Copper-catalyzed oxidative C(sp3)-H/C(sp2)-H cross-coupling en route to carbocyclic rings

Cu(II) carboxylate arene C─H functionalization: Tuning for nonradical pathways

We report carbon-hydrogen acetoxylation of nondirected arenes benzene and toluene, as well as related functionalization with pivalate and 2-ethylhexanoate ester groups, using simple copper(II) [Cu(II)] salts with over 80% yield. By changing the ratio of benzene and Cu(II) salts, 2.4% conversion of benzene can be reached. Combined experimental and computational studies results indicate that the arene carbon-hydrogen functionalization likely occurs by a nonradical Cu(II)-mediated organometallic pathway. The Cu(II) salts used in the reaction can be isolated, recycled, and reused with little change in reactivity. In addition, the Cu(II) salts can be regenerated in situ using oxygen and, after the removal of the generated water, the arene carbon-hydrogen acetoxylation and related esterification reactions can be continued, which leads to a process that enables recycling of Cu(II).

Concomitant Functionalization of Two Different Ketones by Merging Brønsted Acid Catalysis and Radical Relay Coupling

In previous literature, incorporation of functional groups at the α-position of unactivated carbonyl compounds was mainly restricted to one kind of corresponding precursors. Herein, we report the concomitant functionalization of...

Copper-catalyzed selective oxidation of hydrazones through C(sp3)-H functionalization

A simple and mild protocol for copper-catalyzed oxidation of hydrazones at the α-position has been reported. Various substrates are compatible, providing the corresponding products in moderate to good yields. This strategy provides an efficient and convenient solution for the synthesis of carbonyl hydrazone. A free radical pathway mechanism is suggested for the transformation.

Palladium-Catalyzed Intramolecular Cross-Coupling of Unactivated C(sp3)-H and C(sp2)-H Bonds

Direct C-H/C-H coupling represents an appealing method for the construction of C-C bonds, and the cross-coupling of unactivated C(sp³)-H and C(sp²)-H bonds is challenging and remains to be investigated. We have developed the Pd-catalyzed intramolecular coupling of inert C(sp³)-H and C(sp²)-H bonds. The reaction proceeded by o-methyl oxime-directed aryl C(sp²)-H activation and subsequent alkyl C(sp³)-H cleavage, generating C(sp²),C(sp³)-palladacycles as the key intermediates. Dihydrobenzofurans and indanes were formed as the final products.

Copper-Catalyzed Intramolecular Amination of C(sp3)-H Bond of Secondary Amines to Access Azacycles

The cross-coupling of C-N bond directly from inert C-H bonds is an ideal approach to synthesize saturated azacycles due to its high efficiency and atom economy. In this article, a copper-catalyzed intramolecular amination via the cross coupling of C(sp³)-H and N-H bonds of secondary amine has been reported, which exhibit excellent chemo- and regioselectivity, extensive substrate scope, and functional group tolerance in good to excellent yield, offering an efficient pathway to build nitrogen-containing heterocycle skeletons.

Radical cascade synthesis of azoles via tandem hydrogen atom transfer

A radical cascade strategy for the modular synthesis of five-membered heteroarenes (e.g. oxazoles, imidazoles) from feedstock reagents (e.g. alcohols, amines, nitriles) has been developed. This double C-H oxidation is enabled by in situ generated imidate and acyloxy radicals, which afford regio- and chemo-selective β C-H bis-functionalization. The broad synthetic utility of this tandem hydrogen atom transfer (HAT) approach to access azoles is included, along with experiments and computations that provide insight into the selectivity and mechanism of both HAT events.

Transition-metal-free radical relay cyclization of vinyl azides with 1,4-dihydropyridines involving a 1,5-hydrogen-atom transfer: access to α-tetralone scaffolds

The remote C(sp³)–H functionalization enabled by a radical-mediated 1,5-hydrogen-atom transfer (HAT) process using vinyl azides and 1,4-dihydropyridines as precursors has been described.

Remote azidation of C(sp3)–H bonds to synthesize δ-azido sulfonamidesviairon-catalyzed radical relay

A selective remote C–H azidation was developed with an iron catalyst playing dual roles as a radical initiator and terminator.

Enantioselective Copper-Catalyzed Cyanation of Remote C(sp3)-H Bonds Enabled by 1,5-Hydrogen Atom Transfer

The direct functionalization of C(sp³)-H bonds has led to the development of methods to access molecules or intermediates from basic chemicals in an atom- and step-economic fashion. Nevertheless, achieving high levels of chemo-, regio-, and enantioselectivity in these reactions remains challenging due to the ubiquity and low reactivity of C(sp³)-H bonds. Herein, we report an unprecedented protocol for enantioselective cyanation of remote C(sp³)-H bonds. With chiral Box-Cu complex as the catalyst, the reaction of N-fluorosulfonamide furnishes the corresponding products in excellent yields and high enantiomeric excess (ee) under mild reaction conditions. A radical relay pathway involving 1,5-hydrogen atom transfer (1,5-HAT) of N-center radicals followed by enantioselective cyanation of the in situ-formed benzyl radicals is proposed. This enantioselective copper-catalyzed cyanation thus offers insights into an efficient way for the synthesis of bioactive molecules for drug discovery.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

δ C-H (hetero)arylation via Cu-catalyzed radical relay

A radical relay strategy has been developed to enable selective δ C–H arylation. The approach employs a chiral copper catalyst, which serves the dual roles of generating an N-centered radical to promote intramolecular H-atom transfer, and then intercepting a distal C-centered radical for C–C bond formation with (hetero)aryl boronic acids.

A Cu-catalyzed strategy has been developed that harnesses a radical relay mechanism to intercept a distal C-centered radical for C–C bond formation. This approach enables selective δ C–H (hetero)arylation of sulfonamides via intramolecular hydrogen atom transfer (HAT) by an N-centered radical. The radical relay is both initiated and terminated by a Cu catalyst, which enables incorporation of arenes and heteroarenes by cross-coupling with boronic acids. The broad scope and utility of this catalytic method for δ C–H arylation is shown, along with mechanistic probes for selectivity of the HAT mechanism. A catalytic, asymmetric variant is also presented, as well as a method for accessing 1,1-diaryl-pyrrolidines via iterative δ C–H functionalizations.

Radical Cation Salt-initiated Aerobic C-H Phosphorylation of N-Benzylanilines: Synthesis of α-Aminophosphonates

A radical cation salt-initiated phosphorylation of N-benzylanilines was realized through an aerobic oxidation of the sp3 C-H bond, providing a series of α-aminophosphonates in high yields. An investigation of the reaction scope revealed that this mild catalyst system is superior in good functional group tolerance and high reaction efficiency. The mechanistic study implied that the cleavage of the sp3 C-H bond was involved in the rate-determining step.

Copper-catalyzed remote oxidation of alcohols initiated by radical difluoroalkylation of alkenes: facile access to difluoroalkylated carbonyl compounds

A Cu-catalyzed oxidation of alcohols triggered by the radical difluoroalkylation of alkenes has been developed.

Phosphine-catalyzed remote α-C–H bond activation of alcohols or amines triggered by the radical trifluoromethylation of alkenes: reaction development and mechanistic insights

An efficient radical protocol for concomitant functionalization of both alkenes and remote α-C–H bonds of alcohols or amines via 1,5(6,7)-HAT was realized in a highly controlled site-selective manner.

Novel and facile synthesis of 1-benzazepines via copper-catalyzed oxidative C(sp3)–H/C(sp2)–H cross-coupling

A novel synthetic strategy for the facile construction of 1-benzazepines has been developedviacopper-catalyzed oxidative C(sp³)–H/C(sp²)–H cross-coupling.