Iron Catalysis in Organic Synthesis

The coupling of aryl or vinyl halides with terminal acetylenes catalysed by palladium and other transition metals, commonly termed as Sonogashira cross-coupling reaction, is one of the most important and widely used sp2–sp carbon–carbon bond formation reactions in organic synthesis, frequently employed in the synthesis of natural products, biologically active molecules, heterocycles, molecular electronics, dendrimers and conjugated polymers or nanostructures. This critical review focuses on developments in the Sonogashira reaction achieved in recent years concerning catalysts, reaction conditions and substrates (352 references).

Recent advances in Sonogashira reactions

Chemistries that Facilitate Nanotechnology Kim E. Sapsford,† W. Russ Algar, Lorenzo Berti, Kelly Boeneman Gemmill,‡ Brendan J. Casey,† Eunkeu Oh, Michael H. Stewart, and Igor L. Medintz*,‡ †Division of Biology, Department of Chemistry and Materials Science, Office of Science and Engineering Laboratories, U.S. Food and Drug Administration, Silver Spring, Maryland 20993, United States ‡Center for Bio/Molecular Science and Engineering Code 6900 and Division of Optical Sciences Code 5611, U.S. Naval Research Laboratory, Washington, D.C. 20375, United States College of Science, George Mason University, 4400 University Drive, Fairfax, Virginia 22030, United States Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Sacramento, California 95817, United States Sotera Defense Solutions, Crofton, Maryland 21114, United States

Functionalizing nanoparticles with biological molecules: developing chemistries that facilitate nanotechnology.

Over the past 40 years, transition metal catalysis has enabled bond formation between aryl and olefinic (sp(2)) carbons in a selective and predictable manner with high functional group tolerance. Couplings involving alkyl (sp(3)) carbons have proven more challenging. Here, we demonstrate that the synergistic combination of photoredox catalysis and nickel catalysis provides an alternative cross-coupling paradigm, in which simple and readily available organic molecules can be systematically used as coupling partners. By using this photoredox-metal catalysis approach, we have achieved a direct decarboxylative sp(3)-sp(2) cross-coupling of amino acids, as well as α-O- or phenyl-substituted carboxylic acids, with aryl halides. Moreover, this mode of catalysis can be applied to direct cross-coupling of C(sp³)-H in dimethylaniline with aryl halides via C-H functionalization.

/pdf/merging-photoredox-with-nickel-catalysis-coupling-of-a-200ydcysj9.pdf

Merging photoredox with nickel catalysis: Coupling of α-carboxyl sp3-carbons with aryl halides

Conventional oxidations of organic compounds formally transfer hydrogen atoms from the substrate to an acceptor molecule such as oxygen, a metal oxide, or a sacrificial olefin. In acceptorless dehydrogenation (AD) reactions, catalytic scission of C-H, N-H, and/or O-H bonds liberates hydrogen gas with no need for a stoichiometric oxidant, thereby providing efficient, nonpolluting activation of substrates. In addition, the hydrogen gas is valuable in itself as a high-energy, clean fuel. Here, we review AD reactions selectively catalyzed by transition metal complexes, as well as related transformations that rely on intermediates derived from reversible dehydrogenation. We delineate the methodologies evolving from this recent concept and highlight the effect of these reactions on chemical synthesis.

http://science.sciencemag.org/content/sci/341/6143/1229712.full.pdf

Applications of acceptorless dehydrogenation and related transformations in chemical synthesis.

Imines are selectively formed by coupling of nitriles and amines under mild hydrogen pressure. The reaction is catalyzed by a bipyridine-based PNN Ru(II) pincer complex and proceeds under mild, neutral conditions at 4 bar of H2.

Catalytic Coupling of Nitriles with Amines to Selectively Form Imines under Mild Hydrogen Pressure.

The atom-efficient and environmentally benign catalytic hydrogenation of carboxylic acid esters to alcohols has been accomplished in recent years mainly with precious-metal-based catalysts, with few exceptions. Presented here is the first cobalt-catalyzed hydrogenation of esters to the corresponding alcohols. Unexpectedly, the evidence indicates the unprecedented involvement of ester enolate intermediates.

Cobalt-Catalyzed Hydrogenation of Esters to Alcohols: Unexpected Reactivity Trend Indicates Ester Enolate Intermediacy.

Herein, we demonstrated Mn-catalyzed selective C-3 functionalization of indoles with alcohols. The developed catalyst can also furnish bis(indolyl)methanes from the same set of substrates under slightly modified reaction conditions. Mechanistic studies reveal that the C-3 functionalization of indoles is going via a borrowing hydrogen pathway. To highlight the practical utility, a diverse range of substrates including nine structurally important drug molecules are synthesized. Furthermore, we also introduced a one-pot cascade strategy for synthesizing C-3 functionalized indoles directly from 2-aminophenyl ethanol and alcohol.

Well-Defined NNS-Mn Complex Catalyzed Selective Synthesis of C-3 Alkylated Indoles and Bisindolylmethanes Using Alcohols.

The “borrowing hydrogen” (BH) method for C-alkylation reactions using alcohol as alkylating agents is an important synthetic transformation. In this respect, designing cheap and bench stable earth abundant metal catalyst for borrowing hydrogen transformation is a key challenge to be witnessed. Herein we have presented a synthesis of non-phosphine, easily accessible and bench stable SNS−Ni complexes. The Ni-catalyst was successfully applied for the C-alkylation of ketone enolates to α-alkylated ketones. Primary alcohol with different functional groups and various heteroaromatic alcohols are well tolerated. The present catalyst system was efficiently applied to gram scale synthesis and also the green chemistry metrics of the reaction were calculated. The present protocol was also extended successfully for the synthesis of biologically important quinoline moieties. Finally, various control experiments and deuterium labelled experiments suggest that the reaction proceeds via borrowing hydrogen pathway. 

Well‐defined Ni‐SNS Complex Catalysed Borrowing Hydrogenative α‐Alkylation of Ketones and Dehydrogenative Synthesis of Quinolines

Catalytically active Pd nanoparticles have been synthesized in water by a novel reduction of Pd(II) with a Fischer carbene complex where polyethylene glycol (PEG) was used as stabilizer. PEG molecules wrap around the nanoparticles to impart stability and prevent agglomeration, yet leave enough surface area available on the nanoparticle for catalytic activity. Our method is superior to others in terms of rapid generation and stabilization of Pd nanoparticles in water with a cheap, readily available PEG stabilizer. The size of the nanoparticles generated can be controlled by the concentration of PEG in water medium. The size decreased with the increase in the PEG: Pd ratio. This aqueous nano-sized Pd is a highly efficient catalyst for Suzuki, Heck, Sonogashira, and Stille reaction. Water is used as the only solvent for the coupling reactions.

Dipankar Srimani

Papers

Catalytic Coupling of Nitriles with Amines to Selectively Form Imines under Mild Hydrogen Pressure.

Cobalt-Catalyzed Hydrogenation of Esters to Alcohols: Unexpected Reactivity Trend Indicates Ester Enolate Intermediacy.

Well-Defined NNS-Mn Complex Catalyzed Selective Synthesis of C-3 Alkylated Indoles and Bisindolylmethanes Using Alcohols.

Well‐defined Ni‐SNS Complex Catalysed Borrowing Hydrogenative α‐Alkylation of Ketones and Dehydrogenative Synthesis of Quinolines

Size Controlled Synthesis of Pd Nanoparticles in Water and Their Catalytic Application in C—C Coupling Reactions.