Enantioselective synthesis of cyclic carbamimidates via a three-component reaction of imines, terminal alkynes, and p-toluenesulfonylisocyanate using a monophosphine gold(I) catalyst()

Isocyanate-based multicomponent reactions

Since their discovery, multicomponent reactions have attracted significant attention due to their versatility and efficiency. This review aims to explore the latest advancements in isocyanate-based multicomponent reactions and the sophisticated chemical opportunities they present for generating molecules of interest. The added value of the methodologies described, supported by mechanism schemes, as well as scopes of application, will be discussed. These developments will be organised as the main accessible chemical functions and sorted according to their type of MCR (3, 4 or 5-MCR).

A Ligand-Controlled Approach Enabling Gold(I)-Catalyzed Stereoinvertive Glycosylation with Primal Glycosyl ortho-Alkynylbenzoate Donors

A diarylurea-containing phosphine ligand-modulated stereoinvertive O-glycosylation with primal furanosyl and pyranosyl ortho-alkynylbenzoate (ABz) donors under gold(I) catalysis is disclosed. Both α- and β-configured glycosides could be obtained from the corresponding stereochemically pure β- and α-glycosyl donors with high yields and good to excellent stereoselectivities, respectively. This method accommodates a variety of glycosyl donors and alcoholic acceptors, leading to both 1,2-cis and 1,2-trans glycosidic linkages, and has been applied to the convenient preparation of a series of linear arabinan glycans. Mechanistic investigations reveal that the counteranion could bridge the diarylurea residue on the phosphine ligand with the alcoholic acceptor via hydrogen bond interactions, thereby permitting stereoinvertive displacement at the anomeric position.

Electrosynthesis of Cyclic Isoureas and Ureas Through Contiguous Heterofunctionalizations

An efficient synthetic protocol for the selenylated cyclic isoureas was developed using electrochemical activation of diselenides. This sustainable approach permitted transition metal and chemical oxidant-free difunctionalization of olefins and overall access to distinct 1,2,3 triheterofunctionalized carbon skeletons. Excellent functional group tolerance was noticed, allowing the synthesis of a series of cyclic isourea derivatives. In addition, an acid-triggered skeletal isomerization facilitated the synthesis of cyclic urea derivatives from the corresponding cyclic isoureas. Mechanistic investigations, along with voltammetric studies, enabled the postulation of the reaction mechanism.

Gold Catalyzed 7-endo-dig Hydroaminations Yielding 1,4-Diazepineones

Strategies to access the 1,4-diazepindiones heterocyclic core of the TAN-1057 family of natural products revealed a successful gold-catalyzed hydroamination of yneamide tethered amines. The precursor amino-yneamides are derived from easily accessible 1,2-diamines and alkynoic acids and are efficiently cyclized to the corresponding diazepineones.

Gold-based enantioselective bimetallic catalysis

Multimetallic catalysis is a powerful strategy to access complex molecular scaffolds efficiently from easily available starting materials. Numerous reports in the literature have demonstrated the effectiveness of this approach, particularly for capitalizing on enantioselective transformations. Interestingly, gold joined the race of transition metals very late making its use in multimetallic catalysis unthinkable. Recent literature revealed that there is an urgent need to develop gold-based multicatalytic systems based on the combination of gold with other metals for enabling enantioselective transformations that are not possible to capitalize with the use of a single catalyst alone. This review article highlights the progress made in the field of enantioselective gold-based bimetallic catalysis highlighting the power of multicatalysis for accessing new reactivities and selectivities which are beyond the reach of individual catalysts.

Gold-catalyzed multicomponent reactions

Multicomponent reactions (MCRs) have emerged as an important branch in organic synthesis for the creation of complex molecular structures. This review is focused on gold-catalyzed MCRs with a special emphasis on the recent developments.

Cascade Nucleophilic Attack/Addition Cyclization Reactions to Synthesize Oxazolidin-2-imines via (Z)-2-Bromo-3-phenylprop-2-en-1-ols/3-phenylprop-2-yn-1-ols and Diphenyl Carbodiimides

Two concise strategies to synthesize oxazolidin-2-imines by cascade nucleophilic attack/addition cyclization reactions of (Z)-2-bromo-3-phenylprop-2-en-1-ols/3-phenylprop-2-yn-1-ols and diphenyl carbodiimides without a transition-metal catalyst have been developed. The reactions exhibited good substrate applicability tolerance, and a variety of substituted (Z)-4-((Z)-benzylidene)-N,3-diphenyloxazolidin-2-imines were synthesized in moderate to excellent yields with good stereoselectivity. The reports also provided a convenient strategy to synthesize 3-phenylprop-2-yn-1-ols by (Z)-2-bromo-3-phenylprop-2-en-1-ols. The economic and practical methods provide a great advantage for potential industrial synthesis of oxazolidin-2-imines.

Direct Enantioselective Addition of Alkynes to Imines by a Highly Efficient Palladacycle Catalyst

Enantiopure propargylic amines are highly valuable synthetic building blocks. Much effort has been devoted to develop methods for their preparation. The arguably most important strategy is the 1,2-addition of alkynes to imines. Despite remarkable progress, the known methods using Zn and Cu catalysts suffer from the need for high catalyst loadings, typically ranging from 2-60 mol % for neutral aldimine substrates. Here we report a planar chiral Pd complex acting as very efficient catalyst for direct asymmetric alkyne additions to imines, requiring very low catalyst loadings. Turnover numbers of up to 8700 were accomplished. Our investigation suggests that a Pd-acetylide complex is generated as a catalytically relevant intermediate by the aid of an acac ligand acting as internal catalytic base. It is shown that the catalyst is quite stable under the reaction conditions and that product inhibition is not an issue. A total of 39 examples is shown which all yielded almost enantiopure products.

Enantioselective C-H Functionalization Reactions under Gold Catalysis

Transition metal-catalyzed enantioselective functionalization of ubiquitous C-H bonds has proven to be promising field as it offers the construction of chiral molecular complexity in a step- and atom-economical manner. In recent years, gold has emerged as an attractive contender for catalyzing such reactions. The unique reactivities and selectivities offered by gold catalysts have been exploited to access numerous asymmetric transformations based on gold-catalyzed C-H functionalization processes. Herein, this review critically highlights the major advances and discoveries made in the enantioselective C-H functionalization under gold catalysis which is accompanied by mechanistic insights at appropriate places.

Paired electrolysis enabled annulation for the quinolyl-modification of bioactive molecules

A paired electrolysis enabled cascade annulation that enables the efficient synthesis of highly functionalized quinoline-substituted bioactive molecules from readily available starting materials is reported. Using this methodology, two goals, namely, the direct synthesis of quinolines and the introduction of quinoline moieties to bioactive molecules, can be simultaneously achieved in one simple operation. The use of electroreduction for the activation of isatin, together with the further anodic oxidation of KI to catalytically result in a cascade annulation, highlight the unique possibilities associated with electrochemical activation methods. This transformation can tolerate a wide range of functional groups and can also be used as a functionalization tactic in pharmaceutical research as well as other areas.

A Chiral Iron Disulfonate Catalyst for the Enantioselective Synthesis of 2-Amino-2'-hydroxy-1,1'-binaphthyls (NOBINs)

A novel type of chiral redox disulfonate iron complex for asymmetric catalysis is reported. The [Fe((R_a)-BINSate)]⁺ (BINSate = 1,1'-binaphthalene-2,2'-disulfonate) complex effectively promotes the enantioselective oxidative cross-coupling between 2-naphthols (1) and 2-aminonaphthalene derivatives (2), affording optically enriched (R_a)-2-amino-2'-hydroxy-1,1'-binaphthyls (NOBINs) with exceptional yields and enantioselective ratios (up to 99% yield and 96:4 er). The [Fe((R_a)-BINSate)]⁺ catalyst was designed as a chiral version of FeCl₃ with multicoordination sites available for binding the two coupling partners 1 and 2 as well as the oxidant. Our structure-selectivity and activity study, which covered most of the important positions in the NOBIN scaffold, revealed the effect of different substitution patterns on the coupling efficiency and stereoselectivity.

H-Bonded Counterion-Directed Enantioselective Au(I) Catalysis

A new strategy for enantioselective transition-metal catalysis is presented, wherein a H-bond donor placed on the ligand of a cationic complex allows precise positioning of the chiral counteranion responsible for asymmetric induction. The successful implementation of this paradigm is demonstrated in 5-exo-dig and 6-endo-dig cyclizations of 1,6-enynes, combining an achiral phosphinourea Au(I) chloride complex with a BINOL-derived phosphoramidate Ag(I) salt and thus allowing the first general use of chiral anions in Au(I)-catalyzed reactions of challenging alkyne substrates. Experiments with modified complexes and anions, ¹H NMR titrations, kinetic data, and studies of solvent and nonlinear effects substantiate the key H-bonding interaction at the heart of the catalytic system. This conceptually novel approach, which lies at the intersection of metal catalysis, H-bond organocatalysis, and asymmetric counterion-directed catalysis, provides a blueprint for the development of supramolecularly assembled chiral ligands for metal complexes.

Cyanide-Free Synthesis of Air Stable N-Substituted Li and K Cyanamide Salts from Tetrazoles. Applications toward the Synthesis of Primary and Secondary Cyanamides as Precursors to Amidines

A practical two-step synthesis of N,N'-disubstituted cyanamides consists in the low-temperature metalation of N-substituted 5H-tetrazoles that undergo spontaneous cycloreversion at 0 °C releasing dinitrogen, and forming N-metalated cyanamides that can be reacted in situ with a variety of electrophiles. Remarkably, the N-substituted Li and K cyanamides are air stable white solids at room temperature. Addition of lithium organometallics to the N,N'-disubstituted cyanamides provides a new method for accessing N,N'-disubstituted amidines.

Copper-catalyzed asymmetric dearomative alkynylation of isoquinolines

Cu/(Ph-pybox)-catalyzed asymmetric dearomative alkynylation of isoquinolines has been developed, with high yields and enantioselectivities.

Experimental results and computational insight into sequential reactions of β-(2-aminophenyl)-α,β-ynones with aryl isocyanates/benzoyl isothiocyanate

The selective formation of quinazoline vs. benzoxazine  and benzothiazine derivatives from β-(2-aminophenyl)-α,β-ynones and aryl isocyanates/benzoyl isothiocyanate was explored. DFT calculations provide a plausible rationale.

Development of and recent advances in asymmetric A3 coupling

Asymmetric A3 coupling has emerged as an important class of reactions to synthesise chiral propargylamines. In this tutorial review, an up to date progress of this reaction, with significant recent advancements in terms of ligand development, is presented. Applications of asymmetric A3 coupling in natural product synthesis and in tandem processes are also discussed.

AuBr3 -Catalyzed Chemoselective Annulation of Isocyanates with 2H-Azirine

The chemoselective cyclization of isocyanates with 2H-azirine was achieved with AuBr₃ as catalyst. This transfer sets the stage for the synthesis of aromatic oxazole-ureas in a tandem process. The addition of a catalytic amount of phosphite enhances the process enormously. The reaction can also be performed in a one-pot process using benzoyl azide instead of isocyanate under the same conditions. A detailed study on the role of the phosphite that was applied as an additive revealed that only non-coordinated phosphite can reduce gold(III) and that gold(I) coordinated phosphite is not oxidized. Accompanied by the reduction of gold, HBr is generated in situ, which turned out to be the actual promotor in combination with the remaining AuBr₃ . The positive effect of acid can be explained by a strong N-Au coordination, which tends to break more easily in the presence of small amount of protic acid in the reaction solution.

(CH3)2CuLi/Cu(OTf)2 Mediated N- or O-Cyclization of Urea-Tethered Cyclobuta[ b]indolines

A (CH₃)₂CuLi/Cu(OTf)₂ mediated N- or O-cyclization of urea-tethered cyclobuta[ b]indolines has been reported in this paper, giving a new synthetic protocol for the construction of pyrimido[1,6- a]indolone and cyclic imidate derivatives in moderate to good yields with a broad substrate scope under mild conditions. A plausible reaction mechanism has been also proposed on the basis of previous reports and the control experiments.

Recent advances in reactions of aryl sulfonyl isocyanates

Aryl sulfonyl isocyanates are important intermediates in organic synthesis. They are used as electrophilic reagents and easily react with nucleophiles to form amides, sulfonyl ureas, pyrrolidine derivatives, lactams, oxazolidinones, and so on. Although there are some reviews on the applications of isocyanates in chemistry, few are concerned about aryl sulfonyl isocyanates. This review focuses on recent advances related to aryl sulfonyl isocyanates in organic synthesis, mainly including three special characteristics. The mechanisms of the typical reactions are also discussed.

Reactions of secondary propargylamines with heteroallenes for the synthesis of diverse heterocycles

This focused review aims to summarize recent developments in the processes involving additions of secondary propargylamines to various heteroallenes and subsequent transition metal-catalyzed or electrophile-mediated cyclizations.

Highly Enantioselective Synthesis of Propargyl Amides through Rh-Catalyzed Asymmetric Hydroalkynylation of Enamides: Scope, Mechanism, and Origin of Selectivity

Chiral propargyl amides are particularly useful structural units in organic synthesis. The enantioselective synthesis of propargyl amide is highly desirable. Conventional approach involves the use of a stoichiometric amount of metal reagent or chiral auxiliary. In comparison, direct alkynylation with terminal alkyne is attractive because it avoids the use of stoichiometric organometallic reagent. The asymmetric coupling of aldehyde, amine, and alkyne (A³-coupling) provides an efficient method for the synthesis of N-alkyl and N-aryl-substituted propargyl amines, but this strategy is not amenable for the direct enantioselective synthesis of propargyl amide. We have developed a new strategy and report here a Rh-catalyzed asymmetric hydroalkynylation of enamides. Alkynylations occur regioselectively at the α position of an enamide to produce chiral propargyl amides. High yield and enantioselectivity were observed. Previous alkynylation methods to prepare chiral propargyl amine involve the nucleophilic addition to an electron-deficient imine. In contrast, our current approach proceeds through regioselective hydroalkynylation of an electron-rich alkene. Kinetic studies indicated that migratory insertion of the enamide to the rhodium hydride is turnover limiting. Computational studies revealed the origin of regio- and enantioselectivities. This novel strategy provides an efficient method to access chiral propargyl amides directly from terminal alkynes.

One pot conversion of benzophenone imine into the relevant 2-aza-allenium

Interaction of common imines with WCl₆ in organic solvents revealed very unusual features in the context of imine chemistry. Ph₂C[double bond, length as m-dash]NH was converted in one pot into the 2-aza-allenium species [Ph₂C[double bond, length as m-dash]N[double bond, length as m-dash]CPh₂]⁺via N₂ release, with [Ph₂C[double bond, length as m-dash]NH₂]⁺ being a co-product. PhCH[double bond, length as m-dash]N^tBu underwent C-H bond activation resulting in the formation of [C[triple bond, length as m-dash]N] containing derivatives, together with [PhCH[double bond, length as m-dash]NH^tBu]⁺.

Enantioselective Oxidative Homocoupling and Cross-Coupling of 2-Naphthols Catalyzed by Chiral Iron Phosphate Complexes

Novel chiral iron phosphate complexes were prepared as catalysts for asymmetric oxidative coupling reactions. These catalysts were applied for the synthesis of enantio-enriched C₁- and C₂-symmetric BINOLs, in which the 3 and 3' positions are available for chemical modifications. It was proposed that the reaction takes place via an oxidative radical-anion coupling mechanism. A destructive BINOL racemization that competes with the enantioselective oxidative coupling of 2-naphthols was revealed, thereby offering new insights into this highly important reaction.

Synthesis of a Small Library of Imidazolidin-2-ones using Gold Catalysis on Solid Phase

An efficient and high-yielding solid phase synthesis of a small library of imidazolidin-2-ones and imidazol-2-ones was carried out employing a high chemo- and regioselective gold-catalyzed cycloisomerization as a key step. Polymer-supported amino acids derivatized with several alkyne functionalities combined with tosyl- and phenylureas have been subjected to gold-catalysis exhibiting exclusively C-N bond formation. The present work proves the potential of solid phase synthesis and homogeneous gold catalysis as an efficient and powerful synthetic tool for the generation of drug-like heterocycles.

MediaChrom: Discovering a Class of Pyrimidoindolone-Based Polarity-Sensitive Dyes

A small library of six polarity-sensitive fluorescent dyes, nicknamed MediaChrom, was prepared. This class of dyes is characterized by a pyrimidoindolone core fitted out with a conjugated push-pull system and a carboxy linker for a conceivable coupling with biomolecules. The optimized eight-step synthetic strategy involves a highly chemo- and regioselective gold-catalyzed cycloisomerization reaction. The photophysical properties of MediaChrom dyes have been evaluated in-depth. In particular, the MediaChrom bearing a diethylamino as an electron-donating group and a trifluoromethyl as an electron-withdrawing group displays the most interesting and advantageous spectroscopic features (e.g., absorption and emission in the visible range and a good quantum yield). Promising results in terms of sensitivity have been obtained in vitro on this dye as a membrane/lipophilic probe and as a peptide fluorescent label.

Gold-catalyzed tandem reactions of amide-aldehyde-alkyne coupling and cyclization-synthesis of 2,4,5-trisubstituted oxazoles

We report the first cationic gold(i)-catalyzed one-pot reaction of amide, alkyne and aldehyde followed by cyclization, to successfully access highly substituted oxazoles derivatives in good yields. A single catalyst allows the occurring of this multi-step reaction atom- and step-economically, with water as the only theoretical side product.