Direct functionalization of (un)protected tetrahydroisoquinoline and isochroman under iron and copper catalysis: two metals, two mechanisms

DDQ-catalyzed oxidative α-allylation of isochromans under aerobic conditions

The allylation of isochromans at the α-position via aerobic DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) catalysis is described. This process involves the DDQ oxidation of various isochromans under mild conditions to generate oxocarbenium intermediates, which are effectively stabilized in equilibration in the presence of acid before undergoing allylation. Molecular oxygen and tert-butyl nitrite are employed as an environmentally benign oxidant and mediator, respectively, in the catalytic cycle. The transformation of an allylated product to biologically active molecules has also been successfully demonstrated.

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.

Recent Advances in Functionalization of Pyrroles and their Translational Potential

Among the known aromatic nitrogen heterocycles, pyrrole represents a privileged aromatic heterocycle ranging its occurrence in the key component of "pigments of life" to biologically active natural products to active pharmaceuticals. Pyrrole being an electron-rich heteroaromatic compound, its predominant functionalization is legendary to aromatic electrophilic substitution reactions. Although a few excellent reviews on the functionalization of pyrroles including the reports by Baltazzi in 1963, Casiraghi and Rassu in 1995, and Banwell in 2006 are available, they are fragmentary and over fifteen years old, and do not cover the modern aspects of catalysis. A review covering a comprehensive package of direct functionalization on pyrroles via catalytic and non-catalytic methods including their translational potential is described. Subsequent to statutory yet concise introduction, the classical functionalization on pyrroles using Lewis acids largely following an ionic mechanism is discussed. The subsequent discussion follows the various metal-catalyzed C-H functionalization on pyrroles, which are otherwise difficult to implement by Lewis acids. A major emphasize is given on the radical based pyrrole functionalization under metal-free oxidative conditions, which is otherwise poorly highlighted in the literature. Towards the end, the current development of pyrrole functionalization under photocatalyzed and electrochemical conditions is appended. Only a selected examples of substrates and important mechanisms are discussed for different methods highlighting their scopes and limitations. The aromatic nucleophillic substitution on pyrroles (being an electron-rich heterocycle) happened to be the subject of recent investigations, which has also been covered accentuating their underlying conceptual development. Despite great achievements over the past several years in these areas, many challenges and problems are yet to be solved, which are all discussed in summary and outlook.

The synthesis of indeno[de]isochromene derivatives from arylvinyl epoxides and carbonyl compounds via tandem Nazarov and oxa-Pictet–Spengler cyclizations

The conversion of aryl vinyl epoxides and carbonyl compounds in the presence of a catalytic amount of BF3·OEt2 to indeno[de]isochromenes via tandem Nazarov and oxa-Pictet–Spengler cyclizations is described.

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.

Transition metal-free cross-dehydrogenative arylation of unactivated benzylic C–H bonds

The cross-dehydrogenative arylation of benzylic C–H bonds with arenes provides straightforward access to synthetically useful 1,1-diarylmethanes, from readily available starting materials.

Copper-catalysed oxidative α-C(sp3)–H nitroalkylation of (hetero)arene-fused cyclic amines

Under aerobic copper catalysis, a direct α-C(sp³)–H nitroalkylation of N-unsubstituted (hetero)arene-fused cyclic amines with nitroalkanes has been demonstrated.

Solvent-Free C-3 Coupling of Azaindoles with Cyclic Imines

By direct coupling 7-azaindole and cyclic imines, such as 3,4-dihydroisoquinoline, 6,7-dihydrothieno[3,2-c]pyridine, 3,4-dihydro-β-carboline, and 4,5-dihydro-3H-benz[c]azepine, new 3-substituted 7-azaindole derivatives have been synthesized. The reaction was extended to 4-azaindoles and 6-azaindoles, as electron-rich aromatic compounds. The lowest reactivity was observed in the case of C-3 substitution of 5-azaindole. In this case, the aza-Friedel-Crafts reaction took place by using 10 mol % of p-toluenesulfonic acid (p-TSA) as the catalyst. The role of the acid catalyst can be explained by the different pKa values of the azaindoles. All reactions were performed in solvent-free conditions by using both classical heating and microwave irradiation. In all cases, microwave heating proved to be more convenient to synthesize new C-3-substituted azaindole derivatives.

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.

Copper nitrate: a privileged reagent for organic synthesis

Copper has been explored as an ideal candidate for replacing noble metals in organic synthesis, especially for practical large scale preparation. Recent decades have witnessed the renaissance and improvement of copper-catalyzed and copper-mediated organic reactions. Copper nitrate is a common inorganic copper salt which has been proved to be a ubiquitous reactant in organic synthesis due to its commercial availability, stability, inexpensiveness and environmentally benign nature. Copper nitrate could be used as a nitration reagent, oxidant, catalyst or promoter, and Lewis acid as well. Remarkably, great attention has been devoted to the efficient transformation of copper nitrate into functionalized or complicated compounds through various reaction types including cyclization, C-H activation, difunctionalization, nitration, rearrangement and asymmetric synthesis with chiral ligands. Further modification of copper nitrate, such as solid-supported copper nitrate or copper nitrate complexes, extends its applications in organic synthesis. The present review highlights recent advances of copper nitrate in organic synthesis, along with the mechanisms.

Copper-catalyzed highly diastereoselective cross-dehydrogenative coupling between 8-hydroxyisochromanes and 1,3-dicarbonyl compounds

A novel copper-catalyzed highly diastereoselective cross-dehydrogenative coupling reaction for the access of tricyclic chromanes from 8-hydroxyisochromanes and 1,3-dicarbonyl compounds.

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.

2D-MoS2 photocatalyzed cross dehydrogenative coupling reaction synchronized with hydrogen evolution reaction

The chemically exfoliated 2D-MoS₂ has a mixture of 1T (metallic) and 2H (semiconducting) phases.

Direct Arylation of Benzyl Ethers with Organozinc Reagents

A novel C(sp³)-H bond arylation of benzyl ethers with Knochel-type arylzinc reagents has been developed. This transition-metal-catalyst-free reaction proceeds well under mild conditions in a simple and effective manner and enables the synthesis of a wide range of potentially biologically active benzyl ethers by using highly functionalized organozinc reagents as a carbon nucleophile.

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.

Cobalt-catalyzed cross-dehydrogenative coupling of imidazo[1,2-a]pyridines with isochroman using molecular oxygen as the oxidant

A cobalt-catalyzed cross-dehydrogenative coupling of 2-arylimidazo[1,2-a]pyridines with isochroman is developed using molecular oxygen as the oxidant.

Cross-dehydrogenative coupling and oxidative-amination reactions of ethers and alcohols with aromatics and heteroaromatics

Cross-dehydrogenative coupling (CDC) is a process in which, typically, a C-C bond is formed at the expense of two C-H bonds, either catalyzed by metals or other organic compounds, or via uncatalyzed processes. In this perspective, we present various modes of C-H bond-activation at sp3 centers adjacent to ether oxygen atoms, followed by C-C bond formation with aromatic systems as well as with heteroaromatic systems. C-N bond-formation with NH-containing heteroaromatics, leading to hemiaminal ethers, is also an event that can occur analogously to C-C bond formation, but at the expense of C-H and N-H bonds. A large variety of hemiaminal ether-forming reactions have recently appeared in the literature and this perspective also includes this complementary chemistry. In addition, the participation of C-H bonds in alcohols in such processes is also described. Facile access to a wide range of compounds can be attained through these processes, rendering such reactions useful for synthetic applications via Csp3 bond activations.

Facile Construction of Quinoline-2-carboxylate Esters through Aerobic Oxidation of Alkyl 4-Anilinocrotonates Induced by a Radical Cation Salt

A facile construction of quinoline-2-carboxylate esters through an aerobic oxidation of alkyl 4-anilinocrotonates is described. In the presence of dioxygen, sp³ C-H bonds in 4-anilinocrotonates can easily be oxidized by using a catalytic amount of a radical cation salt, providing a radical intermediate. After further oxidation and domino cyclization, the desired quinoline derivatives were afforded in high yields. This reaction provides a new way to construct the pharmaceutically relevant quinoline skeleton, avoiding harsh reaction conditions and tedious starting material synthesis.

Oxidative C–H functionalization of N-carbamoyl 1,2-dihydroquinolines

A modular and efficient method for the synthesis of α-substituted 1,2-dihydroquinolines through an oxidative C–H functionalization strategy is described.

Improved simplicity and practicability in copper-catalyzed alkynylation of tetrahydroisoquinoline

ABSTRACT

Alkynylation reactions of N-protected tetrahydroisoquinolines have been performed using several different protocols of cross dehydrogenative coupling. Initially, a CuCl-catalyzed method was investigated, which worked well with three different N-protecting groups, namely phenyl, PMP, and benzyl and t-BuOOH as oxidant in acetonitrile as solvent. The peroxide could then be replaced by simple air and acetonitrile for water, leading to an overall very environmentally friendly protocol. Finally, a decarboxylative alkynylation protocol starting from alkynoic acids was also developed using again air as oxidant. This avoids the use of gaseous alkynes in the introduction of short-chained alkyne substituents.

GRAPHICAL ABSTRACT