Synthesis of (2H)‐Indazoles from Azobenzenes Using Paraformaldehyde as a One‐Carbon Synthon

Site-selective direct nitration of 2H-indazoles: easy access to 7-nitroindazoles

A new site-selective methodology for C-H nitration of 2H-indazoles has been accomplished at the C7 position using iron(III) nitrate. This strategy enables practical access to an array of 7-nitroindazoles with broad functional group tolerance in good yields. The synthesized products have been proven as valuable synthetic intermediates by demonstrating the synthetic utility. Mechanistic investigations indicate that the reaction goes through a radical pathway.

Pd(II)-Catalyzed Three-Component Synthesis of Furo[2,3-d]pyrimidines from β-Ketodinitriles, Boronic Acids, and Aldehydes

A Pd(II)-catalyzed three-component synthesis of 2,4,6-triarylfuro[2,3-d]pyrimidines from β-ketodinitriles, boronic acids, and aldehydes has been developed. The participation of both nitrile (-CN) groups led to the concurrent construction of furo-pyrimidine via the formation of C-C, C═C, C-O, C-N, and C═N bonds. The compounds show excellent photoluminescence properties with absorption maxima ranging from 348 to 387 nm and emission from 468 to 533 nm. The synthetic utility of the protocol was further demonstrated through a few postsynthetic manipulations.

Paraformaldehyde as C1 Synthon: Electrochemical Three-Component Synthesis of Tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones in Aqueous Ethanol

A green and practical method for the electrochemical synthesis of tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones through the three-component reaction of quinoxalin-2(1H)-ones, N-arylglycines and paraformaldehyde was reported. In this strategy, EtOH played dual roles (eco-friendly solvent and waste-free pre-catalyst) and the in situ generated ethoxide promoted triple sequential deprotonations.

Dimeric Manganese-Catalyzed Direct Nucleophilic Addition of C(sp2)-H Bonds to Inert Aldehydes

An efficient direct nucleophilic addition reaction of C(sp²)-H bonds to aldehydes catalyzed by a dimeric manganese has been developed. This reaction has a broad range of substrates, and high yields were also obtained with inert aliphatic aldehydes as substrates. A dimeric Mn₂(CO)₈Br₂ was proven to be a more efficient catalyst precursor than the monomeric Mn(CO)₅Br.

Recent Strategies in Transition-Metal-Catalyzed Sequential C–H Activation/Annulation for One-Step Construction of Functionalized Indazole Derivatives

Designing new synthetic strategies for indazoles is a prominent topic in contemporary research. The transition-metal-catalyzed C–H activation/annulation sequence has arisen as a favorable tool to construct functionalized indazole derivatives with improved tolerance in medicinal applications, functional flexibility, and structural complexity. In the current review article, we aim to outline and summarize the most common synthetic protocols to use in the synthesis of target indazoles via a transition-metal-catalyzed C–H activation/annulation sequence for the one-step synthesis of functionalized indazole derivatives. We categorized the text according to the metal salts used in the reactions. Some metal salts were used as catalysts, and others may have been used as oxidants and/or for the activation of precatalysts. The roles of some metal salts in the corresponding reaction mechanisms have not been identified. It can be expected that the current synopsis will provide accessible practical guidance to colleagues interested in the subject.

Synthesis of Pyranones: Ru-Catalyzed Cascade Reaction via Vinylic C-H Addition to Glyoxylate

The efficient synthesis of pyranones is presented by a three-component cascade reaction from readily available acrylic acids, ethyl glyoxylate, and p-toluenesulfonamide under ruthenium catalysis. For the first time, the nucleophilic addition of the vinylic C-H bond of acrylic acids across aldehyde is achieved, and the intramolecular cyclization as well as subsequent second insertion to aldehyde form the substituted butenolides. The elimination of sulfonamides occurs at higher temperature to give the pyranones.

Metal-free regioselective C-H amination for the synthesis of pyrazole-containing 2H-indazoles

A general and practical regioselective approach for the C-H amination of 2H-indazoles under transition-metal-free conditions was developed. A series of substrates were tested showing eminent functional group tolerance and affording the C-N functionalization products in good to excellent yields. Mechanism studies revealed that a radical process was involved in this transformation.

N-Nosyl-O-bromoethyl hydroxylamine acts as a multifunctional formaldehyde, formaldimine, and 1,2-oxazetidine surrogate for C–C and C–O bond-forming reactions

Hydroxylamine could chemo- and stereoselectively produce α-hydroxymethyl and α-aminomethyl ketones and chiral α-alkoxyl and α-aminomethyl carboxylates in good yields.

Base-controlled divergent synthesis of vinyl sulfones from (benzylsulfonyl)benzenes and paraformaldehyde

A tuneable metal-free protocol for the selective preparation of α-substituted vinyl sulfone and (E)-vinyl sulfone derivatives has been described. The base played an important role in the selectivity control of transformation.