Direct C2-Alkylation of Azoles with Alcohols and Ethers through Dehydrogenative Cross-Coupling under Metal-Free Conditions

Metal-Free Oxidative C-C Bond Formation through C-H Bond Functionalization

The formation of C-C bonds embodies the core of organic chemistry because of its fundamental application in generation of molecular diversity and complexity. C-C bond-forming reactions are well-known challenges. To achieve this goal through direct functionalization of C-H bonds in both of the coupling partners represents the state-of-the-art in organic synthesis. Oxidative C-C bond formation obviates the need for prefunctionalization of both substrates. This Minireview is dedicated to the field of C-C bond-forming reactions through direct C-H bond functionalization under completely metal-free oxidative conditions. Selected important developments in this area have been summarized with representative examples and discussions on their reaction mechanisms.

Base-mediated direct fluoroalkenylation of 2-phenyl-1,3,4-oxadiazole, benzothiazole and benzoxazole with gem-difluoroalkenes

Direct α-fluorovinylation of 2-phenyl-1,3,4-oxadiazole, benzothiazole and benzoxazole with gem-difluoroalkenes under the assistance of KHMDS or NaH at room temperature was developed.

Copper-catalyzed oxidative alkenylation of C(sp3)–H bonds via benzyl or alkyl radical addition to β-nitrostyrenes

A new method for the preparation of (E)-β-alkylstyrene derivatives has been developed via the addition of benzyl or alkyl radicals to β-nitrostyrenes using di-tert-butyl peroxide (DTBP) as the oxidant in the presence of Cu powder catalyst.

Metal-free direct difunctionalization of alkenes with I2O5 and P(O)–H compounds leading to β-iodophosphates

A novel and efficient procedure for direct difunctionalization of alkenes with I₂O₅ and P(O)–H compounds has been developed under metal-free conditions.

Oxidative cross-coupling of pyridine N-oxides and ethers between C(sp2)–H/C(sp3)–H bonds under transition-metal-free conditions

An oxidative cross-coupling of pyridine N-oxides and ethers between C(sp²)–H/C(sp³)–H bonds under transition metal free conditions was developed.

Regioselective synthesis of triazoles via base-promoted oxidative cycloaddition of chalcones with azides in aqueous solution

A base-promoted oxidative cycloaddition of chalcones with azides was developed for the regioselective synthesis of trisubstituted triazoles under transition-metal-free conditions.

Metal-free cascade construction of C–C bonds by activation of inert C(sp3)–H bonds

Metal-free catalytic activation of inert C(sp³)–H/elimination of SO₂/C–C bond formation via a free radical process were achieved in sequence.

Cu-catalyzed oxidative Povarov reactions between N-alkyl N-methylanilines and saturated oxa- and thiacycles

Cu-catalyzed oxidative Povarov reactions between N,N-dialkylanilines and saturated oxa- or thiacycles with tert-butyl hydroperoxide (TBHP) are described.

Iron-catalyzed direct α-arylation of ethers with azoles

The direct α-arylation of cyclic and acyclic ethers with azoles has been achieved, which features a novel iron-catalyzed cross-dehydrogenative coupling (CDC) process.

Structurally diverse α-substituted benzopyran synthesis through a practical metal-free C(sp3)-H functionalization

A trityl ion-mediated practical C-H functionalization of a variety of benzopyrans with a wide range of nucleophiles (organoboranes and C-H molecules) at ambient temperature has been disclosed. The metal-free reaction has an excellent functional group tolerance and high chemoselectivity and displays a broad scope with respect to both benzopyran and nucleophile partners, efficiently affording a collection of benzopyrans bearing diverse skeletons and α-functionalities in one step.

Practical and highly selective C-H functionalization of structurally diverse ethers

A trityl ion mediated C-H functionalization of ethers with a wide range of nucleophiles at ambient temperature has been developed. The reaction displays high chemoselectivity and good functional group tolerance. The protocol also exhibits excellent regio- and diastereoselectivities for the unsymmetric ethers, thus stereoselectively generating highly functionalized disubstituted 2,5-trans tetrahydrofurans (THF), 2,6-trans tetrahydropyrans (THP), 2,6-trans dihydropyrans (DHP), and 1,3-trans isochromans, and highlighting the capacity of the protocol in complex molecule synthesis.

Efficient imidation of C(sp3)–H bonds adjacent to oxygen atoms of aryl ethers under metal-free conditions

A metal-free intermolecular oxidative C–N formation reaction of aryl ethers with saccharins was realized for the first time.

Palladium-catalyzed thiolation of alkanes and ethers with arylsulfonyl hydrazides

A new method for the preparation of alkyl aryl sulfides through direct oxidative thiolation of alkanes or ethers with arylsulfonyl hydrazides using DTBP as an oxidant catalyzed by Pd(OAc)₂ has been reported. The C–H bonds in various alkanes or ethers were successfully converted into C–S bonds with moderate to good yields.

KOAc-promoted alkynylation of α-C–H bonds of ethers with alkynyl bromides under transition-metal-free conditions

KOAc-promoted α-position C–H activation and alkynylation of ethers with alkynyl bromides to 2-alkynyl ethers was developed under transition-metal-free conditions.

The cross-dehydrogenative coupling of C(sp3)-H bonds: a versatile strategy for C-C bond formations

Over the last decade, substantial research has led to the introduction of an impressive number of efficient procedures which allow the selective construction of CC bonds by directly connecting two different CH bonds under oxidative conditions. Common to these methodologies is the generation of the reactive intermediates in situ by activation of both CH bonds. This strategy was introduced by the group of Li as cross-dehydrogenative coupling (CDC) and discloses waste-minimized synthetic alternatives to classic coupling procedures which rely on the use of prefunctionalized starting materials. This Review highlights the recent progress in the field of cross-dehydrogenative C sp 3C formations and provides a comprehensive overview on existing procedures and employed methodologies.

Iron-catalyzed decarboxylative alkenylation of cycloalkanes with arylvinyl carboxylic acids via a radical process

A Fe(acac)₃-catalyzed decarboxylative coupling of 2-(aryl)vinyl carboxylic acids with cycloalkanes was developed by using DTBP as an oxidant through a radical process. This reaction tolerates a wide range of substrates, and products are obtained in good to excellent yields (71–95%). The reaction also shows excellent stereoselectivity, and only trans-isomers are obtained.