Oxidants Controlled C-H Bond Functionalization of N-Aryltetrahydroisoquinolines: The Construction of the Quaternary Carbon Center and Cleavage of the C-N Bond

Initiated by triarylamine radical cation salt (TBPA), the direct C-H bond functionalization of α-N-aryltetrahydroisoquinoline esters was smoothly realized, giving a series of α-hydroxylated derivatives with a quaternary carbon center in good yields. Differently, in the presence of tert-butyl nitrite (TBN), the C-N single bond was cleaved to keto esters. The mechanistic study revealed that these reactions were mediated by a similar mechanism, in which the N-nitrosation might provide a driving force to the C-N bond cleavage.

Direct Formation of C-C, C-N, and C-O Bonds in Dihydroquinazolines via Hypervalent Iodine(III)-Mediated sp3 C-H Functionalization

A hypervalent iodine(III)-mediated cross-dehydrogenative coupling reaction for the direct formation of C-C, C-N, and C-O bonds in dihydroquinazolines has been developed. This one-pot method allows for the synthesis of C4-disubstituted dihydroquinazolines as well as C4-spirolactam, spirolactone, and spiroindene dihydroquinazolines in moderate to high yields.

[2 + 3] Annulative Coupling of Tetrahydroisoquinolines with Aryliodonio diazo compounds To Access 1,2,4-Triazolo[3,4-a]isoquinolines

Base promoted one-pot annulative coupling of 1,2,3,4-tetrahydroisoquinolines (THIQs) with hypervalent iodine(III) species aryliodonio diazo compounds has been devised for the direct construction of 1,2,4-triazolo[3,4-a]isoquinoline derivatives at room temperature in open air for the first time. This approach involves [2 + 3] cascade annulation of nucleophilic THIQ with an electrophilic aryliodonio diazo compound via N-H and α-C1(sp³)-H difunctionalization of THIQ.

TEMPO-Promoted Mono- and Bisimidation of Tertiary Anilines: Synthesis of Symmetric and Unsymmetric N-Mannich Bases

A TEMPO-promoted method was developed for the synthesis of symmetric bis-N-Mannich bases via sequential activation of two α,α'-amino C(sp³)-H bonds of N,N-dimethylanilines under mild conditions. This methodology was further extended for monoimidation of α-amino-functionalized methylanilines to give unsymmetric N-Mannich bases in good to high yields. Several control experiments were performed, and the coupling reaction outcomes indicated that the oxoammonium (TEMPO⁺) species is involved in the reaction.

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.

Electrochemical Synthesis of Allylic Amines from Terminal Alkenes and Secondary Amines

Allylic amines are valuable synthetic targets en route to diverse biologically active amine products. Current allylic C-H amination strategies remain limited with respect to the viable N-substituents. Herein, we disclose a new electrochemical process to prepare aliphatic allylic amines by coupling two abundant starting materials: secondary amines and unactivated alkenes. This oxidative transformation proceeds via electrochemical generation of an electrophilic adduct between thianthrene and the alkene substrates. Treatment of these adducts with aliphatic amine nucleophiles and base provides allylic amine products in high yield. This synthetic strategy is also amenable to functionalization of feedstock gaseous alkenes at 1 atm. In the case of 1-butene, high Z-selective crotylation is observed. This strategy, however, is not limited to the synthesis of simple building blocks; complex biologically active molecules are suitable as both alkene and amine coupling partners. Preliminary mechanistic studies implicate vinylthianthrenium salts as key reactive intermediates.

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

Quinoline-Fused Lactones via Tandem Oxidation Cyclization: Metal-Free sp3 C-H Functionalization

A unique lactonization of 2-methyl-3-acyl-4-phenylquinolines using PhIO as the oxidant and selectfluor as an additive is reported. The reaction occurs under ambient conditions through tandem oxidation and cyclization of sp³ C-H bonds under metal-free conditions. The heterocycle-fused lactones are obtained in moderate to good yield.

Photo-induced sp3 C-H bond arylation, cyanation and nitroalkylation of tetrahydroisoquinolines (THIQs) under visible light irradiation using a combination of NHPI and Rose Bengal

This work reports a sustainable protocol for α-arylation of tetrahydroisoquinolines (THIQs) with aryl diazonium salts using a combination of a cheap and stable oxidant, N-hydroxyphthalimide (NHPI), and an inexpensive dye photosensitizer, Rose Bengal (RB), for the first time. The reaction was performed at ambient temperature using green LED light irradiation under an oxygen atmosphere. The developed α-arylation protocol shows a diverse substrate scope for various THIQs and aryl diazonium salts. The work also explores nitroalkylation and cyanation under the developed reaction conditions. A possible SET mechanism is involved which was confirmed by ESI-MS analysis of the reaction mixture.

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 organophotocatalyzed C-H bond functionalization reactions

Over the last decade, a variety of methodologies for the direct functionalization of C-H bonds have been developed. Among others, visible light photoredox reactions have recently emerged as one of the most efficient and highly selective processes for the direct introduction of a functionality into organic molecules. Easy reaction setups, as well as mild reaction conditions, make this approach superior to other methodologies applying transition metals or strong oxidants, in terms of both costs and substrate and functional group tolerance. In this review, the recent developments in organophotocatalyzed C-H bond functionalization reactions are presented.

Dehydrogenative C(sp3)–H bond functionalization of tetrahydroisoquinolines mediated by organic oxidants under mild conditions

DDQ and IBX are effective SET oxidants of N-aryl-protected and unprotected THIQs. Mannich and Strecker type functionalizations ensued in an overall CDC reaction.

Combination of RuCl3·xH2O with PEG – a simple and recyclable catalytic system for direct arylation of heteroarenes via C–H bond activation

A simple and recyclable catalytic system for direct arylation of heteroarenes via C–H bond activation was developed with a relatively inexpensive RuCl₃·xH₂O as a catalyst and PEG-400 as a green medium without any additive or ligand.

Recent progress in mild Csp3–H bond dehydrogenative or (mono-) oxidative functionalization

This review summarizes recent advances in mild and green dehydrogenative and mono-oxidative Csp³–H bond functionalization reactions, considering both new approaches and the re-elaboration of known methodologies.

Copper-catalyzed asymmetric sp3 C-H arylation of tetrahydroisoquinoline mediated by a visible light photoredox catalyst

This report describes a highly enantioselective oxidative sp³ C-H arylation of N-aryltetrahydroisoquinolines (THIQs) through a dual catalysis platform. The combination of the photoredox catalyst, [Ir(ppy)₂(dtbbpy)]PF₆, and chiral copper catalysts provide a mild and highly effective sp³ C-H asymmetric arylation of THIQs.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Cross-dehydrogenative regioselective Csp3–Csp2coupling of enamino-ketones followed by rearrangement: an amazing formation route to acridine-1,8-dione derivatives

A novel route for the construction of acridine-1,8-dione derivatives through CDC coupling of enamino-ketones followed by rearrangement.

Synthesis of substituted tetrahydroisoquinolines by lithiation then electrophilic quench

Lithiation then electrophilic quench of tetrahydroisoquinolines provides access to 1-substituted products. Removal of the N-Boc group allows rapid access to natural products such as (±)-crispine A.

One-pot functionalisation of N-substituted tetrahydroisoquinolines by photooxidation and tunable organometallic trapping of iminium intermediates

Nucleophilic trapping of iminium salts generated via oxidative functionalisation of tertiary amines is well established with stabilised carbon nucleophiles. The few reports of organometallic additions have limited scope of substrate and organometallic nucleophile. We report a novel, one-pot methodology that functionalises N-substituted tetrahydroisoquinolines by visible light-assisted photooxidation, followed by trapping of the resultant iminium ions with organometallic nucleophiles. This affords 1,2-disubstituted tetrahydroisoquinolines in moderate to excellent yields.