Sulfuryl Chloride as an Efficient Initiator for the Metal-Free Aerobic Cross-Dehydrogenative Coupling Reaction of Tertiary Amines

Synthetic Methods for Azaheterocyclic Phosphonates and Their Biological Activity: An Update 2004-2024

The increasing importance of azaheterocyclic phosphonates in the agrochemical, synthetic, and medicinal field has provoked an intense search in the development of synthetic routes for obtaining novel members of this family of compounds. This updated review covers methodologies established since 2004, focusing on the synthesis of azaheterocyclic phosphonates, of which the phosphonate moiety is directly substituted onto to the azaheterocyclic structure. Emphasizing recent advances, this review classifies newly developed synthetic approaches according to the ring size and providing information on biological activities whenever available. Furthermore, this review summarizes information on various methods for the formation of C-P bonds, examining sustainable approaches such as the Michaelis-Arbuzov reaction, the Michaelis-Becker reaction, the Pudovik reaction, the Hirao coupling, and the Kabachnik-Fields reaction. After analyzing the biological activities and applications of azaheterocyclic phosphonates investigated in recent years, a predominant focus on the evaluation of these compounds as anticancer agents is evident. Furthermore, emerging applications underline the versatility and potential of these compounds, highlighting the need for continued research on synthetic methods to expand this interesting family.

Strategies Toward the Catalyst-Free α-C-H Functionalizations of Tertiary Amines

α-C-H functionalization of tertiary amines has been a highly studied field for the past two decades because several important nitrogen containing heterocycles or compounds can be synthesized through this strategy. Though transition metal catalysts and some metal-free catalysts are mainly used for these reactions, a few catalyst-free reactions have recently been efficiently performed. Catalyst-free reactions are cost-effective, less sensitive to air/moisture, easier to operate, have a simple purification process, and are relatively environment-friendly. In this article, we have summarized all the α-C-H functionalization reactions of tertiary amines performed without using any external catalysts. The content of this article will undoubtedly encourage readers to do more work in this area.

Direct phosphorylation of benzylic C-H bonds under transition metal-free conditions forming sp3C-P bonds

Direct phosphorylation of benzylic C-H bonds was achieved in a biphasic system under transition metal-free conditions. A selective radical/radical sp3C-H/P(O)-H cross coupling was proposed, and various substituted toluenes were applicable. The transformation provided a promising method for constructing sp3C-P bonds.

The Photocatalyst-Free Cross-Dehydrogenative Coupling Reaction Enabled by Visible-Light Direct Excitation of Substrate

A new photocatalyst-free strategy for the cross-dehydrogenative C-C and C-P coupling reaction has been described. This protocol provides a concise method to synthesize various 1-substituted tetrahydroisoquinoline (THIQ) derivatives enabled by visible-light direct excitation of substrates without using any photocatalyst. Moreover, a wide substrate scope demonstrated good synthetic versatility and practicality.

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 Synthesis of α-Amino Nitriles from Sulfonamides via Base-Mediated C-H Cyanation

Herein, we disclose a transition-metal-free reaction system that enables α-cyanation of sulfonamides through C-H bond cleavage for the preparation of α-amino nitriles, including difficult-to-access all-alkyl α-tertiary scaffolds. More than 50 substrate examples prove a wide functional group tolerance. Additionally, its synthetic practicality is highlighted by gram-scalability and the late-stage modification of natural compounds. Mechanistic experiments suggest that this process involves in situ formation of an imine intermediate via base-promoted elimination of HF.

Synergistic promoting effect of ball milling and Fe(ii) catalysis for cross-dehydrogenative-coupling of 1,4-benzoxazinones with indoles

In this work, a novel C(sp3)–C(sp2) cross-dehydrogenative-coupling method is developed to react benzoxazin-2-one derivatives with various indoles. As a result, combined use of ball milling and Fe(ii) catalysis leads to rapid coupling of 1,4-benzoxazinones with derivatives of indole. Under the conditions, derivatives of 1 couple with various indoles at room temperature to produce good yields of the desired compounds within 0.5–2 h time period. Thus, derivatives of both starting materials couple smoothly under relatively mild conditions to give good yields of 3.

Mechanistic Studies on Bioinspired Aerobic C-H Oxidation of Amines with an ortho-Quinone Catalyst

We report herein our mechanistic studies of the ortho-quinone-catalyzed aerobic oxidation of primary, secondary, and tertiary amines. Two different catalytic pathways were discovered for the reductive half reactions: for primary amines, the reaction was found to proceed via a transamination pathway, while the reactions with secondary amines and tertiary amines proceeded via hydride transfer. We also found that the amine substrates could significantly promote the regeneration of the ortho-quinone catalyst in the oxidative half reaction, in which a proton transfer occurs between the amine substrates and catechol derivatives (the reduced form of the ortho-quinone catalyst).

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.

Direct cross-coupling between alkenes and tetrahydrofuran with a platinum-loaded titanium oxide photocatalyst

A Pt-loaded TiO₂ photocatalyst successfully catalyzed the direct cross-coupling between various alkenes and tetrahydrofuran (THF) without any additional oxidizing agent.

Catalyst-Free Cross-Dehydrogenative Coupling Strategy Using Air as an Oxidant: Synthesis of α-Aminophosphonates

α-Aminophosphonates are synthesized by employing unfunctionalized starting materials using cross-dehydrogenative coupling strategy. This method does not require any catalyst and proceeds in the presence of open air as the only oxidant. Mechanistic studies revealed that the reaction is nucleophile dependent and specific to dialkyl phosphite and N-aryl tetrahydroisoquinoline derivatives.

Direct vinylogous oxidative cross-dehydrogenative coupling of 4-nitroisoxazoles with N-aryl tetrahydroisoquinolines in water under air conditions

Using water as the solvent and air as the oxidant, a direct catalytic vinylogous cross-dehydrogenative coupling reaction was developed.

Nitroxyl-Radical-Catalyzed Oxidative Coupling of Amides with Silylated Nucleophiles through N-Halogenation

A nitroxyl-radical-catalyzed oxidative coupling reaction between amines with an N-protecting electron-withdrawing group (EWG) and silylated nucleophiles was developed to furnish coupling products in high yields, thus opening up new frontiers in organocatalyzed reactions. This reaction proceeded through the activation of N-halogenated amides by a nitroxyl-radical catalyst, followed by carbon-carbon coupling with silylated nucleophiles. Studies of the reaction mechanism indicated that the nitroxyl radical activates N-halogenated amides, which are generated from N-EWG-protected amides and a halogenation reagent, to give the corresponding imines.

The Photocatalyzed Aza-Henry Reaction of N-Aryltetrahydroisoquinolines: Comprehensive Mechanism, H(•)- versus H(+)-Abstraction, and Background Reactions

The cross-dehydrogenative coupling (CDC) reaction of N-aryltetrahydroisoquinolines (THIQ) is one of the most exploited photocatalytic transformation and a test reaction for an exceptional variety of catalysts. However, its mechanism remained unclear concerning involved intermediates, reactive pathways of the amine radical cation and the influence of oxygen and the light source. Therefore, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and synthetic methods were combined to provide a comprehensive picture of the reaction mechanism using Ru(bpy)3Cl2 as a photocatalyst under aerobic and anaerobic conditions. The reaction profiles and involved intermediates were monitored and analyzed by NMR spectroscopy. Several intermediates contributing to product formation were identified, the iminium ion, the hydroperoxide and dimer of THIQ, and a new ring opened intermediate, cleaved at the benzylic C-N bond. Mechanistic evidence is given that under anaerobic conditions preferentially the α-amino radical is formed by deprotonation, in contrast to the formation of iminium ions via H(•)-abstraction in the presence of oxygen. Further, the light-induced background reaction in the absence of the catalyst was studied in detail, revealing that the product formation rate is correlated to the intensity and wavelength of the light source and that oxygen is essential for an efficient conversion. The reaction rate and efficiency is comparable to previously reported photocatalytic systems, performed under aerobic conditions in combination with intense blue light sources. Thus, the multitude of reaction parameters investigated reveals the preference for hydrogen atom or proton abstraction in photoreactions and allows to assess the influence of experimental conditions on the mechanistic pathways.

Metal-free C(sp(3))-H functionalization: oxidative carbo-oxygenation of α-diazo carbonyls via radical dediazotization

A novel three-component carbo-oxygenation of α-diazo carbonyls for flexible synthesis of unprecedented α-aminooxy-β-amino ketones has been established through metal-free C(sp(3))-H functionalization from readily accessible N,N-dimethylanilines and N-hydroxyphthalimide. The reaction pathway involves an in situ-generated phthalimide N-oxyl radical-triggered dediazotization/radical coupling sequence, leading to C-O and C-C bond formation.

Dehydrogenative coupling involving P(O)–H bonds: a powerful way for the preparation of phosphoryl compounds

This Frontier highlights the recent progress in the preparation of organophosphorus compounds via transition metal-catalysed dehydrogenative couplings of P(O)H compounds with Z–H compounds.

A simple and highly diasteroselective approach for the vicinal dichlorination of functional olefins

An efficient strategy for triphenylphosphine oxide-catalyzed vicinal dichlorination of functional olefins was developed using sulfuryl chloride as the source of chloride.

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.

Versatile and highly efficient oxidative C(sp3)–H bond functionalization of tetrahydroisoquinoline promoted by bifunctional diethyl azodicarboxylate (DEAD): scope and mechanistic insights

The oxidant–base bifunctionality of DEAD realized highly reliable cross dehydrogenative coupling of tetrahydroisoquinoline derivatives via aminal intermediates.