Synthesis of α-Fluoro-α-nitroarylacetates via Vicarious Nucleophilic Substitution of Hydrogen

Chlorine in an Organic Molecule, a Universal Promoter-Workhorse-Of Reactions

Due to the electronic configuration of the atom and charge of the nucleus, the chlorine in organic molecules can exert a variety of effects. It can depart as a chloride anion in the process of substitution and elimination, facilitates the abstraction of protons and stabilizes generated carbanions, exerts moderate stabilizing effect of carbenes, carbocations and radicals. There are frequent cases where chlorine substituent promotes more than one transformation. These rich effects of chlorine substituent will be illustrated by examples of our work.

Recent Advances in Monofluorinated Carbenes, Carbenoids, Ylides, and Related Species

The synthesis of monofluorinated compounds is of great interest because of the vast applications of organofluorine compounds. Recently, the introduction of monofluorocarbene synthons has emerged as an important strategy for the synthesis of fluorine-containing products. In contrast to direct fluorination, in which C-F bonds are formed, the use of monofluorinated carbenes and related reactive species involves C-C or C-X bond formation while delivering valuable fluorine atoms into the target structure. Owing to increased knowledge on carbon-carbon and carbon-heteroatom bond formations, monofluorinated carbenes have enormous potential for the synthesis of organofluorine compounds, which, in our opinion, has not yet been fully exploited. This review summarizes the recent advances in the synthetic applications of monofluorinated carbenes, carbenoids, ylides, and related species.

Synthesis of quinoxaline derivatives via aromatic nucleophilic substitution of hydrogen

The electrophilic nature of quinoxaline has been explored in the vicarious nucleophilic substitution (VNS) of hydrogen with various carbanions as nucleophiles in an attempt to develop a general method for functionalizing the heterocyclic ring. Only poorly stabilized nitrile carbanions were found to give the VNS products. 2-Chloroquinoxaline gave products of S_NAr of chlorine preferentially. A variety of quinoxaline derivatives containing cyanoalkyl, sulfonylalkyl, benzyl or ester substituents, including fluorinated ones, have been prepared in the VNS reactions with quinoxaline N-oxide.

The Coupling of Carbon and Nitrogen Substituents with Nitroarenes：Vicarious Nucleophilic Substitution of Hydrogen in Nitroarenes

Abstract:

In recent years, nitroarenes have been extensively exploited as green, efficient electrophilic arylation reagents used in a variety of organic syntheses. Transition-metal-free cross-coupling reactions and vicarious nucleophilic substitution (VNS) have become practical and reliable synthetic methods to access momentous func-tionalized organic compounds. Thus, the introduction of various substituents into nitroarenes has attracted considerable attention as important synthetic approach-es due to their simplicity and practicality. In this review, we comprehensively summarize the coupling of several carbon and nitrogen substituents with ni-troarenes via nucleophilic substitution under mild conditions, followed by the ap-plication of these transversions in the construction of carbon and heterocyclic rings.

Cobalt-Catalyzed Regioselective para-Amination of Azobenzenes via Nucleophilic Aromatic Substitution of Hydrogen

The metal-catalyzed nucleophilic aromatic substitution of hydrogen (S_NArH) via coordination of the substituent on the aromatic ring to the metal catalyst, in terms of reactivity, substrate type, and reaction selectivity, complements the transition metal-catalyzed C-H functionalization that proceeds via C-H metalation but remains an elusive target. Described herein is the development of an unprecedented cobalt-catalyzed para-selective amination of azobenzenes, which is essentially a metal-promoted S_NArH process as revealed by Hammett analysis, thus illustrating the concept that coordination of the substituent on the arene ring to the metal catalyst may result in electrophilic activation of the arene ring toward S_NArH. This cobalt-catalyzed protocol allows the use of a variety of both aliphatic amines and anilines as aminating reagents, tolerates electronically diverse substituents of azobenzene, and furnishes the corresponding products in good yields with a regiospecific selectivity for para-amination.

Rhodium-catalysed ortho-alkynylation of nitroarenes

The ortho-alkynylation of nitro-(hetero)arenes takes place in the presence of a Rh(iii) catalyst to deliver a wide variety of alkynylated nitroarenes regioselectively. These interesting products could be further derivatized by selective reduction of the nitro group or palladium-catalysed couplings. Experimental and computational mechanistic studies demonstrate that the reaction proceeds via a turnover-limiting electrophilic C-H metalation ortho to the strongly electron-withdrawing nitro group.

Introduction of Carbon Substituents into Nitroarenes via Nucleophilic Substitution of Hydrogen: New Developments

Nucleophilic substitution of hydrogen in nitroarenes has become a powerful synthetic tool for functionalization of these important organic substrates, complementary to other modern methods. In this review we present new developments in the area of introduction of alkyl and functionalized alkyl substituents into nitroarene rings via nucleo­philic substitution of hydrogen, followed by application of these processes in the construction of carbo- and heterocyclic rings. Finally, new developments in the investigation of the mechanism of SNArH are summarized.

1 Introduction

2 Alkylation and Haloalkylation

3 Functionalized Carbon Substituents

4 Formation of Carbo- and Heterocyclic Rings

5 Mechanistic Aspects of SNArH

6 Conclusion

Medium effect (water versus MeCN) on reactivity and reaction pathways for the SNAr reaction of 1-aryloxy-2,4-dinitrobenzenes with cyclic secondary amines

A kinetic study on SNAr reactions of 1-aryloxy-2,4-dinitrobenzenes (1a–1h) with a series of cyclic secondary amines in 80 mol% water – 20 mol% DMSO at 25.0 ± 0.1 °C is reported. The plots of kobsd versus amine concentration curve upward except for the reactions of substrates possessing a strong electron-withdrawing group in the leaving aryloxide with strongly basic piperidine. The curved plots indicate that the reactions proceed through both uncatalytic and catalytic routes. Linear Brønsted-type plots have been obtained for the uncatalyzed and catalyzed reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with βnuc = 0.84 and 0.78, respectively. The Yukawa–Tsuno plot for the uncatalyzed reactions of 1a–1h with piperidine results in an excellent linear correlation with ρ = 1.66 and r = 0.31. In contrast, rate constants for catalyzed reactions are independent of the electronic nature of the substituent in the leaving group. The current SNAr reactions have been proposed to proceed via a zwitterionic intermediate (MC±) that partitions to products through uncatalytic and catalytic routes. The catalyzed reaction from MC± has been concluded to proceed through a concerted mechanism with a six-membered cyclic transition state (TScycl) rather than via a stepwise pathway with a discrete anionic intermediate (MC−), the traditionally accepted mechanism. Medium effects on the reactivity and reaction mechanism are discussed. Particularly, hydrogen bonding of the amines to water precludes formation of kinetically significant dimers found in some aprotic solvents; no explicit role for water in the catalytic transition state is required or proposed. The specific stabilization of the leaving aryloxides substituted with strong electron-withdrawing groups accounts for the lack of the catalytic pathway in these systems (1a–1c) with piperidine nucleophile.