Photocatalytic Decarboxylative [2 + 2 + m] Cyclization of 1,7-Enynes Mediated by Tricyclohexylphosphine and Potassium Iodide

Photocatalytic Bicyclization of Indole-Tethered 1,6-Enynes for Diastereoselective Synthesis of Pyrrolo[3,2,1-jk]carbazoles

A visible-light-driven photocatalytic protocol is established for the diastereoselective synthesis of pyrrolo[3,2,1-jk]carbazoles via a radical-triggered multicomponent bicyclization reaction starting from readily available indole-tethered 1,6-enynes and α-benzyl-α-bromomalonates under mild conditions. This photocatalytic approach exhibits a wide substrate compatibility and excellent tolerability toward various functional groups and boasts the benefit of efficient ring formation and chemical bond creation.

Photocatalytic, α-Aminoalkyl Radical-Mediated, Methylene-Extrusive Ring-Closing Transformation of o-Alkynyl and o-Cyano Acrylamides

Herein we report a visible-light-induced, α-aminoalkyl radical-mediated cascade reaction of 1,7-enynes that establishes a unique ring-closing enyne transformation pathway which occurs with concomitant loss of a methylene moiety. The α-aminoalkyl radical derived from N,N-dimethylaniline was demonstrated to be a traceless promoter of enyne reorganization leading to 4-alkylquinolinones. The reaction can also be extended to nitrile-substituted acrylamide systems, leading to carbostyrils. Experiments with deuterated N,N-dimethylaniline-d6 (PhN(CD3)2) established the involvement of 1,5-H atom transfer in the mechanism.

Metal-free radical bicyclization/chloroalkylarylation of 1,6-enynes with chloroalkanes

Free radical initiated bicyclization of 1,6-enynes with chloralkanes, is achieved via selective activation of the C(sp3)-H bond of the chloralkane, resulting in diverse polychlorinated/chlorinated polyheterocycles. Two kinds of transformations and a scaled-up experiment were performed to test the synthetic importance of the organic chlorides. Finally, a range of radical inhibition operations and radical clock tests were explored to support the reaction process.

Visible-Light-Initiated Catalyst-Free Radical Annulation Reactions of 1,6-Enynes and Aryl Sulfonyl Bromide to Assemble Sulfonation/Bromination Succinimide Derivatives

In the present study, the environment-friendly visible-light-promoted strategy is used to perform an efficient, simple, and straightforward photocatalytic succinimide derivative synthesis from the reaction of 1,6-enynes and aryl sulfonyl bromide at room temperature under air ambient conditions. This method features mild conditions, broad substrate scope, high yields, and excellent configurational selectivity. In addition, all the atoms of the substrates involved in the reaction converge in the product structures, showing a high atomic economy. Moreover, the most important characteristic of this study is that no photocatalyst and additives are used, while the key factor that triggers the reaction is visible light, indicating that this study has an important practical value.

Visible-light-induced decarboxylative cascade cyclization of acryloylbenzamides with N-hydroxyphthalimide esters via EDA complexes

A visible-light-induced decarboxylative cascade reaction of acryloylbenzamides with alkyl N-hydroxyphthalimide (NHP) esters for the synthesis of various 4-alkyl isoquinolinediones mediated by triphenylphosphine (PPh3) and sodium iodide (NaI) was developed. This operationally simple protocol proceeded via the photoactivation of electron donor-acceptor (EDA) complexes between N-hydroxyphthalimide esters and NaI/PPh3, resulting in multiple carbon-carbon bond formations without the use of precious metal complexes or synthetically elaborate organic dyes, which provided an alternative practical approach to synthesize diverse isoquinoline-1,3(2H,4H)-dione derivatives.

Redox-Neutral Cyclization of 2-Isocyanobiaryls through Photoredox/PPh3 Dual Catalysis

The photoredox/PPh3-mediated cyclization of 2-isocyanobiaryls has been developed. A substantial range of functional-group-rich phenanthridine derivatives were synthesized at room temperature in a highly selective and atom-economic manner. Mechanistic studies suggested that the cyclization process is probably mediated both by Ph3P radical cation with key 1,2-hydride transfer and hydrogen atom generated through O-H bond homolytic cleavage of Ph3P-OH radical intermediate.

Recent advancements in iodide/phosphine-mediated photoredox radical reactions

Photoredox catalysis plays a crucial role in contemporary synthetic organic chemistry. Since the groundbreaking work of Shang and Fu on photocatalytic decarboxylative alkylations in 2019, a wide range of organic transformations, such as alkylation, alkenylation, cyclization, amination, iodination, and monofluoromethylation, have been progressively achieved using a combination of iodide and PPh3. In this review, we primarily focus on summarizing the recent advancements in inexpensive and readily available iodide/phosphine-mediated photoredox radical transformations.

Palladium-Catalyzed Carbonylative Synthesis of Polycyclic 3,4-Dihydroquinolin-2(1H)-one Scaffolds Containing Perfluoroalkyl and Carbonyl Units

A palladium-catalyzed one-pot two-step radical mediated carbonylative cyclization of 1,7-enynes with perfluoroalkyl iodides and Mo(CO)₆ has been developed for the construction of polycyclic 3,4-dihydroquinolin-2(1H)-one scaffolds. This method realizes a facile synthesis of various polycyclic 3,4-dihydroquinolin-2(1H)-one derivatives containing perfluoroalkyl and carbonyl units in high yields. Moreover, modifications of several bioactive molecules were demonstrated using this protocol.

Visible-Light-Promoted Decarboxylative Alkylation/Cyclization of Vinylcycloalkanes

An efficient strategy for visible-light-promoted decarboxylative alkylation of vinylcyclopropanes with alkyl N-(acyloxy)phthalimide esters through the dual C-C bond and single N-O bond cleavage, employing triphenylphosphine and lithium iodide as the photoredox system to synthesize 2-alkylated 3,4-dihydronaphthalenes, has been established. This alkylation/cyclization involves a radical process and undergoes a sequence of N-(acyloxy)phthalimide ester single-electron reduction, N-O bond cleavage, decarboxylative, alkyl radical addition, C-C bond cleavage, and intramolecular cyclization. Moreover, using the photocatalyst Na₂-Eosin Y instead of triphenylphosphine and lithium iodide, the vinyl transfer products are acquired when vinylcyclobutanes or vinylcyclopentanes are utilized as alkyl radical receptors.

Radical bicyclization of 1,6-enynes with sulfonyl hydrazides by the use of TBAI/TBHP in the aqueous phase

A novel 5-exo-dig/6-endo-trig bicyclization of 1,6-enynes with sulfonyl hydrazides in the aqueous phase using the cheap and available tetrabutylammonium iodide (TBAI)-tert-butyl hydroperoxide (TBHP) combined system is reported. The resulting reaction of diverse nitrogen- and oxygen-polyheterocycles displays high chemical selectivity, high step-economy, and a moderate substrate scope. Moreover, iodosulfonylation can be realized by modulating the structure of the 1,6-enynes.

NaI/PPh3-catalyzed visible-light-mediated decarboxylative radical cascade cyclization of N-arylacrylamides for the efficient synthesis of quaternary oxindoles

A practical NaI/PPh3-catalyzed decarboxylative radical cascade cyclization of N-arylacrylamides with redox-active esters is described, which is mediated by visible light irradiation. A wide range of substrates bearing different substituents and derived from ubiquitous carboxylic acids, including α-amino acids, were synthesized and examined under this very mild, efficient, and cost effective transition-metal-free synthetic method. These afforded various functionalized oxindoles featuring a C3 quaternary stereogenic center. Mechanistic experiments suggest a radical mechanism.

Radical annulation using a radical reagent as a two-carbon unit

Radical annulation has emerged as one of the most efficient and straightforward methods for synthesizing cyclic/polycyclic compounds as the core structures can be constructed through a single procedure comprising multiple bond-forming steps. Particularly, radical annulation using a radical reagent as a cyclization partner and two-carbon unit greatly expands the diversity of cyclic skeletons and the functionality of radical reagents. We herein have highlighted the representative processes reported in the past decade for radical annulation using a radical reagent as a two-carbon unit, including [2 + 2 + 2], [3 + 2], [4 + 2], and [5 + 2] modes, with an emphasis on their reaction mechanisms. These studies not only pave the way toward annulation but also provide insight into the exploration of a new reaction mode for radical chemistry.

Recent Advances in Employing Catalytic Donors and Acceptors in Electron Donor-Acceptor Complex Photochemistry

Electron donor-acceptor (EDA) complexes provide a means to initiate radical reactions under visible light irradiation using substrates that do not absorb visible light individually. Catalytic approaches to complex formation are vital for advancing this synthetic strategy as it decouples the complexation and photogeneration of radicals from substrate functionalization, a limitation inherent to stoichiometric approaches that restricts structural diversity. This Synopsis highlights recent developments in EDA complex photochemistry in which either the donor or acceptor are employed catalytically.

Synthesis of quinazoline by decarboxylation of 2-aminobenzylamine and α-keto acid under visible light catalysis

Quinazoline compounds demonstrate a variety of physiological and pharmacological activities. However, the most common syntheses require large quantities of oxidants, high temperature, and other extreme conditions. In this study, quinazoline compounds were synthesized from the condensation of α-keto acid and 2-aminobenzylamine and then decarboxylation under blue LED irradiation at room temperature without transition metal catalysts or additives. Therefore, we demonstrated that by using α-keto acid as the acyl source, decarboxylation can be realized under blue LED without oxidants, in a simple, mild, and environmentally friendly process.

Alkylation/Ipso-cyclization of Active Alkynes Leading to 3-Alkylated Aza- and Oxa-spiro[4,5]-trienones

A method for the preparation of 3-alkylated spiro[4.5]trienones via alkylation/ipso-cyclization of activated alkynes with 4-alkyl-DHPs under transition-metal-free conditions is proposed. This alkylation successively undergoes the generation of alkyl radicals, addition of alkyl radicals to the alkynes, and intramolecular ipso-cyclization. The mechanism studies suggest that the alkylation/ipso-cyclization involves a radical process. This ipso-cyclization procedure shows a series of advantages, such as accessibility, mild conditions, high efficiency, greater safety, and an environmentally friendly method.

Electron donor–acceptor complex-catalyzed photoredox reactions mediated by DIPEA and inorganic carbonates

A DIPEA–NHPI ester–inorganic carbonate catalytic EDA complex is reported as an efficient and sustainable radical generation platform for developing photocatalytic reactions.

Visible-light-mediated decarboxylative alkylation of 2-pyridone derivatives via a C3-selective C-H functionalization

A direct C-H functionalization approach to access C3-alkylated 2-pyridone derivatives is reported. This study utilizes N-hydroxyphthalimide (NHPI) esters of various carboxylic acids as sources of alkyl radicals by reductive cleavage under photocatalytic reaction conditions. The carbon-carbon bond formation occurred site-selectively at C3 of 2-pyridone to give the desired products in moderate to good yields. This method enables a faster access to C3-alkylated pyridone compounds which can be applied to the synthesis of small molecule drugs.

Photocatalytic Decarboxylative [3 + 2] and [4 + 2] Annulation of Enynals and γ,σ-Unsaturated N-(Acyloxy)phthalimides by NaI/PPh3 Catalysis

A practical and eco-friendly strategy for the radical-mediated decarboxylative [3 + 2] and [4 + 2] annulation of enynals and γ,σ-unsaturated N-(acyloxy)phthalimides through the photoactivation of an electron donor-acceptor (EDA) complex has been developed. A wide range of primary, secondary, and tertiary alkyl N-hydroxyphthalimide (NHP) esters can be used as suitable substrates for the synthesis of fused ketones without any transition-metal catalysts or oxidants. This protocol features a broad substrate scope, excellent selectivity, and clean reaction conditions.

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Visible-light-induced iodine-anion-catalyzed decarboxylative/deaminative C–H alkylation of enamides

An efficient method for photo-induced iodine-anion-catalyzed C–H alkylation of enamides has been developed. Redox-active esters and Katritzky salts of amino acids are amenable, successfully delivering various functionalized enamides.