Direct C2-Alkylation of Azoles with Alcohols and Ethers through Dehydrogenative Cross-Coupling under Metal-Free Conditions

Minisci-Type Dehydrogenative Coupling of N-Heteroaromatic Rings with Inert C(sp3)-H Enabled by a Visible-Light-Catalyzed Intermolecular Hydrogen Atom Transfer Process

The Minisci-type dehydrogenative coupling of N-heteroaromatic rings with inert C-H or Si-H partners via visible-light-catalyzed hydrogen atom transfer has been reported. This methodology allows the coupling reactions to be carried out in water as a solvent under air atmospheric conditions with visible-light illumination. A wide range of inert C-H and Si-H partners could be directly coupled with various N-aromatic heterocycles to deliver products in good to excellent yields.

Visible light-induced direct alkylation of the purine C8-H bond with ethers

The one-step visible light-induced direct alkylation of the C₈-H bond for purine derivatives by ethers was developed using Eosin Y as the photocatalyst and t-BuOOH as the oxidant at room temperature. This method describes the coupling of the α-C of the ether to the C₈ of purine. Of particular interest is that substrates include purines with various functional groups and even unprotected 9H-purines. The protocol provides an effective method for the synthesis of 8-alkylpurine derivatives with high atom economy and high regioselectivity.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Heteroarylation of unactivated C-H bonds suitable for late-stage functionalization

The late-stage introduction of diverse heterocycles onto complex small molecules enables efficient access to new medicinally relevant compounds. An attractive approach to such a transformation would utilize the ubiquitous aliphatic C-H bonds of a complex substrate. Herein, we report a system that enables direct C-H heteroarylation using a stable, commercially available O-alkenylhydroxamate with heterocyclic sulfone partners. The C-H heteroarylation proceeds efficiently with a range of aliphatic substrates and common heterocycles, and is a rare example of heteroarylation of strong C-H bonds. Importantly, the present approach is amenable to late-stage functionalization as the substrate is the limiting reagent in all cases.

Regioselective C6-H Hydroxyalkylation of Purines and Purine Nucleosides via α-C-H Functionalization of Alcohols at Room Temperature

The highly regioselective C⁶-H hydroxylalkylation of purines and purine nucleosides within 10 min via the α-C(sp³)-H functionalization of alcohols at room temperature is reported here for the first time. The reaction tolerated various functional groups, which have the potential for further modification to afford other valuable molecules. The reported method avoids metal catalysts, light, and protecting groups, giving a direct strategy to access 6-substitued alkylated purines and nucleosides with pharmaceutical bioactivities.

Phenanthrenequinone (PQ) catalyzed cross-dehydrogenative coupling of alkanes with quinoxalin-2(1H)-ones and simple N-heteroarenes under visible light irradiation

A direct and convenient strategy to 3-alkylquinoxalin-2(1H)-ones and other alkyl N-heteroarenes via a photocatalyzed alkylation of quinoxalin-2(1H)-ones and other N-heterocycles with commercially available, low-cost alkanes under ambient conditions using phenanthrenequinone (PQ) as a photocatalyst was developed. This transformation has advantages of environment-friendly protocol, mild conditions, good functional-group tolerance, and high yields of products.

Photocatalyst-, metal- and additive-free, regioselective radical cascade sulfonylation/cyclization of benzimidazoles derivatives with sulfonyl chlorides induced by visible light

Herein, an environmental and practical protocol for the visible-light-triggered regioselective radical cascade sulfonylation/cyclization of unactivated alkenes towards synthesis of polycyclic benzimidazoles containing sulfone group has been developed. Notably, the control...

Enantioselective α-Arylation of Primary Alcohols under Sequential One-Pot Catalysis

Secondary benzylic alcohols and diarylmethanols are common structural motifs of biologically active and medicinally relevant compounds. Here we report their enantioselective synthesis by α-arylation of primary aliphatic and benzylic alcohols under sequential catalysis integrating a Ru-catalyzed hydrogen transfer oxidation and a Ru-catalyzed nucleophilic addition. The method can be applied to various alcohols and aryl nucleophiles tolerating a range of functional groups, including secondary alcohols, ketones, alkenes, esters, NH amides, tertiary amines, aryl halides, and heterocycles.

Lewis acid mediated, mild C-H aminoalkylation of azoles via three component coupling

This manuscript reports the development of a mild, highly functional group tolerant and metal-free C–H aminoalkylation of azoles via a three-component coupling approach. This method enables the C–H functionalization of diverse azole substrates, such as oxazoles, benzoxazoles, thiazoles, benzothiazoles, imidazoles, and benzimidazoles. DFT calculations identify a key deprotonation equilibrium in the mechanism of the reaction. Using DFT as a predictive tool, the C–H aminoalkylation of initially unreactive substrates (imidazoles/benzimidazoles) can be enabled through an in situ protecting/activating group strategy. The DFT-supported mechanistic pathway proposes key interactions between the azole substrate and the Lewis acid/base pair TBSOTf/EtNⁱPr₂ that lead to azole activation by deprotonation, followed by C–C bond formation between a carbene intermediate and an iminium electrophile. Two diverse approaches are demonstrated to explore the amine substrate scope: (i) a DFT-guided predictive analysis of amine components that relates reactivity to distortion of the iminium intermediates in the computed transition state structures; and (ii) a parallel medicinal chemistry workflow enabling synthesis and isolation of several diversified products at the same time. Overall, the presented work enables a metal-free approach to azole C–H functionalization via Lewis acid mediated azole C–H deprotonation, demonstrating the potential of a readily available, Si-based Lewis acid to mediate new C–C bond formations.

Lewis acid mediated activation enables mild, metal-free, and highly functional group tolerant C–H aminoalkylation of diverse azoles via three-component coupling.

An expeditious microwave assisted one-pot sequential route to pyrido fused imidazo[4,5-c] quinolines in green media

A novel one-pot sequential route for the synthesis of pyrido[2′,1′:2,3]imidazo[4,5-c]quinolines via microwave-assisted sequential reaction of 2-aminopyridines, 2-bromophenacyl bromides, and aldehydes has been developed in green media.

C(sp3)-H Monoarylation of Methanol Enabled by a Bidentate Auxiliary

With the assistance of a practical directing group (COAQ), the first catalytic protocol for the palladium-catalyzed C(sp³)-H monoarylation of methanol has been developed, offering an invaluable synthesis means to establish extensive derivatives of crucial arylmethanol functional fragments. Furthermore, the gram-scale reaction, broad substrate scope, excellent functional group compatibility, and even the practical synthesis of medicines further demonstrate the usefulness of this strategy.

Free-Radical-Promoted Dehydrogenative Coupling of Polyfluorinated Alcohol with Quinone, Chromone, and Coumarin

A free-radical-mediated dehydrogenative cross-coupling reaction of polyfluorinated alcohol with quinone, coumarin, and chromone was developed. It provides a sustainable and practical strategy for installation of fluorine atom into organic molecules by using polyfluorinated alcohols.

Direct intramolecular double cross-dehydrogentive-coupling (CDC) cyclization of N-(2-pyridyl)amidines under metal-free conditions

A facile transition-metal-free protocol to form 2-iminoimidazo[1, 2-a]-pyridines bearing a -CHBr₂ group and an aza-quaternary carbon center at the 3 position from N-(2-pyridyl)amidines substrates, in which the new heterocyclic skeletons constructed from amidines via radical reactions or nucleophilic substitution reactions are promoted only by CBr₄ under mild conditions, is demonstrated. The reactions were realized by intramolecular CDC reaction involving C-N and C-C bond formation via the sequential C(sp³)-H bifunctionalization mode on the same carbon atom under mild conditions. Moreover, this work also provides an excellent and representative example for CBr₄ as an efficient reagent to initiate radical reactions under initiator-free conditions or to give rise to nucleophilic substitution reactions only by base.

The Ag-promoted α-C-H arylation of alcohols

We herein report the functionalization of α-C-H in alcohols through cross-dehydrogenative coupling reactions. Selectfluor was used as a mild oxidant. In situ-generated HF participated in the reaction and no external strong acid was necessary. A variety of heteroaryl-substituted alcohols were achieved with good yields and with good functional group tolerance.

Palladium-catalyzed direct C2-arylation of azoles with aromatic triazenes

A highly efficient palladium-catalyzed arylation of azoles at the C2-position using 1-aryltriazenes as aryl reagents was developed. Azoles including oxazoles, thiazoles, imidazoles, 1,3,4-oxadiazoles, and oxazolines could react with 1-aryltriazenes smoothly to generate the corresponding products in good to excellent yields, and various substitution patterns were tolerated toward the reaction.

Base-promoted domino radical cyclization of 1,6-enynes

A good regioselective, high atom-economical and transition-metal-free method for the synthesis of α-functionalized ether derivatives via the domino radical cyclization of 1,6-enynes is described. A series of α-functionalized ether derivatives could be easily obtained in good yields with wide functional group tolerance by using less toxic and inexpensive Cs₂CO₃ as the base. The control experiment results show that the reaction involves a radical process. This strategy provides a regioselective way toward the formation of dual C-C bonds in one step.

Transition-metal free oxidative C-H etherification of acylanilines with alcohols through a radical pathway

A transition metal free approach for the synthesis of methyl/ethyl aryl ether via oxidative C-H etherification of acylanilines with alcohols has been developed. Various acylanilines are compatible under standard conditions, giving the corresponding products in moderate to good yields. This strategy avoids transition-metal catalyst and excessive alcohol, providing a simple and reliable alternative method for the synthesis of methyl/ethyl aryl ether. Control experiments reveal that a radical mechanism is involved in this transformation.

K2S2O8-Mediated Hydroxyalkylation of Benzothiazoles with Alcohols in Aqueous Solution

The K2S2O8-mediated hydroxyalkylation of 2H-benzothiazoles with aliphatic alcohols in aqueous solution was described. The mild and convenient protocol generated a series of hydroxyalkylated benzothiazoles in moderate to good yields. Besides, benzimidazole and ethers were also compatible in this reaction, leading to corresponding C2 ether-substituted heteroarenes.