Catalytic access to α-oxo gold carbenes by N-O bond oxidants

Cu(i)- and Au(i)-catalyzed regioselective oxidation of diynes: divergent synthesis of N-heterocycles

The efficient and divergent construction of two types of valuable N-heterocycle is achieved easily, with the first example of the generation of α-oxo copper carbenes via copper-catalyzed oxidation of non-polarized alkynes.

Gold-Catalyzed Oxidative Aminocyclizations of Propargyl Alcohols and Propargyl Amines to Form Two Distinct Azacyclic Products: Carbene Formation versus a 3,3-Sigmatropic Shift of an Initial Intermediate

Gold-catalyzed oxidations of propargyl alcohols with nitrones by using a P(tBu)₂ (o-biphenyl)Au⁺ catalyst, afforded bicyclic annulation products from the Mannich reactions of gold enolates. The same reactions of propargyl amines with nitrones by using the same gold catalyst gave distinct oxoarylation products. Our DFT calculations indicate that oxidation of propargyl alcohols with nitrones by using electron-rich gold catalysts lead only to gold carbenes, which can generate gold enolates or oxoarylation intermediates with enolate species having a barrier smaller than that of oxoarylation species.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Brønsted acid-mediated reactions of ynamides

Over the past two decades, the development and application of ynamide chemistry have received more and more attention. Ynamides have proven to be versatile reagents for organic synthesis, and have been widely applied to the rapid assembly of a diverse range of structurally complex N-containing molecules, especially the valuable N-heterocycles. In comparison with the well-established transition metal-catalyzed reactions of ynamides, metal-free ynamide transformations have relatively seldom been exploited. Recently, Brønsted acid-mediated reactions of ynamides represent significant advances in ynamide chemistry. This review summarizes the latest trends and developments of Brønsted acid-mediated reactions of ynamides, including cycloaddition, cyclization, intramolecular alkoxylation-initiated rearrangement, oxygen atom transfer reactions and hydro-heteroatom addition reactions.

Regioselective C-H Functionalization of Heteroarene N-Oxides Enabled by a Traceless Nucleophile

Although N-alkenoxyheteroarenium salts have been widely used as umpoled synthons with nucleophilic (hetero)arenes, the use of electron-poor heteroarenes has remained unexplored. To overcome the inherent electron deficiency of quinolinium salts, a traceless nucleophile-triggered strategy was designed, wherein the quinolinium segment is converted into a dearomatized intermediate, thereby allowing simultaneous C8-functionalization of quinolines at room temperature. Experimental and computational studies support the traceless operation of a nucleophile, which enables the previously inaccessible transformation of N-alkenoxyheteroarenium salts. Remarkably, the generality of this strategy has been further demonstrated by broad applications in the regioselective C-H functionalization of other electron-deficient heteroarenes such as phenanthridine, isoquinoline, and pyridine N-oxides, offering a practical tool for the late-stage functionalization of complex biorelevant molecules.

Transition Metal-Catalyzed Tandem Reactions of Ynamides for Divergent N-Heterocycle Synthesis

ConspectusYnamides are electron-rich heteroatom-substituted alkynes with a C-C triple bond directly attached to the amide group. Importantly, this amide group is able to impose an electronic bias, thus resulting in the highly regioselective attack of this polarized alkyne by a large variety of nucleophiles. Over the past two decades, catalytic reactions of ynamides have experienced dramatic developments, especially those catalyzed by transition metals. As a result, ynamides have been widely applied to the rapid and efficient assembly of versatile structurally complex N-containing molecules, especially in an atom-economic and stereoselective way.On the basis of newly developed ynamide preparations and new alkyne transformations, we first developed oxidation-initiated tandem reactions of ynamides such as zinc-catalyzed ynamide oxidation/C-H functionalization and copper-catalyzed ynamide oxidation/carbene metathesis, leading to divergent synthesis of isoquinolones, β-carbolines, and pyrrolo[3,4-c]quinolin-1-ones. Importantly, this protocol represents the first non-noble-metal-catalyzed intermolecular oxidation of alkynes by N-oxide type oxidants, and the related overoxidation could be dramatically inhibited in this non-noble-metal catalysis. Then, we achieved gold-catalyzed amination-initiated tandem reactions of ynamides via α-imino gold carbenes for efficient construction of various 2-aminoindoles, 3-amino-β-carbolines, and 2-aminopyrroles, where two new types of nitrene transfer reagents (benzyl azides and isoxazoles) were discovered. In particular, the use of isoxazoles as nitrene transfer reagents for atom-economic generation of α-imino metal carbenes has also been elegantly exploited by Hashmi, Liu, and many other groups, providing ready access to a wide range of functionalized N-heterocycles. Moreover, we revealed that donor/donor copper carbenes could be generated via copper-catalyzed diyne cyclization under mild conditions. These novel copper carbenes could undergo asymmetric C-H insertion, cyclopropanation, and formal [3 + 2] cycloaddition to produce diverse chiral polycyclic pyrroles with good to excellent enantioselectivities. Thus, this strategy may open new avenues in catalytic asymmetric reaction of ynamides, which remain largely unexplored and deserve more attention. Meanwhile, we also accomplished the efficient and practical synthesis of medium-sized lactams by yttrium-catalyzed cascade cyclization of allyl alcohol-tethered ynamides and the combination of radical chemistry based on visible-light photoredox catalysis with ynamide chemistry for divergent synthesis of useful 2-benzhydrylindoles and 3-benzhydrylisoquinolines.In this Account, we describe a panoramic picture of our recent contributions since 2015 to the development and application of ynamide chemistry in organic synthesis via transition metal-catalyzed tandem reactions by focusing on the tetrafunctionalization of ynamides. These studies provide not only efficient and attractive methods for divergent synthesis of valuable N-heterocycles but also some new insights into the exploration of alkyne chemistry and metal carbene chemistry.

Cu(I)-Catalyzed Oxidative Cyclization of Enynamides: Regioselective Access to Cyclopentadiene Frameworks and 2-Aminofurans

An efficient Cu(I)-catalyzed oxidative cyclization of alkynyl-tethered enynamides for the construction of fused bicyclic cyclopentadiene derivatives is disclosed. The cascade proceeds through alkyne oxidation, carbene/alkyne metathesis, and formal (3 + 2) cycloaddition. Employing aryl-tethered enynamides as starting materials, substituted 2-aminofurans can be exclusively formed.

Copper-Catalyzed Azide-Ynamide Cyclization to Generate α-Imino Copper Carbenes: Divergent and Enantioselective Access to Polycyclic N-Heterocycles

Here an efficient copper-catalyzed cascade cyclization of azide-ynamides via α-imino copper carbene intermediates is reported, representing the first generation of α-imino copper carbenes from alkynes. This protocol enables the practical and divergent synthesis of an array of polycyclic N-heterocycles in generally good to excellent yields with broad substrate scope and excellent diastereoselectivities. Moreover, an asymmetric azide-ynamide cyclization has been achieved with high enantioselectivities (up to 98:2 e.r.) by employing BOX-Cu complexes as chiral catalysts. Thus, this protocol constitutes the first example of an asymmetric azide-alkyne cyclization. The proposed mechanistic rationale for this cascade cyclization is further supported by theoretical calculations.

Nitrene Transfer and Carbene Transfer in Gold Catalysis

Catalytic transformations involving metal carbenes are considered one of the most important aspects of homogeneous transition metal catalysis. Recently, gold-catalyzed generation of gold carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. This Review summarizes the advances in the gold-catalyzed nitrene-transfer reactions of alkynes with nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils, and gold-catalyzed carbene-transfer reactions, involving oxygen atom-transfer reactions of alkynes with nitro compounds, nitrones, sulfoxides, and pyridine N-oxides, through the presumable α-imino gold carbene and α-oxo gold carbene intermediates, respectively. Gold-catalyzed processes are reviewed by highlighting their product diversity, selectivity, and applicability, and the mechanistic rationale is presented where possible.

Aminooxygenation of Ynamides with N-Hydroxybenzotriazoles: Synthesis of α-Benzotriazolyl Carbonyl Compounds

Addition of N-hydroxybenzotriazoles to ynamides causes spontaneous rearrangement, resulting in α-benzotriazolyl imides. The transformation proceeded at rt in the absence of any catalyst but could be efficiently catalyzed by Zn(OTf)₂. Crossover experiments confirmed that the rearrangement is an intramolecular process, most likely via a concerted mechanism. However, heating the mixture above 110 °C resulted in isomerization of N2 into N1 product, via heterolytic C-N bond dissociation. This tandem addition-rearrangement sequence provides an efficient and atom-economical synthetic route for the synthesis of α-benzotriazolyl carbonyl compounds.

Gold(iii)-catalyzed azide-yne cyclization/O-H insertion cascade reaction for the expeditious construction of 3-alkoxy-4-quinolinone frameworks

A gold-catalyzed 6-endo-dig azide-yne cyclization/O-H insertion cascade reaction of azide-tethered alkynes with alcohols has been developed, and it provides an expeditious access to 3-alkoxy-4-quinoline derivatives in good to high yields under mild and neutral reaction conditions with broad substrate generality. The utility of this method is emphasized by a scalable experiment and concise total synthesis of a bioactive natural product Leiokinine A, and other bioactive quinoline analogs.

Gold-Catalyzed Oxidative Cascade Cyclization of 1,3-Diynamides: Polycyclic N-Heterocycle Synthesis via Construction of a Furopyridinyl Core

A facile and practical approach to construct a furopyridinyl motif through a gold-catalyzed cascade cyclization of easily accessible diynamides is described. This strategy offers a straightforward approach to furo[2,3-c]isoquinoline and 6H-furo[3',2':5,6]pyrido[3,4-b]indole derivatives. The reaction could build up four new bonds and two additional heteroaromatic rings via a single operation. The heterocyclic products show promising blue luminous performance with fluorescence quantum yields up to 75%.

Recent Progress in N-Heterocyclic Carbene Gold-Catalyzed Reactions of Alkynes Involving Oxidation/Amination/Cycloaddition

Recent rapid development in homogeneous gold catalysis affords an alternative and particularly thriving strategy for the generation of gold carbenes through gold-catalyzed oxidation/amination/cycloaddition of alkynes, while it avoids the employment of hazardous and potentially explosive diazo compounds as starting materials for carbene generation. In addition to facile and secure operation, gold carbenes generated in this strategy display good chemoselectivity distinct from other metal carbenes produced from the related diazo approach. N-heterocyclic carbene (NHC) gold is a special metal complex that can be used as ancillary ligands, which provides enhanced stability and can also act as an efficient chiral directing group. In this review, we will present an overview of these recent advances in alkyne oxidation/amination/cycloaddition by highlighting their specificity and applicability, aiming to facilitate progress in this very exciting area of research.

α-Imino Gold Carbene Intermediates from Readily Accessible Sulfilimines: Intermolecular Access to Structural Diversity

Catalytic approaches to pharmaceutically important bioactive skeletons through gold carbene intermediates have experienced a dramatic development in the last decade. Although various carbene precursors continue to play an important role in heterocyclic syntheses, these reagents are associated with some drawbacks in terms of functional group tolerance, synthetic methods and safety limitations. A new generation of nitrene transfer reagents was established in 2019: the sulfilimines. These are safe, inexpensive and readily available. They can conveniently be stored and handled, and thus represent ideal reagents for the fast and modular modification of scaffolds and the preparation of libraries by intermolecular reactions of two components. Both the practical methods for synthesizing sulfilimines and the versatility of these ylidic species in gold-catalyzed preparation of structural diversity, for both heterocycles and carbocycles, will be outlined in this Concept article.

Hydration of alkynes catalyzed by [Au(X)(L)(ppy)]X in the green solvent γ-valerolactone under acid-free conditions: the importance of the pre-equilibrium step

Stable and robust [Au(H₂O)(NHC)(ppy)](X)₂ successfully catalyses the hydration of alkynes in GVL, under acid-free conditions. DFT calculation and NMR measurements suggest that pre-equilibrium is the key step of the whole process.

Zinc-Catalyzed Alkyne-Carbonyl Metathesis of Ynamides with Isatins: Stereoselective Access to Fully Substituted Alkenes

A zinc-catalyzed intermolecular alkyne-carbonyl metathesis reaction of ynamides with isatins followed by an amide to ester conversion has been developed, which produces the indolone derivatives with a fully substituted alkene species in good to high yields. The salient features of this reaction include the following: mild reaction conditions, an inexpensive zinc catalyst, a broad substrate scope, the excellent regiocontrol and stereoselectivity, and amenable to the gram scale.

Synthesis of gold allyloxysulfonium complexes and elimination to form an α,β-unsaturated aldehyde

Treatment of the gold vinyl carbene/allylic cation complex (E)-[(IPr)AuC(H)C(H)C(4-C6H4OMe)2]+ OTf- with sulfoxides at -95 °C formed the corresponding gold allyloxysulfonium complexes [(IPr)AuC(H)(OSR2)C(H)[double bond, length as m-dash]C(4-C6H4OMe)2]+ OTf- [R = Me, -(CH2)4-, Ar] in ≥95 ± 5% NMR yield. Allyloxysulfonium gold complexes underwent elimination at or below room temperature to form 3,3-bis(4-methoxyphenyl)acrylaldehyde in ≥67% yield.

β-Oxidation of Ynamides into N,O-Acetals by mCPBA: Application in Enantioselective Intermolecular Transacetalization

Oxidation of ynamides by mCPBA led to β-oxygenation and resulted in formation of carbonyl compounds with α-N,O-acetal functionality. These N,O-acetals are formed in high yields and can be stored indefinitely at room temperature. Yet, they can be activated by a chiral Brønsted acid and underwent an enantioselective transacetalization into a α-N,O-acetal. Subsequent diastereoselective transformations occurred with exceptional selectivity according to Felkin-Anh model.

Dual gold catalysis - an update

This review summarizes the recent developments in the field of dual gold activation chemistry. New developments including synthetic strategies, latest mechanistic insights, computational studies and the identification and isolation of key intermediates, are discussed.

Generation of Donor/Donor Copper Carbenes through Copper-Catalyzed Diyne Cyclization: Enantioselective and Divergent Synthesis of Chiral Polycyclic Pyrroles

The generation of metal carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. However, most of these transformations are based on the use of noble-metal catalysts and successful examples of such an asymmetric version are still very scarce. Here a copper-catalyzed enantioselective cascade cyclization of N-propargyl ynamides is reported, enabling the practical and atom-economical construction of diverse chiral polycyclic pyrroles in generally good to excellent yields with wide substrate scope and excellent enantioselectivities (up to 97:3 e.r.). Importantly, this protocol represents the first copper-catalyzed asymmetric diyne cyclization. Moreover, mechanistic studies revealed that the generation of donor/donor copper carbenes is presumably involved in this 1,5-diyne cyclization, which is distinctively different from the related gold catalysis, and thus it constitutes a novel way for the generation of donor/donor metal carbenes.

Gold-Catalyzed Hydroalkoxylation/Povarov Reaction Cascade of Alkynols with N-Aryl Imines: Synthesis of Tetrahydroquinolines

A one-pot gold-catalyzed hydroalkoxylation/Povarov reaction cascade of alkynols with N-aryl imines or in situ generated iminium has been developed. The protocol provides a facile access to a series of fused tricyclic tetrahydroquinolines with a broad substrate scope using readily available materials under mild conditions. The unique mechanistic feature is the dual function of the gold catalyst, which first catalyzed the intramolecular hydroalkoxylation of alkynols, and upon the formation of dihydrofuran species, promoted the following Povarov reaction with high stereoselectivity.

Bis(imino)pyridine iron complexes for catalytic carbene transfer reactions

The bis(imino)pyridine iron complex, for the first time, is developed as an effective metal carbene catalyst for carbene transfer reactions of donor-acceptor diazo compounds. Its broad catalytic capability is demonstrated by a range of metal carbene reactions, from cyclopropanation, cyclopropenation, epoxidation, and Doyle-Kirmse reaction to O-H insertion, N-H insertion, and C-H insertion reactions. The asymmetric cyclopropanation of styrene and methyl phenyldiazoacetate was successfully achieved by the new chiral bis(imino)pyridine iron catalyst, which delivers a new gateway for the development of chiral iron catalysis for metal carbene reactions.

Synthesis of γ-substituted carbonyl compounds from DMSO-mediated oxidation of enynamides: mechanistic insights and carbon- and hetero-functionalizations

1,3-Enynamides underwent oxygenative coupling with carbon- and heteroatom nucleophiles with high remote selectivity. Kinetic analysis revealed a continuum mechanism between concerted S_N2′′ and via a carbocation, depending on the nucleophiles used.

Oxidative coupling of 1,3-enynamides using DMSO as a terminal oxidant has been developed. Carbon as well as unmodified heteroatom nucleophiles, including aliphatic alcohols, thiols, and hydrazides, could be efficiently alkylated at the γ-position in a highly regioselective fashion. The kinetic analysis suggested a nucleophile-dependent mechanism ranging from a concerted S_N2′′ to a carbocationic mechanism. Thus, the remote site-selectivity was ascribed to the partial positive charge developing at the terminal carbocationic center.