Nonprecious Metals Catalyzing Hydroamination and C–N Coupling Reactions

Photo- and Cobalt-Catalyzed Synthesis of Heterocycles via Cycloisomerization of Unactivated Olefins

We report a general, intramolecular cycloisomerization of unactivated olefins with pendant nucleophiles. The reaction proceeds under mild conditions and tolerates ethers, esters, protected amines, acetals, pyrazoles, carbamates, and arenes. It is amenable to N-, O-, as well as C-nucleophiles, yielding a number of different heterocycles including, but not limited to, pyrrolidines, piperidines, oxazolidinones, and lactones. Use of both a benzothiazinoquinoxaline as organophotocatalyst and a Co-salen catalyst obviates the need for stoichiometric oxidant or reductant. We showcase the utility of the protocol in late-stage drug diversification and synthesis of several small natural products.

Group IV Complexes with Sterically Congested N-Aryl-adamantylcarbamidate Ligands

Metalation of N-(2,6-dibenzhydryl-4-tolyl)adamantane-1-carboxamide (1, Ar*N(H)-C(O)-Ad) with M(NMe2)4 (M = Ti, Zr, Hf) yields amidate complexes Ar*N=C(Ad)-O-Ti(NMe2)3 (2) as well as bis(amidate) compounds (Ar*N=C(Ad)-O)2M(NMe2)2 (M = Zr (3), Hf (4)). In 2, the amidate ligand acts as a monodentate base via the oxygen atom with the Ti center in a slightly distorted tetrahedral environment. The steric requirement of the amidate ligand stabilizes the small coordination number of four of the Ti atom. In congeners 3 and 4, two bidentate amidate ligands exist in the coordination spheres, leading to hexacoordinate group IV metal atoms. The small bite angles of the Zr- and Hf-bound amidate ligands lead to severe distortion of the octahedral environments of the Zr and Hf centers. Titanium compound 2 is an unsuitable choice to catalyze hydrofunctionalization of alkynes with amines and phosphane oxides and despite the significantly smaller pKa value of the carboxylic amide, formation of carboxamide 1 is the dominant reaction upon addition of amines or phosphane oxides to release intramolecular steric strain introduced by the very bulky adamantylamidato ligand.

Synthesis of α-Aminophosphonates by Umpolung-Enabled Cu-Catalyzed Regioselective Hydroamination

A copper-catalyzed regioselective hydroamination of α,β-unsaturated phosphonates has been developed to form corresponding α-aminophosphonates of interest in medicinal chemistry. The introduction of an umpolung, electrophilic amination strategy with the hydroxylamine derivative is the key to achieving the α-amination regioselectivity, which is otherwise difficult under the conventional nucleophilic hydroamination conditions with the parent amine. Asymmetric synthesis with a chiral bisphosphine ligand and application to a related silylamination reaction are also described.

Metal-Free Aerobic C-N Bond Formation of Styrene and Arylamines via Photoactivated Electron Donor-Acceptor Complexation

This study processes a facile and green approach for the Markovnikov-selective hydroamination of styrene with naphthylamine through irradiation with UV LED light (365 nm) via an electron donor-acceptor complexation between naphthylamines and oxygen in situ. This protocol showcases the synthetic potential for aerobic C-N bond formation without using a metal catalyst and photosensitizer. Three naphthylamines were examined and afforded desired C-N bond formation product in moderate yield.

Palladium-Catalyzed Asymmetric Sequential Hydroamination of 1,3-Enynes: Enantioselective Syntheses of Chiral Imidazolidinones

Pd-catalyzed sequential hydroamination of readily available 1,3-enynes is reported. The redox-neutral process provides an efficient route to synthesize a broad scope of imidazolidinones, thiadiazolidines, and imidazolidines. Asymmetric sequential hydroamination generates a series of synthetically valuable, enantioenriched imidazolidinones. Mechanistic studies revealed that the transformation occurred via an intermolecular enyne hydroamination pathway to give an allene intermediate. Subsequent intramolecular hydroamination of the allene intermediate proceeded under the Curtin-Hammett principle to provide enantioenriched imidazolidinone products.

Hydroamination, Aminoboration, and Carboamination with Electrophilic Amination Reagents: Umpolung-Enabled Regio- and Stereoselective Synthesis of N-Containing Molecules from Alkenes and Alkynes

Nitrogen (N) is ubiquitously found in bioactive molecules, pharmaceutical agents, and organic functional materials. Accordingly, development of new C-N bond-forming catalysis has been one of the long-standing research subjects in synthetic organic chemistry. In this Perspective, recent advances in highly selective amination reactions with electrophilic amination reagents are described: by taking advantage of the concept of nitrogen umpolung, otherwise challenging aminofunctionalizations, such as hydroamination, aminoboration, and carboamination, of readily available feedstock-like alkenes and alkynes are possible, giving densely functionalized complex and often chiral alkylamines with high selectivity. The scope, limitations, and reaction mechanism are briefly summarized.

Copper supported silica-based nanocatalysts for CuAAC and cross-coupling reactions

Recent advances in Cu/SiO2-based heterogeneous catalysts for click reaction, C–N, C–S, and C–O coupling reactions are reviewed and summarized.

Electrophilic Aminating Agents in Total Synthesis

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C−N bond‐forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

Copper-Catalyzed Lactamization of (E)-2-(2-Bromophenyl)-3-arylacrylamides for the Synthesis of (E)-3-Arylideneindolin-2-ones

A copper-catalyzed, ligand-free intramolecular C-N coupling of (E)-2-(2-bromophenyl)-3-arylacrylamides has been developed. This protocol provides an efficient and practical synthetic route for the biologically important (E)-3-arylideneindolin-2-ones from o-bromophenylacetic acids and aromatic or conjugated alkenyl aldehydes. Readily available starting materials, mild and noble metal-free conditions, high efficiency, and good tolerability for phenolic hydroxyl groups make this approach attractive and applicable.

Photocatalytic intermolecular anti-Markovnikov hydroamination of unactivated alkenes with N-hydroxyphthalimide

A visible-light-induced/phosphite-promoted anti-Markovnikov hydroamination of alkenes with N-hydroxyphthalimide was successfully realized, which was initiated by a proton-coupled electron transfer to enable direct cleavage of its N–O bond.

Zinc salt-catalyzed reduction of α-aryl imino esters, diketones and phenylacetylenes with water as hydrogen source

The zinc salt-catalyzed reduction of α-aryl imino esters, diketones and phenylacetylenes with water as hydrogen source and zinc as reductant was successfully conducted. The presented method provides a low-cost, environmentally friendly and practical preparation of α-aryl amino esters, α-hydroxyketones and phenylethylenes. By using D2O as deuterium source, the corresponding products were obtained in high efficiency with excellent deuterium incorporation rate, which gives a cheap and safe tool for access to valuable deuterium-labelled compounds.

CuH-Catalyzed Olefin Functionalization: From Hydroamination to Carbonyl Addition

In organic synthesis, ligand-modified copper(I) hydride (CuH) complexes have become well-known reagents and catalysts for selective reduction, particularly toward Michael acceptors and carbonyl compounds. Recently, our group and others have found that these hydride complexes undergo migratory insertion (hydrocupration) with relatively unactivated and electronically unpolarized olefins, producing alkylcopper intermediates that can be leveraged to forge a variety of useful bonds. The resulting formal hydrofunctionalization reactions have formed the basis for a resurgence of research in CuH catalysis. This Account chronicles the development of this concept in our research group, highlighting its origin in the context of asymmetric hydroamination, evolution to more general C-X bond-forming reactions, and applications in the addition of olefin-derived nucleophiles to carbonyl derivatives.Hydroamination, the formal insertion of an olefin into the N-H bond of an amine, is a process of significant academic and industrial interest, due to its potential to transform widely available alkenes and alkynes into valuable complex amines. We developed a polarity-reversed strategy for catalytic enantioselective hydroamination relying on the reaction of olefins with CuH to generate chiral organocopper intermediates, which are intercepted by electrophilic amine reagents. By engineering the auxiliary ligand, amine electrophile, and reaction conditions, the scope of this method has since been extended to include many types of olefins, including challenging internal olefins. Further, the scope of amine reagents has been expanded to enable the synthesis of primary, secondary, and tertiary amines as well as amides, N-alkylated heterocycles, and anilines. All of these reactions exhibit high regio- and stereoselectivity and, due to the mild conditions required, excellent tolerance for heterocycles and polar functional groups.Though the generation of alkylcopper species from olefins was originally devised as a means to solve the hydroamination problem, we soon found that these intermediates could react efficiently with an unexpectedly broad range of electrophiles, including alkyl halides, silicon reagents, arylpalladium species, heterocycles, and carbonyl derivatives. The general ability of olefins to function as precursors for nucleophilic intermediates has proved particularly advantageous in carbonyl addition reactions because it overcomes many of the disadvantages associated with traditional organometallic reagents. By removing the need for pregeneration of the nucleophile in a separate operation, CuH-catalyzed addition reactions of olefin-derived nucleophiles feature improved step economy, enhanced functional group tolerance, and the potential for catalyst control over regio- and stereoselectivity. Following this paradigm, feedstock olefins such as allene, butadiene, and styrene have been employed as reagents for asymmetric alkylation of ketones, imines, and aldehydes.

Scope and Limitations of the s-Block Metal-Mediated Pudovik Reaction

The hydrophosphorylation of phenylacetylene with di(aryl)phosphane oxides Ar₂P(O)H (Pudovik reaction) yields E/Z‐isomer mixtures of phenylethenyl‐di(aryl)phosphane oxides (1). Alkali and alkaline‐earth metal di(aryl)phosphinites have been studied as catalysts for this reaction with increasing activity for the heavier s‐block metals. The Pudovik reaction can only be mediated for di(aryl)phosphane oxides whereas P‐bound alkyl and alcoholate substituents impede the P−H addition across alkynes. The demanding mesityl group favors the single‐hydrophosphorylated products 1‐Ar whereas smaller aryl substituents lead to the double‐hydrophosphorylated products 2‐Ar. Polar solvents are beneficial for an effective addition. Increasing concentration of the reactants and the catalyst accelerates the Pudovik reaction. Whereas Mes₂P(O)H does not form the bis‐phosphorylated product 2‐Mes, activation of an ortho‐methyl group and cyclization occurs yielding 2‐benzyl‐1‐mesityl‐5,7‐dimethyl‐2,3‐dihydrophosphindole 1‐oxide (3).

Integrating CuO-Fe2 O3 Nanocomposites and Supramolecular Assemblies of Phenazine for Visible-Light Photoredox Catalysis

A photoredox catalytic ensemble consisting of CuO-Fe₂ O₃ nanocomposites and oligomeric derivative of phenazine has been developed. The prepared system acts as an efficient photoredox catalyst for C-N bond formation reaction via SET mechanism under 'green' conditions (aerial environment, mixed aqueous media, recyclable), requiring less equivalents of base and amine substrate. The present study demonstrates the significant role of supramolecular assemblies as photooxidants and reductants upon irradiation and their important contribution towards the activation of the metallic centre through energy transfer and electron transfer pathways. The potential of oligomer 4: CuO-Fe₂ O₃ has also been explored for C-C bond formation reactions via the Sonogashira protocol.

Hydroamination of isocyanates and isothiocyanates by alkaline earth metal initiators supported by a bulky iminopyrrolyl ligand

Synthesis of heteroleptic and homoleptic alkaline earth metal complexes supported by bulky bis-iminopyrrolyl ligands are reported. The catalytic hydroamination of isocyanates and isothiocyanates with aryl amines using calcium complex is presented.

Alkali metal and stoichiometric effects in intermolecular hydroamination catalysed by lithium, sodium and potassium magnesiates

Main group bimetallic complexes, while being increasingly used in stoichiometric deprotonation and metal-halogen exchange reactions, have not yet made a significant impact in catalytic applications. This paper explores the ability of alkali metal magnesiates to catalyse the intermolecular hydroamination of alkynes and alkenes using sytrene and diphenylacetylene as principle setting model substrates. By systematically studying the role of the alkali-metal and the formulation of the heterobimetallic precatalyst, this study establishes higher order potassium magnesiate [(PMDETA)₂K₂Mg(CH₂SiMe₃)₄] (7) as a highly effective system capable of catalysing hydroamination of styrene and diphenylacetylene with several amines while operating at room temperature. This high reactivity contrasts with the complete lack of catalytic ability of neutral Mg(CH₂SiMe₃)₂, even when harsher reaction conditions are employed (24 h, 80 °C). A pronounced alkali metal effect is also uncovered proving that the alkali metal (Li, Na, or K) is not a mere spectating counterion. Through stoichiometric reactions, and structural and spectroscopic (DOSY NMR) investigations we shed some light on the potential reaction pathway as well as the constitution of key intermediates. This work suggests that the enhanced catalytic activity of 7 can be rationalised in terms of the superior nucleophilic power of the formally dianionic magnesiate {Mg(NR₂)₄}^2- generated in situ during the hydroamination process, along with the ability of potassium to engage in π-interactions with the unsaturated organic substrate, enhancing its susceptibility towards a nucleophilic attack by the amide anion.

Bis(trimethylsilyl)amide complexes of s-block metals with bidentate ether and amine ligands

The synthesis of the bis(trimethylsilyl)amide complexes of alkali and alkaline-earth metals with bidentate ether and amine bases 1,2-bis(dimethylamino)ethane (tmeda), dimethyl-methoxyethylamine (dmmea), and 1,2-dimethoxyethane (dme) succeedsviaaddition of these bases to coligand-free complexes orvialigand exchange of thf adducts.

Synthesis and catalytic activity of tridentate N-(2-pyridylethyl)-substituted bulky amidinates of calcium and strontium

Sterically protected tridentate amidinates of calcium have been prepared to study the intramolecular hydroamination of an aminoalkene.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Synthesis of α-Aminophosphines by Copper-Catalyzed Regioselective Hydroamination of Vinylphosphines

A copper-catalyzed net hydroamination of vinylphosphine boranes with hydrosilanes and O-benzoylhydroxylamines has been developed. The reaction proceeds regioselectively to form the corresponding α-aminophosphine boranes of potent interest in medicinal and pharmaceutical chemistry. This copper catalysis is based on an umpolung, electrophilic amination strategy and provides a new electrophilic amination approach to α-aminophosphine derivatives. Additionally, although still preliminary, asymmetric synthesis has also been achieved by judicious choice of a chiral bisphosphine-ligated copper complex.