Transition-Metal-Free Decarboxylative Arylation of 2-Picolinic Acids with Arenes under Air Conditions

Radical and ionic meta -C–H functionalization of pyridines, quinolines, and isoquinolines

Carbon-hydrogen (C−H) functionalization of pyridines is a powerful tool for the rapid construction and derivatization of many agrochemicals, pharmaceuticals, and materials. Because of the inherent electronic properties of pyridines, selective meta -C−H functionalization is challenging. Here, we present a protocol for highly regioselective meta -C−H trifluoromethylation, perfluoroalkylation, chlorination, bromination, iodination, nitration, sulfanylation, and selenylation of pyridines through a redox-neutral dearomatization-rearomatization process. The introduced dearomative activation mode provides a diversification platform for meta-selective reactions on pyridines and other azaarenes through radical as well as ionic pathways. The broad scope and high selectivity of these catalyst-free reactions render these processes applicable for late-stage functionalization of drugs.

Recent Advances in the Transition-Metal-Free Arylation of Hetero­arenes

Transition-metal-free arylation reactions have attracted considerable attention for economic and environmental reasons over the past 40 years. In recent years, much effort has been made to develop efficient transition-metal-free approaches for the arylation of heteroarenes. Covering the literature from 2015 to early 2021, this review aims to provide a thorough overview of the synthetic and mechanistic aspects of these atom-economical and environmentally benign reactions.

1 Introduction

2 Arylation of Pre-functionalized Heteroarenes

2.1 Arylation of Heteroaryl Halides

2.2 Decarboxylative Arylation of Heteroarenes

3 Direct C–H Arylation of Heteroarenes

3.1 C(sp2)–H Arylation

3.2 C(sp3)–H Arylation

4 N-Arylation of Heteroarenes

5 Summary and Outlook

A practical route to 2-iodoanilines via the transition-metal-free and base-free decarboxylative iodination of anthranilic acids under oxygen

A new approach for synthesizing 2-iodoanilines via the transition-metal-free and base-free decarboxylative ortho-C–H iodination of anthranilic acids with a combination of KI and I₂ as the halogen donor and catalyst under oxygen has been developed.

α-Hydroxy acid as an aldehyde surrogate: metal-free synthesis of pyrrolo[1,2-a]quinoxalines, quinazolinones, and other N-heterocycles via decarboxylative oxidative annulation reaction

A metal-free and efficient procedure for the synthesis of pyrrolo[1,2-a]quinoxalines, quinazolinones, and indolo[1,2-a]quinoxaline has been developed. The key features of our method include the in situ generation of aldehyde from α-hydroxy acid in the presence of TBHP (tert-butyl hydrogen peroxide), and further condensation with various amines, followed by intramolecular cyclization and subsequent oxidation to afford the corresponding quinoxalines, quinazolinones derivatives in moderate to high yields.

Palladium-Catalyzed Cascade Reactions of δ-Ketonitriles with Arylboronic Acids: Synthesis of Pyridines

This study presents the first example of the Pd-catalyzed cascade reactions of 5-oxohexanenitrile with arylboronic acids, affording important synthon 2-methylpyridines that can be further translated through C(sp³)-H functionalization to construct pyridine derivatives. Furthermore, this chemistry allows 5-oxo-5-arylpentanenitrile to react with arylboronic acids to provide unsymmetrical 2,6-diarylpyridines. This protocol paves the way for the practical and atom economical syntheses of valuable pyridines with broad functional groups in moderate to excellent yields under mild conditions.

Metal-free reductive coupling of aliphatic aldehydes/ketones with 4-cyanopyridines: expanded scope and mechanistic studies

A practical B₂pin₂ mediated reductive coupling of 4-cyanopyridine with aliphatic aldehydes/ketones has been established. This metal-free protocol provides a convenient route to construct a wide range of C₄-pyridine-functionalized alcohols.

Transition-Metal-Free Deconstructive Lactamization of Piperidines

One of the major challenges in organic synthesis is the activation or deconstructive functionalization of unreactive C(sp³ )-C(sp³ ) bonds, which requires using transition or precious metal catalysts. We present here an alternative: the deconstructive lactamization of piperidines without using transition metal catalysts. To this end, we use 3-alkoxyamino-2-piperidones, which were prepared from piperidines through a dual C(sp³ )-H oxidation, as transitory intermediates. Experimental and theoretical studies confirm that this unprecedented lactamization occurs in a tandem manner involving an oxidative deamination of 3-alkoxyamino-2-piperidones to 3-keto-2-piperidones, followed by a regioselective Baeyer-Villiger oxidation to give N-carboxyanhydride intermediates, which finally undergo a spontaneous and concerted decarboxylative intramolecular translactamization.