Recent advances in the Rh-catalyzed cascade arene C-H bond activation/annulation toward diverse heterocyclic compounds

Rh(iii)-catalyzed highly site- and regio-selective alkenyl C-H activation/annulation of 4-amino-2-quinolones with alkynes via reversible alkyne insertion

3,4-Fused 2-quinolone frameworks are important structural motifs found in natural products and biologically active compounds. Intermolecular alkenyl C-H activation/annulation of 4-amino-2-quinolone substrates with alkynes is one of the most efficient methods for accessing such structural motifs. However, this is a formidable challenge because 4-amino-2-quinolones have two cleavable C-H bonds: an alkenyl C-H bond at the C3-position and an aromatic C-H bond at the C5-position. Herein, we report the Rh(iii)-catalyzed highly site-selective alkenyl C-H functionalization of 4-amino-2-quinolones to afford 3,4-fused 2-quinolones. This method has a wide substrate scope, including unsymmetrical internal alkynes, with complete regioselectivity. Several control experiments using an isolated key intermediate analog suggested that the annulation reaction proceeds via reversible alkyne insertion involving a binuclear Rh complex although alkyne insertion is generally recognized as an irreversible process due to the high activation barrier of the reverse process.

Rh(III)-Catalyzed [3 + 2] Annulation/Pinacol Rearrangement Reaction of Enaminones with Iodonium Ylides: Direct Synthesis of 2-Spirocyclo-pyrrol-3-ones

A novel Rh(III)-catalyzed cascade alkenyl C-H activation/[3 + 2] annulation/pinacol rearrangement reaction of enaminones with iodonium ylides has been developed. This methodology provides a new and straightforward synthetic strategy to afford highly functionalized 2-spirocyclo-pyrrol-3-ones in satisfactory yield from readily available starting materials under mild conditions. Moreover, gram-scale reactions and further derivatization experiments are implemented to demonstrate the potential utility of this developed approach.

Rhodium-Catalyzed Isoquinoline Synthesis Using Vinyl Selenone as Oxidizing Acetylene Surrogate

Isoquinoline is a privileged structure in many bioactive compounds and valuable ligands. Transition-metal-catalyzed oxidative annulation of imine derivatives has become a promising synthetic method; however, catalytic synthesis of 3,4-nonsubstituted isoquinolines by formal acetylene annulation has been scarce to date. Herein, we introduce vinyl selenone as an effective acetylene surrogate for the Rh-catalyzed annulative coupling under mild conditions. The Se fragment can be recovered as diselenide and recycled. The product can readily be converted to 1-aminoisoquinolines.

Asymmetric [5+1] Annulation via C-H Activation/1,4-Rh Migration/Double Bond Shift Using a Transformable Pyridazine Directing Group

N-Heterocycle-assisted C-H activation/annulation reactions have provided new concepts for the construction and transformation of azacycles. In this work, we disclose a [5+1] annulation reaction using a novel transformable pyridazine directing group (DG). The DG-transformable reaction mode led to the construction of a new heterocyclic ring accompanied by transformation of the original pyridazine directing group via a C-H activation/1,4-Rh migration/double bond shift pathway, affording the skeleton of pyridazino[6,1-b]quinazolines with a good substrate scope under mild conditions. Diverse fused cyclic compounds can be achieved by derivatization of the product. The asymmetric synthesis of the skeleton was also realized to afford the enantiomeric products with good stereoselectivity.

Three-component synthesis of N-naphthyl pyrazoles via Rh(III)-catalyzed cascade pyrazole annulation and Satoh-Miura benzannulation

The synthesis of N-naphthyl pyrazoles has been realized by the direct three-component reactions of enaminones, aryl hydrazine hydrochlorides and internal alkynes via Rh(III) catalysis. The synthetic reactions employing simple substrates lead to simultaneous construction of dual cyclic moieties, including a pyrazole ring and a phenyl ring, via sequential formation of two C-N and three C-C bonds.

Ru(II)-Catalyzed One-Pot Synthesis of 1,2-Hydropyridines via a Three-Component Reaction

A ruthenium(II)-catalyzed one-pot synthesis of highly substituted 1,2-dihydropyridines (DHPs) via a three-component reaction system has been realized. The reaction is conducted using a simple Ru(II) catalyst without the addition of specific ligands. The catalytic system exhibits good functionality tolerance with a wide range of starting materials. The DHPs obtained can be easily converted into tetrahydropyridines and azabicyclo[4.2.0]octa-4,7-dienes by subsequent reduction or [2 + 2] cycloaddition reaction.

Free Amine-Directed Ru(II)-Catalyzed Redox-Neutral [4 + 2] C-H Activation/Annulation of Benzylamines with Sulfoxonium Ylides

An external oxidant free Ru(II)-catalyzed C-H functionalization/annulation of primary benzylamines with sulfoxonium ylides has been developed for the synthesis of isoquinolines. The reaction utilizes free amine as a directing group, which is generally considered to be a poor directing group. This work presents the first example of Ru-catalyzed C-H functionalization of benzylamines under redox-neutral conditions. The detection of the amine-directed ruthenacyclic intermediate using high-resolution mass spectrometry corroborated the involvement of free amine as a directing group.

Diverse reactivity of alkynes in C-H activation reactions

Alkynes occupy a prominent role as a coupling partner in the transition metal-catalysed directed C-H activation reactions. Due to low steric requirements and linear geometry, alkynes can effectively coordinate with metal d-orbitals. This makes alkynes one of the most successful coupling partners in terms of the number of useful transformations. Remarkably, by changing the reaction conditions and transition-metals from 5d to 3d, the pattern of reactivity of alkynes also changes. Due to the varied reactivity of alkynes, such as alkenylation, annulation, alkylation, and alkynylation, they have been extensively used for the synthesis of valuable organic molecules. Despite enormous explorations with alkynes, there are still a lot more possible ways by which they can be made to react with M-C bonds generated through C-H activation. Practically there is no limit for the creative use of this approach. In particular with the development of new high and low valent first-row metal catalysts, there is plenty of scope for this chemistry to evolve as one of the most explored areas of research in the coming years. Therefore, a highlight article about alkynes is both timely and useful for synthetic chemists working in this area. Herein, we have highlighted the diverse reactivity of alkynes with various transition metals (Ir, Rh, Ru, Pd, Mn, Fe, Co, Ni, Cu) and their applications, along with some of our thoughts on future prospects.

Selective Synthesis of C4-Functionalized Benzofurans by Rhodium-Catalyzed Vinylene Transfer: Computational Study on the Cyclopentadienyl Ligand

Benzofuran is a privileged structure in many bioactive compounds; however, the controlled synthesis of C2,C3-nonsubstituted benzofurans has been scarce. In particular, cumbersome multistep processes are inevitable for the most inaccessible C4-substituted isomers. Herein, we report a Rh-catalyzed direct vinylene annulation of readily available m-salicylic acid derivatives with vinylene carbonate to achieve selective construction of C4-substituted benzofurans. The Weinreb amide directing group facilitated the following product derivatization. The reaction mechanism was investigated by DFT calculations.

Regio- and Diastereoselective [3 + 2]-Spiroannulation of Benzoxazines with Chalcones: A Rh(III)-Catalyzed Redox-Neutral Approach to α-Aroyl Spiro-Indanamines

We report an atom-economic Rh(III)-catalyzed [3 + 2]-spiroannulation reaction between cyclic ketimines and α,β-unsaturated carbonyl compounds, allowing the synthesis of novel spirocycles with concomitant generation of three stereogenic centers in one pot. The reaction does not require any silver additives or external oxidants and is believed to proceed in a redox-neutral manner. A broad substrate scope with good functional group tolerance permitted the synthesis of a vast spectrum of spirocyclic 1,4-benzoxazine derivatives containing polysubstituted α-aroyl-indanamines in good to excellent yields with high diastereoselectivity.

Rh-Catalyzed [4 + 2] Annulation with a Removable Monodentate Structure toward Iminopyranes and Pyranones by C-H Annulation

The Rh-catalyzed reactions of N-pyridinyl enaminones with internal alkynes leading to the synthesis of iminopyranes via a key C-H bond activation and subsequent tautomeric O-H bond cleavage are reported. Moreover, the pyridine ring in the amino group acts as an auxiliary monodentate site for this annulation and can be easily removed by a simple hydrolysis to afford pyranones.

Directing Group Strategies in Rhodium-Catalyzed C-H Amination

Construction of a carbon-nitrogen bond is one of the most prevalent operations in nature and organic synthesis. The resulting amino compounds are privileged structural fragments in natural products, pharmaceutical drugs,...

Rh(III)-Catalyzed Cascade Annulation to Produce N-acetyl Chain of Spiropyrroloisoquinoline Derivatives

A new rhodium(III)-catalyzed three-component multistep cascade spirocyclization approach was developed to synthesize nolvel N-acetyl chain of spiropyrroloisoquinoline derivatives using oxadiazoles as the directing group. This one-pot reaction also isolates aryloxadiazole...

Application of a transient directing strategy in cyclization reactions via C–H activation

This review introduces seven types of cyclization reactions via C–H activation using a transient directing strategy.

Metal-/solvent-free oxidative [4+2]/[3+2] annulation of 2-ethynylbenzaldehydes with arylalkenes: Facile synthesis of benzo[a]fluoren-5-ones

We report here a DTBP-mediated oxidative [4+2]/[3+2] annulation between 2-alkynylbenzaldehydes and terminal arylalkenes enabled by aldehyde C(sp2)-H functionalization for assembling a range of benzo[a]fluoren-5-ones with high step-economy, simple operations under...

Recent advances in rhodium-catalyzed C(sp2)-H (hetero)arylation

Arylation is a common behaviour in organic synthesis for the construction of complex structures, especially the biaryls. Among those reported arylation procedures, transition-metal-catalyzed direct C(sp²)-H arylation has been rapidly developed in recent decades and has become a reliable alternative to traditional cross-coupling procedures using organometallic reagents. Great achievements in rhodium-catalyzed C(sp²)-H arylation have been witnessed during the last decade. Aryl halides, simple arenes, aryl boronic acids, arylsilanes, aryl aldehyde, aryl carboxylic acid, diazides, etc. were successfully utilized as arylating reagents under rhodium-catalyzed conditions. In this review, recent achievements in rhodium-catalyzed arylations through C(sp²)-H bond activation were summarized together with the mechanism discussions.

Rhodium and Iridium Mediated C-H and O-H Bond Activation of Two Schiff Base Ligands: Synthesis, Characterization and Catalytic Properties of the Organometallic Complexes

Reaction of [Rh(PPh₃)₃Cl] with two Schiff base ligands, viz. N-(2′-hydroxyphenyl)furan-2-aldimine (H₂L¹) and N-(2′-hydroxyphenyl)thiophene-2-aldimine (H₂L²), in refluxing toluene affords organorhodium complexes of type [Rh(PPh₃)₂(L)Cl] (L = L¹ and L²). Similar reaction with [Ir(PPh₃)₃Cl] yields organoiridium complexes of type [Ir(PPh₃)₂(L) (H)] (L = L¹ and L²). Crystal structures of [Rh(PPh₃)₂(L¹)Cl] and [Ir(PPh₃)₂(L²) (H)] have been determined, where the imine ligands are found to bind to the metal centers as CNO-donors. Structures of [Rh(PPh₃)₂(L²)Cl] and [Ir(PPh₃)₂(L¹) (H)] have been optimized by density functional theory method. Formation of the organometallic complexes is believed to proceed via C-H and O-H bond activation of the imine ligands. All four complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows an oxidation on the positive side of SCE and a reduction on the negative side. The organoiridium complexes are found to efficiently catalyze Suzuki-type C-C cross coupling reactions.