Ni- and Fe-Based Cross-Coupling Reactions

Much effort has been devoted to developing new methods using Ni catalysts for the cross-coupling reaction of alkyl electrophiles with organometallic reagents, and significant achievements in this area have emerged during the past two decades. Nickel catalysts have enabled the coupling reaction of not only primary alkyl electrophiles, but also sterically hindered secondary and tertiary alkyl electrophiles possessing b-hydrogens with various organometallic reagents to construct carbon skeletons. In addition, Ni catalysts opened a new era of asymmetric crosscoupling reaction using alkyl halides. Recent progress in nickel-catalyzed crosscoupling reaction of alkyl electrophiles with sp3-, sp2-, and sp-hybridized organometallic reagents including asymmetric variants as well as mechanistic insights of nickel catalysis are reviewed in this chapter.

The traditional transition metal-catalyzed cross-coupling reaction, although well suited for C(sp2)–C(sp2) cross-coupling, has proven less amenable toward coupling of C(sp3)-hybridized centers, particularly using functional group tolerant reagents and reaction conditions. The development of photoredox/Ni dual catalytic methods for cross-coupling has opened new vistas for the construction of carbon–carbon bonds at C(sp3)-hybridized centers. In this chapter, a general outline of the features of such processes is detailed.

The Ni-catalyzed reductive coupling of alkyl/aryl with other electrophiles has evolved to be an important protocol for the construction of C–C bonds. This chapter first emphasizes the recent progress on the Ni-catalyzed alkylation, arylation/ vinylation, and acylation of alkyl electrophiles. A brief overview of CO₂ fixation is also addressed. The chemoselectivity between the electrophiles and the reactivity of the alkyl substrates will be detailed on the basis of different Nicatalyzed conditions and mechanistic perspective. The asymmetric formation of C(sp³)–C(sp²) bonds arising from activated alkyl halides is next depicted followed by allylic carbonylation. Finally, the coupling of aryl halides with other C(sp²)– electrophiles is detailed at the end of this chapter.

The sustainable utilization of available feedstock materials for preparing valuable compounds holds great promise to revolutionize approaches in organic synthesis. In this regard, the implementation of abundant and inexpensive carbon dioxide (CO₂) as a C1 building block has recently attracted considerable attention. Among the different alternatives in CO₂ fixation, the preparation of carboxylic acids, relevant motifs in pharmaceuticals and agrochemicals, is particularly appealing, thus providing a rapid and unconventional entry to building blocks that are typically prepared via waste-producing protocols. While significant advances have been realized, the utilization of simple unsaturated hydrocarbons as coupling partners in carboxylation events is undoubtedly of utmost academic and industrial relevance, as two available feedstock materials can be combined in a catalytic fashion. This review article aims to describe the main achievements on the direct carboxylation of unsaturated hydrocarbons with CO₂ by using cheap and available Ni or Fe catalytic species.

Nickel-catalyzed cross-coupling reactions of aryl esters, carbamates, carbonates, ethers and arenols are reviewed. Carbon–oxygen bonds in these phenol derivatives cannot be activated by palladium, a typical cross-coupling catalyst, but a low valent nickel species in conjunction with a strong r-donor ligand is uniquely effective for achieving this. The review is organized primarily by substrate class and secondarily by coupling partners, encompassing organometallics, heteroatom nucleophiles, C–H bonds and many others. Although the reactions in this category are covered thoroughly, each reaction is described only briefly, so that it is possible to quickly overview the spectrum of nickel-catalyzed cross-coupling reactions of inert phenol derivatives. The robustness of inert phenol derivatives under typically used catalytic conditions as well as their utility as a directing group allow unique synthetic applications of these new C–O cross-coupling reactions, which is also included in cases where appropriate. Mechanistic aspects of C–O bond activation by nickel are also summarized, highlighting their diversity compared with the C–X bond activation involved in conventional cross-coupling processes.

Catalytic C–H functionalization using transition metals has received significant interest from organic chemists because it provides a new strategy to construct carbon–carbon bonds and carbon–heteroatom bonds in highly functionalized, complex molecules without pre-functionalization. Recently, inexpensive catalysts based on transition metals such as copper, iron, cobalt, and nickel have seen more use in the laboratory. This review describes recent progress in nickelcatalyzed aromatic C–H functionalization reactions classified by reaction types and reaction partners. Furthermore, some reaction mechanisms are described and cutting- edge syntheses of natural products and pharmaceuticals using nickel-catalyzed aromatic C–H functionalization are presented.

Iron-catalyzed C–H activation has recently emerged as an increasingly powerful tool for the step-economical transformation of unreactive C–H bonds. Particularly, the recent development of low-valent iron catalysis has set the stage for novel C–H activation strategies via chelation assistance. The low-cost, natural abundance, and low toxicity of iron prompted its very recent application in organometallic C–H activation catalysis. An overview of the use of iron catalysis in C–H activation processes is summarized herein up to May 2016.

Cross-dehydrogenative coupling (CDC), which enables the formation of carbon–carbon (C–C) and C–heteroatom bonds from the direct coupling of two C–H bonds or C–H/X–H bonds, represents a new state of the art in the field of organic chemistry. Iron, a prominent metal, has already shown its versatile application in chemical synthesis. This review attempts to provide a comprehensive understanding of the evolution of cross-dehydrogenative coupling via iron catalysis, as well as its application in synthetic chemistry.

Over the last decades, iron-catalyzed cross-couplings have emerged as an important tool for the formation of C–C bonds. A wide variety of alkenyl, aryl, and alkyl (pseudo)halides have been coupled to organometallic reagents, the most currently used being Grignard reagents. Particular attention has been devoted to the development of iron catalysts for the functionalization of alkyl halides that are generally challenging substrates in classical cross-couplings. The high functional group tolerance of iron-catalyzed cross-couplings has encouraged organic chemists to use them in the synthesis of bioactive compounds. Even if some points remain obscure, numerous studies have been carried out to investigate the mechanism of iron-catalyzed cross-coupling and several hypotheses have been proposed.