Synthesis of 1,3-Cycloalkadienes from Cycloalkenes: Unprecedented Reactivity of Oxoammonium Salts

Access to cyclohexadiene and benzofuran derivatives via catalytic arene cyclopropanation of α-cyanodiazocarbonyl compounds

The arene cyclopropanation between diazo compounds and benzene is well known to produce a tautomeric mixture of norcaradiene and cycloheptatriene in favour of the latter species. Nevertheless, previous studies have suggested that the initially formed norcaradiene can be stabilized by a C-7 cyano group with prevention of its 6π-electrocyclic ring opening. According to this feature, a synthetic route to functionalized cyclohexadienes has been designed using α-cyanodiazoacetates and α-diazo-β-ketonitriles as the starting materials, respectively. The Rh2(esp)2-catalyzed arene cyclopropanation of α-cyanodiazoacetates in benzene afforded the expected 7-alkoxycarbonyl-7-cyanonorcaradienes as isolable compounds, which then served as templates for the second cyclopropanation with ethyl diazoacetate or α-cyanodiazocarbonyls to enable the formation of bis(cyclopropanated) adducts. Their subsequent treatment with SmI2 triggered a double ring-opening process, allowing for the generation of 1,4- and/or 1,3-cyclohexadienes as either regio- or diastereomeric mixtures. On the other hand, the norcaradienes generated from phenyl- or methyl-substituted α-diazo-β-ketonitriles were found to undergo an in situ rearrangement to yield dihydrobenzofurans that could be converted to benzofuran derivatives by DDQ oxidation.

Asymmetric Synthesis of Nidulalin A and Nidulaxanthone A: Selective Carbonyl Desaturation Using an Oxoammonium Salt

Nidulaxanthone A is a dimeric, dihydroxanthone natural product that was isolated in 2020 from Aspergillus sp. Structurally, the compound features an unprecedented heptacyclic 6/6/6/6/6/6/6 ring system which is unusual for natural xanthone dimers. Biosynthetically, nidulaxanthone A originates from the monomer nidulalin A via stereoselective Diels-Alder dimerization. To expedite the synthesis of nidulalin A and study the proposed dimerization, we developed methodology involving the use of allyl triflate for chromone ester activation, followed by vinylogous addition, to rapidly forge the nidulalin A scaffold in a four-step sequence which also features ketone desaturation using Bobbitt's oxoammonium salt. An asymmetric synthesis of nidulalin A was achieved using acylative kinetic resolution (AKR) of chiral, racemic 2H-nidulalin A. Dimerization of enantioenriched nidulalin A to nidulaxanthone A was achieved using solvent-free, thermolytic conditions. Computational studies have been conducted to probe both the oxoammonium-mediated desaturation and (4 + 2) dimerization events.

Comprehensive Structural and Electronic Properties of 2-Azaadamantane N-Oxyl Derivatives Correlated with Their Catalytic Ability

Herein, a comprehensive kinetic study is performed to compare the catalytic efficiency of 2-azaadamantane N-oxyl (AZADO) derivatives with that of 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) used as radical catalysts in the aerobic oxidation of l-menthol. Furthermore, the correlation between the catalytic activity and structural/electronic parameters of AZADOs and TEMPO is elucidated. The reaction rate constants achieved with several AZADO derivatives exhibit moderate relationships with spectroscopic parameters, such as the hyperfine coupling constant of the N atom (AN) and NO stretching vibration frequency (νNO) observed in electron spin resonance and infrared spectra, respectively. The planarity C-(NO)-C angle (φ) at the N atom, determined by density functional theory (DFT) calculations, also strongly correlates with the AN and νNO. Moreover, the bond order of NO, which strongly depends on the structural and electronic properties of NO radicals, correlates with radical activity; thus, the radical activity can be predicted by DFT calculations, thereby accelerating the synthesis of new AZADO derivatives without requiring alcohol oxidation experiments.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Reactivity regulation for olefin metathesis-catalyzing ruthenium complexes with sulfur atoms at the terminal of 2-alkoxybenzylidene ligands

For regulating the olefin metathesis (OM) activity of the Hoveyda-Grubbs second-generation complex (HG-II), the structural modification of the benzylidene ligand is a useful strategy. This paper reports the effect of a chalcogen atom placed at the end of the benzylidene group on the catalytic properties of HG-II derivatives, using complexes with a thioether or ether component in the benzylidene ligand (ortho-Me-E-(CH₂)₂O-styrene; E = S, O). Nuclear magnetic resonance and X-ray crystallographic analyses of the complex with a thioether moiety (E = S) proved the (O,S)-bidentate and trans-dichlorido coordination for the complex. A stoichiometric ligand exchange between HG-II and the benzylidene ligand (E = S) produced the corresponding complex with an 86% yield, confirming higher stability of the complex (E = S) than that of HG-II. Despite the bidentate chelation, the complex (E = S) exhibited OM catalytic activity, indicating the exchangeability of the S-chelating ligand with an olefinic substrate. The green solution color, a characteristic of HG-II derivatives, was retained after the complex (E = S)-mediated OM reactions, indicating high catalyst durability. Conversely, the complex (E = O) rapidly initiated OM reactions; however, it showed low catalyst durability. In the OM reactions conducted in the presence of methanol, the complex (E = S) exhibited higher yields than the complex (E = O) and HG-II: the S-coordination increased the catalyst tolerance to methanol. A coordinative atom (such as sulfur) placed at the terminal of the benzylidene ligand can precisely regulate the reactivity of HG-II derivatives.

Sequential click modification of a lithium-ion endohedral fullerene connecting small molecules through a dieneazide linker

A versatile method for the chemical modification of a lithium-ion endohedral fullerene (Li⁺@C₆₀) to connect various small molecules is described. The designed dieneazide linker enables the facile connection of Li⁺@C₆₀ with small molecules bearing a terminal alkyne via Huisgen annulation and a subsequent Diels-Alder reaction. This strategy significantly expands the diversity of small molecules to be attached by Li⁺@C₆₀.

Chemoselective Oxidation of Alcohols in the Presence of Amines Using an Oxoammonium Salt

The oxidation of alcohols in the presence of reactive amines employing the commercially available oxoammonium cation, "Bobbitt's salt" is described. The oxidation is accomplished under acidic conditions and subsequent treatment with a suitable base affords a convenient one-pot method to access imines in good to excellent isolated yields (74-99%).

Convergent and Scalable Synthesis of the ABCDE-Ring Fragment of Caribbean Ciguatoxin C-CTX-1

Convergent and scalable synthesis of the ABCDE-ring fragment of Caribbean ciguatoxin C-CTX-1, the major causative toxin for ciguatera poisoning in the Caribbean Sea and the Northeast Atlantic areas, is described in detail. The key features of the synthesis include an iterative use of 2,2,6,6-tetramethyl piperidine 1-oxyl (TEMPO)/PhI(OAc)₂-mediated oxidative lactonization and Suzuki-Miyaura coupling en route to the DE-ring system and a convergent fragment coupling to form the ABCDE-ring skeleton via the Suzuki-Miyaura coupling strategy.

Oxoammonium Salt-Promoted Multifunctionalization of Saturated Cyclic Amines Based On β-Oxo Cyclic Iminium Ion Intermediates

Herein we describe a convenient method for multiple C(sp³)-H bond functionalization of saturated cyclic amines through oxoammonium salt-promoted oxidation to afford a β-oxo cyclic iminium ion as a key intermediate, followed by cascade addition with thiocyanate and diverse N-, O-, and S-containing nucleophiles in the green solvent and EtOH. Notably, chiral spiro azapolyheterocycles were prepared enantioselectively (>20:1 dr, up to 99% ee) when cysteine or serine esters were used as substrates. Moreover, the concise late-stage modification of several natural product derivatives was accomplished using this method.

TEMPO as a Hydrogen Atom Transfer Catalyst for Aerobic Dehydrogenation of Activated Alkanes to Alkenes

2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO) has been extensively utilized as a radical scavenger or an oxidation catalyst. In contrast, TEMPO as a hydrogen atom transfer (HAT) catalyst has rarely been studied. Here, we report that TEMPO, as the HAT catalyst, homolytically cleaves benzylic or allylic C-H bonds to give the corresponding alkyl radicals. Benefiting from the dual roles played by TEMPO as the HAT catalyst and the radical scavenger, the highly challenging aerobic dehydrogenation of activated alkanes to alkenes is successfully developed.

Hydrohalogenation of Unactivated Alkenes Using a Methanesulfonic Acid/Halide Salt Combination

The hydrochlorination, hydrobromination, and hydroiodination of unactivated alkenes using methanesulfonic acid and inorganic halide salts (CaCl2, LiBr, LiI) in acetic acid are reported. This approach uses readily available and inexpensive reagents to provide the alkyl halides in up to 99% yield. An example of deuteriochlorination using deuterated acetic acid as the solvent is also demonstrated.

Stereoselective Synthesis of cis-2-Ene-1,4-diones via Aerobic Oxidation of Substituted Furans Catalyzed by ABNO/HNO3

We report a highly efficient and selective catalytic system, ABNO (9-azabicyclo-[3.3.1]nonane N-oxyl)/HNO₃, for the aerobic oxidation of substituted furans to cis-2-ene-1,4-diones under mild reaction conditions using oxygen as the oxidant. The catalyst system is amenable to various substituted (mon-, di-, and tri-) furans and tolerates diverse functional groups, including cyano, nitro, naphthyl, ketone, ester, heterocycle, and even formyl groups. Based on the control and ¹⁸O-labeling experiments, the possible mechanism of the oxidation is proposed.

Dehydrogenative Aza-[4 + 2] Cycloaddition of Amines with 1,3-Dienes via Dual Catalysis

We describe a synergistic utilization of copper catalysis and proton-transfer catalysis that enables an atom- and step-economical aza-[4 + 2] cycloaddition reaction of readily available N-arylamino carbonyl compounds with simple 1,3-dienes. The reaction proceeds smoothly under an air atmosphere and produces water as the sole side product. Whereas the amines can directly serve as the C- and N-atom donors, this operationally simple protocol provides green, rapid, and efficient access to 1,2,3,6-tetrahydropyridines with a broad scope.

Tandem elimination-oxidation of tertiary benzylic alcohols with an oxoammonium salt

Tertiary benzylic alcohols react with oxoammonium salts, undergoing a tandem elimination/allylic oxidation to provide an allylic ether product in a single step. This mode of reactivity provides a rapid entry into allylic ethers from certain benzylic tertiary alcohols. The allylic ether may be cleaved under reductive conditions to reveal the allylic alcohol.

CuCl/TMEDA/nor-AZADO-catalyzed aerobic oxidative acylation of amides with alcohols to produce imides

Although aerobic oxidative acylation of amides with alcohols would be a good complement to classical synthetic methods for imides (e.g., acylation of amides with activated forms of carboxylic acids), to date, there have been no reports on oxidative acylation to produce imides. In this study, we successfully developed, for the first time, an efficient method for the synthesis of imides through aerobic oxidative acylation of amides with alcohols by employing a CuCl/TMEDA/nor-AZADO catalyst system (TMEDA = teramethylethylendiamine; nor-AZADO = 9-azanoradamantane N-oxyl). The proposed acylation proceeds through the following sequential reactions: aerobic oxidation of alcohols to aldehydes, nucleophilic addition of amides to the aldehydes to form hemiamidal intermediates, and aerobic oxidation of the hemiamidal intermediates to give the corresponding imides. This catalytic system utilizes O2 as the terminal oxidant and produces water as the sole by-product. An important point for realizing this efficient acylation system is the utilization of a TMEDA ligand, which, to the best of our knowledge, has not been employed in previously reported Cu/ligand/N-oxyl systems. Based on experimental evidence, we consider that plausible roles of TMEDA involve the promotion of both hemiamidal oxidation and regeneration of an active CuII-OH species from a CuI species. Here promotion of hemiamidal oxidation is particularly important. Employing the proposed system, various types of structurally diverse imides could be synthesized from various combinations of alcohols and amides, and gram-scale acylation was also successful. In addition, the proposed system was further applicable to the synthesis of α-ketocarbonyl compounds (i.e., α-ketoimides, α-ketoamides, and α-ketoesters) from 1,2-diols and nucleophiles (i.e., amides, amines, and alcohols).

IBX as a catalyst for dehydration of hydroperoxides: green entry to α,β-unsaturated ketones via oxygenative allylic transposition

A catalytic transformation of allylic hydroperoxides into α,β-unsaturated carbonyl compounds using IBX as a dehydration catalyst is described.

Arenophile-Mediated Dearomative Reduction

A dearomative reduction of simple arenes has been developed which employs a visible-light-mediated cycloaddition of arenes with an N-N-arenophile and in situ diimide reduction. Subsequent cycloreversion or fragmentation of the arenophile moiety affords 1,3-cyclohexadienes or 1,4-diaminocyclohex-2-enes, compounds that are not synthetically accessible using existing dearomatization reactions. Importantly, this strategy also provides numerous opportunities for further derivatization as well as site-selective functionalization of polynuclear arenes.