Reductive Cyclization of Unactivated Alkyl Chlorides with Tethered Alkenes under Visible‐Light Photoredox Catalysis

The chemical inertness of abundant and commercially available alkyl chlorides precludes their widespread use as reactants in chemical transformations. Presented in this work is a metallaphotoredox methodology to achieve the catalytic intramolecular reductive cyclization of unactivated alkyl chlorides with tethered alkenes. The cleavage of strong C(sp³)?Cl bonds is mediated by a highly nucleophilic low‐valent cobalt or nickel intermediate generated by visible‐light photoredox reduction employing a copper photosensitizer. The high basicity and multidentate nature of the ligands are key to obtaining efficient metal catalysts for the functionalization of unactivated alkyl chlorides.     

Visible‐Light‐Induced Carbo‐2‐pyridylation of Electron‐Deficient Alkenes with Pyridinium Salts

A simple and practical visible‐light‐induced carbo‐2‐pyridylation of electron‐deficient alkenes with readily available N‐benzoylmethylpyridinium bromides is reported. More than 40 examples are presented and proceed in greater than 80 % yield (on average) with excellent regio‐ and diastereoselectivities.     

Alkyne Versus Ynamide Reactivity: Regioselective Radical Cyclization of Yne‐Ynamides

Ynamides are typically more reactive than simple alkynes and olefins. However, a serendipitous observation revealed a rare case where the reactivity of simple alkynes exceeds that of ynamides. This led to the development of a unique sulfur‐radical‐triggered cyclization of yne‐tethered ynamides, which involves attack of the alkyne by a thiyl radical followed by cyclization with the ynamide. A wide range of novel 4‐thioaryl pyrroles that could tolerate common functional moieties and N‐protecting groups were expediently constructed by this strategy. The current method contrasts with the typical cyclization of yne‐ynamides, which involves the attack of the alkyne moiety by the ynamide core. Control experiments and DFT calculations supported the participation of the sulfur radical in the reaction and the regioselective cyclization. The synthetic potential of the substituted pyrroles is also discussed.     

β‐Selective Aroylation of Activated Alkenes by Photoredox Catalysis

Late‐stage synthesis of α,β‐unsaturated aryl ketones remains an unmet challenge in organic synthesis. Reported herein is a photocatalytic non‐chain‐radical aroyl chlorination of alkenes by a 1,3‐chlorine atom shift to form β‐chloroketones as masked enones that liberate the desired enones upon workup. This strategy suppresses side reactions of the enone products. The reaction tolerates a wide array of functional groups and complex molecules including derivatives of peptides, sugars, natural products, nucleosides, and marketed drugs. Notably, addition of 2,6‐di‐tert‐butyl‐4‐methyl‐pyridine enhances the quantum yield and efficiency of the cross‐coupling reaction. Experimental and computational studies suggest a mechanism involving PCET, formation and reaction of an α‐chloro‐α‐hydroxy benzyl radical, and 1,3‐chlorine atom shift.     

Metal‐Catalyzed Sequential Formation of Distant Bonds in Organic Molecules: Palladium‐Catalyzed Hydrosilylation/Cyclization of 1,n‐Dienes by Chain Walking

Sequential formation of distant bonds in organic molecules was achieved for the palladium‐catalyzed hydrosilylation/cyclization of various 1,n‐dienes by chain walking of the metal catalyst. The reaction was applicable to various 1,n‐dienes, including a 1,13‐diene, to form a cyclopentane ring as well as a carbon–silicon bond at a remote site. The use of “nondissociative” chain walking provides a fascinating strategy in organic synthesis to functionalize distant parts of organic molecules, in a particular order, within a catalytic cycle by effectively moving the reactive center from a remote position.     

Cyclopropanation of Terminal Alkenes through Sequential Atom‐Transfer Radical Addition/1,3‐Elimination

An operationally simple method to affect an atom‐transfer radical addition of commercially available ICH₂Bpin to terminal alkenes has been developed. The intermediate iodide can be transformed in a one‐pot process into the corresponding cyclopropane upon treatment with a fluoride source. This method is highly selective for the cyclopropanation of unactivated terminal alkenes over non‐terminal alkenes and electron‐deficient alkenes. Due to the mildness of the procedure, a wide range of functional groups such as esters, amides, alcohols, ketones, and vinylic cyclopropanes are well tolerated.     

Thiol-Ene Cationic and Radical Reactions: Cyclization,Step-Growth,and Concurrent Polymerizations for Thioacetal and Thioether Units

Thiol-ene cationic and radical reactions were conducted for 1:1 addition between a thiol and vinyl ether, and also for cyclization and step-growth polymerization between a dithiol and divinyl ether. p-Toluenesulfonic acid (PTSA) induced a cationic thiol-ene reaction to generate a thioacetal in high yield, whereas 2,2′-azobisisobutyronitrile resulted in a radical thiol-ene reaction to give a thioether, also in high yield. The cationic and radical addition reactions between a dithiol and divinyl ether with oxyethylene units yielded amorphous poly(thioacetal)s and crystalline poly(thioether)s, respectively. Under high-dilution conditions, the cationic and radical reactions resulted in 16- and 18-membered cyclic thioacetal and thioether products, respectively. Furthermore, concurrent cationic and radical step-growth polymerizations were realized using PTSA under UV irradiation to produce polymers having both thioacetal and thioether linkages in the main chain.     

Iron‐Catalyzed Intermolecular 1,2‐Difunctionalization of Styrenes and Conjugated Alkenes with Silanes and Nucleophiles

The first iron‐catalyzed 1,2‐difunctionalization of styrenes and conjugated alkenes with silanes and either N or C, using an oxidative radical strategy, is described. Employing FeCl₂ and di‐tert ‐butyl peroxide allows divergent alkene 1,2‐difunctionalizations, including 1,2‐aminosilylation, 1,2‐arylsilylation, and 1,2‐alkylsilylation, which rely on a wide range of nucleophiles, namely, amines, amides, indoles, pyrroles, and 1,3‐dicarbonyls, thus providing a powerful platform for producing diverse silicon‐containing alkanes.     

Visible‐Light‐Induced [4+2] Annulation of Thiophenes and Alkynes to Construct Benzene Rings

The [4+2] annulation represents an elegant and versatile synthetic protocol for the construction of benzene rings. Herein, a strategy for visible‐light induced [4+2] annulation of thiophenes and alkynes, to afford benzene rings, is presented. Under simple and mild reaction conditions, the ready availability and structural diversity of thiophenes and alkynes permit the facile synthesis of several substituted aromatic rings. Valuable drugs and amino acids are also well tolerated. Moreover, DFT calculations explain the high regioselectivity of the reaction.