Alkene-derived radical cations are versatile reactive intermediates and have been widely applied in the construction of complex functionalized molecules and cyclic systems for chemical synthesis. ...Therefore, the synthetic application of these alkene-derived radical cations represents a powerful and green tool that can be used to achieve the functionalization of alkenes partially because the necessity of stoichiometric external chemical oxidants and/or hazardous reaction conditions is eliminated. This review summarizes the recent advances in the synthetic applications of the electro-/photochemical alkene-derived radical cations, emphasizing the key single-electron oxidation steps of the alkenes, the scope and limitations of the substrates, and the related reaction mechanisms. Using electrocatalysis and/or photocatalysis, single electron transfer (SET) oxidation of the C&z.dbd;C bonds in the alkenes occurs, generating the alkene-derived radical cations, which sequentially enables the functionalization of translocated radical cations to occur in two ways: the first involves direct reaction with a nucleophile/radical or two molecules of nucleophiles to realize hydrofunctionalization, difunctionalization and cyclization; and the second involves the transformation of the alkene-derived radical cations into carbon-centered radicals using a base followed by radical coupling or oxidative nucleophilic coupling.
This review covers the recent progress in electro-/photo-catalytic alkene-derived radical cation chemistry for organic synthesis, including synthetic strategies, plausible mechanisms and further research outlook.
A new metal‐free radical 5‐exo‐dig cyclization of phenol‐linked 1,6‐enynes with O2, 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), and tBuONO is described. With this general method, carbonylated ...benzofurans can be accessed through incorporation of two oxygen atoms into the product from O2 and TEMPO through dioxygen activation and oxidative cleavage of the NO bond, respectively.
Benzofurans are obtained by the tBuONO‐initiated radical 5‐exo‐dig cyclization of enynes under mild and metal‐free conditions. The two oxygen atoms that constitute the newly formed carbonyl groups of the benzofuran system originate from O2 and 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), respectively.
The 1, n -enyne annulation reaction has emerged as one of the most powerful and straightforward tools to build carbo- and hetero-cyclic frameworks that are found in numerous natural products, ...pharmaceuticals and functional materials. Although the 1, n -enyne annulation methods have been well documented to date, there is a tremendous challenge with current methodologies for simultaneously incorporating external functional groups into the resulting cyclic systems. Recent advances in the radical-mediated oxidative 1, n -enyne annulation strategy involving C–H functionalization have been proven to be an ideal alternative to overcome these disadvantages. Such radical-mediated oxidative 1, n -enyne annulation can be accomplished by two different C–H functionalization modes: One proceeds through generation of the carbon-centered radicals from C–H bond direct oxidative cleavage and their subsequent addition across the CC bond or CC bond enabling the 1, n -enyne annulation; the other employs the C–H bonds as the radical acceptors to terminate the initial oxidative radical-triggered annulation of 1, n -enyne. In addition, during many annulation processes the inherent C–H bonds of 1, n -enynes were functionalized. Here, we summarize recent progress in radical-mediated oxidative annulations of 1, n -enynes involving two different conceptual C–H functionalization strategies and the inherent C–H functionalization with an emphasis on the scope, limitations and mechanisms of these different reactions.
A cobalt‐promoted electrochemical 1,2‐diarylation of alkenes with electron‐rich aromatic hydrocarbons via direct dual C−H functionalizations is described, which employs a radical relay strategy to ...produce polyaryl‐functionalized alkanes. Simply by using graphite rod cathode instead of platinum plate cathode, chemoselectivity of this radical relay strategy is shifted to the dehydrogenative 2+2+2 cycloaddition via 1,2‐diarylation, annulation, and dehydrogenation cascades leading to complex 11,12‐dihydroindolo2,3‐acarbazoles. Mechanistical studies indicate that a key step for the radical relay processes is transformations of the aromatic hydrocarbons to the aryl sp2‐hybridized carbon‐centered radicals via deprotonation of the corresponding aryl radical cation intermediates with bases.
Direct C−H functionalizations enable electrochemical diarylation of alkenes with aromatic hydrocarbons using a radical relay strategy toward polyaryl‐functionalized compounds. Chemoselectivity of the strategy relies on choice of both anode and cathode, which is shifted to the dehydogenative 2+2+2 cycloaddition of alkenes with indoles when using graphite rod as the anode and cathode.
A new metal‐free oxidative radical 2+2+1 carbocyclization of benzene‐linked 1,n‐enynes with two C(sp3)H bonds adjacent to the same heteroatom is described. This method achieves two C(sp3)H ...oxidative functionalizations and an annulation, thus providing efficient and general access to a variety of fused five‐membered carbocyclic hydrocarbons.
An oxidative functionalization of two C(sp3)H bonds adjacent to a same heteroatom for the 2+2+1 carbocyclization with benzene‐linked 1,n‐enynes is presented. This method successfully proceeds using a radical mechanism and provides straightforward access to a variety of fused five‐membered carbocyclic hydrocarbons. TBPB=tert‐butyl perbenzoate.
A mild and general visible light photoredox catalysis-induced intermolecular three-component alkene 1,2-diarylation involving aryl C(sp2)-H functionalization is described. The key to controlling the ...chemoselectivity toward alkene 1,2-diarylation is the employment of a 2,2'-bipyridine base, thus allowing the formation of two new C(sp3)-C(sp2) bonds via aryl radical formation from aryldiazonium salts, addition across the Cdouble bond, length as m-dashC bonds, and aryl C(sp2)-H functionalization cascades.
A new electrochemical intermolecular 1,2‐aminosulfonylation of alkenes with sulfinates and amines is achieved by utilizing balanced three‐component interactions and reactivity differentiation. This ...strategy can be applicable to a wide range of amines, including primary and secondary amines, thus enabling alkene aminosulfonylation for producing diverse functionalized 2‐sulfonylethan‐1‐amines without the need of additive redox catalysts, metal catalysts and chemical oxidants.
Radical‐initiated difunctionalization of alkenes is one of the most important methods in organic synthesis and medicinal chemistry, which can be applied to synthesize value complex compounds as well ...as structural motifs that found in bioactive natural products and pharmaceuticals. In recent years, impressive progress have been made in this area with ideal silver catalysis. Here, we summarize recent advances in silver‐mediated radical difunctionalization of alkenes for the formation of diverse bonds, including 1) two‐component radical difunctionalization reactions enabled by an intramolecular cyclization process toward various cyclic compounds and 2) three‐component radical difunctionalization reactions leading to complex linear compounds. These silver‐mediated radical alkene difunctionalization transformations are general initiated by different radicals, such as carbon‐, oxygen‐, sulfur‐, phosphinyl‐, and halogen‐center radicals, followed by terminated with nucleophiles to form two new bonds in a single reaction.
Ag catalysis: Recent advances in the silver‐mediated intermolecular radical 1,2‐difunctionalization of alkenes are summarized. These reaction are classified by the substrate type and radical regents. Two new C−C/C−C bonds, C−C/C−X bonds (X=F, O, S…) and C−X/C−X bonds (X=O, Br, I…) could be constructed in a single reaction.
For straightforward access to various substituted 1,1-diarylalkanes a photoredox-catalyzed and copper-promoted 1,2-alkylarylation reaction of styrenes has been developed, which uses α-carbonyl alkyl ...bromides and N,N-disubstituted anilines as functionalization reagents. In this radical difunctionalization reaction, α-carbonyl alkyl bromides, including primary-, secondary- and tertiary-α-bromoalkyl ketone esters, malonic esters and cycloalkane were transformed to the corresponding 1,1-diarylalkanes in moderate to good yields at room temperature. Notably, this transformation provided a new route for the C-H alkylation of N,N-disubstituted anilines with high para-selectivity beyond the typical Friedel-Crafts alkylation.
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.
Iron clad: By employing FeCl2 and di‐tert‐butyl peroxide (DTBP), divergent alkene 1,2‐difunctionalization reactions, including 1,2‐aminosilylation, 1,2‐arylsilylation, and 1,2‐alkylsilylation, are achieved by using different nucleophiles. The method provides straightforward and practical access to 1‐amino‐2‐silylalkanes and other functionalized silicon‐containing alkanes with broad substrate scope and high selectivity.
