Nucleophilic character of alkyl radicals—V

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Hydrogen Atom Transfer Driven Enantioselective Minisci Reaction of Alcohols

Catalytic enantioselective Minisci reactions have recently been developed but all instances so far utilize α-amino radical coupling partners. We report a substantial evolution of the enantioselective Minisci reaction that enables α-hydroxy radicals to be used, providing valuable enantioenriched secondary alcohol products. This is achieved through the direct oxidative coupling of two C-H bonds on simple alcohol and pyridine partners through a hydrogen atom transfer (HAT)-driven approach: a challenging process to achieve due to the numerous side reactions that can occur. Our approach is highly regioselective as well as highly enantioselective. Dicumyl peroxide, upon irradiation with 390 nm light, serves as both HAT reagent and oxidant whilst selectivity is controlled by use of a chiral phosphoric acid catalyst. Computational and experimental evidence provide mechanistic insight as to the origin of selectivity, revealing a stereodetermining deprotonation step distinct from the analogous reaction of amide-containing substrates.

Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

Photoredox chemistry with organic or transition metal agents has been reviewed in earlier years, but such is the pace of progress that we will overlap very little with earlier comprehensive reviews. This review first presents an overview of the area of research and then examines recent examples of C-C, C-N, C-O and C-S bond formations via radical intermediates with transition metal and organic radical promoters. Recent successes with Birch reductions are also included. The transition metal chemistry will be restricted to photocatalysts based on the most widely used metals, Ru and Ir, but includes coupling chemistries that take advantage of low-valent nickel, or occasionally copper, complexes to process the radicals that are formed. Our focus is on developments in the past 10 years (2011-2021). This period has also seen great advances in the chemistry of organic photoredox reagents and the review covers this area. The review is intended to present highlights and is not comprehensive.

KIO4 -mediated Selective Hydroxymethylation/Methylenation of Imidazo-Heteroarenes: A Greener Approach

Herein, we report a KIO₄ -mediated, sustainable and chemoselective approach for the one-pot C(sp² )-H bond hydroxymethylation or methylenation of imidazo-heteroarenes with formaldehyde, generated in situ via the oxidative cleavage of ethylene glycol or glycerol (renewable reagents) through the Malaprade reaction. In the presence of ethylene glycol, a series of 3-hydroxymethyl-imidazo-heteroarenes was obtained in good to excellent yields. These compounds are important intermediates to access pharmaceutical drugs, e.g., Zolpidem. Furthermore, by using glycerol, bis(imidazo[1,2-a]pyridin-3-yl)methane derivatives were selectively obtained in good to excellent yields.

Hydroxymethylation of quinolines via iron promoted oxidative C-H functionalization: synthesis of arsindoline-A and its derivatives

Herein, we report a mild and efficient hydroxymethylation of quinolines via an iron promoted cross-dehydrogenative coupling reaction under external acid free conditions. Various hydroxyalkyl substituted quinolines were achieved in excellent yields with well tolerated functional groups. Importantly, a few of the hydroxylmethylated quinolines were further transformed into respective aldehydes, and were successfully utilized for the synthesis of alkaloid arsindoline-A and its derivatives.

External oxidant-free alkylation of quinoline and pyridine derivatives

A novel and efficient method for the generation of alkyl radicals and the alkylation of quinoline and pyridine derivatives under mild conditions has been developed. This strategy allows the direct alkylation of heteroaromatics in the absence of an external oxidant. A preliminary mechanistic study suggests that the present reaction probably proceeds via an intermolecular HAT process.

Carbon nitride as a heterogeneous visible-light photocatalyst for the Minisci reaction and coupling to H2 production

Cyanamide functionalised carbon nitride powder is reported as a photocatalyst for direct Minisci-type coupling of heteroarenes with ethers, alcohols, and amides using atmospheric oxygen as the oxidant at room temperature. This mild protocol displays broad substrate scope, good functional group tolerance and the catalyst can be easily isolated and reused for several cycles. It thereby addresses the two major limitations of previously reported photoredox-mediated Minisci reactions: (i) use of expensive and potentially harmful non-recyclable photocatalysts, and (ii) requirement of a stoichiometric amount of strong chemical oxidant. Finally, using platinum as a co-catalyst with the carbon nitride allows this light-mediated reaction to generate two value-added products under an anaerobic atmosphere - functionalised heteroarenes and H₂ fuel.

Metal- and Acid-Free C-H Formylation of Nitrogen Heterocycles: Using Trioxane as an Aldehyde Equivalent Enabled by an Organic-Soluble Oxidant

A metal-free, innate, and practical C-H formylation of nitrogen heterocycles using trioxane as a formyl equivalent is reported. This reaction provides a mild and robust method for modifying medicinally relevant heterocycles with an aldehyde handle. The use of an organic soluble oxidant, tetrabutylammonium persulfate, is critical in promoting the desired coupling.

The photochemical alkylation and reduction of heteroarenes

The functionalization of heteroarenes has been integral to the structural diversification of medicinally active molecules such as quinolines, pyridines, and phenanthridines. Electron-deficient heteroarenes are electronically compatible to react with relatively nucleophilic free radicals such as hydroxyalkyl. However, the radical functionalization of such heteroarenes has been marked by the use of transition-metal catalyzed processes that require initiators and stoichiometric oxidants. Herein, we describe the photochemical alkylation of quinolines, pyridines and phenanthridines, where through direct excitation of the protonated heterocycle, alcohols and ethers, such as methanol and THF, can serve as alkylating agents. We also report the discovery of a photochemical reduction of these heteroarenes using only iPrOH and HCl. Mechanistic studies to elucidate the underlying mechanism of these transformations, and preliminary results on catalytic methylations are also reported.

Transition metal-free, visible-light mediated synthesis of 1,10-phenanthroline derived ligand systems

A broad range of 1,10-phenanthroline substrates was efficiently C-H functionalised, providing rapid, gram-scale access to substituted heteroaromatic cores of broad utility. Furthermore, this C-H functionalisation pathway was extended to the synthesis of previously inaccessible, ultra-soluble, 2,9-bis-triazinyl-1,10-phenanthroline (BTPhen) ligands for advanced nuclear fuel cycles.

Direct Cα-heteroarylation of structurally diverse ethers via a mild N-hydroxysuccinimide mediated cross-dehydrogenative coupling reaction

An important challenge in the Cα-heteroarylation of ethers is the requirement of a large excess amount of ethers (that are used as solvents in many cases) to achieve effective transformations. This drawback has significantly restricted the Cα-heteroarylation of ethers to the use of simple and easily accessible ether substrates. To overcome this limitation, a new, efficient, N-hydroxysuccinimide (NHS) mediated, mild and metal-free CDC strategy for the direct Cα-heteroarylation of diverse ethers has been developed. Different to our previous benzaldehyde mediated photoredox Cα-heteroarylation, we have identified NHS as a new and efficient mediator without using light. A distinct non-photoredox engaged hydrogen-atom-transfer (HAT) mechanism that used a nitrogen-centered radical cation produced from NHS is initially revealed. Notably, only 5-10 equivalents of ethers as coupling partners are used, which allows for structurally diverse and complex ethers to engage in this process, to create highly medicinally relevant Cα-heteroarylated ethers. Furthermore, more structurally diverse heterocyclics can serve as reactants for this process.

C-H Functionalization of Azines

Azines, which are six-membered aromatic compounds containing one or more nitrogen atoms, serve as ubiquitous structural cores of aromatic species with important applications in biological and materials sciences. Among a variety of synthetic approaches toward azines, C-H functionalization represents the most rapid and atom-economical transformation, and it is advantageous for the late-stage functionalization of azine-containing functional molecules. Since azines have several C-H bonds with different reactivities, the development of new reactions that allow for the functionalization of azines in a regioselective fashion has comprised a central issue. This review describes recent advances in the C-H functionalization of azines categorized as follows: (1) S_NAr reactions, (2) radical reactions, (3) deprotonation/functionalization, and (4) metal-catalyzed reactions.

Introduction of Quinolines and Isoquinolines onto Nonactivated α-C-H Bond of Tertiary Amides through a Radical Pathway

Treatment of quinolines and isoquinolines with benzoyl peroxide in tertiary amides, such as N,N-dimethylacetamide, N,N-dimethylpropionamide, and N-acetylpyrrolidine, etc., under irradiation with a Hg lamp in the temperature range of 35 °C to 40 °C gave C-C-bonded quinolines and isoquinolines bearing amide groups with high regioselectivity in good to moderate yields, respectively, under transition-metal-free conditions.

Direct α-arylation of ethers through the combination of photoredox-mediated C-H functionalization and the Minisci reaction

The direct α-arylation of cyclic and acyclic ethers with heteroarenes has been accomplished through the design of a photoredox-mediated CH functionalization pathway. Transiently generated α-oxyalkyl radicals, produced from a variety of widely available ethers through hydrogen atom transfer (HAT), were coupled with a range of electron-deficient heteroarenes in a Minisci-type mechanism. This mild, visible-light-driven protocol allows direct access to medicinal pharmacophores of broad utility using feedstock substrates and a commercial photocatalyst.

Cross-dehydrogenative coupling of α-C(sp3)–H of ethers/alkanes with C(sp2)–H of heteroarenes under metal-free conditions

A metal-free cross-dehydrogenative coupling method between α-oxyalkyl/alkyl radicals and electron deficient heteroarenes is reported.

Novel S1P(1) receptor agonists--part 3: from thiophenes to pyridines

In preceding communications we summarized our medicinal chemistry efforts leading to the identification of potent, selective, and orally active S1P1 agonists such as the thiophene derivative 1. As a continuation of these efforts, we replaced the thiophene in 1 by a 2-, 3-, or 4-pyridine and obtained less lipophilic, potent, and selective S1P1 agonists (e.g., 2) efficiently reducing blood lymphocyte count in the rat. Structural features influencing the compounds' receptor affinity profile and pharmacokinetics are discussed. In addition, the ability to penetrate brain tissue has been studied for several compounds. As a typical example for these pyridine based S1P1 agonists, compound 53 showed EC50 values of 0.6 and 352 nM for the S1P1 and S1P3 receptor, respectively, displayed favorable PK properties, and penetrated well into brain tissue. In the rat, compound 53 maximally reduced the blood lymphocyte count for at least 24 h after oral dosing of 3 mg/kg.

The cross-dehydrogenative coupling of C(sp3)-H bonds: a versatile strategy for C-C bond formations

Over the last decade, substantial research has led to the introduction of an impressive number of efficient procedures which allow the selective construction of CC bonds by directly connecting two different CH bonds under oxidative conditions. Common to these methodologies is the generation of the reactive intermediates in situ by activation of both CH bonds. This strategy was introduced by the group of Li as cross-dehydrogenative coupling (CDC) and discloses waste-minimized synthetic alternatives to classic coupling procedures which rely on the use of prefunctionalized starting materials. This Review highlights the recent progress in the field of cross-dehydrogenative C sp 3C formations and provides a comprehensive overview on existing procedures and employed methodologies.

Structure-activity relationship of 2-[[(2-pyridyl)methyl]thio]-1H- benzimidazoles as anti Helicobacter pylori agents in vitro and evaluation of their in vivo efficacy

A relationship between the structure of 21 2-[[(2-pyridyl)methyl]thio]-1H-benzimidazoles (6) and their anti Helicobacter pylori activity expressed as minimum bactericidal concentration (MBC) values is described. Observed MBCs ranged from 256 to 1 microg/mL. The structure-activity relationship (SAR) showed that larger and more lipophilic compounds, especially compounds with such substituents in the 4-position of the pyridyl moiety, generally had lower MBC values. Four new compounds that were predicted to be potent by the established SAR model were synthesized and tested. One such compound, i.e., 2-[[(4-[(cyclopropylmethyl)oxy]-3-methyl-2-pyridyl)methyl]thio]-1H-be nzimidazole (18), was tested for in vivo efficacy in a mouse Helicobacter felismodel (125 micromol/kg bid given orally for 4 days, n = 4). Unfortunately, antibacterial activity could not be clearly demonstrated in this model. Instead a potent acid secretion inhibition was observed. This finding was attributed to the methylthio compound being oxidized to the corresponding methyl sulfinyl derivative, i.e., a proton pump inhibitor, in vivo. Although the antibacterial activity had the potential of decreasing H. felis cell counts in vivo the proton pump inhibitory effect became dominant and actually promoted H. felis cell growth. Hence, we conclude that the antibacterial utility of the 2-[[(2-pyridyl)methyl]thio]-1H-benzimidazoles (6) as a compound class is compromised by their propensity to become proton pump inhibitors upon metabolic oxidation in vivo.