyl-substituted azaarenes under mild conditions without the need of prefunctionalization of 2-alkylazaarenes or using the specialized arylating agents.

Metal-Free Sequential C(sp2)-H/OH and C(sp3)-H Aminations of Nitrosoarenes and N-Heterocycles to Ring-Fused Imidazoles

Hydrogen bond assisted ortho-selective C(sp²)-H amination of nitrosoarenes and subsequent α-C(sp³)-H functionalization of aliphatic amines is achieved under metal-free conditions. The annulation of nitrosoarenes and 2-hydroxy-C-nitroso compounds with N-heterocycles provides a facile excess to a wide range of biologically relevant ring-fused benzimidazoles and structurally novel polycyclic imidazoles, respectively. Nucleophilic aromatic hydrogen substitution (S_NArH) was found to be preferred over classical S_NAr reaction during the C(sp²)-H amination of halogenated nitrosoarenes.