The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to “traditional” precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal–ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands.

The last two decades have seen an impressive improvement of the use of iron as a fascinating and valuable alternative transition metal in homogeneous catalysis in terms of sustainability and economy. It was efficiently used in catalytic organic synthetic transformations, which in particular include the reduction of unsaturated bonds. This review summarizes the fast development and the recent advances in selective reductions of olefins, alkynes, carbonyl and carboxylic derivatives, imines, and nitro compounds promoted by iron catalysts. The topical hydrogen-borrowing reactions and hydroboration of unsaturated compounds are also reported. It is hoped that this account not only provides an overview of the state of the art in iron catalysis but also stimulates the development of superior greener catalytic systems in the near future.

Iron Catalysis in Reduction and Hydrometalation Reactions.

Recent Advances in Hydrometallation of Alkenes and Alkynes via the First Row Transition Metal Catalysis

Cobalt has become increasingly attractive in homogeneous catalysis because of its unique characteristics and outstanding catalytic performance in addition to being cheap and earth-abundant. Hydride transfer processes are involved in a broad range of organic transformations that allow the facile preparation of various useful chemicals and synthetic building blocks. These reactions have continuously received great attention both from academia and industry. In this perspective, we review homogeneous cobalt-catalyzed hydride transfer reactions according to the classified reaction types and provide a comprehensive overview of the design, synthesis, and reactivity of cobalt catalysts, their catalytic applications, and reaction mechanisms.

Hydride Transfer Reactions Catalyzed by Cobalt Complexes.

In industries and academic laboratories, several late transition metal-catalyzed prerequisite reactions are widely performed during single and multistep synthesis. However, besides the desired products, these reactions lead to the generation of numerous chemical waste materials, by-products, hazardous gases, and other poisonous materials, which are discarded in the environment. This is partly responsible for the creation of global warming, resulting in climate adversities. Thus, the development of environmentally benign, cheap, easily accessible, and earth-abundant metal catalysts is desirable to minimize these issues. Certainly, iron is one of the most important metals belonging to this family. The field of iron catalysis has been explored in the last two-three decades out of its rich chemistry depending on its oxidation states and ligand cooperation. Moreover, this field has been enriched by the promising development of iron-catalyzed reactions namely, C–H bond activation, including organometallic C–H activation and C–H functionalization via outer-sphere pathway, cross-dehydrogenative couplings, insertion reactions, cross-coupling reactions, hydrogenations including hydrogen borrowing reactions, hydrosilylation and hydroboration, addition reactions and substitution reactions. Thus, herein an inclusive overview of these reaction have been well documented.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

Transition metal catalyzed asymmetric hydrofunctionalization of readily available unsaturated hydrocarbons presents one of the most straightforward and atom-economic protocols to access valuable optically active products. For decades, noble transition metal catalysts have laid the cornerstone in this field, on account of their superior reactivity and selectivity. In recent years, from an economical and sustainable standpoint, first-row, earth-abundant transition metals have received considerable attention, due to their high natural reserves, affordable costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed hydrofunctionalization reactions have also gained much interest and been investigated gradually. However, since chiral ligand libraries for earth-abundant transition-metal catalysis are limited to date, the development of highly enantioselective versions remains a significant challenge.This Account summarizes our recent efforts in developing suitable chiral ligands for iron and cobalt catalysts and their applications in the highly enantioselective hydrofunctionalization reactions (hydroboration and hydrosilylation) of alkenes and alkynes. In ligand design, we envisioned that chiral unsymmetric NNN-tridentate (UNT) ligand scaffolds could promote these enantioselective transformations with earth-abundant metals. Therefore, several types of chiral UNT ligands were designed and prepared in our laboratory, utilizing readily available natural amino acids as chiral sources. In the very beginning, chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated through the rational combination of nitrogen-containing ligand scaffolds. After a systematic survey of the ligand effects, the imine moiety in the rigid OIP ligands was replaced by a conformationally more flexible amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine (OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline iminopyridine (TIP) ligands were prepared by altering the oxygen atom of oxazoline with nitrogen and sulfur linkers, respectively. To further expand the chiral ligand library, other tridentate ligands containing a twisted pincer, anionic, and nonrigid backbone were also designed and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA) and imidazoline phenyl picolinamide (ImPPA). The efficacy of these chiral UNT ligands for asymmetric induction in iron and cobalt catalysis has been demonstrated through asymmetric hydrofunctionalization of alkenes and asymmetric sequential hydrofunctionalization of alkynes, which exhibit excellent reactivity as well as high chemo-, regio-, and stereoselectivity with broad functional group tolerance. Notably, highly regio- and enantioselective hydrofunctionalization of challenging substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic alkenes, was also achieved. Furthermore, the development of asymmetric sequential isomerization/hydroboration of internal alkenes and sequential hydrofunctionalization of alkynes further demonstrates the synthetic power of these catalytic systems. The chiral enantioenriched products obtained by these methodologies could be potentially utilized in organic synthesis, medicinal chemistry, and materials science. We believe that our continuous efforts in this field would be beneficial to the development of asymmetric earth-abundant metal catalysis.

Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes

Recent years have witnessed the growing interest in the remote functionalization of alkenes for it offers a strategy to activate the challenging C-H bonds distant from the initiation point via alkene isomerization/functionalization. However, the catalytic enantioselective isomerization/functionalization with one single transition metal catalyst remains rare. Here we report a highly regio- and enantioselective cobalt-catalyzed remote C-H bond borylation of internal alkenes via sequential alkene isomerization/hydroboration. A chiral ligand featured twisted pincer, anionic, and non-rigid characters is designed and used for this transformation. This methodology, which is operationally simple using low catalyst loading without additional activator, shows excellent enantioselectivity and can be used to convert various internal alkenes with regio- and stereoisomers to valuable chiral secondary organoboronates with good functional group tolerance.

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Asymmetric remote C-H borylation of internal alkenes via alkene isomerization.

Highly regio- and enantioselective intermolecular hydroamination of alkenes is a challenging process potentially leading to valuable chiral amines. Hydroamination of alkenes via metal-catalyzed hydrogen atom transfer (HAT) with good regioselectivity and functional group tolerance has been reported, however, high enantioselectivity has not been achieved due to the lack of suitable ligands. Here we report a ligand-promoted cobalt-catalyzed Markovnikov-type selective radical hydroamination of alkenes with diazo compounds. This operationally simple protocol uses unsymmetric NNN-tridentate (UNT) ligand, readily available alkenes and hydrosilanes to construct hydrazones with good functional group tolerance. The hydrazones can undergo nitrogen–nitrogen bond cleavage smoothly to deliver valuable amine derivatives. Additionally, asymmetric intermolecular hydroamination of unactivated aliphatic terminal alkenes using chiral N-imidazolinylphenyl 8-aminoquinoline (IPAQ) ligands has also been achieved to afford chiral amine derivatives with good enantioselectivities. Asymmetric intermolecular hydroamination of alkenes is a challenging process, potentially leading to useful chiral amines. Here, the authors report unsymmetric NNN tridentate ligands promoting the cobalt-catalyzed radical hydroamination of alkenes via hydrogen atom transfer, also in an asymmetric fashion.

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Zhaoyang Cheng

Papers

Iron- and Cobalt-Catalyzed Asymmetric Hydrofunctionalization of Alkenes and Alkynes

Asymmetric remote C-H borylation of internal alkenes via alkene isomerization.

Ligand-promoted cobalt-catalyzed radical hydroamination of alkenes.

Iron-Catalyzed, Markovnikov-Selective Hydroboration of Styrenes

Recent advances in metal-catalysed asymmetric sequential double hydrofunctionalization of alkynes