Organic Peroxides in Transition-Metal-Free Cyclization and Coupling Reactions (C-C) via Oxidative Transformation

Transition-metal-free transformations are recognized as green and sustainable methods for constructing carbon-carbon bonds in organic synthesis. This review describes the application of six organic peroxides, including tert-butyl hydroperoxide (TBHP), di-tert-butyl peroxide (DTBP), tert-butyl peroxybenzoate (TBPB), benzoyl peroxide (BPO), dialauroyl peroxide (DLP), and diguyl peroxide (DCP), in C-C bond construction, highlighting selected examples and mechanisms of challenging transformations. Each section concludes with a detailed overview of suitable reagents for various coupling reactions and strengths and weaknesses of the reported works. This work aims to inspire further innovations in transition-metal-free oxidative transformations, promoting sustainable and eco-friendly chemical processes and paving the way for new peroxide-based organic synthesis methods.

BF3 ⋅Et2 O-Catalyzed Selective C-4 Alkylation of Isoquinolin-1(2H)-ones Employing p-Quinone Methides

The direct C-4 alkylation of isoquinolin-1(2H)-one moiety is a challenging transformation in organic synthesis. Here we present a practical and efficient synthesis of C-4 alkylated isoquinolin-1(2H)-ones through conjugate addition of isoquinolin-1(2H)-ones to p-quinone methides for the first time. The process is facilitated by Lewis acid catalysis and this operationally straightforward, mild, metal-free and one-pot transformation provides a wide range of C-4 alkylated isoquinolin-1(2H)-ones at ambient temperature in good to excellent yields.

Iron-Catalyzed Ring Opening of Cyclopropanols and Their 1,6-Conjugate Addition to p-Quinone Methides

A novel iron-catalyzed ring opening of cyclopropanols and their 1,6-conjugate addition to p-quinone methides for accessing substituted phenols is disclosed. In this protocol, various cyclopropanols are converted to alkyl radicals and undergo 1,6-conjugate addition to p-quinone methides toward C-C bond formation. The salient features of this methodology include operationally simple and mild reaction conditions, environmentally benign protocol, high efficiency, inexpensive catalyst, good to excellent yield, and a wide range of substrate scope.

Ammonium Chloride-Mediated Trifluoromethylthiolation of p-Quinone Methides

Ammonium chloride-mediated trifluoromethylthiolation of p-quinone methides is reported using inexpensive and bench stable AgSCF₃ as a nucleophilic trifluoromethylthiolating (-SCF₃) reagent. This method is an efficient strategy for the construction of the benzylic C(sp³)-SCF₃ bond to synthesize trifluoromethylthio-diarylmethane derivatives by 1,6-conjugate addition/aromatization under mild reaction conditions without any metal catalyst, oxidants, or additives. This is the first report of trifluoromethylthiolation of p-quinone methides. In addition, di-trifluoromethylthiolation of δ-chloro-p-quinone methide and scalability are demonstrated.

Recent developments in 1,6-addition reactions of para-quinone methides (p-QMs)

In this review, we provide a comprehensive overview of recent progress in this rapidly growing field by summarizing the 1,6-conjugate addition and annulation reactions of p-QMs with consideration of their mechanisms and applications.

InCl3-catalyzed 5-exo-dig cyclization/1,6-conjugate addition of N-propargylamides with p-QMs to construct oxazole derivatives

An InCl₃-catalyzed tandem intramolecular 5-exo-dig cyclization/1,6-conjugate addition/aromatization of N-propargylamides with p-QMs to produce oxazoles tethering diarylmethane has been successfully developed.

Visible-light-promoted radical cross-coupling of para-quinone methides with N-substituted anilines: an efficient approach to 2,2-diarylethylamines

A series of 2,2-diarylethylamines were accessed via visible-light-promoted radical cross-coupling of p-QMs with N-alkyl anilines.

Catalyst- and reagent-free 1,6-hydrophosphonylation of p-quinone methides: a practical approach for the synthesis of diarylmethyl phosphine oxides

We developed a catalyst-, reagent-, and additive-free protocol with 100% atom economy for the synthesis of diarylmethyl phosphine oxides via 1,6-hydrophosphonylation of p-quinone methides using water as a green solvent. The reaction showed broad scope with excellent functional group tolerance. The practicability of this method was demonstrated by carrying out the reaction on the gram scale whereby product was obtained in high yield by the filtration technique avoiding chromatographic purification.

Metal-free, radical 1,6-conjugated addition of cyclic ethers with para-quinone methides (p-QMs)

An efficient method for metal-free C–C bond formation between p-quinone methides (p-QMs) and cyclic ethers via a radical pathway to afford substituted diarylmethanes and triarylmethanes or to effect the α-alkylation of the cyclic ethers has been developed.

Copper-catalyzed oxidative coupling of quinoxalin-2(1H)-ones with alcohols: access to hydroxyalkylation of quinoxalin-2(1H)-ones

An efficient protocol for the synthesis of hydroxyl-containing quinoxalin-2(1H)-ones has been developed via the copper-catalyzed cross-coupling reaction of quinoxalin-2(1H)-ones with alcohols with moderate to good yields.

1,4-Conjugate addition/esterification of ortho-quinone methides in a multicomponent reaction

A novel 1,4-conjugate addition/esterification of ortho-quinone methides in a multicomponent reaction has been developed for facilely accessing 3,3-diarylpropanamides.