Double C−H Activation: The Palladium-Catalyzed Direct C-Arylation of Xanthines with Arenes

Q-Tube®-Assisted Alkylation and Arylation of Xanthines and Other N-H-Containing Heterocycles in Water

In this paper, a simple and clean process for the alkylation and arylation of nitrogen-containing heterocycles is reported. The reactions were conducted using the Q-tube® as a non-conventional technology, in water as a green solvent, at overboiling temperature. The developed strategy was used to improve two steps in the total synthesis of caffeine, as reported by Narayan, and then extended to the preparation of N-decorated xanthines. Finally, piperidine, methyl piperazine, and isatine were proven to be suitable substrates for the protocol proposed herein.

Microwave-assisted modified synthesis of C8-analogues of naturally occurring methylxanthines: Synthesis, biological evaluation and their practical applications

An efficient, microwave-assisted, oxidant-interceded, transition-metal-free, cross-dehydrogenative C_sp²-C_sp³ coupling of C₈-Caffeine 2/Theobromine 3/theophylline 4 with substituted aliphatic alcohols 11a-lvia CH bond activation for the preparation of series of substituted C₈-(hydroxymethyl) Caffeine 12a-l/theobromine 13a-c/theophylline 14a-b has been developed using microwave irradiation upto 98% yield. The reaction proceeds smoothly in the presence of tert-butyl hydroperoxide (TBHP) under solvolysis condition at 120 °C for 20 min to corresponding substituted C₈-(hydroxymethyl)-methylxanthine derivatives in good to excellent yields. The good substrate scope, control experiments, gram-scale synthesis, and practical synthetic transformations further highlights the practicality of this methodology. These C₈-(hydroxymethyl) Caffeine 12a-l, 13a-c and 14a-b have been found to show promising in vitro antioxidant as well as antiplatelet activities.

An atom-economical and regioselective metal-free C-5 chalcogenation of 8-aminoquinolines under mild conditions

A general and simple metal-free protocol for expedient C-H functionalization leading to the regioselective generation of C-5 chalcogenated 8-aminoquinoline analogues in up to 90% yield at room temperature (25 °C) has been established. This methodology is an eco-friendly approach to the atom-economical utilization of diaryl/dialkyl chalcogenides for direct access to chalcogenated quinolines and is scalable to the gram scale without considerable decrease in the yield of the product. It represents a practical alternative to the existing metal-catalyzed functionalization of 8-aminoquinoline derivatives with broad functional group tolerance. The controlled experiments suggest that the reaction possibly proceeds through an ionic pathway at room temperature. Furthermore, the potentiality for the functionalization of free amines in chalcogenated-8-aminoquinolines provides an attractive perspective for further elaboration of the amine substituent through chemical manipulations. The applicability of the standardized method has been augmented through late-stage antimalarial drug diversification of primaquine analogues.

Recent developments in palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C–Z (Z = B, Si, Sn, S, N, C, H) bonds in bi(hetero)aryl synthesis

This review article describes the palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C_(Ar)–Z (Z = B, Si, Sn, S, N, C, H) bonds for facile bi(hetero)aryl synthesis in the past 10 years.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Palladium-catalyzed C–H olefination of uracils and caffeines using molecular oxygen as the sole oxidant

The palladium-catalyzed oxidative C–H olefination of uracils or caffeines with alkenes using an atmospheric pressure of molecular oxygen as the sole oxidant has been disclosed.

Density functional theory investigation on Pd-catalyzed cross-coupling of azoles with aryl thioethers

The mechanism and the origin of chemoselectivity of Pd-catalyzed C–H/C–S activation have been studied by density functional theory.

Palladium-catalyzed regioselective C-2 arylation of 7-azaindoles, indoles, and pyrroles with arenes

A palladium-catalyzed regioselective C-2 arylation of 7-azaindoles, indoles, and pyrroles with arenes has been developed, which features substrate dependent acid concentration.

Direct C–H heteroarylation of azoles with 1,2-di(pyrimidin-2-yl)disulfides through C–S cleavage of disulfides

C–C bond construction via C–H bond functionalization of oxazoles/thiazoles and C–S bond cleavage of di(hetero)aryl disulfides are described.

Rh(iii)-catalyzed oxime ether-directed heteroarylation of arene through oxidative C–H/C–H cross-coupling

A rhodium(iii)-catalyzed dehydrogenative cross-coupling reaction has been developed for the synthesis of bi(hetero)aryl scaffolds by using oxime ethers as the directing group. This protocol features a relatively broad substrate scope and a good tolerance of functional groups.

Direct arylation of N-heteroarenes with aryldiazonium salts by photoredox catalysis in water

A highly effective visible light-promoted "radical-type" coupling of N-heteroarenes with aryldiazonium salts in water has been developed. The reaction proceeds at room temperature with [Ru(bpy)3 ]Cl2 ⋅6 H2 O as a photosensitizer and a commercial household light bulb as a light source. Pyridine and a variety of substituted pyridines are effective substrates under these reaction conditions, and only monosubstituted products are formed with different regioselectivities. Using aqueous formic acid as solvent, an array of xanthenes, thiazole, pyrazine, and pyridazine are compatible with this new arylation approach. The broad substrate scope, mild reaction conditions, and use of water as reaction solvent make this procedure a practical and environmentally friendly method for the synthesis of compounds containing aryl-heteroaryl motifs.

Palladium catalysed cross-dehydrogenative-coupling of 1,3,5-trialkoxybenzenes with simple arenes

The cross-dehydrogenative coupling of two arene C–H positions is reported, generating multi-ortho-substituted biaryls under Pd catalysis.