Ionic Diamine Rhodium Complex Catalyzed Reductive N-Heterocyclization of 2-Nitrovinylarenes

Intramolecular CH-Hydrogen Bonding During the Dissociation of the Oxaphosphetane Intermediate Facilitates Z/E-Selectivity in Wittig Olefination

Herein, DFT studies corroborating experimental results revealed that the shortest intramolecular hydrogen bonding distance of cis/trans-oxaphosphetane (OPA) oxygen with the CH-hydrogen of a triphenylphosphine phenyl ring provides good evidence for the attained olefin Z/E-selectivity in Wittig olefination of the studied examples. 2-Nitrobenzaldehyde, 3-nitrobenzaldehyde, 2-nitro-3-bromobenzaldehyde, 2-nitro-5-bromobenzaldehyde and 2-nitro-5-arylbenzaldehydes provided Z-nitrostilbenes with (2-chloro-4-hydroxy-3-methoxy-5-(methoxycarbonyl)benzyl) triphenylphosphonium chloride as the major products. However, 4-nitrobenzaldehyde and 2-nitro-6-bromobenzaldehydes furnished E-nitrostilbenes as the major products in high yields. Furthermore, the DFT computed intramolecular CH1/CH2-hydrogen bond distances with Cl/NO2 of selected stilbene derivatives were in good agreement with intramolecular hydrogen bond distances measured from single-crystal X-ray diffraction measurements.

The Development and Mechanistic Study of an Iron-Catalyzed Intramolecular Nitroso Ene Reaction of Nitroarenes

An intramolecular iron-catalyzed nitroso ene reaction was developed to afford six- or seven-membered N-heterocycles from nitroarenes using an earth abundant iron catalyst and phenylsilane as the terminal reductant. The reaction can be triggered using as little as 3 mol % of iron(II) acetate and 3 mol % of 4,7-dimethoxyphenanthroline as the ligand. The scope of the reaction is broad tolerating a range of electron-releasing or electron-withdrawing substituents on the nitroarene, and the ortho-substituent can be modified to diastereoselectively construct benzoxazines, dihydrobenzothiazines, tetrahydroquinolines, tetrahydroquinoxalines, or tetrahydrobenzooxazepines. Mechanistic investigations indicated that the reaction proceeds via a nitrosoarene intermediate, and kinetic analysis of the reaction revealed a first-order rate dependence in catalyst-, nitroarene-, and silane concentration, and an inverse kinetic order in acetate was observed. The difference in rates between PhSiH3 and PhSiD3 was found to be 1.50 ± 0.09, and investigation of the temperature dependence of the reaction rate revealed that the activation parameters to be ΔH‡ = 13.5 kcal•mol-1 and ΔS‡ = -39.1 cal•mol-1•K-1. These data were interpreted to indicate that the turnover-limiting step to be hydride transfer from iron to the coordinated nitroarene, which occurs through an ordered transition state with little Fe-H bond breaking.

A switchable redox annulation of 2-nitroarylethanols affording N-heterocycles: photoexcited nitro as a multifunctional handle

The efficient transformation of nitroaromatics to functional molecules such as N-heterocycles has been an attractive and significant topic in synthesis chemistry. Herein, a photoexcited nitro-induced strategy for switchable annulations of 2-nitroarylethanols was developed to construct N-heterocycles including indoles, N-hydroxyl oxindoles and N-H oxindoles. The metal- and photocatalyst-free reaction proceeds through intramolecular redox C-N coupling of branched hydroxyalkyl and nitro units, which is initiated by a double hydrogen atom abstraction (d-HAA) process. The key to the switchable reaction outcomes is the mediation of a diboron reagent by its favorable oxy-transfer reactivity to in situ generated nitroso species. The utility of this protocol was well demonstrated by broad substrate scope, excellent yields, functional group tolerance and wide applications. Finally, detailed mechanistic studies were performed, and kinetic isotope effect (KIE) experiments indicate that the homolysis of the C-H bond is involved in the rate-determining step.

Annulation strategies for diverse heterocycles via the reductive transformation of 2-nitrostyrenes

Reduction of the stable nitro group is a fundamental and widely used transformation for the construction of complex and functionalized heterocyclic architectures. The unfolding of the reactivity of the nitro group in the 2-nitrostyrene moiety not only triggers the formation of carbon-nitrogen bonds, but also offers the opportunity for annulation and heteroannulation, thereby providing a cascade process for the synthesis of highly conjugated natural and unnatural molecules. In this review, we comprehensively discuss the use of 2-nitrostyrene motifs in the synthesis of various N-heterocycles. We offer readers an overview of the synthetic achievements achieved to date, highlighting their important features, reactivities, and mechanisms.

Palladium-Catalyzed Intramolecular Heck/Aminocarbonylation of Alkene-Tethered Iodobenzenes with Nitro Compounds: Synthesis of Carbamoyl-Substituted Benzoheterocycles

A palladium-catalyzed intramolecular Heck/aminocarbonylation of alkene-tethered iodobenzenes with nitro compounds has been developed for the synthesis of carbamoyl-substituted benzoheterocycles. Using Mo(CO)₆ as a solid CO source, no external reductant or additives were needed in this procedure. Both nitroarenes and nitroalkanes were well tolerated. A range of carbamoyl-substituted dihydrobenzofurans and indolines were prepared in moderate to high yields.

Synthesis of Cyclic Peptides in SPPS with Npb-OH Photolabile Protecting Group

Significant efforts have been made in recent years to identify more environmentally benign and safe alternatives to side-chain protection and deprotection in solid-phase peptide synthesis (SPPS). Several protecting groups have been endorsed as suitable candidates, but finding a greener protecting group in SPPS has been challenging. Here, based on the 2-(o-nitrophenyl) propan-1-ol (Npp-OH) photolabile protecting group, a structural modification was carried out to synthesize a series of derivatives. Through experimental verification, we found that 3-(o-Nitrophenyl) butan-2-ol (Npb-OH) had a high photo-release rate, high tolerance to the key conditions of Fmoc-SPPS (20% piperidine DMF alkaline solution, and pure TFA acidic solution), and applicability as a carboxyl-protective group in aliphatic and aromatic carboxyl groups. Finally, Npb-OH was successfully applied to the synthesis of head–tail cyclic peptides and side-chain–tail cyclic peptides. Moreover, we found that Npb-OH could effectively resist diketopiperazines (DKP). The α-H of Npb-OH was found to be necessary for its photosensitivity in comparison to 3-(o-Nitrophenyl)but-3-en-2-ol (Npbe-OH) during photolysis-rate verification.

Synthesis of Indoles by Palladium-Catalyzed Reductive Cyclization of β-Nitrostyrenes with Phenyl Formate as a CO Surrogate

The reductive cyclization of suitably substituted organic nitro compounds by carbon monoxide is a very appealing technique for the synthesis of heterocycles because of its atom efficiency and easiness of separation of the only stoichiometric byproduct CO2, but the need for pressurized CO has hampered its diffusion. We have recently reported on the synthesis of indoles by reductive cyclization of o-nitrostyrenes using phenyl formate as a CO surrogate, using a palladium/1,10-phenanthroline complex as catalyst. However, depending on the desired substituents on the structure, the use of β-nitrostyrenes as alternative reagents may be advantageous. We report here the results of our study on the possibility to use phenyl formate as a CO surrogate in the synthesis of indoles by reductive cyclization of β-nitrostyrenes, using PdCl2(CH3CN)2 + phenanthroline as the catalyst. It turned out that good results can be obtained when the starting nitrostyrene bears an aryl substituent in the alpha position. However, when no such substituent is present, only fair yield of indole can be obtained because the base required to decompose the formate also catalyzes an oligo-polymerization of the starting styrene. The reaction can be performed in a single glass pressure tube, a cheap and easily available piece of equipment.

Counterion Control of t-BuO-Mediated Single Electron Transfer to Nitrostilbenes to Construct N-Hydroxyindoles or Oxindoles

tert-Butoxide unlocks new reactivity patterns embedded in nitroarenes. Exposure of nitrostilbenes to sodium tert-butoxide was found to produce N-hydroxyindoles at room temperature without an additive. Changing the counterion to potassium changed the reaction outcome to yield solely oxindoles through an unprecedented dioxygen-transfer reaction followed by a 1,2-phenyl migration. Mechanistic experiments established that these reactions proceed via radical intermediates and suggest that counterion coordination controls whether an oxindole or N-hydroxyindole product is formed.

Reductive Cleavage of Unactivated Carbon-Cyano Bonds under Ammonia-Free Birch Conditions

A general protocol for the reductive cleavage of unactivated carbon-cyano bonds in aliphatic nitriles has been achieved under single-electron-transfer conditions using Na/15-crown-5/H₂O. Electron is supplied by the electride derived from bench-stable sodium dispersions and recoverable 15-crown-5. H₂O provides the proton source and suppresses the reduction of aromatic moieties. Compared with the Na/NH₃ electride system generated under traditional Birch conditions, this ammonia-free electride system is more practical and features better reactivity and chemoselectivity for the decyanations of a broad range of aliphatic nitriles.

Intramolecular Reductive Cyclization of o-Nitroarenes via Biradical Recombination

A visible-light-induced/thiourea-mediated intramolecular cyclization of o-nitroarenes under mild conditions is realized for the first time, which provides an efficient and environmentally friendly way to access pharmaceutical relevant quinazolinone derivatives. The reaction can be easily extended to gram level by using a continuous-flow setup with high efficiency. Mechanistic investigation including control experiments, transient fluorescence, UV-vis spectra, and DFT calculations suggests that the formation of active biradical intermediates via intramolecular single electron transfer (SET) is key stage in the catalytic cycle.

Regiocontrolled Synthesis of α-Sulfonylmethyl o-Nitrostyrenes via ZnI2-Mediated Sulfonylation and AgNO2/Pd(PPh3)4-Promoted o-Nitration

We report herein the AgNO₂/Pd(PPh₃)₄-promoted regiocontrolled o-nitration of α-sulfonylmethylstyrenes in MeNO₂ with good yields. The o-nitration process provides a series of sulfonyl o-nitrostyrenes. Substituted α-sulfonylmethylstyrenes were synthesized from ZnI₂-mediated sulfonylation of substituted α-methylstyrenes and sodium sulfinates (RSO₂Na) in MeCN with good to excellent yields. The structures of the key products were confirmed by X-ray crystallography. A plausible mechanism has been proposed herein.

Homogeneous rhodium(i)-catalysis in de novo heterocycle syntheses

Rh(i)-catalysed reactions often employ mild reaction conditions and offer excellent functional group tolerance, making them ideal transformations for the preparation of complex molecules. This review surveys examples of these synthetically useful transformations as applied to the synthesis of various heterocycles.

An efficient and selective microwave-assisted Claisen-Schmidt reaction for the synthesis of functionalized benzalacetones

A simple and direct method for the Claisen-Schmidt reaction to prepare functionalized α,β-unsaturated ketones has been developed. Microwave irradiation of aldehydes with acetone produces benzalacetones selectively without self-condensation product in very short reaction times and good yields. Graphical AbstractGraphics for use in the Table of Contents.