Removal of synthetic dyes from wastewaters: a review.

Metal-catalyzed regiodivergent reactions allow control over regioselectivity in the synthesis of a wide range of organic products. Starting from the same material, it is possible to prepare different regioisomers just by appropriately choosing the catalyst or by modifying the reaction conditions. Therefore, these regiodivergent methodologies should be included as the key factor in the concept of efficiency and atom economy in synthetic organic chemistry. The synthetic potential of this subject has been demonstrated mainly in addition reactions to unsaturated carbon–carbon bonds, allylic and propargylic nucleophilic substitutions, C–H activation reactions, cross-couplings, and intramolecular or intermolecular cyclizations. This review article overviews the development and rationalization of regiodivergence in these fundamental reactions in the last 15 years.

Metal-catalyzed regiodivergent organic reactions

The first enantioselective Satoh-Miura-type reaction is reported. A variety of C-N axially chiral N-aryloxindoles have been enantioselectively synthesized by an asymmetric rhodium-catalyzed dual C-H activation reaction of N-aryloxindoles and alkynes. High yields and enantioselectivities were obtained (up to 99 % yield and up to 99 % ee). To date, it is also the first example of the asymmetric synthesis of C-N axially chiral compounds by such a C-H activation strategy.

Enantioselective Synthesis of C-N Axially Chiral N-Aryloxindoles by Asymmetric Rhodium-Catalyzed Dual C-H Activation.

Indazole-containing derivatives represent one of the most important heterocycles in drug molecules. Diversely substituted indazole derivatives bear a variety of functional groups and display versatile biological activities; hence, they have gained considerable attention in the field of medicinal chemistry. This review aims to summarize the recent advances in various methods for the synthesis of indazole derivatives. The current developments in the biological activities of indazole-based compounds are also presented.

Recent Advances in Indazole-Containing Derivatives: Synthesis and Biological Perspectives

Alkylation of arenes is one of the most fundamental transformations in chemical synthesis and leads to privileged scaffolds in many areas of science. Classical methods for the introduction of alkyl groups to arenes are mostly based on the Friedel-Crafts reaction, radical additions, metalation, or prefunctionalization of the arene: these methods, however, suffer from limitations in scope, efficiency, and selectivity. Moreover, they are based on the innate reactivity of the starting arene, favoring the alkylation at a certain position and rendering the introduction of alkyl chains at other positions much more challenging. This can be addressed by the use of a directing group that facilitates, in the presence of a metal catalyst, the regioselective alkylation of a C-H bond. These directed alkylations of C-H bonds in arenes will be comprehensively summarized in this Review.

Beyond Friedel and Crafts: Directed Alkylation of C-H Bonds in Arenes.

We have developed inter- and intramolecular C-H alkenylation reactions of pyrazoles. The catalyst, derived from Pd(OAc)2 and pyridine, enabled the oxidative alkenylation of pyrazoles containing a variety of functional groups at the C4 position. Activated alkenes, including acrylate, acrylamide, and styrene derivatives, and enamides could be installed in this process. The sequential C-H alkenylation and cyclization reactions gave rise to fused bicyclic pyrazoles, providing a new strategy to annulate readily available pyrazole compounds.

Direct C–H Alkenylation of Functionalized Pyrazoles

Synthesis of Fluorescent Indazoles by Palladium-Catalyzed Benzannulation of Pyrazoles with Alkynes

Regioselective C4-, C5-, and di-alkenylations of pyrazoles were achieved. An electrophilic Pd catalyst generated by trifluoroacetic acid (TFA) and 4,5-diazafluoren-9-one (DAF) leads to C4-alkenylation, whereas KOAc and mono-protected amino acid (MPAA) ligand Ac-Val-OH give C5-alkenylation. A combination of palladium acetate, silver carbonate, and pivalic acid affords dialkenylation products. Annulation through sequential alkenylation, thermal 6π-electrocyclization, and oxidation gives functionalized indazoles. This comprehensive strategy greatly expands the range of readily accessible pyrazole and indazole derivatives, enabling useful regiodivergent C-H functionalization of pyrazoles and other heteroaromatic systems.

Ligand‐controlled Regiodivergent C−H Alkenylation of Pyrazoles and its Application to the Synthesis of Indazoles

This review describes recent advances in transition-metal-catalyzed C–H functionalization reactions of pyrazoles to form new C–C and C–heteroatom bonds on the pyrazole ring. In contrast to traditional cross-coupling reactions that require pre-functionalized pyrazoles, direct C–H functionalization reactions provide access to a wide range of functionalized pyrazoles in a single step. Various strategies are presented and the efficiency and regioselectivity are described with respect to the types of functional groups that are installed.

Hyun Tae Kim

Papers

Direct C–H Alkenylation of Functionalized Pyrazoles

Synthesis of Fluorescent Indazoles by Palladium-Catalyzed Benzannulation of Pyrazoles with Alkynes

Ligand‐controlled Regiodivergent C−H Alkenylation of Pyrazoles and its Application to the Synthesis of Indazoles

Transition-metal-catalyzed C–H functionalization of pyrazoles

Synthesis of Bidentate Nitrogen Ligands by Rh-Catalyzed C-H Annulation and Their Application to Pd-Catalyzed Aerobic C-H Alkenylation.