Divergent Catalytic Approach from Cyclic Oxime Esters to Nitrogen-Containing Heterocycles with Group 9 Metal Catalysts

Stable 2-Azaallyl Salts as a Bridge between 2H-Azirines and Densely Functionalized 2H-Pyrroles

An efficient protocol for the synthesis of stable 2-azaallyl anion salts by the reaction of alkyl 2-bromo-2H-azirine-2-carboxylates with trimethylsilyl cyanide/Bu4NF has been developed. The domino reaction proceeds in four steps via the cleavage of the azirine C-C bond to provide the tetrabutylammonium salts of stereochemically pure 2-azaallyl anions having U-configuration relative to the cyano groups. The anions with an ortho-substituted aryl group or styryl group exist as a mixture of two geometrical isomers across the N2-C3 bond. 2-Azaallyl anion salts have been shown to be convenient substrates for the one-pot synthesis of densely functionalized 2H-pyrroles by the alkylation-cyclization sequence.

Construction of Indium(III)-Organic Framework Based on a Flexible Cyclotriphosphazene-Derived Hexacarboxylate as a Reusable Green Catalyst for the Synthesis of Bioactive Aza-Heterocycles

The constant demand for eco-friendly methods of synthesizing complex organic compounds inspired researchers to design and develop modern, highly efficient heterogeneous catalytic systems. Herein, In-HCPCP metal-organic framework (SRMIST-1), a heterogeneous Lewis acid catalyst containing less toxic indium and eco-friendly robust cyclotriphosphazene and exhibiting notable chemical and thermal stability, durable catalytic activity, and exceptional reusability was produced through the reaction between indium(III) nitrate hydrate and hexakis(4-carboxylatophenoxy)-cyclotriphosphazene. In the SRMIST-1 structure, secondary building units {InO7} are assembled by a connection of η2- and η1-carboxylic oxo atoms from different HCPCP ligands, forming a three-dimensional network. The occurrence of regularly distributed In(III) sites in SRMIST-1 confers superior reactivity on the catalyst toward the synthesis of 2,3-dihydroquinazolin-4(1H)-ones and 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxides by the cyclization reaction of 2-aminobenzamides and 2-aminobenzenesulphonamides with aldehydes under optimized reaction conditions, respectively. The notable features of this method include broad functional group compatibility, low catalyst loading (1-5 mol %), mild reaction conditions, easy workup procedures, good to excellent reaction yields, ethanol as a green solvent, reusability of the catalyst (five cycles), and economic attractiveness, which is mainly due to sustainability of SRMIST-1 as a reusable green catalyst. Our findings demonstrate that the highly reactive and reusable green catalyst finds widespread applications in medicinal chemistry.

Cu(II)-Catalyzed Aminocyclization of N-Propargyl Hydrazones to Substituted Pyrazolines

An efficient route for the copper(II)-catalyzed synthesis of substituted pyrazolines from readily accessible N-propargyl hydrazones has been reported under open flask conditions via intramolecular C-N bond formation. N-acyl and N-tosyl-substituted pyrazolines have been prepared in moderate to excellent yields. Mechanistic investigations using NMR, high-resolution mass spectrometry (HRMS), and Hammett analyses suggest that the Cu(II) catalyst generally acts as a Lewis acid to form an iminium-ion intermediate via cyclization, which afforded the desired pyrazolines upon hydrolysis. One progesterone receptor antagonist has also been synthesized utilizing this reaction methodology.

Synthesis of 3-Azabicyclo[3.1.0]hexane Derivates

3-Azabicyclo[3.1.0]hexanes are an important class of nitrogen-containing heterocycles that have been found to be key structural features in a wide range of biologically active natural products, drugs, and agrochemicals. As a cutting-edge area, the synthesis of these derivatives has made spectacular progress in recent decades, with various transition-metal-catalyzed and transition-metal-free catalytic systems being developed. In this review, we provide an overview of recent advances in the efficient methods for the synthesis of 3-azabicyclo[3.1.0]hexane derivatives since 2010, emphasizing the scope of substrates and synthesis' applications, as well as the mechanisms of these reactions.

Stereoretentive Regio- and Enantioselective Allylation of Isoxazolinones by a Planar Chiral Palladacycle Catalyst

The catalytic allylic substitution is one of the most important tools in asymmetric synthesis to form C-C bonds in an enantioselective way. While high efficiency was previously accomplished in terms of enantio- and regiocontrol using different catalyst types, a strong general limitation is a very pronounced preference for the formation of allylic substitution products with (E)-configured C=C double bonds. Herein, we report that with a planar chiral palladacycle catalyst a diastereospecific reaction outcome is achieved using isoxazolinones and allylic imidates as substrates, thus maintaining the C=C double bond geometry of the allylic substrates in the highly enantioenriched products. DFT calculations show that the reactions proceed via an S_N 2 mechanism and not via π-allyl Pd complexes. Crucial for the high control is the stabilization of the allylic fragment in the S_N 2 transition state by π-interactions with the phenyl substituents of the pentaphenylferrocenyl catalyst core.

Non-Decarboxylative Ruthenium-Catalyzed Rearrangement of 4-Alkylidene-isoxazol-5-ones to Pyrazole- and Isoxazole-4-carboxylic Acids

Treatment of 4-(2-hydroaminoalkylidenyl)- and 4-(2-hydroxyalkylidenyl)-substituted isoxazol-5(4H)-ones with catalytic amounts of [RuCl2(p-cymene)]2, without any additive, afforded pyrazole- and isoxazole-4-carboxylic acids, respectively. The presence of an intramolecular H-bond in these substrates was the key to divert the classical mechanism toward a ring-opening non-decarboxylative path that is expected to generate a vinyl Ru-nitrenoid intermediate, the cyclization of which affords the rearranged products. A gram scale protocol demonstrated the synthetic applicability of this transformation.

Polysubstituted Indole Synthesis via Palladium/Norbornene Cooperative Catalysis of Oxime Esters

Polysubstituted indoles are prevalent in pharmaceuticals, agrochemicals, and organic materials. Presented herein is the fact that polyfunctionalized indoles can be efficiently constructed from easily accessible oxime esters and aryl iodides, involving a palladium/norbornene synergistic synthesis. The reaction is enabled by a unique class of electrophiles in palladium/norbornene cooperative catalysis, which are oxime esters derived from simple ketone. The broad substrate scope and high functional group tolerance could make this method attractive for the synthesis of polysubstituted indoles.

Enantioenriched γ-Aminoalcohols, β-Amino Acids, β-Lactams, and Azetidines Featuring Tetrasubstituted Fluorinated Stereocenters via Palladacycle-Catalyzed Asymmetric Fluorination of Isoxazolinones

Enantiopure fluorine containing β-amino acids are of large biological and pharmaceutical interest. Strategies to prepare β-amino acid derivatives possessing a F-containing tetrasubstituted stereocenter at the α-C atom in a catalytic asymmetric sense are rare, in particular using an enantioselective electrophilic C-F bond formation. In the present study, a highly enantioselective palladacycle-catalyzed fluorination of isoxazolinones was developed. It is demonstrated that isoxazolinones are useful precursors toward enantiopure β-amino acid derivatives by diastereo- and chemoselective reduction. The formed γ-aminoalcohols served as valuable precursors toward β-amino acids, β-amino acid esters, and β-lactams, all featuring tetrasubstituted fluorinated stereocenters. In addition, by this work, enantioenriched fluorinated azetidines were accessible for the first time.

Mechanistic Studies of the Pd- and Pt-Catalyzed Selective Cyclization of Propargyl/Allenyl Complexes

Following the discovery of an unusual transition-metal-catalyzed reaction, the elucidation of the underlying mechanism is essential to understand the characteristic reactivity of the metal. We previously reported a synthetic method for tricyclic indoles using Pt-catalyzed Friedel-Crafts-type C-H coupling. In this reaction, the Pt catalyst selectively formed a seven-membered ring, but the Pd catalyst only afforded a six-membered ring. However, the reasons for the different selectivities caused by Pd and Pt were unclear. We performed density functional theory (DFT) calculations and experimental studies to reveal the origin of the different behaviors of the two metals. The calculations revealed that the formation of the six- and seven-membered rings proceeds via η¹-allenyl and η³-propargyl/allenyl complexes, respectively. A molecular orbital analysis of the η³-propargyl/allenyl complex revealed that, for the platinum complex, the energy required to convert the unoccupied molecular orbital on the reactive carbon into the lowest unoccupied molecular orbital (LUMO) was lower than that for the palladium complex. In addition, DFT calculations revealed that the combination of platinum and bis[2-(diphenylphosphino)phenyl] ether (DPEphos) reduced the activation energy of the seven-membered cyclization in comparison with palladium or PPh₃. Additional experimental studies, including NMR studies and stoichiometric reactions, support the aforementioned examination.

Highly Regioselective Tandem Reaction of Ene-Yne-Oxazolones Induced by H-Phosphonates: Construction of Phosphinylindane Derivatives

A highly regioselective divergent approach for the phosphine-containing indane/indene derivatives from the ene-yne-oxazolone precursors was reported. An insight into the reaction mechanism involving the phospha-1,4-addition followed by 5-exo-dig ring closure with a concomitant C-P/C-C bond formation was also proposed. This promising protocol utilized H-phosphonate as the phosphonating reagent in a silver-catalyzed or base-mediated cascade cyclization to construct the corresponding phosphorylated spiroindenoxazolones and amidoindenes, respectively, in good yields (up to 88% yield).

Nickel-Catalyzed Migratory Hydrocyanation of Internal Alkenes: Unexpected Diastereomeric-Ligand-Controlled Regiodivergence

A regiodivergent nickel-catalyzed hydrocyanation of a broad range of internal alkenes involving a chain-walking process is reported. When appropriate diastereomeric biaryl diphosphite ligands are applied, the same starting materials can be converted to either linear or branched nitriles with good yields and high regioselectivities. DFT calculations suggested that the catalyst architecture determines the regioselectivity by modulating electronic and steric interactions. In addition, moderate enantioselectivities were observed when branched nitriles were produced.

Theoretical study of rhodium- and cobalt-catalyzed decarboxylative transformations of isoxazolones: origin of product selectivity

DFT calculations were performed to elucidate the origins of catalyst-controlled product selectivity in the decarboxylative transformations of isoxazolones.

Copper(II)-Mediated Intramolecular Cyclopropanation of Ketene N,X-Acetals (X = S, O, N) under Mild Conditions

CuBr₂-mediated intramolecular oxidative cyclopropanation of α-oxo ketene N,S-acetals was efficiently achieved to afford 2-thioalkyl-3-azabicyclo-[3.1.0]hexenes under mild conditions. 2-Oxyalkyl- and 2-aminoallyl-3-azabicyclo-[3.1.0]hexenes were also obtained from the corresponding α-oxo ketene N,O- and N,N-acetals. Further C-S bond transformations led to diverse 2-aryl-substituted 3-azabicyclo[3.1.0]hexene derivatives in good to high yields.

Gold-Catalyzed Hydroamination of Propargylic Alcohols: Controlling Divergent Catalytic Reaction Pathways To Access 1,3-Amino Alcohols, 3-Hydroxyketones, or 3-Aminoketones

A versatile approach to the valorization of propargylic alcohols is reported, enabling controlled access to three different products from the same starting materials. First, a general method for the hydroamination of propargylic alcohols with anilines is described using gold catalysis to give 3-hydroxyimines with complete regioselectivity. These 3-hydroxyimines can be reduced to give 1,3-amino alcohols with high syn selectivity. Alternatively, by using a catalytic quantity of aniline, 3-hydroxyketones can be obtained in high yield directly from propargylic alcohols. Further manipulation of the reaction conditions enables the selective formation of 3-aminoketones via a rearrangement/hydroamination pathway. The utility of the new chemistry was exemplified by the one-pot synthesis of a selection of N-arylpyrrolidines and N-arylpiperidines. A mechanism for the hydroamination has been proposed on the basis of experimental studies and density functional theory calculations.

Divergent Conversion of N-Acyl-isoxazol-5(2 H)-ones to Oxazoles and 1,3-Oxazin-6-ones Using Photoredox Catalysis

The fragmentation of N-acyl-isoxazol-5-ones using visible light photoredox catalysis has been disclosed. The catalyst-controlled divergent mechanisms, namely the oxidative and reductive quenching catalytic cycle, are utilized. Various oxazoles and 1,3-oxazin-6-ones are selectively obtained from the same isoxazol-5-one skeleton under mild conditions.

Developments towards synthesis of N-heterocycles from amidines via C–N/C–C bond formation

This review focuses on the synthesis of N-heterocycles using amidines as starting materials, with an emphasis on the mechanisms of these reactions via C–N/C–C bond formation.

Three-component bis-heterocycliation for synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles

A cooperative base system has been developed for the novel three-component synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles via bis-heterocyclization of methylketoxime acetates.

Recent Progress on Cyclic Nitrenoid Precursors in Transition-Metal-Catalyzed Nitrene-Transfer Reactions

Nitrene-transfer reactions are powerful synthetic tools for the direct incorporation of nitrogen atoms into organic molecules. The discovery of novel nitrene-transfer reactions has been dominantly supported not only by improvements in transition-metal catalysts but also by the employment of novel precursors of nitrenoids. Since pioneering work involving the use of organic azides and iminoiodinanes as practical synthetic tools for nitrogen-containing compounds was reported, a new approach using various N-heterocycles containing strain energy or a weak bond has emerged. In this review, we briefly summarize the history of nitrene-transfer chemistry from the viewpoint of its precursors. In particular, the use of N-heterocycles such as 2H-azirines, 1,4,2-dioxazol-5-ones, 1,2,4-oxadiazol-5-ones, isoxazol-5(4H)-ones, and isoxazoles is comprehensively described, showing the recent remarkable progress in this chemistry.