Access to β-Lactams via Iron-Catalyzed Olefin Oxyamidation Enabled by the π-Accepting Phthalocyanine Ligand

Mechanistic insights into photocatalytic nitrene transfer reactions of benzoyl azide in the presence of phosphoric acid

Computational studies were carried out to shed light on the mechanism of photocatalytic nitrene transfer reactions of benzoyl azide in the presence of phosphoric acid. The formed H-bonding complex of benzoyl azide with H3PO4 would readily undergo single electron reduction with the excited *Ru(II) photocatalyst, followed by sequential proton transfer and N2 dissociation to give a key N-centered radical. The energy transfer mechanistic pathway leading to a protonated nitrenium species, however, might not be feasible. The formed N-radical underwent different types of reactions with N-methylpyrrole and alkene, respectively, which was also rationalized.

4-Exo-Trig Radical Cyclization: A Promising Approach to Four-Membered Rings

The 4-exo-trig radical cyclization represents a formidable challenge owing to the unfavorable thermodynamics associated with the formation of highly strained four-membered rings. Stimulated by the explosive advancements of radical chemistry over the past decades, significant progresses have been made in this area, including the SmI2-mediated 4-exo-trig carbonyl-alkene cyclization, n-Bu3SnH-mediated 4-exo cyclization of functionalized alkenes or alkynes, transition metal-catalyzed 4-exo-trig ring closures, and visible light-induced 4-exo-trig cyclization cascade, providing efficient protocols to overcome the inherent limitations and expand the synthetic utility of this methodology. Representative examples, reaction mechanism, scope and limitations, and synthetic applications are presented and discussed in detail. We believe that the systematic review of recent advances on 4-exo-trig radical cyclization will provide more insights in this field, and may enable further development of this cyclization process for the concise synthesis of four-membered carbo- and heterocycles that are difficult to access via traditional methods.

Nitrene-mediated glycosylation with thioglycoside donors under metal catalysis

Glycosylation chemistry plays a pivotal role in glycoscience. Recent substantial developments have poised the field to address emerging challenges related to sustainability, cost efficiency, and robust applicability in complex substrate settings. The transition from stoichiometric activation to metal-catalyzed methods promises enhanced chemoselectivity and greater precision in controlling glycosidic bond breakage and formation, key to overcoming existing obstacles. Here, we report a nitrene-mediated glycosylation strategy using regular aryl sulfide glycosyl donors and easily accessible 3-methyl dioxazolone as an activator under the catalysis of iron or ruthenium. The iron-catalyzed system demonstrates exceptional catalytic reactivity, requiring as little as 0.1 mole % of catalyst at room temperature, and works well for complex peptide substrates. The ruthenium-catalyzed system can accommodate acid-sensitive functional groups and challenging low-reactivity acceptors. Mechanistic investigations have unveiled unusual multistep pathways involving sulfur imidation of sulfide donors via nitrene transfer and sulfur-to-oxygen rearrangement of N-acyl sulfilimines for the nitrene-mediated activation of sulfide donors.

Fe-Catalyzed α-C(sp3)-H Amination of N-Heterocycles

Nitrogen-heterocycles are privileged structures in both marketed drugs and natural products. On the other hand, C-H amination reactions furnish unconventional and straightforward approaches for the construction of C-N bonds. Yet, most of the known methods rely on precious metal catalysts. Herein we report a site-selective intermolecular C(sp3)-H amination of N-heterocycles, catalyzed by inexpensive FeCl2, which allows the functionalization of a wide range of pharmaceutically relevant cyclic amines. The C-H amination occurs site-selectively in α-position to the nitrogen atom, even when weaker C-H bonds are present, and furnishes Troc-protected aminals or amidines. The method employs the N-heterocycle as limiting reagent and is applicable to the late-stage functionalization of complex molecules. Its synthetic potential was further illustrated through the derivatization of α-aminated products and the application to a concise total synthesis of the reported structure for senobtusin. Mechanistic studies allowed to propose a plausible reaction mechanism involving a turnover-limiting Fe-nitrene generation followed by fast H atom transfer and radical rebound.

Visible-Light-Promoted Enantioselective α-Amidation of Aldehydes by Harnessing Organo-Iron Dual Catalysis

The strategic integration of organocatalysis with transition-metal catalysis to achieve otherwise unattainable stereoselective transformations may serve as a powerful synthetic tool. Herein, we present a synthetically versatile α-amidation of aldehydes by leveraging dual iron and chiral enamine catalysis in an enantioselective manner (up to >99:1 er). Experimental and computational studies have led us to propose a new mechanistic platform, wherein visible-light-promoted LMCT generates [Fe(II)Cl3-], which effectively activates dioxazolones to form an iron-acylnitrenoid radical that inserts into chiral enamine intermediates.

Nitrene-Mediated Enantioselective Intramolecular Olefin Oxyamination to Access Chiral γ-Aminomethyl-γ-Lactones

Attaching a nitrene precursor to an intramolecular nucleophile allows for a catalytic asymmetric intramolecular oxyamination of alkenes in which the nucleophile adds in an endocyclic position and the amine in an exocyclic fashion. Using chiral-at-ruthenium catalysts, chiral γ-aminomethyl-γ-lactones containing a quaternary carbon in γ-position are provided in high yields (up to 99 %) and with excellent enantioselectivities (up to 99 % ee). DFT calculations support the possibility of both a singlet (concerted oxyamination of the alkene) and triplet pathway (stepwise oxyamination) for the formation of the predominant stereoisomer. γ-Aminomethyl-γ-lactones are versatile chiral building blocks and can be converted to other heterocycles such as δ-lactams, 2-oxazolidinones, and tetrahydrofurans.

Ferrocene catalyzed carbohydroxylation of alkenes using H2O and cycloketone oxime esters

A cyanoalkyl-hydroxylation reaction of aryl alkenes has been successfully devised, employing ferrocene as a catalyst for the addition of a cycloketone oxime ester and H2O across the double bond of the alkene. This environmentally friendly approach employs a solvent mixture consisting of water and demonstrates redox neutrality, along with exceptional regio- and chemoselectivity, leading to the formation of diverse distal hydroxy-nitrile compounds. Moreover, this research presents noteworthy contributions in terms of late-stage functionalization of complex molecules and offers valuable insights into the mechanistic aspects of the reaction.

Iron-catalyzed dual decarboxylative coupling of α-amino acids and dioxazolones under visible-light to access amide derivatives

An iron-catalyzed decarboxylative C-N coupling of α-amino acids with dioxazolones is described herein to synthesize amide derivatives under visible-light. The desired products can be given in good to excellent yields under simple, mild, and oxidant-free conditions. This protocol provides a practical route for the transformation of α-amino acids to the corresponding amides. Computational studies were carried out to shed light on the mechanism of this reaction.

Rh(III)-catalyzed Intra- and Intermolecular 3,4-Difunctionalization of 1,3-Dienes via Rh(III)-π-allyl Amidation with 1,4,2-Dioxazolones

We herein report a modular strategy, which enables Rh(III)-catalyzed diastereoselective 3,4-amino oxygenation and diamination of 1,3-dienes using different O- and N-nucleophiles in combination with readily available 3-substituted 1,4,2-dioxazolones (78 examples, 37-91% yield). Previous attempts to functionalize the internal double bond rested on the use of plain alcoholic solvents as nucleophilic coupling partners thus dramatically limiting the scope of this transformation. We have now identified hexafluoroisopropanol as a non-nucleophilic solvent which allows the use of diverse nucleophiles and greatly expands the scope, including an unprecedented amino hydroxylation to selectively install valuable, unprotected β-amino alcohols across 1,3-dienes. Moreover, various elaborate alcohols prove to be compatible providing unique access to complex organic molecules. Finally, this method is employed in a series of intramolecular reactions to deliver valuable nitrogen heterocycles as well as γ- and δ-lactones.

Endo-Selective Intramolecular Alkyne Hydroamidation Enabled by NiH Catalysis Incorporating Alkenylnickel Isomerization

Intramolecular alkyne hydroamidation represents a straightforward approach for the access to synthetically valuable cyclic enamides. Despite some advances made in this realm, the ability to attain a precise regiocontrol still remains challenging, especially for endo cyclization that leads to six-membered and larger azacyclic rings. Herein, we report a NiH-catalyzed intramolecular hydroamidation of alkynyl dioxazolones that allows for an excellent endo selectivity, thus affording a range of six- to eight-membered endocyclic enamides with a broad scope. Mechanistic investigations revealed that Ni(I) catalysis is operative in the current system, proceeding via regioselective syn-hydronickelation, alkenylnickel E/Z isomerization, and Ni-centered inner-sphere nitrenoid transfer. In particular, the key alkenylnickel isomerization step, which previously lacked mechanistic understandings, was found to take place through the η²-vinyl transition state. The synthetic value of this protocol was demonstrated by diastereoselective modifications of the obtained endocyclic enamides to highly functionalized δ-lactam scaffolds.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.