A heterobimetallic Ga/Yb-Schiff base complex for catalytic asymmetric alpha-addition of isocyanides to aldehydes

Organocatalytic Atroposelective Construction of Pentatomic Heterobiaryl Diamines through Arylation of 5-Aminoisoxazoles with Azonaphthalenes

An efficient catalytic asymmetric Michael-type reaction of azonaphthalenes with 5-aminoisoxazoles has been developed. The reaction was based on the utilization of a chiral phosphoric acid as the catalyst, delivering a large panel of axially chiral heterobiaryl diamines in generally good yields with excellent enantioselectivities. The gram-scale reaction and postmodification of the chiral product demonstrated their potentials in the synthesis of chiral catalysts and ligands. This approach not only provides a useful method for the construction of pentatomic heterobiaryl scaffolds but also offers new members to the axially chiral diamine family with promising applications in synthetic and medicinal chemistry.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

The 100 facets of the Passerini reaction

This perspective aims at celebrating the 100th anniversary of the discovery of the Passerini three component reaction. After being nearly neglected for many years, now this reaction has become quite popular, thanks to the achievements of the last 30 years, which have revealed several chances of exploitation in organic synthesis. Though not being comprehensive, this review means to show the various ways that have been used in order to expand the utility of the Passerini reaction. Post-MCR transformations to give heterocycles or peptidomimetics, variants through single component replacement, stereochemical issues, and applications in total syntheses will be especially covered.

Silica gel-immobilised chiral 1,2-benzenedisulfonimide: a Brønsted acid heterogeneous catalyst for enantioselective multicomponent Passerini reaction

A chiral heterogeneous catalyst derivative of (−)-4,5-dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide is proven here to be efficient in a three-component asymmetric Passerini protocol, carried out in a deep eutectic solvent. Reaction conditions are mild and green, while enantioselectivity is excellent. The catalyst was easily recovered and reused with no decrease in its catalytic activity.

A chiral heterogeneous catalyst derivative of (−)-4,5-dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide is proven here to be efficient in a three-component asymmetric Passerini protocol, carried out in a deep eutectic solvent.

Catalytic Enantioselective Isocyanide-Based Reactions: Beyond Passerini and Ugi Multicomponent Reactions

The development of catalytic enantioselective isocyanide-based reactions is currently of great interest because the resulting products are valuable in organic synthesis, pharmacological chemistry, and materials science. This review assembles and comprehensively summarizes the recent achievements in this rapidly growing area according to the reaction types. Special attention is paid to the advantages, limitations, possible mechanisms, and synthetic applications of each reaction. In addition, a personal outlook on the opportunities for further exploration is given at the end.

Brønsted Acid Organocatalyzed Three-Component Hydroamidation Reactions of Vinyl Ethers

Hydroamidation of carbon-carbon double bonds is an attractive strategy for installing nitrogen functionality into molecular scaffolds and, with it, increasing molecular complexity. To date, metal-based approaches have dominated this area of chemical synthesis, despite the drawbacks of air and moisture sensitivity, limited functional group tolerance, toxicity, and/or high cost often associated with using metals. Here, in offering an alternative solution, we disclose an operationally simple, metal-free, one-pot, regioselective, multicomponent synthetic procedure for the hydroamidation of carbon-carbon double bonds. This method features mild reaction conditions and utilizes isocyanides and vinyl ethers for the rapid and modular synthesis of privileged α-oxygenated amide scaffolds. In unraveling the mechanistic underpinning of this non-metal-based reactivity, we present kinetic solvent isotope effect studies, variable time normalization analysis, and density functional theory computations offering insight into the mechanism of this multistep catalytic hydroamidation process.

Passerini-type reaction of boronic acids enables α-hydroxyketones synthesis

Multicomponent reactions (MCRs) facilitate the rapid and diverse construction of molecular scaffolds with modularity and step economy. In this work, engagement of boronic acids as carbon nucleophiles culminates in a Passerini-type three-component coupling reaction towards the synthesis of an expanded inventory of α-hydroxyketones with skeletal diversity. In addition to the appealing features of MCRs, this protocol portrays good functional group tolerance, broad substrate scope under mild conditions and operational simplicity. The utility of this chemistry is further demonstrated by amenable modifications of bioactive products and pharmaceuticals as well as in the functionalization of products to useful compounds.

Multicomponent reactions enable the rapid construction of diverse molecular scaffolds with modularity and step economy. In this work, the authors report the use of boronic acids as carbon nucleophiles in a Passerini-type three-component coupling reaction towards an expanded inventory of α-hydroxyketones.

Controlling the stereochemistry in 2-oxo-aldehyde-derived Ugi adducts through the cinchona alkaloid-promoted electrophilic fluorination

In this report, we introduce a new strategy for controlling the stereochemistry in Ugi adducts. Instead of controlling stereochemistry directly during the Ugi reaction we have attempted to stereodefine the chiral center at the peptidyl position through the post-Ugi functionalization. In order to achieve this, we chose to study 2-oxo-aldehyde-derived Ugi adducts many of which partially or fully exist in the enol form that lacks the aforementioned chiral center. This in turn led to their increased nucleophilicity as compared to the standard Ugi adducts. As such, the stereocenter at the peptidyl position could be installed and stereodefined through the reaction with a suitable electrophile. Towards this end, we were able to deploy an asymmetric cinchona alkaloid-promoted electrophilic fluorination producing enantioenriched post-Ugi adducts fluorinated at the peptidyl position.

Metal-free oxidative C(sp3)–H functionalization: a facile route to quinoline formaldehydes from methyl-azaheteroarenes

A facile protocol for the synthesis of quinoline formaldehydes via direct oxidative C–H bonds functionalization of methyl-azaheteroarenes in the presence of I2–DMSO has been described. This method is metal-free and easy to operate. This reaction provided a convenient route for the preparation of a range of important quinoline formaldehydes.

Asymmetric Catalysis Using Chiral Salen-Metal Complexes: Recent Advances

Chiral salen-metal complexes are among the most versatile asymmetric catalysts and have found utility in fields ranging from materials chemistry to organic synthesis. These complexes are capable of inducing chirality in products formed from a wide variety of chemical processes, often with close to perfect stereoinduction. Salen ligands are tunable for steric as well as electronic properties, and their ability to coordinate a large number of metals gives the derived chiral salen-metal complex very broad utility in asymmetric catalysis. This review primarily summarizes developments in chiral salen-metal catalysis over the last two decades with particular emphasis on those applications of importance in asymmetric synthesis.

C-H Functionalization of Amines via Alkene-Derived Nucleophiles through Cooperative Action of Chiral and Achiral Lewis Acid Catalysts: Applications in Enantioselective Synthesis

Catalytic transformations of α-amino C-H bonds to afford valuable enantiomerically enriched α-substituted amines, entities that are prevalent in pharmaceuticals and bioactive natural products, have been developed. Typically, such processes are carried out under oxidative conditions and require precious metal-based catalysts. Here, we disclose a strategy for an enantioselective union of N-alkylamines and α,β-unsaturated compounds, performed under redox-neutral conditions, and promoted through concerted action of seemingly competitive Lewis acids, B(C₆F₅)₃, and a chiral Mg-PyBOX complex. Thus, a wide variety of β-amino carbonyl compounds may be synthesized, with complete atom economy, through stereoselective reaction of an in situ-generated enantiomerically enriched Mg-enolate and an appropriate electrophile.

Still Unconquered: Enantioselective Passerini and Ugi Multicomponent Reactions

The Passerini three-component (P-3CR) and the Ugi four-component (U-4CR) are two of the most prominent isocyanide-based multicomponent reactions (IMCRs). The P-3CR transforms isocyanides, aldehydes (ketones), and carboxylic acids to α-acyloxy carboxamides, while the U-4CR converts isocyanides, aldehydes (ketones), amines, and carboxylic acids to α-acetamido carboxamides. Conversion of the high energy formal divalent isocyano carbon into a tetravalent amide carbonyl carbon provides the driving force for these reactions. While the prototypical P-3CR and U-4CR provide linear adducts, many heterocycles and macrocycles are now readily synthesized by modifying these truly versatile reactions. As one stereocenter is generated by the nucleophilic addition of the isocyanide to the carbonyl and imine functions, the search for enantioselective versions of these reactions has become a much sought after goal among synthetic chemists. This seemingly trivial endeavor turns out to be extremely difficult to achieve, in sharp contrast to the remarkable progress documented in the field of asymmetric synthesis in general and catalytic enantioselective nucleophilic addition to C═X bond in particular. Since Denmark's first report in 2003 on the catalytic enantioselective Passerini two-component reaction of isocyanides with aldehydes, several Lewis acid (LA) and Brønsted acid-catalyzed enantioselective protocols have been developed. However, it is fair to say that truly catalytic enantioselective P-3CR and U-4CR with wide application scope remain elusive. In this Account, we summarize the progress recorded in this field over the past 15 years. We entered the field by investigating the enantioselective reaction of α-isocyanoacetamides with aldehydes and imines, which was previously developed in our lab for the synthesis of functionalized 5-aminooxazoles. Our initial experimental results, in conjunction with Dömling's and Schreiber's earlier findings, prompted us to assume that the low turnover number in LA-catalyzed asymmetric IMCRs is a main hurdle for enantioselectivity. We speculated that the LA incapable of forming chelates would be the catalyst of choice for enantioselectivity, the rational being that the P-3CR and the U-4CR afforded bidentate intermediates (α-hydroxy imidates, α-amino imidates) and products (α-acyloxy carboxamides, α-acetamido carboxamides) from nonchelating inputs. Therefore, the transfer of catalyst from these chelating intermediates or products to the monocoordinating starting materials would be difficult, hence the problem with catalyst turnover. This working hypothesis turned out to be a valuable guide that allowed us to develop Al-salen and Al-phosphate-catalyzed enantioselective P-3CR and enantioselective construction of chiral heterocycles such as oxazoles and tetrazoles. Nevertheless, all our attempts to apply these LA catalysts to the Ugi reaction failed. Indeed, to date, no reports on the successful LA-catalyzed asymmetric Ugi-type reactions exist in the literature. However, significant progress has been made in recent years employing organocatalysts. We developed a chiral phosphoric acid (CPA)-catalyzed enantioselective three-component synthesis of 2-(1-aminoalkyl)-5-aminooxazoles, a four-component synthesis of epoxy-tetrahydropyrrolo[3,4- b]pyridin-5-ones and a Ugi four-center, three-component reaction of isocyanides, anilines, and 2-formylbenzoic acids for the synthesis of isoindolinones. Other groups have found that chiral dicarboxylic acid and BOROX are effective catalysts for truncated Ugi three-component reactions.

To each his own: isonitriles for all flavors. Functionalized isocyanides as valuable tools in organic synthesis

The term functionalized isocyanides refers to all those isocyanides in which a neighbouring functional group can finely tune the reactivity of the isocyano group or can be exploited in post-functionalization processes. In this manuscript, we have reviewed all the isocyanides in which the pendant functional group causes either deviation from or reinforces the normal reactivity of the isocyano group and categorized them to highlight their common features and differences. An analysis of their synthetic potential and the possible unexplored directions for future research studies is also addressed.

Dinuclear PhosphoiminoBINOL-Pd Container for Malononitrile: Catalytic Asymmetric Double Mannich Reaction for Chiral 1,3-Diamine Synthesis

A phosphoiminoBINOL ligand was designed to form a dinuclear metal complex that could hold a malononitrile molecule. The dinuclear bis(phosphoimino)binaphthoxy-Pd₂(OAc)₂ complex catalyzed a double Mannich reaction of N-Boc-imines with malononitrile to give chiral 1,3-diamines with high enantioselectivity. The rational asymmetric catalyst, which smoothly introduces the first coupling product to the second coupling reaction while avoiding the reverse reaction, facilitates the over-reaction into a productive reaction process.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

An Efficient Nickel-Catalyzed Asymmetric Oxazole-Forming Ugi-Type Reaction for the Synthesis of Chiral Aryl-Substituted THIQ Rings

A nickel-catalyzed asymmetric oxazole-forming Ugi reaction of C,N-cyclic azomethine imines and isonitriles is disclosed. The reported protocol proceeds smoothly, and gives the corresponding adducts, which contain two important pharmaceutically active ring-systems (tetrahydroquinoline and oxazole rings), in good yields and excellent enantioselectivities by employing an easily accessible chiral diamine as a ligand. This simple and efficient strategy provides easy access to a series of C1-substituted aryl tetrahydroisoquinolines.

A N,N'-dioxide/Mg(OTf)2 complex catalyzed enantioselective α-addition of isocyanides to alkylidene malonates

A simple and efficient catalytic asymmetric α-addition of isocyanides to alkylidene malonates was realized using a chiral N,N′-dioxide/Mg^II catalyst. A variety of 2-alkyl-5-aminoxazoles were obtained in up to 99% yield and 96% ee, and could be converted to imides and dipeptides.

A highly efficient catalytic asymmetric α-addition of isocyanides to alkylidene malonates was accomplished. The process was based on the utilization of a chiral N,N′-dioxide/Mg^II catalyst, delivering a variety of 2-alkyl-5-aminooxazoles in up to 99% yield and 96% ee under mild reaction conditions. Besides, a chiral imide and dipeptide could be easily obtained by ring-opening of the oxazole product, both of which are important structural motifs for many biologically active compounds. Based on the experimental investigations and previous work, a possible transition state model was proposed.

A one-pot O-sulfinative Passerini/oxidation reaction: synthesis of α-(sulfonyloxy)amide derivatives

We have developed a one-pot O-sulfinative Passerini/oxidation reaction, in which a combination of an aldehyde, an isocyanide, and a sulfinic acid react, followed by the addition of mCPBA as an oxidant to give the corresponding α-(sulfonyloxy)amides in high yields. This reaction is the first reported demonstration of an isocyanide-based multicomponent reaction using a sulfinic acid in place of a carboxylic acid. A wide range of aldehydes and isocyanides are applicable to this reaction.

Enantioselective Epoxidation of Styrene by Manganese Chiral Schiff Base Complexes Immobilized on MCM-41

Two chiral Mn^III complexes synthesized and characterized by single-crystal X-ray diffraction were grafted onto mesoporous silica MCM-41. The MCM-41-immobilized solid chiral catalysts were characterized by various spectrochemical and physicochemical methods (XRD, FTIR, ²⁹ Si NMR, TGA, UV/Vis/DRS, EPR, cyclic voltammetry, SEM, BET). Both the homogeneous and heterogeneous Mn^III chiral complexes are found to be effective catalysts for epoxidation of styrene. MCM-41-supported chiral catalysts are found to retain their catalytic activity up to five cycles. When an ionic liquid was implemented to enhance the catalytic recyclability of the homogeneous catalysts, they can be recycled up to three times. However, after the third cycle the catalytic activities of the complexes were found to decrease due to rupturing of the chiral catalyst.

AlCl3-catalyzed O-alkylative Passerini reaction of isocyanides, cinnamaldehydes and various aliphatic alcohols for accessing α-alkoxy-β,γ-enamides

The inexpensive Lewis acid AlCl₃ was found to be an efficient catalyst for the O-alkylative Passerini reaction of isocyanides, cinnamaldehydes and alcohols.

One-pot synthesis of 2-amino-4(3H)-quinazolinones via ring-opening of isatoic anhydride and palladium-catalyzed oxidative isocyanide-insertion

An efficient and practical two-step process has been developed for the synthesis of 2-amino-4(3H)-quinazolinones via ring-opening of isatoic anhydride and palladium-catalyzed oxidative isocyanide-insertion in one pot. This regioselective procedure could construct a wide range of 2-amino-4(3H)-quinazolinones in moderate to excellent yields. Furthermore, the methodology also had distinct advantages of easily accessible starting materials and operational simplicity.