Enantioselective Copper-Catalyzed Electrophilic Dearomative Azidation of β-Naphthols

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

Practical and regioselective halonitrooxylation of olefins to access β-halonitrates

Organic nitrates, as effective donors of the signaling molecule nitric oxide, are widely applied in the pharmaceutical industry. However, practical and efficient methods for accessing organic nitrates are still scarce, and achieving high regiocontrol in unactivated alkene difunctionalization remains challenging. Here we present a simple and practical method for highly regioselective halonitrooxylation of unactivated alkenes. The approach utilizes TMSX (X: Cl, Br, or I) and oxybis(aryl-λ3-iodanediyl) dinitrates (OAIDN) as sources of halogen and nitrooxy groups, with 0.5 mol % FeCl3 as the catalyst. Remarkably, high regioselectivity in the halonitrooxylation of aromatic alkenes can be achieved even without any catalyst. This protocol features easy scalability and excellent functional group compatibility, providing a range of β-halonitrates (127 examples, up to 99% yield, up to >20:1 rr). Notably, 2-iodoethyl nitrate, a potent synthon derived from ethylene, reacts smoothly with a variety of functional units to incorporate the nitrooxy group into the desired molecules.

Enantioselective copper-catalyzed azidation/click cascade reaction for access to chiral 1,2,3-triazoles

Chiral 1,2,3-triazoles are highly attractive motifs in various fields. However, achieving catalytic asymmetric click reactions of azides and alkynes for chiral triazole synthesis remains a significant challenge, mainly due to the limited catalytic systems and substrate scope. Herein, we report an enantioselective azidation/click cascade reaction of N-propargyl-β-ketoamides with a readily available and potent azido transfer reagent via copper catalysis, which affords a variety of chiral 1,2,3-triazoles with up to 99% yield and 95% ee under mild conditions. Notably, chiral 1,5-disubstituted triazoles that have not been accessed by previous asymmetric click reactions are also prepared with good functional group tolerance.

Nitrogen atom insertion into arenols to access benzazepines

Advances in site-selective molecular editing have enabled structural modification on complex molecules. However, thus far, their applications have been restricted to C-H functionalization chemistry. The modification of the underlying molecular skeleton remains limited. Here, we describe a skeletal editing approach that provides access to benzazepine structures through direct nitrogen atom insertion into arenols. Using widely available arenols as benzazepine precursors, this alternative approach allowed the streamlined assembly of benzazepines with broad functional group tolerance. Experimental mechanistic studies support a reaction pathway involving dearomatizative azidation and then aryl migration. This study further highlights the potential for carbon-nitrogen transmutation sequences through combinations with oxidative carbon atom deletion, providing an alternative for the development of N-heteroarenes and demonstrating significant potential in materials chemistry.

Enantioselective catalytic radical decarbonylative azidation and cyanation of aldehydes

Empowered by the ubiquity of carbonyl functional groups in organic compounds, decarbonylative functionalization was prevalent in the construction of complex molecules. Under this context, asymmetric decarbonylative functionalization has emerged as an efficient pathway to accessing chiral motifs. However, ablation of enantiomeric control in a conventional 2e transition metal-catalyzed process was notable because of harsh conditions (high temperatures, etc.) that are usually required. To address this challenge and use readily accessible aldehyde directly, we report the asymmetric radical decarbonylative azidation and cyanation. Diverse aldehydes were directly used as alkyl radical precursor, engaging in the subsequent inner-sphere or outer-sphere ligand transfer where functional motifs (CN and N3) could be incorporated in excellent site- and enantioselectivity. Mild conditions, broad scope, excellent regioselectivity (driven by polarity-matching strategy), and enantioselectivity were shown for both transformations. This radical decarbonylative strategy using aldehydes as alkyl radical precursor has offered a powerful reaction manifold in asymmetric radical transformations to construct functional motifs regio- and stereoselectively.

Hypervalent Iodine(III) Reagents with Transferable Primary Amines: Structure and Reactivity on the Electrophilic α-Amination of Stabilized Enolates

A new family of hypervalent iodine reagents containing transferable primary amine groups is described. Benziodoxolone-based reagents were synthesized on the gram-scale through operationally simple reactions in up to quantitative yields. These bench-stable solids were characterized by X-ray analysis and successfully employed in the α-amination of indanone-based β-ketoesters in up to 83% yield. Mechanistic studies indicate a substitution mechanism involving an electrophilic amine.

Organocatalytic asymmetric azidation of sulfoxonium ylides: mild synthesis of enantioenriched α-azido ketones bearing a labile tertiary stereocenter

Disclosed here is a catalytic asymmetric azidation reaction for the efficient synthesis of α-azido ketones bearing a labile tertiary stereocenter. With a superb chiral squaramide catalyst, a mild asymmetric formal H–N₃ insertion of α-carbonyl sulfoxonium ylides proceeded with excellent efficiency and enantioselectivity. This organocatalytic process not only complements the previous α-azidation approaches for the formation of quaternary stereocenters and mostly for 1,3-dicarbonyl compounds, but also has advantages over the well-known metal-catalyzed asymmetric carbene insertion chemistry using α-diazocarbonyl compounds. Detailed mechanistic studies via control reactions and NMR studies provided important insights into the reaction pathway, which features reversible protonation and dynamic kinetic resolution. The curiosity in mechanism also led to the development of a simplified alternative protocol with a cheaper HN₃ source.

An organocatalytic asymmetric H–N₃ insertion of α-carbonyl sulfoxonium ylides has been developed, providing efficient access to α-azido ketones bearing labile tertiary stereocenters and complementing the metal carbene insertion chemistry.

Synthesis of amidines via iron-catalyzed dearomative amination of β-naphthols with oxadiazolones

An efficient and convenient method for the synthesis of amidines bearing a β-naphthalenone moiety catalyzed by cheap iron(ii) chloride is presented by employing oxadiazolones as the nitrene precursors.

Asymmetric catalytic nitrooxylation and azidation of β-keto amides/esters with hypervalent iodine reagents

Chiral Lewis acid-catalyzed enantioselective nitrooxylation and azidation of cyclic and acyclic β-keto amides/esters with hypervalent iodine(iii) reagents.

Electrophilic Aminating Agents in Total Synthesis

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C-N bond-forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

Cu-Catalyzed Chemodivergent, Stereoselective Propargylic Dearomatization and Etherification of 2-Naphthols

The first stereoselective propargylic dearomatization of 2-naphthol derivatives is reported using a chiral Cu^II-L10 complex. The reaction shows chemodivergent reactivity and produced propargyl dearomatization and etherification product for differently substituted 2-naphthols. Both the reactions generate the desired products in high yields with excellent chemo- and stereoselectivities (up to 99% ee, dr = 9:1) by using only 2 mol % catalyst loading. Dearomatization products contain a contiguous all-carbon quaternary-tertiary stereocenter and a terminal alkyne functionality.

Dearomatizing Amination Reactions

Dearomatization reactions allow the direct synthesis of structurally complex sp³ -rich molecules from readily available "flat" precursors. Established dearomatization processes commonly involve the formation of new C-C bonds, whereas methods that enable the introduction of C-N bonds have received less attention. Because of the privileged position of nitrogen in drug discovery, significant recent methodological efforts have been directed towards addressing this deficiency. Consequently, a variety of new processes are now available that allow the direct preparation of sp³ -rich amino-containing building blocks and scaffolds. This review gives an overview of C-N bond forming dearomatization reactions, particularly with respect to scaffold assembly processes. The discussion gives historical context, but the main focus is on selected methods that have been reported recently.

Manganese-catalyzed Electro-oxidative Azidation-annulation Cascade to Access Oxindoles and Quinolinones

An environmentally benign and proficient electro-oxidative tandem azidation-radical cyclization strategy is reported. Manganese-catalyzed electrochemical reaction in an undivided cell at room temperature and the use of NaN₃ as the cheapest azide source are the key features of this protocol. Using this approach, a series of oxindole and quinolinone derivatives are synthesized in high yields. The synthesized azide functionality was efficiently converted to various valuable derivatives.

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.

Zinc-catalyzed asymmetric nitrooxylation of β-keto esters/amides with a benziodoxole-derived nitrooxy transfer reagent

Zinc-catalyzed asymmetric nitrooxylation to afford a series of α-nitrooxy β-keto esters/amides in high yields and with low to moderate enantioselectivities has been disclosed.

Copper and manganese co-mediated cascade aza-Michael addition/cyclization and azidation of 1,3-enynes: regioselective synthesis of fully substituted azido pyrroles

A Cu and Mn co-mediated aerobic oxidative cyclization and azidation reaction of 1,3-enynes with amines and trimethylsilyl azide has been developed to synthesize fully substituted azido pyrroles.

Direct enantioselective Mannich reactions of α-azido cyclic ketones: asymmetric construction of chiral azides possessing an α-quaternary stereocenter

Direct enantioselective Mannich reactions of α-azido cyclic ketones with aldimines are realized through chiral phosphoric acid catalysis, which generate chiral azides possessing an α-quanternary stereocenter with complete regioselectivities and high diastereoselectivities and enantioselectivities.