Tetra-n-butylammonium Iodide Catalyzed C–H Azidation of Aldehydes with Thermally Stable Azidobenziodoxolone

SOMOphilic alkyne vs radical-polar crossover approaches: The full story of the azido-alkynylation of alkenes

We report the detailed background for the discovery and development of the synthesis of homopropargylic azides by the azido-alkynylation of alkenes. Initially, a strategy involving SOMOphilic alkynes was adopted, but only resulted in a 29% yield of the desired product. By switching to a radical-polar crossover approach and after optimization, a high yield (72%) of the homopropargylic azide was reached. Full insights are given about the factors that were essential for the success of the optimization process.

Reaction of Amide and Sodium Azide for the Synthesis of Acyl Azide, Urea, and Iminophosphorane

Amides reacted with NaN₃ to give the acyl azides in DMF at 25 °C and produce the symmetrical ureas in THF/H₂O at 80 °C via the sequential reaction of acyl substitution and Curtius rearrangement. All acyl azides were also obtained from the secondary amides via sequential reaction of p-toluenesulfonyl chloride and NaN₃. In addition, keto-stabilized iminophosphoranes were prepared from a one-pot reaction of amides, NaN₃, and phosphines.

Divergent functionalization of aldehydes photocatalyzed by neutral eosin Y with sulfone reagents

While aldehydes represent a classic class of electrophilic synthons, the corresponding acyl radicals are inherently nucleophilic, which exhibits umpolung reactivity. Generation of acyl radicals typically requires noble metal catalysts or excess oxidants to be added. Herein, we report a convenient and green approach to access acyl radicals, capitalizing on neutral eosin Y-enabled hydrogen atom transfer (HAT) photocatalysis with aldehydes. The generated acyl radicals underwent SOMOphilic substitutions with various functionalized sulfones (X-SO2R') to deliver value-added acyl products. The merger of eosin Y photocatalysis and sulfone-based SOMOphiles provides a versatile platform for a wide array of aldehydic C-H functionalizations, including fluoromethylthiolation, arylthiolation, alkynylation, alkenylation and azidation. The present protocol features green characteristics, such as being free of metals, harmful oxidants and additives; step-economic; redox-neutral; and amenable to scale-up assisted by continuous-flow technology.

Catalyst-free synthesis of substituted pyridin-2-yl, quinolin-2-yl, and isoquinolin-1-yl carbamates from the corresponding hetaryl ureas and alcohols

A novel catalyst-free synthesis of N-pyridin-2-yl, N-quinolin-2-yl, and N-isoquinolin-1-yl carbamates utilizes easily accessible N-hetaryl ureas and alcohols. The proposed environmentally friendly technique is suitable for the good-to-high yielding synthesis of a wide range of N-pyridin-2-yl or N-quinolin-2-yl substituted carbamates featuring electron-donating and electron-withdrawing groups in the azine rings and containing various primary, secondary, and even tertiary alkyl substituents at the oxygen atom (48-94%; 31 examples). The DFT calculation and experimental study showed that the reaction proceeds through the intermediate formation of hetaryl isocyanates. The method can be applied to obtain N-isoquinolin-1-yl carbamates, although in lower yields, and ethyl benzo[h]quinolin-2-yl carbamate has also been successfully synthesized (68%).

Copper-Catalyzed Coupling of Amines with Carbazates: An Approach to Carbamates

A new approach for the preparation of carbamates via the copper-catalyzed cross-coupling reaction of amines with alkoxycarbonyl radicals generated from carbazates is described. This environmentally friendly protocol takes place under mild conditions and is compatible with a wide range of amines, including aromatic/aliphatic and primary/secondary substrates.

Divergent functionalization of α,β-enones: catalyst-free access to β-azido ketones and β-amino α-diazo ketones

A simple and practical method for the azidation of β-fluoroalkyl α,β-unsaturated ketones to access a wide variety of fluorinated nitrogenous carbonyl compounds is developed. Different from existing precedents, neither a metallic nor an organic catalyst was involved in our strategy. Judicious choice of solvents allows for the modulation of the reaction outcomes, delivering β-azido ketones or β-amino α-diazo ketones. The reaction system features environmental friendliness, mild conditions, simplicity and excellent functional group tolerance.

Palladium(II)-Catalyzed Enantioselective Azidation of Unactivated Alkenes

The first Pd-catalyzed enantioselective azidation of unactivated alkenes has been established by using readily accessible 1-azido-1,2-benziodoxol-3(1H)-one (ABX) as an azidating reagent, which affords a wide variety of structurally diverse 3-N₃ -substituted piperidines in good yields with excellent enantioselectivity. The reaction features good functional-group compatibility and mild reaction conditions. Notably, both an electrophilic azidating reagent and the sterically bulky chiral pyridinyl-oxazoline (Pyox) ligand are crucial to the successful reaction.

Dirhodium(II)-Catalyzed C(sp2 )-H Azidation of Benzaldehydes

Multiple steps are needed to achieve the C-H functional of aromatic aldehyde, since the C-H functional reaction usually occurs preferentially at the aldehydic C-H bond over the aryl C-H bond. We report an efficient azidation method mediated by dirhodium(II) catalysts to achieve the direct aryl azidation of aromatic aldehydes avoiding the simultaneous use of protected aldehydes and prefunctionalized arenes. The regioselectivity of this method is similar to those of typical aromatic electrophilic substitution reactions. The resulting azidobenzaldehyde products are versatile building blocks or precursors for the synthesis of many biologically active compounds. The mechanism studies indicate that the one-electron oxidative intermediate Rh₂ ^(II,III) N₃ is responsible for the azide transfer.

Chiral Derivatives of 2-Aminotribenzotriquinacene: Synthesis and Optical Resolution

Starting from a hitherto unknown 2-aminotribenzotriquinacene, several 2-amino-3-X-substituted TBTQ derivatives, all bearing a single ortho-difunctionalized indane wing, were synthesized as rigid and chiral building blocks for the potential construction of complex supramolecular architectures. Efficient access to two pairs of enantiomeric TBTQ derivatives, namely, the peripheral ortho-nitroaniline (X = NO₂) and the related anthranilic acid (X = CO₂H), was developed using chiral auxiliaries as the resolving reagents. The structure of the intermediate diastereomers was confirmed by ¹H and ¹³C NMR spectroscopy, high-resolution mass spectroscopy (HRMS), and polarimetry. The absolute configuration of the optically active derivatives was confirmed by quantum chemical time-dependent density functional theory (TD-DFT) calculations of the theoretical electronic circular dichroism (ECD) spectra and by single-crystal X-ray structure analysis of a synthesis intermediate.

Methods for direct C(sp2)–H bonds azidation

Direct functionalization of C–H bonds has attracted great attention in recent years from the perspectives of atom and step economy.

Preparation, structure, and reactivity of bicyclic benziodazole: a new hypervalent iodine heterocycle

A new bicyclic organohypervalent iodine heterocycle derivative of benziodazole was prepared by oxidation of 2-iodo-N,N’-diisopropylisophthalamide with m-chloroperoxybenzoic acid under mild conditions. Single crystal X-ray crystallography of this compound revealed a five-membered bis-heterocyclic structure with two covalent bonds between the iodine atom and the nitrogen atoms. This novel benziodazole is a very stable compound with good solubility in common organic solvents. This compound can be used as an efficient reagent for oxidatively assisted coupling of carboxylic acids with alcohols or amines to afford the corresponding esters or amides in moderate yields.

The Curtius rearrangement: mechanistic insight and recent applications in natural product syntheses

The Curtius rearrangement is a versatile reaction in which a carboxylic acid can be converted to an isocyanate through an acyl azide intermediate under mild conditions. The resulting stable isocyanate can then be readily transformed into a variety of amines and amine derivatives including urethanes and ureas. There have been wide-ranging applications of the Curtius rearrangement in the synthesis of natural products and their derivatives. Also, this reaction has been extensively utilized in the synthesis and application of a variety of biomolecules. In this review, we present mechanistic studies, chemical methodologies and reagents for the synthesis of isocyanates from carboxylic acids, the conversion of isocyanates to amines and amine derivatives, and their applications in the synthesis of bioactive natural products and their congeners.

Sulfoximidoyl-Containing Hypervalent Iodine(III) Reagents: 1-Sulfoximidoyl-1,2-benziodoxoles

Treatment of NH-sulfoximines with pseudocyclic benziodoxole triflate results in N-I bond formation leading to a new type of sulfoximidoyl-containing hypervalent iodine(III) reagents in high yields. Degradation studies revealed a pronounced stability in both the solid state and in solution.

A Chemoselective N-Arylation Reaction of 2-Aminopyridine Derivatives with Arynes

A chemoselective N-arylation reaction of 2-aminopyridine derivatives with arynes in good to excellent yields has been described. The N-arylation products could be further applied to the facile construction of benzoisoquinuclidines and isoquinuclidines as well as pyrido[1,2-a]benzimidazoles.

Iodine(III) Reagents in Radical Chemistry

The chemistry of hypervalent iodine(III) compounds has gained great interest over the past 30 years. Hypervalent iodine(III) compounds show valuable ionic reactivity due to their high electrophilicity but also express radical reactivity as single electron oxidants for carbon and heteroatom radical generation. Looking at ionic chemistry, these iodine(III) reagents can act as electrophiles to efficiently construct C–CF₃, X–CF₃ (X = heteroatom), C–R_f (R_f = perfluoroalkyl), X–R_f, C–N₃, C–CN, S–CN, and C–X bonds. In some cases, a Lewis or a Bronsted acid is necessary to increase their electrophilicity. In these transformations, the iodine(III) compounds react as formal “CF₃⁺”, “R_f⁺”, “N₃⁺”, “Ar⁺”, “CN⁺”, and “X⁺” equivalents. On the other hand, one electron reduction of the I(III) reagents opens the door to the radical world, which is the topic of this Account that focuses on radical reactivity of hypervalent iodine(III) compounds such as the Togni reagent, Zhdankin reagent, diaryliodonium salts, aryliodonium ylides, aryl(cyano)iodonium triflates, and aryl(perfluoroalkyl)iodonium triflates. Radical generation starting with I(III) reagents can also occur via thermal or light mediated homolysis of the weak hypervalent bond in such reagents. This reactivity can be used for alkane C–H functionalization. We will address important pioneering work in the area but will mainly focus on studies that have been conducted by our group over the last 5 years.

We entered the field by investigating transition metal free single electron reduction of Togni type reagents using the readily available sodium 2,2,6,6-tetramethylpiperidine-1-oxyl salt (TEMPONa) as an organic one electron reductant for clean generation of the trifluoromethyl radical and perfluoroalkyl radicals. That valuable approach was later successfully also applied to the generation of azidyl and aryl radicals starting with the corresponding benziodoxole (Zhdankin reagent) and iodonium salts. In the presence of alkenes as radical acceptors, vicinal trifluoromethyl-, azido-, and arylaminoxylation products result via a sequence comprising radical addition to the alkene and subsequent TEMPO trapping. Electron-rich arenes also react with I(III) reagents via single electron transfer (SET) to give arene radical cations, which can then engage in arylation reactions. We also recognized that the isonitrile functionality in aryl isonitriles is a highly efficient perfluoroalkyl radical acceptor, and reaction of R_f-benziodoxoles (Togni type reagents) in the presence of a radical initiator provides various perfluoroalkylated N-heterocycles (indoles, phenanthridines, quinolines, etc.). We further found that aryliodonium ylides, previously used as carbene precursors in metal-mediated cyclopropanation reactions, react via SET reduction with TEMPONa to the corresponding aryl radicals. As a drawback of all these transformations, we realized that only one ligand of the iodine(III) reagent gets transferred to the substrate. To further increase atom-economy of such conversions, we identified cyano or perfluoroalkyl iodonium triflate salts as valuable reagents for stereoselective vicinal alkyne difunctionalization, where two ligands from the I(III) reagent are sequentially transferred to an alkyne acceptor.

Finally, we will discuss alkynyl-benziodoxoles as radical acceptors for alkynylation reactions. Similar reactivity was found for the Zhdankin reagent that has been successfully applied to azidation of C-radicals, and also cyanation is possible with a cyano I(III) reagent. To summarize, this Account focuses on the design, development, mechanistic understanding, and synthetic application of hypervalent iodine(III) reagents in radical chemistry.

Iridium(iii) catalyzed regioselective amidation of indoles at the C4-position using weak coordinating groups

The C4-aminated indole scaffold is frequently encountered in several natural products and biologically active compounds.

Green and practical transition metal-free one-pot conversion of substituted benzoic acids to anilines using tosyl azide

A simple and efficient method for conversion of 2-iodo/2-nitro benzoic acids and dihydropyranone-fused benzoic acids into corresponding anilines, using tosyl azide, under transition metal-free conditions was developed. Steric and electronic effects play crucial role.

Copper-catalyzed radical reactions of 2-azido-N-arylacrylamides with 1-(trifluoromethyl)-1,2-benziodoxole and 1-azidyl-1,2-benziodoxole

The tandem addition/cyclization reactions of 2-azido-N-arylacrylamides with 1-(trifluoromethyl)-1,2-benziodoxole and 1-azidyl-1,2-benziodoxole-derived trifluoromethyl radicals and azidyl radicals were investigated.

Copper-catalyzed intermolecular chloroazidation of α,β-unsaturated amides

A copper catalyzed three component chloroazidation of α,β-unsaturated amides has been performed using azidoiodine(iii) reagent and SOCl₂ as azide and chlorine sources, respectively.