Transformable Sulfoximine Assisted One-Pot Double Annulation of Vinylic C–H Bonds with Unactivated Alkynes

A rhodium-catalyzed cascade C-H activation/annulation strategy for the expeditious assembly of pyrrolidinedione-fused 1,2-benzothiazines

A cascade annulation strategy triggered by rhodium(III)-catalyzed C-H activation has been reported for the expeditious assembly of pyrrolidinedione-fused 1,2-benzothiazines from free NH-sulfoximines with maleimides under mild conditions. Without the need for inert atmosphere protection, a broad range of sulfoximines with maleimides were well tolerated, producing diverse fused-thiazine derivatives in moderate to good yields. Additionally, the late-stage transformation of the target product demonstrated the potential synthetic value of this protocol.

Switchable Regioselective C-H Activation/Annulation of Acrylamides with Alkynes for the Synthesis of 2-Pyridones

A catalyst-based switchable regioselective C-H activation/annulation of acrylamides with propargyl carbonates has been developed, delivering C5 or C6 alkenyl substituted 2-pyridones. This robust protocol proceeds with a broad substrate scope and good functional group tolerance under redox-neutral reaction conditions. More significantly, this reaction is highly effective with previously challenging unsymmetrical alkynes, including unbiased alkyl-alkyl substituted alkynes, with perfect and switchable regioselectivity. Additionally, mechanistic studies and DFT calculations were performed to shed light on the switchable regioselectivity.

Sulfoximine Assisted C-H Activation and Annulation via Vinylene Transfer: Access to Unsubstituted Benzothiazines

In this study, we report the synthesis of unsubstituted 1,2-benzothiazines through a redox-neutral Rh(III)-catalyzed C-H activation and [4+2]-annulation of S-aryl sulfoximines with vinylene carbonate. Notably, the introduction of an N-protected amino acid ligand significantly enhances the reaction rate. The key aspect of this redox-neutral process is the utilization of vinylene carbonate as an oxidizing acetylene surrogate and an efficient vinylene transfer agent. This vinylene carbonate enables the cyclization with the sulfoximine motifs, successfully forming a diverse array of 1,2-benzothiazine derivatives in moderate to good yields. Importantly, this study highlights the potential of Rh(III)-catalyzed C-H activation and [4+2]-annulation reactions for the synthesis of optically pure 1,2-benzothiazines with high enantiomeric purity.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Multiple annulations of inert C(sp2)-H bonds with alkynes

Transition-metal catalyzed directing group (DG) assisted annulation of inert C-H bonds leads to the formation of complex molecular frameworks from readily accessible substrates. Thus, multiple annulation of less functionalized substrates with unsaturated species leads to the construction of structurally diverse fused poly(hetero)cycles. The directed inert C(arene)-H bond activation and the mode of TM-migration in this process could enabled obatining L-type [involves DG heteroatom, o-C(arene)-H bond, and C(arene)-H bond of aryl-motif in alkyne], Y-type [involves two heteroatoms of the DG and o-,o'-C(arene)-H bonds], and B-type [involves o-C(arene)-H bond and m-C(arene)-H bond] π-extended annulation products. The coordination preference of the DG heteroatom makes the transformation chemo- and regio-selective. This article underlines the conceptual development of unsymmetrical multiple annulation of arene C(sp²)-H bonds with alkynes, which is exceedingly appealing and highly important.

Rhodium-Catalyzed C-H Annulation of Free Anilines with Vinylene Carbonate as a Bifunctional Synthon

Chemical transformation with vinylene carbonate as an emerging synthetic unit has recently attracted considerable attention. This report is a novel conversion pattern with vinylene carbonate, in which such a vibrant reagent unprecedentedly acts as a difunctional coupling partner to complete the C-H annulation of free anilines. From commercially available substrates, this protocol leads to the rapid construction of synthetically versatile 2-methylquinoline derivatives (43 examples) with excellent functionality tolerance.

Synthesis of Six-Membered N-Heterocycle Frameworks Based on Intramolecular Metal-Free N-Centered Radical Chemistry

The formation of intramolecular C-N bond represents a powerful strategy in organic transformation as the great importance of N-heterocycles in the fields of natural products and bioactive molecules. This personal account describes the synthesis of cyclic phosphinamidation, benzosultam, benzosulfoximine, phenanthridine and their halogenated compounds through transition-metal-free intramolecular oxidative C-N bond formation. Mechanism study reveals that N-X bond is initially formed under the effect of hypervalent halogen, which is very unstable and easily undergoes thermal or light homolytic cleavage to generate nitrogen radical. Then the nitrogen radical is trapped by the arene to give aryl radical. Rearomatization of aryl radical under the oxidant to provide corresponding N-heterocycle. Under suitable conditions, the N-heterocycles can be further transformed to halogenated N-heterocycles.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

20 Years of Forging N-Heterocycles from Acrylamides through Domino/Cascade Reactions

Acrylamides are versatile building blocks that are easily obtained from readily available starting materials. During the last 20 years, these valuable substrates bearing a nucleophilic nitrogen atom and an electrophilic double bond have proven to be efficient domino partners, leading to a wide variety of complex aza-heterocycles of synthetic relevance. In this non-exhaustive review, metal-free and metal-triggered reactions followed by an annulation will be presented; these two approaches allow good modulation of the reactivity of the polyvalent acrylamides.

1 Introduction

2 Metal-Free Annulations

2.1 Domino Reactions Triggered by a Michael Addition

2.2 Domino Reactions Triggered by an Aza-Michael Addition

2.3 Domino Processes Triggered by an Acylation Reaction

2.4 Domino Reactions Triggered by a Baylis–Hillman Reaction

2.5 Cycloadditions and Domino Reactions

2.6 Miscellaneous Domino Reactions

3 Metal-Triggered/Mediated Annulations

3.1 Zinc-Promoted Transformations

3.2 Rhodium-Catalyzed Functionalization/Annulation Cascades

3.3 Cobalt-Catalyzed Functionalization/Annulation Cascades

3.4 Ruthenium-Catalyzed Functionalization/Annulation Cascades

3.5 Iron-Catalyzed Functionalization/Annulation Cascades

3.6 Palladium-Catalyzed Functionalization/Annulation Cascades

3.7 Copper-Catalyzed Transformations

3.8 Transition Metals Acting in Tandem in Domino Processes

4 Radical Cascade Reactions

5 Conclusion

Recent advances in chelation-assisted site- and stereoselective alkenyl C–H functionalization

This review highlights recent advances in vicinal- and geminal-group-directed alkenyl C–H functionalizations which proceeded by endo- and exo-cyclometallation.

Transition Metal-Catalyzed Intermolecular Cascade C-H Activation/Annulation Processes for the Synthesis of Polycycles

Polycycles are abundantly present in numerous advanced chemicals, functional materials, bioactive molecules and natural products. However, the strategies for the synthesis of polycycles are limited to classical reactions and transition metal-catalyzed cross-coupling reactions, requiring pre-functionalized starting materials and lengthy synthetic operations. The emergence of novel approaches shows great promise for the fields of organic/medicinal/materials chemistry. Among them, transition metal-catalyzed C-H activation followed by intermolecular annulation reactions prevail, due to their straightforward manner with high atom- and step-economy, providing rapid, concise and efficient methods for the construction of diverse polycycles. Several strategies have been developed for the synthesis of polycycles, relying on sequential multiple C-H activation/annulation, or combination of C-H activation/annulation and further interaction with a proximal group, or merger of C-H activation with a cycloaddition reaction, or in situ formation of the directing group. These are attractive, efficient, step- and atom-economic methods starting from commercially available materials. This Minireview will provide an introduction to transition metal-catalyzed C-H activation for the synthesis of polycycles, helping researchers to discover indirect connections and reveal hidden opportunities. It will also promote the discovery of novel synthetic strategies relying on C-H activation.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

Ruthenium(II)-Catalyzed Double Annulation of Quinones: Step-Economical Access to Valuable Bioactive Compounds

Double ruthenium(II)-catalyzed alkyne annulations of quinones were accomplished. Thus, a strategy is reported that provides step-economical access to valuable quinones with a wide range of applications. C-H/N-H activations for alkyne annulations of naphthoquinones provided challenging polycyclic quinoidal compounds by forming four new bonds in one step. The singular power of the thus-obtained compounds was reflected by their antileukemic activity.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Catalyst and solvent switched divergent C-H functionalization: oxidative annulation of N-aryl substituted quinazolin-4-amine with alkynes

The development of site-selective C-H functionalizations/annulations is one of the most challenging practices in synthetic organic chemistry particularly for substrates bearing several similarly reactive C-H bonds. Herein, we describe catalyst and solvent controlled ortho/peri site-selective oxidative annulation of C-H bonds of N-aryl substituted quinazolin-4-amines with internal alkynes. The ortho C-H selective annulation was observed using Pd-catalyst in DMF to give indole-quinazoline derivatives, while, Ru-catalyst in PEG-400 favoured the peri C-H bond annulation exclusively to furnish pyrido-quinazoline derivatives.

Rhodium(III)-Catalyzed Redox-Neutral Coupling of α-Trifluoromethylacrylic Acid with Benzamides through Directed C-H Bond Cleavage

A rhodium(III)-catalyzed redox-neutral coupling of α-trifluoromethylacrylic acid with bezamides proceeds smoothly accompanied by amide-directed C-H bond cleavage to produce β-[2-(aminocarbonyl)phenyl]-α-trifluoromethylpropanoic acid derivatives. One of the products can be transformed to a trifluoromethyl substituted heterocyclic compound. In addition, the redox-neutral coupling of α-trifluoromethylacrylic acid with related aromatic substrates possessing a nitrogen-containing directing group can also be conducted under similar conditions.

Ruthenium(ii)-catalyzed intermolecular annulation of alkenyl sulfonamides with alkynes: access to bicyclic sultams

A ruthenium-catalyzed allylic C(sp3)-H activation strategy has been employed to develop an intermolecular coupling of alkenyl sulfonamides with alkynes. This protocol features the diastereoselective construction of [3.3.0] and [4.3.0] bicyclic sultams in one step.

Palladium-Catalyzed Decarboxylative ortho-C(sp2 )-H Aroylation of N-Sulfoximine Benzamides at Room Temperature

A palladium-catalyzed method for the decarboxylative ortho C-H acylation of N-sulfoximine benzamides is developed at room temperature. The catalytic method enables easy access to various functionalized 2-aroylaromatic carboxylic acid derivatives in good isolated yields. Based on our mechanistic studies, a Pd(II)/Pd(IV) catalytic cycle that involves aroyl radical intermediate is proposed for the reaction.

Recent Advancements in Transition-Metal-Catalyzed One-Pot Twofold Unsymmetrical Difunctionalization of Arenes

Transition-metal-catalyzed direct C-H bond activation reactions have been embraced as a powerful synthetic tool to access diverse functionalized arenes. However, site-selective incorporation of multiple distinct functionalities in an arene has always been a formidable challenge. Recent efforts from the synthetic community have disclosed a few dynamic synthetic approaches to fabricate multifunctionalized arenes in one-pot using a single catalytic system. These reports manifested the immense potential of such approaches to expedite contemporary organic synthesis towards building molecular complexity. In this minireview, we have illustrated the recent progress in this area, highlighting the contribution from several synthetic chemists including our group.

Rhodium(III)-Catalyzed C(sp2)-H Functionalization of Cyclobutenes. Access to Cyclobuta[c]pyridones and -pyridines

O-(ω-Alkynyl) hydroxamates derived from cyclobutenyl carboxylic acids were identified as viable substrates in intramolecular rhodium(III)-catalyzed heteroannulations, which led to diversely substituted cyclobuta[c]pyridones. Further functionalization of the resulting cyclobutapyridones enabled the synthesis of cyclobuta[c]pyridines and other nitrogen heterocycles after electrocyclic ring opening of the four-membered ring.

Combining the Ugi-azide multicomponent reaction and rhodium(III)-catalyzed annulation for the synthesis of tetrazole-isoquinolone/pyridone hybrids

An efficient sequence based on the Ugi-azide reaction and rhodium(III)-catalyzed intermolecular annulation has been established for the preparation of tetrazole-isoquinolone/pyridone hybrids. Several N-acylaminomethyltetrazoles were reacted with arylacetylenes to form the hybrid products in moderate to very good yields. The method relies on the capacity of the rhodium catalyst to promote C(sp2)-H activation in the presence of a suitable directing group. The Ugi-azide reaction provides broad molecular diversity and enables the introduction of the tetrazole moiety, which may further assist the catalytic reaction by coordinating the metal center. The scope of the isoquinolones is very wide and may be extended to the preparation of complex compounds having heterocyclic moieties such as pyridone, furan, thiophene and pyrrole, as well as the corresponding benzo-fused derivatives. The developed procedure is simple, reproducible and does not require inert conditions.

Construction of Pyranoisoquinolines via Ru(II)-Catalyzed Unsymmetrical Double Annulation of N-Methoxybenzamides with Unactivated Alkynes

A ruthenium (Ru)-catalyzed double annulation of easily accessible N-methoxybenzamide derivatives with unactivated alkynes for the synthesis of unusual 6,6-fused pyranoisoquinolines is described. Both ortho-C-H bonds of arenes and the N- and O-moieties of N-methoxybenzamides are involved in the construction of four [(C-C)-(C-N) and (C-C)-(C-O)] bonds in one step under single catalytic conditions. The unsymmetrical annulation of N-methoxybenzamides with two distinct alkynes is also demonstrated. The oxidizable directing group N-methoxyamine (NHOMe) assists the unsymmetrical double annulations of arenes [that use both N- and O-heteroatoms] in a single operation. This synthetic method features excellent substrate scope and tolerates a wide range of functional groups. Peripheral modification of pyranoisoquinolines for the construction of complex heterocyclic compounds is also demonstrated.

Intramolecular cascade annulation triggered by rhodium(III)-catalyzed sequential C(sp2)-H activation and C(sp3)-H amination

A rhodium(III)-catalyzed intramolecular oxidative annulation of O-substituted N-hydroxyacrylamides for the construction of indolizinones via sequential C(sp²)–H activation and C(sp³)–H amination has been developed. This approach shows excellent functional-group tolerance. The synthesized scaffold forms the core of many natural products with pharmacological relevance.

Co-catalyzed ortho-C–H functionalization/annulation of arenes and alkenes with alkynylsilanes: access to isoquinolone and pyridone motifs

A method for cobalt-catalyzed ortho-C–H functionalization annulation of arenes and alkenes with alkynylsilanes assisted by 8-aminoquinolyl auxiliary has been described. Alkynylsilanes were employed as the coupling partners to react with a broad range of benzamides and acrylamides, affording the corresponding isoquinolone and pyridone derivatives in moderate to high yields. It is worth noting that the silyl group in the final products can be retained or removed by switching the reaction conditions.

A method for cobalt-catalyzed ortho-C–H functionalization annulation of arenes and alkenes with alkynylsilanes assisted by 8-aminoquinolyl auxiliary.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Rhodium-Catalyzed [4 + 1] Cyclization via C-H Activation for the Synthesis of Divergent Heterocycles Bearing a Quaternary Carbon

The development of an efficient approach to construct fused polycyclic systems bearing a quaternary carbon center represents a great challenge to synthetic chemistry. Herein, we report a Rh(III)-catalyzed [4 + 1] annulation of propargyl alcohols with various heterocyclic scaffolds under an air atmosphere. Diverse fused heterocycles containing a quaternary carbon center were obtained in moderate to good yields. Additionally, this method features a high atom-economy, metal oxidant free, simple operation, and compatibility with various functional groups.

An Orchestrated Unsymmetrical Annulation Episode of C(sp2)-H Bonds with Alkynes and Quinones: Access to Spiro-isoquinolones

A nontrivial Ru-catalyzed one-pot sequential oxidative coupling of a (hetero)arene/vinylic/chromene system with alkyne and quinone is presented; the methyl phenyl sulfoximine (MPS) directing group is vital. This cyclization forms four (two C-C and two C-N) bonds in a single operation and produces unusual spiro-fused-isoquinolones with a broad scope. The release of phenyl methyl sulfoxide makes the MPS group transformable. A deuterium scrambling study sheds light on the reaction path.