LED-induced Ru-photoredox Pd-catalyzed C-H arylation of (6-phenylpyridin-2-yl)pyrimidines and heteroaryl counterparts

N-heterocycles are essential building blocks and scaffolds in medicinal chemistry. A Pd-catalyzed, Ru-photoredox-mediated C-H arylation is applied herein, for converting a series of functionality-inclusive (6-phenylpyridin-2-yl)pyrimidines to single arylated derivatives, using phenyldiazonium tetrafluoroborate as aryl source. This green chemistry-compliant transformation is induced by LED light. The drug-like modular substrates are constructed via combination of Biginelli multi-component condensation and Suzuki C-C cross-coupling, in order to strategically install, adjacent to the Ph-ring intended to undergo C-H arylation, a (6-pyridin-2-yl)pyrimidine that plays the role of a chelating directing moiety for the C-H arylation catalyst. The scope has been demonstrated on a series of 26 substrates, comprising diverse Ph-ring substituents and substitution patterns, as well as with 13 different aryl donors. Substrates in which the Ph-ring (arylation acceptor) was replaced by an electron-rich heteroaryl counterpart (2-/3-thiophene or -benzofuran) have also been examined and found to undergo arylation regioselectively. End-product conformations afford interesting motifs for occupying 3D chemical space, as implied by single-crystal X-ray diffraction, which has allowed the elucidation of six structures of aryl derivatives and one of an unprecedented pyrimidine-pyridine-benzofuran carbopalladated complex, believed to be a C-H activation derivative.

Substituent-controlled regioselective arylation of carbazoles using dual catalysis

Regioselective arylation of carbazoles is reported using dual palladium-photoredox catalysis. Controlled monoarylation and diarylation of symmetrical and unsymmetrical carbazoles were achieved under mild reaction conditions with a broad substrate scope and functional group tolerance. Steric and electronic control the regioselectivity of the arylation of unsymmetrical carbazoles. Late-stage functionalization of a caprofen drug derivative and large-scale synthesis of mono- and di-arylated carbazoles were demonstrated to showcase the synthetic versatility of the method. Finally, we also showcased the synthesis of hyellazole analogues (a marine alkaloid) in a short route using our strategy.

Dearomative bis-functionalization of quinoxalines and bis-N-arylation of (benz)imidazoles via Cu(II)-mediated addition of boronic acids

A Cu(OTf)₂-mediated regioselective dearomative aryl-hydroxylation across the C(sp²)N bond of 2-aryl quinoxalines and bis-N-arylation of (benz)imidazoles were developed using aryl boronic acids. For the dearomative aryl-hydroxylation, the C-center should be electrophilic (ca. 0.08), the N-center nucleophilic (ca. -0.50), and the C(sp²)N bond polarized (Δe = 0.609).

N-Directed Pd-Catalyzed Photoredox-Mediated C–H Arylation for Accessing Phenyl-Extended Analogues of Biginelli/Suzuki-Derived Ethyl 4-Methyl-2,6-diphenylpyrimidine-5-carboxylates

The availability and application of direct, functional group-compatible C–H activation methods for late-stage modification of small-molecule bioactives and other valuable materials remains an ongoing challenge in organic synthesis. In the current study, we demonstrate that a LED-activated, photoredox-mediated, Pd(OAc)2-catalyzed C–H arylation, employing a phenyldiazonium aryl source and either tris(2,2′-bipyridine)ruthenium(II) or (2,2′-bipyridine)bis[3,5-di-fluoro-2-[5-(trifluoromethyl)-2-pyridinyl-kN][phenyl-kC]iridium(III) as photoredox initiator, may successfully produce unprecedented mono- and bis-phenyl derivatives of functionality-rich 2,6-diphenylpyrimidine substrates at room temperature. The series of 19 substrates employed herein, which share the biologically-relevant 4-methyl-2,6-diphenylpyrimidine-5-carboxylate scaffold, were generated via a synthetic route involving (3-component) Biginelli condensation, oxidative dehydrogenation of the obtained 3,4-dihydropyrimidin-2(1H)-one to 2-hydroxypyrimidine, O-sulfonylation, and Suzuki-Miyaura C–C cross-coupling. Submission of these substrates to pyrimidine-N-atom-directed C–H arylation conditions led to regioselective phenylation at the ortho site(s) of the pyrimidine-C2-connected phenyl ring, revealing substituent-dependent electronic and steric effects. A focused library of 18 mono- and 10 bis-phenyl derivatives was generated. Its members exhibit interesting 3D and peripheral substitution features that render them promising for evaluation in drug discovery efforts.

Decoding Directing Groups and Their Pivotal Role in C-H Activation

Synthetic organic chemistry has witnessed a plethora of functionalization and defunctionalization strategies. In this regard, C-H functionalization has been at the forefront due to the multifarious applications in the development of simple to complex molecular architectures and holds a brilliant prospect in drug development and discovery. Despite been explored tremendously by chemists, this functionalization strategy still enjoys the employment of novel metal catalysts as well metal-free organic ligands. Moreover, the switch to photo- and electrochemistry has widened our understanding of the alternative pathways via which a reaction can proceed and these strategies have garnered prominence when applied to C-H activation. Synthetic chemists have been foraging for new directing groups and templates for the selective activation of C-H bonds from a myriad of carbon-hydrogen bonds in aromatic as well as aliphatic systems. As a matter of fact, by varying the templates and directing groups, scientists found the answer to the challenge of distal C-H bond activation which remained an obstacle for a very long time. These templates have been frequently harnessed for selectively activating C-H bonds of natural products, drugs, and macromolecules decorated with multiple C-H bonds. This itself was a challenge before the commencement of this field as functionalization of a site other than the targeted site could modify and hamper the biological activity of the pharmacophore. Total synthesis and pharmacophore development often faces the difficulty of superfluous reaction steps towards selective functionalization. This obstacle has been solved by late-stage functionalization simply by harnessing C-H bond activation. Moreover, green chemistry and metal-free reaction conditions have seen light in the past few decades due to the rising concern about environmental issues. Therefore, metal-free catalysts or the usage of non-toxic metals have been recently showcased in a number of elegant works. Also, research groups across the world are developing rational strategies for directing group free or non-directed protocols that are just guided by ligands. This review encapsulates the research works pertinent to C-H bond activation and discusses the science devoted to it at the fundamental level. This review gives the readers a broad understanding of how these strategies work, the execution of various metal catalysts, and directing groups. This not only helps a budding scientist towards the commencement of his/her research but also helps a matured mind searching out for selective functionalization. A detailed picture of this field and its progress with time has been portrayed in lucid scientific language with a motive to inculcate and educate scientific minds about this beautiful strategy with an overview of the most relevant and significant works of this era. The unique trait of this review is the detailed description and classification of various directing groups and their utility over a wide substrate scope. This allows an experimental chemist to understand the applicability of this domain and employ it over any targeted substrate.

Transition-Metal-Catalyzed C–H Arylation Using Organoboron Reagents

Aryl rings are ubiquitous in the core of numerous natural product and industrially important molecules and thus their facile synthesis is of major interest in the scientific community and industry. Although multiple strategies enable access to these skeletons, metal-catalyzed C–H activation is promising due to its remarkable efficiency. Commercially available organoboron reagents, a prominent arylating partner in the cross-coupling domain, have also been utilized for direct arylation. Organoborons are bench-stable, inexpensive, and readily available coupling partners that promise regioselectivity, chemodivergence, cost-efficiency, and atom-economy without requiring harsh and forcing conditions. This critical, short review presents a summary of all major studies of arylation using organoborons in transition-metal catalysis since 2005.

1 Introduction

2 Arylation without Directing Group Assistance

2.1 Palladium Catalysis

2.2 Iron Catalysis

2.3 Gold Catalysis

3 Arylation with Directing Group Assistance

3.1 Palladium Catalysis

3.2 Ruthenium Catalysis

3.3 Rhodium Catalysis

3.4 Nickel Catalysis

3.5 Cobalt Catalysis

3.6 Copper Catalysis

4 Conclusion

Recent Advances in C–H Bond Functionalization with Ruthenium-Based Catalysts

The past decades have witnessed rapid development in organic synthesis via catalysis, particularly the reactions through C–H bond functionalization. Transition metals such as Pd, Rh and Ru constitute a crucial catalyst in these C–H bond functionalization reactions. This process is highly attractive not only because it saves reaction time and reduces waste,but also, more importantly, it allows the reaction to be performed in a highly region specific manner. Indeed, several organic compounds could be readily accessed via C–H bond functionalization with transition metals. In the recent past, tremendous progress has been made on C–H bond functionalization via ruthenium catalysis, including less expensive but more stable ruthenium(II) catalysts. The ruthenium-catalysed C–H bond functionalization, viz. arylation, alkenylation, annulation, oxygenation, and halogenation involving C–C, C–O, C–N, and C–X bond forming reactions, has been described and presented in numerous reviews. This review discusses the recent development of C–H bond functionalization with various ruthenium-based catalysts. The first section of the review presents arylation reactions covering arylation directed by N–Heteroaryl groups, oxidative arylation, dehydrative arylation and arylation involving decarboxylative and sp3-C–H bond functionalization. Subsequently, the ruthenium-catalysed alkenylation, alkylation, allylation including oxidative alkenylation and meta-selective C–H bond alkylation has been presented. Finally, the oxidative annulation of various arenes with alkynes involving C–H/O–H or C–H/N–H bond cleavage reactions has been discussed.

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.

Synthesis of Unsymmetrical 2,6-Diarylanilines by Palladium-Catalyzed C-H Bond Functionalization Methodology

3,5-Dimethylpyrazole was employed as a monodentate directing group for palladium-catalyzed ortho-sp² C-H arylation with aryl iodides. The reaction shows good functional group tolerance and outstanding selectivity for mono- ortho-arylation. Ozonolysis of ortho-arylated arylpyrazoles gave acylated biphenylamines that were further arylated to afford unsymmetrically substituted 2,6-diarylacetanilides.

Highly meta-selective halogenation of 2-phenylpyridine with a ruthenium(i) catalyst

A practical method for PhI(OCOCF₃)₂ mediated direct meta-C–H monohalogenation of 2-phenylpyridine and its derivatives via an Ru(i) catalyzed reaction is reported.