Nitroso-azomethine(ene) reaction enabled annulations of nitrosoarenes, azomethines and alkenes

An unprecedented example of a nitroso-azomethine(ene) reaction is reported. Nitroso-azomethine(ene) reaction-mediated unprecedented annulation of nitrosoarenes, azomethines, and alkenes to furnish arylquinolines via arene functionalization of nitrosoarene has been developed. DFT studies provided mechanistic insights into the newly developed nitroso-azomethine(ene) reaction.

A sequential Friedländer and anionic benzannulation strategy for the regiodefined assembly of unsymmetrical acridines

An efficient sequential double-annulation strategy has been developed to afford a series of unsymmetrical acridines with high yield and regioselectivity for the first time. This simple protocol enables the sequential assembly of two aromatic rings from simple starting materials. The reaction proceeds via modified Friedländer annulation and subsequent base-mediated benzannulation with acrylates as Michael acceptors. A range of substrate scope and functional group tolerance is observed. Late-stage synthetic modification is also explored to access novel unsymmetrical acridines in good yield.

Photoredox-Catalyzed Cascade sp2 C-H Bond Functionalization to Construct Substituted Acridine with Diarylamine and Hypervalent Iodine(III) Reagents

A photocatalyzed cascade double C-C formation via sp² C-H bond activation of diarylamines with hypervalent iodine diazo reagents was developed. A variety of diarylamines and hypervalent iodine(III) reagents were tolerated well, and a range of substituted acridines with yields ranging from moderate to excellent was provided efficiently. The protocol introduces diazo groups onto diarylamines and enables subsequent late-stage assembly point functionalization with the diazonium structure, forming two new C-C bonds in a sequential fashion.

A practical route to arylated dihydroacridine derivatives via nickel boride mediated intramolecular reductive cyclization-concomitant dehydration

A facile and highly efficient route towards 3-aryl-1,2-dihydroacridine derivatives from an aldol adduct of o-nitrobenzaldehyde and cyclohexenone derivatives has been described.

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.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Transient directing ligands for selective metal-catalysed C–H activation

C-H activation is a 'simple-to-complex' transformation that nature has perfected over millions of years of evolution. Transition-metal-catalysed C-H activation has emerged as an expeditious means to expand the chemical space by introducing diverse functionalities. Notably, among the strategies to selectively cleave a particular C-H bond, the catalytic use of a small molecule as co-catalyst to generate a transient directing group, which provides a balance between step economy and chemical productivity, has gained immense attention in recent years. This allows one to convert a desired C-H bond irrespective of its geometrical or stereochemical configuration. This Review describes the various transient directing groups used in C-H activation and explains their mechanistic significance.

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

The Rh-catalyzed C-H bond activation/annulation provides a new strategy for the synthesis of new frameworks. In this review, we summarize the recent research on the Rh-catalyzed cascade arene C-H bond activation/annulation toward diverse heterocyclic compounds. The application, scope, limitations and mechanism of these transformations are also discussed.

Scandium(III) Triflate-Catalyzed Reaction of Aroyl-Substituted Donor-Acceptor Cyclopropanes with 1-Naphthylamines: Access to Dibenzo[c,h]acridines

The reaction of aroyl-substituted donor-acceptor (D-A) cyclopropanes with two equivalents of 1-naphthylamines in the presence of a catalytic amount of scandium(III) triflate provides access to dibenzo[c,h]acridines. The key steps of the transformation are the formation of nucleophilic ring-opening products from the D-A cyclopropanes and 1-naphthylamines and their subsequent fragmentation and cyclization. The method has a reasonable substrate scope, and the products are formed in 50-70% yields.

Lewis acid catalyzed reactivity switch: pseudo three-component annulation of nitrosoarenes and (epoxy)styrenes

A Lewis acid catalyzed annulation reaction via arene functionalization of nitrosoarenes and C-C cleavage of (epoxy)styrene to provide arylquinolines is reported. The Lewis acid catalyst altered the annulation pattern providing arylquinolines instead of oxazolidines. The reaction with styrene resulted in a mixture of 2,4-diarylquinoline and 4-arylquinoline, while only 3-arylquinoline was formed from the reaction of epoxystyrene.

Pd-Catalyzed Approach for Assembling 9-Arylacridines via a Cascade Tandem Reaction of 2-(Arylamino)benzonitrile with Arylboronic Acids in Water

A novel palladium-catalyzed protocol for the synthesis of 9-arylacridines via tandem reaction of 2-(arylamino)benzonitrile with arylboronic acids in water has been developed with good functional group tolerance. The present synthetic route could be readily scaled up to gram quantity without difficulty. This methodology was further extended to the synthesis of a 4'-OH derivative, which showed estrogenic biological activity. Preliminary mechanistic experiments showed that this transformation involves a nucleophilic addition of aryl palladium species to the nitrile to generate an aryl ketone intermediate followed by an intramolecular Friedel-Crafts acylation and dehydration to acridines.

tert-Butyl Bromide-Promoted Intramolecular Cyclization of 2-Arylamino Phenyl Ketones and Its Combination with Cu-Catalyzed C-N Coupling: Synthesis of Acridines at Room Temperature

Herein, a facile intramolecular cyclization of 2-arylamino phenyl ketones is established to supersede the traditional high-temperature, strongly acidic conditions and achieve 9-substituted acridines, by virtue of the combination of 2,2,2-trifluoroethanol and tert-butyl bromide. This protocol can be merged well with the preceding Cu-catalyzed intermolecular Chan-Evans-Lam cross-coupling reactions, therefore enabling pot-economic modular synthesis of 9-substituted acridines from readily available 2-amino phenyl ketones and aryl boronic acids at room temperature.

Dirhodium(II)-Catalyzed C(sp2 )-H Azidation of Benzaldehydes

Multiple steps are needed to achieve the C-H functional of aromatic aldehyde, since the C-H functional reaction usually occurs preferentially at the aldehydic C-H bond over the aryl C-H bond. We report an efficient azidation method mediated by dirhodium(II) catalysts to achieve the direct aryl azidation of aromatic aldehydes avoiding the simultaneous use of protected aldehydes and prefunctionalized arenes. The regioselectivity of this method is similar to those of typical aromatic electrophilic substitution reactions. The resulting azidobenzaldehyde products are versatile building blocks or precursors for the synthesis of many biologically active compounds. The mechanism studies indicate that the one-electron oxidative intermediate Rh₂ ^(II,III) N₃ is responsible for the azide transfer.

Photocatalytic site-selective C-H difluoroalkylation of aromatic aldehydes

The direct photocatalyzed para-selective CAr-H difluoroalkylation of aromatic aldehyde derivatives has been accomplished using a newly explored catalytic system. In addition, when using para-substituted benzaldehydes as substrates, ortho-selective CAr-H difluoroalkylation was also accomplished. It is worth noting that all the above site-selectivity is opposite to traditional Friedel-Crafts reactions of aromatic aldehydes. The preliminary mechanistic investigations indicate that an electrophilic difluoroalkyl radical is involved in the catalytic cycle.

Chelation-directed remote meta-C-H functionalization of aromatic aldehydes and ketones

We disclose herein the development of a versatile 1,2-diol directing template for palladium-catalyzed remote meta-C-H functionalization of aromatic aldehydes and ketones. In situ-generation of acetals and ketals, as well as removal afterwards, makes the C-H bond functionalization process more straightforward and efficient. This also represents the first example of chelation-directed meta-C-H functionalization of aromatic aldehydes and ketones.

Synthesis of Mono-N-Methyl Aromatic Amines from Nitroso Compounds and Methylboronic Acid

The selective synthesis of mono-N-methyl aromatic amines was achieved by the reaction of aromatic nitroso compounds with methylboronic acid promoted by triethylphosphite under transition metal-free conditions. The target compounds are constructed efficiently without overmethylation, under environmentally benign reaction conditions that do not require bases or reductants and therefore are of interest in pharmaceutical, agricultural, and chemical industries.

Transient Ligand-Enabled Transition Metal-Catalyzed C-H Functionalization

Transition metal-catalyzed C-H bond functionalization is among the most efficient and powerful strategies in synthetic organic chemistry to derivatize otherwise inert sites of organic molecules for the construction of C-C and C-heteroatom bonds. However, additional steps are often required to install the directing groups to realize selective C-H bond functionalization of the substrates. These tedious steps run counter to the step-economical nature of the C-H activation. In contrast, direct functionalization of the substrate by using transient ligands avoids the unnecessary steps for the pre-functionalization of the substrates. This Minireview provides a short overview of the major progress made in this field for C-H functionalization at sp² and sp³ carbon centers with different transient working modes, including covalent, hydrogen, and ionic bonds.

Rhodium(III)-Catalyzed Intramolecular Olefin Hydroarylation of Aromatic Aldehydes Using a Transient Directing Group

A Rh(III)-catalyzed intramolecular olefin hydroarylation of aromatic aldehydes with a transient directing group has been described. The bidentate directing groups in situ generated from aromatic aldehydes and β-alanine could enable the subsequent C-H activation and hydroarylation with excellent site selectivities and high functional group compatibility. The further conversion of the aldehyde group showcased the broad application prospects of this methodology.

Palladium Catalyzed meta-C-H Functionalization of Masked Aromatic Aldehydes

Palladium catalyzed meta-C-H functionalization enabled by transient mediators has the potential to extend the utility of directed ortho-C-H functionalization to remote positions. However, there have been no reports of palladium catalyzed meta-C-H functionalization of aromatic aldehyde derivatives, which are highly versatile intermediates in organic synthesis. Herein we report the development of a directing group that, in the presence of a norbornene derived mediator and an appropriate pyridone ligand, allows palladium catalyzed meta-C-H functionalization of masked aromatic aldehydes. Mechanistic insight regarding the impact of the directing group length on this catalysis is also discussed.

Transient imines as ‘next generation’ directing groups for the catalytic functionalisation of C–H bonds in a single operation

This review describes recent developments in the use of catalytic transient directing groups, through imine linkages, which in combination with transition metal catalysts provide streamlined C–H functionalisation processes.

Active methylene compounds (AMCs) controlled facile synthesis of acridine and phenanthridine from morita Baylis–Hillman acetate

We carried out simple and facile syntheses of acridines and phenanthridines from MBH acetates of 2-chloro-quinoline-3-carbaldehyde with active methylene compounds (AMCs).