Mechanochemical C-H Arylation and Alkylation of Indoles Using 3 d Transition Metal and Zero-Valent Magnesium

Although the 3 d transition-metal catalyzed C-H functionalization have been extensively employed to promote the formation of valuable carbon-carbon bonds, the persistent problems, including the use of sensitive Grignard reagents and the rigorous operations (solvent-drying, inert gas protection, metal pre-activation and RMgX addition rate control), still leave great room for further development of sustainable methodologies. Herein, we report a mechanochemical technology toward in-situ preparation of highly sensitive organomagnesium reagents, and thus building two general 3 d transition-metal catalytic platforms that enables regioselective arylation and alkylation of indoles with a wide variety of halides (including those containing post transformable functionalities and heteroaromatic rings). This mechanochemical strategy also brings unique reactivity and high step-economy in producing functionalized N-free indole products.

Ruthenium-Catalyzed Monoselective C-H Methylation and d 3-Methylation of Arenes

Site-selective installation of C-Me bonds remains a powerful and sought-after tool to alter the chemical and pharmacological properties of a molecule. Direct C-H functionalization provides an attractive means of achieving this transformation. Such protocols, however, typically utilize harsh conditions and hazardous methylating agents with poor applicability toward late-stage functionalization. Furthermore, highly monoselective methylation protocols remain scarce. Herein, we report an efficient monoselective, directed ortho-methylation of arenes using N,N,N-trimethylanilinium salts as noncarcinogenic, bench-stable methylating agents. We extend this protocol to d ₃-methylation in addition to the late-stage functionalization of pharmaceutically active compounds. Detailed kinetic studies indicate the rate-limiting in situ formation of MeI is integral to the observed reactivity.

Manganese-Catalyzed C(sp2)-H Alkylation of Indolines and Arenes with Unactivated Alkyl Bromides

Selective C(sp 2 ) - H bond alkylation of indoline, carbazole and (2-pyridinyl)arenes with unactivated alkyl bromides is achieved using MnBr 2 catalyst in the absence of an external ligand. The alkylation uses a simple LiHMDS base and avoids the necessity of Grignard reagent, unlike other Mn-catalyzed C - H functionalization. This reaction proceeded either through a five- or a less-favored six-membered metallacycle, and tolerated diverse functionalities, including alkenyl, alkynyl, silyl, aryl ether, pyrrolyl, indolyl, carbazolyl and alkyl bearing fatty alcohol and polycyclic-steroid moieties. Alkylation follows a single electron transfer (SET) pathway involving 1e oxidative addition of alkyl bromide and a rate-limiting C-H metalation.

NBMA Promotes Spermatogenesis by Mediating Oct4 Pathway

Non-obstructive azoospermia is one of the most common causes of male infertility, but there is still no specific treatment drug. Given that the Oct4 (Octamer-binding transcription factor 4) has an important regulatory effect on spermatogenesis, activating it can effectively promote spermatogenesis, so it is of great value to develop Oct4-targeted drug design and elucidating its mechanism of action. Here, we screened out the Oct4-targeted drug molecule NBMA (N-benzyl-4-methoxy-2-(1-(4-(trifluoromethyl)phenyl)vinyl)aniline) by computer-assisted technology, and found that it has a significant promoting effect on spermatogenesis in the established mouse azoospermia model. Subsequently, through transcriptome sequencing and enrichment analysis, real-time fluorescent quantitative PCR (qPCR) and western blot experiments revealed that NBMA promotes the differentiation of spermatogonial stem cells by activating the Oct4 pathway, thereby promoting spermatogenesis. This study proves that NBMA is a molecule with great potential to be developed as a therapeutic drug for azoospermia. It also shows that computer-assisted, chemical and biological multidisciplinary methods play a very important role in innovative drug discovery.

Sustainable manganese catalysis for late-stage C-H functionalization of bioactive structural motifs

The late-stage C–H functionalization of bioactive structural motifs is a powerful synthetic strategy for accessing advanced agrochemicals, bioimaging materials, and drug candidates, among other complex molecules. While traditional late-stage diversification relies on the use of precious transition metals, the utilization of 3d transition metals is an emerging approach in organic synthesis. Among the 3d metals, manganese catalysts have gained increasing attention for late-stage diversification due to the sustainability, cost-effectiveness, ease of operation, and reduced toxicity. Herein, we summarize recent manganese-catalyzed late-stage C–H functionalization reactions of biologically active small molecules and complex peptides.

Eco-friendly methoximation of aromatic aldehydes and ketones using MnCl2.4H2O as an easily accessible and efficient catalyst

Methoximes are important as a class of intermediates and products, among fine chemicals and specialties. The development of a new, facile and efficient method for their synthesis is reported. The methoximes were properly accessed from the corresponding aromatic aldehydes and ketones in good to excellent yields, under mild conditions, employing the inexpensive and environmentally friendly MnCl2.4H2O as a catalyst (at low loading and without the addition of ligand), in EtOH at 50°C. The scope of the process was systematically assessed.

Iron-Catalyzed Ortho C-H Homoallylation of Aromatic Ketones with Methylenecyclopropanes

We report here a C-H homoallylation reaction of aromatic ketones with methylenecyclopropanes (MCPs) only using a catalytic amount of Fe(PMe₃)₄. A variety of aromatic ketones and MCPs are applicable to the reaction to form ortho-homoallylated aromatic ketones selectively via regioselective scission of the three-membered rings. The homoallylated products are amenable to further elaborations, providing functionalized 1,2-dihydronaphthalenes.

Installing the “magic methyl” – C–H methylation in synthesis

Following notable cases of remarkable potency increases in methylated analogues of lead compounds, this review documents the state-of-the-art in C–H methylation technology.

Mn(ii)-Catalysed ortho-alkenylation of aromatic amines and its application in reproductive diseases

A Mn(ii)-catalysed ortho-alkenylation of aromatic amines and its application in reproductive diseases were developed. The use of MnCl₂ was critical for the ortho-alkenylation of aromatic amines. The general applicability of this procedure was highlighted by the synthesis of 27 vinylanilines, with good regioselectivities. The value of our approach in practical applications was investigated by studying the effects of one of the compounds 3m on 8 week-old adult male rats with azoospermia as a mammalian model. The results show that a small amount of sperm will gradually be produced in the epididymis and testes by treatment of 8 week-old adult male rats with azoospermia with 1 mg kg⁻¹3m after two weeks, while treatment with 10 mg kg⁻¹3m led to obvious sperm production. Notably, if we increase the dose to 100 mg kg⁻¹, there will be a lot of sperm production in the epididymis and testes after two weeks of treatment. The results of this study will be of great significance in research on drugs for treating azoospermia and oligospermia diseases.

A Mn(ii)-catalysed ortho-alkenylation of aromatic amines and its application in reproductive diseases were developed.

Nickela-electrocatalyzed Mild C-H Alkylations at Room Temperature

Direct alkylations of carboxylic acid derivatives are challenging and particularly nickel catalysis commonly requires high reaction temperatures and strong bases, translating into limited substrate scope. Herein, nickel-catalyzed C-H alkylations of unactivated 8-aminoquinoline amides have been realized under exceedingly mild conditions, namely at room temperature, with a mild base and a user-friendly electrochemical setup. This electrocatalyzed C-H alkylation displays high functional group tolerance and is applicable to both the primary and secondary alkylation. Based on detailed mechanistic studies, a nickel(II/III/I) catalytic manifold has been proposed.

MnBr2-Catalyzed Direct and Site-Selective Alkylation of Indoles and Benzo[h]quinoline

Manganese-catalyzed regioselective C-H alkylation of indoles and benzo[h]quinoline with a variety of unactivated alkyl iodides is reported. Unlike other Mn-catalyzed C-H functionalization, this protocol does not require a Grignard reagent base and employs a simple and inexpensive MnBr₂ as a catalyst. This method tolerates diverse functionalities, including fluoro, chloro, bromo, iodo, alkenyl, alkynyl, pyrrolyl, and carbazolyl groups. The alkylation proceeds through a single-electron transfer pathway comprising reversible C-H manganesation and involving an alkyl radical intermediate.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

Synthesis of 3-(Methylsulfonyl)benzo[ b]thiophenes from Methyl(2-alkynylphenyl)sulfanes and Sodium Metabisulfite via a Radical Relay Strategy

A radical relay strategy for the generation of 3-(methylsulfonyl)benzo[ b]thiophenes through a reaction of methyl(2-alkynylphenyl)sulfanes with sodium metabisulfite in the presence of a photocatalyst under visible light irradiation is developed. This photoinduced sulfonylation proceeds efficiently under mild conditions by using a catalytic amount of sodium methylsulfinate as an initiator. During the reaction process, the methyl radical generated in situ is the key intermediate, which undergoes a radical relay with the combination of sulfur dioxide to afford methylsulfonyl-containing compounds.

MnCl2-Catalyzed C-H Alkylation on Azine Heterocycles

Low-valent manganese-catalyzed C-H alkylation of pyridine derivatives with both primary and challenging secondary alkyl halides was established by amide assistance. The strategy provided expedient access to alkylated pyridines with wide functional group tolerance and ample scope through weak chelation. Mechanistic studies provided strong support for a rate-determining C-H activation and a SET-type C-X scission.

Manganese(ii)-catalysed dehydrogenative annulation involving C–C bond formation: highly regioselective synthesis of quinolines

An inexpensive nontoxic manganese(ii)-catalysed dehydrogenative annulation was developed for C–C bond formation.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

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.

Continuous Visible-Light Photoflow Approach for a Manganese-Catalyzed (Het)Arene C-H Arylation

Manganese photocatalysts enabled versatile room-temperature C-H arylation reactions by means of continuous visible-light photoflow, thus allowing for efficient C-H arylations in 30 minutes with ample scope. The robustness of the manganese-catalyzed photoflow strategy was shown by visible light-induced gram-scale synthesis, clearly outperforming the batch performance.

Inert C-H Bond Transformations Enabled by Organometallic Manganese Catalysis

Traditional organic synthesis relies heavily on the transformations of various preinstalled functional groups, such as cross-coupling reactions using organohalides and organometallic reagents. The strategy of C-H activation enables the direct formation of C-C/C-X (X = heteroatom) bonds from inert C-H bonds, which can enhance the atom- and step-economy of organic synthesis. To date, precious metals have overwhelmingly dominated the C-H activation field; however, the rarity and high cost of these metals necessitate the development of more sustainable catalysts. In this regard, catalysts based on manganese are highly desirable owing to the abundant reserve of manganese in the earth's crust and its economic benefits, low toxicity, and potentially unique reactivity. Although the first stoichiometric manganese-mediated C-H activation reaction was reported as early as 1970, manganese-catalyzed C-H activation reactions are largely underdeveloped. How to construct an efficient catalytic cycle for manganese in C-H activation reactions remains as a key issue to be addressed. In this Account, we summarize our recent advances in the manganese-catalyzed transformations of inert C-H bonds. To overcome the challenges associated with building manganese-based catalytic cycles, we developed two novel strategies, namely, synergy between manganese catalysts and bases and between manganese catalysts (with or w/o bases) and acids. By implementing the former strategy, we developed cooperative manganese/base catalytic systems that facilitate a new mode of C-H bond activation by manganese via a redox-neutral base-assisted deprotonation mechanism. As such, the requirement for the tedious preparation of MnR(CO)₅ complexes (R = Me, Bn, Ph) in stoichiometric reactions was eliminated, and a series of manganese-catalyzed C-H activation reactions of arenes with various reaction partners having C≡C and C═C bonds were achieved. Through the latter strategy of synergy between manganese catalysts (with or w/o bases) and acids, we disclosed a "dual activation" mode for performing manganese-catalyzed C-H bond transformations, that is, merging C-H activation by manganese catalysts and C-X multiple bond activation by Lewis acids. Consequently, the scope of C-H substrates could be expanded to include challenging ketones and olefinic C-H compounds. Additionally, the range of reaction partners could be significantly broadened to include those bearing more polarized C═O, C═N, and C≡N bonds such as aldehydes, imines, and nitriles. Remarkably, the innate reactivity of different C-H bonds in ketones could be reversed by manganese catalysis, and the reactions could even be carried out at room temperature. Our findings provide guiding information for the future development of manganese-catalyzed C-H activation reactions and beyond. Related important contributions from other groups are mentioned, and the remaining challenges and future perspective in this emerging area are also presented.

N-Heterocyclic carbene–chromium-catalyzed alkylative cross-coupling of benzamide derivatives with aliphatic bromides

Described here is a chromium-catalyzed alkylative cross-coupling of benzamide derivatives with aliphatic electrophiles under mild conditions.

Recent advances in manganese-catalysed C–H activation: scope and mechanism

Manganese catalysed C–H activation has emerged as a promising green alternative to transition metal mediated processes.