Minisci‐Type Alkylation of N ‐Heteroarenes by N ‐(Acyloxy)phthalimide Esters Mediated by a Hantzsch Ester and Blue LED Light

Expanding the Chemical Space of Electrophilic β-Glycosyl β-Lactams through Photoinduced Diastereoselective Functionalization

Herein, we present a photoinduced diastereoselective C-3 functionalization of electrophilic β-glycosyl β-lactams. The developed protocol is simple, mild, and scalable and explores the use of 3-exomethylene β-lactams as reaction partners in a Giese type reaction. The key nucleophilic alkyl radical is generated by a photoinduced electron transfer process in the EDA complex formed by NHPI and Hantzsch esters. The diastereoselective hydrogen atom transfer to the β-lactam radical intermediate enables the synthesis of various N-phenyl β-glycosyl β-lactams.

Radical-Mediated α-tert-Alkylation of Aldehydes by Consecutive 1,4- and 1,3-(Benzo)thiazolyl Migrations

The direct alkylation of the α-position of aldehydes is an effective method for accessing a wide range of structurally diverse aldehydes, yet tert-alkylation has proven to be a challenging task. In this study, we present a novel radical-mediated tert-alkylation approach targeting the α-position of aldehydes, enabling the synthesis of complex aliphatic aldehydes. The transformation is initiated by the interaction between an in situ generated enamine intermediate and α-bromo sulfone, forming an electron donor-acceptor (EDA) complex, followed by consecutive 1,4- and 1,3-functional group migrations. This protocol operates under metal-free and mild photochemical conditions, delivering a broad scope of products and providing new mechanistic insights into radical rearrangement reactions.

Metal- and light-free decarboxylative direct C-H alkylation of heteroarenes at room temperature

This study reports a metal- and light-free decarboxylative C-H alkylation of heteroarenes at room temperature. The reaction generates various primary, secondary, and tertiary alkyl radicals and functionalizes seven different privileged scaffolds widely present in bioactive molecules. During this process, one equivalent of hypervalent iodine(III) carboxylates (HICs) plays dual roles as an alkyl radical precursor and an oxidant.

Towards a General Access to 1-Azaspirocyclic Systems via Photoinduced, Reductive Decarboxylative Radical Cyclizations

A convenient and versatile approach to important 1-azaspirocyclic systems relevant to medicinal chemistry and natural products is reported herein. The main strategy relies on a reductive decarboxylative cyclization of redox-active esters which can be rapidly assembled from abundant cyclic azaacids and tailored acceptor sidechains, with a focus on alkyne acceptors enabling the generation of useful exo-alkene moieties. Diastereoconvergent variants were studied and could be achieved either through remote stereocontrol or conformational restriction in bicyclic carbamate substrates. Two sets of metal-free photocatalytic conditions employing inexpensive eosin Y were disclosed and studied experimentally to highlight key mechanistic divergences.

Three-Component Aminoheteroarylation of Alkenes via Photoinduced EDA Complex Activation

A catalyst-free approach for the multicomponent aminoheteroarylation reaction of alkenes with N-aminopyridinium salts and heteroarenes is herein described. The reaction shows good functional group tolerance and allows the generation of valuable β-heteroarylethylamines in satisfying yields. In this transformation, N-aminopyridinium salts and heteroarenes are utilized to generate electron donor-acceptor complexes, which undergo a single-electron transfer process upon light irradiation to form key amidyl radicals and heteroaryl radical cations. The amidyl radical is subsequently captured by alkenes, followed by a Minisci-type reaction to yield the desired β-heteroarylamines as products.

Photoinduced Decarboxylative C3-H Alkylation of Quinoxalin-2(1H)-ones

An efficient, catalyst- and additive-free, visible-light-driven radical C3-H alkylation of quinoxalin-2(1H)-one derivatives has been developed. This reaction utilizes alkyl-NHP-esters as an alkyl radical donor and quinoxalin-2(1H)-one derivatives as an alkyl radical acceptor. The operationally simple protocol works under mild reaction conditions and tolerates a variety of functional groups. Furthermore, the synthetic utility of the methodology was successfully implemented for synthesizing biologically relevant C3-alkyl substituted quinoxalin-2(1H)-one derivatives.

Applications of Hantzsch Esters in Organocatalytic Enantioselective Synthesis

Hantzsch esters (1,4-dihydropyridine dicarboxylates) have become, in this century, very versatile reagents for enantioselective organic transformations. They can act as hydride transfer agents to reduce, regioselectively, a variety of multiple bonds, e.g., C=C and C=N, under mild reaction conditions. They are excellent reagents for the dearomatization of heteroaromatic substances, and participate readily in cascade processes. In the last few years, they have also become useful reagents for photoredox reactions. They can participate as sacrificial electron and hydrogen donors and when 4-alkyl or 4-acyl-substituted, they can act as alkyl or acyl radical transfer agents. These last reactions may take place in the presence or absence of a photocatalyst. This review surveys the literature published in this area in the last five years.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Hydrogen Atom Transfer Driven Enantioselective Minisci Reaction of Alcohols

Catalytic enantioselective Minisci reactions have recently been developed but all instances so far utilize α-amino radical coupling partners. We report a substantial evolution of the enantioselective Minisci reaction that enables α-hydroxy radicals to be used, providing valuable enantioenriched secondary alcohol products. This is achieved through the direct oxidative coupling of two C-H bonds on simple alcohol and pyridine partners through a hydrogen atom transfer (HAT)-driven approach: a challenging process to achieve due to the numerous side reactions that can occur. Our approach is highly regioselective as well as highly enantioselective. Dicumyl peroxide, upon irradiation with 390 nm light, serves as both HAT reagent and oxidant whilst selectivity is controlled by use of a chiral phosphoric acid catalyst. Computational and experimental evidence provide mechanistic insight as to the origin of selectivity, revealing a stereodetermining deprotonation step distinct from the analogous reaction of amide-containing substrates.

A practical and sustainable two-component Minisci alkylation via photo-induced EDA-complex activation

An operationally simple, open-air, and efficient light-mediated Minisci C-H alkylation method is described, based on the formation of an electron donor-acceptor (EDA) complex between nitrogen-containing heterocycles and redox-active esters. In contrast to previously reported protocols, this method does not require a photocatalyst, an external single electron transfer agent, or an oxidant additive. Achieved under mildly acidic and open-air conditions, the reaction incorporates primary-, secondary-, and tertiary radicals, including bicyclo[1.1.1]pentyl (BCP) radicals, along with various heterocycles to generate Minisci alkylation products in moderate to good yields. Additionally, the method is exploited to generate a stereo-enriched, hetereoaryl-substituted carbohydrate.

Visible-Light-Driven Site-selective Alkylation of Benzo Core of Coumarins

An unprecedented, straightforward photochemical platform for efficient site-selective C-H alkylation of C7 position of benzocore via the cross coupling between coumarins and NHPI ester, employing Ru(II) as photocatalyst in visible...