Nickel-catalyzed oxidative thiolation of α-amino carbonyl compounds with thiols

An efficient oxidative thiolation of α-amino carbonyl compounds with thiols by the catalysis of an Earth-abundant nickel salt is disclosed for the first time. A variety of alkyl thiols and (hetero)aryl thiols underwent the reaction well with α-amino ketones and an α-amino ester to produce the desired α,α-aminothiocarbonyl compounds in good to excellent yields under ligand- and base-free conditions.

Copper-Catalyzed Synthesis of N-Fused Quinolines via C(sp3)-H Activation-Radical Addition-Cyclization Cascade

A novel copper-catalyzed cyclization reaction for the synthesis of pyrazolo[1,5-a]quinoline, triazolo[1,5-a]quinoline, and pyrrolo[1,2-a]quinoline derivatives is described. The process is initiated by di-tert-butyl peroxide-mediated C(sp3)-H activation to generate the α-functionalized radical, which supervenes a cascade radical addition/cyclization sequence to access the N-fused quinolines in good yields with broad functional group tolerance.

Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp3)-H to construct C-C bonds

Ether derivatives are widespread as essential building blocks in various drugs, natural products, agrochemicals, and materials. Modern economy requires developing green strategies with improved efficiency and reduction of waste. Due to its atom and step-economy, the cross-dehydrogenative coupling (CDC) reaction has become a major strategy for ether functionalization. This review covers C-H/C-H cross-coupling reactions of ether derivatives with various C-H bond substrates via non-noble metal catalysts (Fe, Cu, Co, Mn, Ni, Zn, Y, Sc, In, Ag). We discuss advances achieved in these CDC reactions and hope to attract interest in developing novel methodologies in this field of organic chemistry.

Uranyl nitrate as a recyclable homogeneous photocatalyst for selective cross-coupling of N-substituted amines and indoles

A homogeneous photocatalytic recyclable system for the selective radical-radical cross-coupling of N-substituted amines and indoles has been established. This system could conduct in water or acetonitrile, featuring the reuse of uranyl nitrate as the recyclable photocatalyst via a simple extraction. With this mild strategy in hand, good to excellent yields of cross-coupling products could be achieved even under the irradiation of sunlight, including 26 natural product derivatives and 16 natural product inspired re-engineered compounds. A radical-radical cross-coupling mechanism was newly proposed based on experimental evidence and reported literature. This strategy has been also applied to a gram scale synthesis to demonstrate its practical utility.

On-resin Cα-functionalization of N-arylglycinyl peptides with boronic acids

A late-stage α-C-H functionalization reaction of resin-bound, electron-rich N-aryl peptides with boronic acid nucleophiles under mild conditions is reported. We explore the impact of the N-arylglycinyl peptide structure on reactivity, and present a scope of the optimized reaction where both the peptide sequence and nature of boronic acid derivatives are varied.

Construction of Fluorinated Amino Acid Derivatives via Cobalt-Catalyzed Oxidative Difunctionalization of Cyclic Ethers

Via difunctionalization of the α- and β-sites of cyclic ethers, we herein demonstrate a new synthetic method for the efficient construction of novel fluorinated γ-amino acid esters by employing a CoBr₂/m-CPBA catalyst system. Several cyclic ethers were transformed in combination with a vast range of amines and ethyl trifluoropyruvate into the desired products under mild conditions, making this method a practical platform to enrich the library of fluorinated amino acid derivatives from cost-effective and readily available feedstocks.

Visible-Light-Driven Photoredox-Catalyzed C(sp3)-C(sp3) Cross-Coupling of N-arylamines with Cycloketone Oxime Esters

A novel photoredox-catalyzed C(sp3)-C(sp3) cross-coupling between N-arylamines and cycloketone oxime esters under mild conditions has been accomplished. The redox-neutral reaction proceeds good functional group tolerance and excellent regioselectivity without any...

Copper-Catalyzed Direct Allenylation of Inactive Cyclic Ethers

We report here a direct allenylation reaction of inactive cyclic ethers. The reaction proceeds through a copper-catalyzed 1,4-difunctionalization of 1,3-enynes, with cyano group installed at the allenes simultaneously. This methodology shows a broad functional group compatibility to 1,3-enynes. Diversified allene-modified cyclic ether derivatives were synthesized with high regioselectivity under mild conditions.

Cyanation of glycine derivatives

We report a catalytic oxidative C-H cyanation of glycine derivatives using a simple copper(i) catalyst with NFSI as an oxidant via a radical process to furnish α-cyano glycine derivatives, which are useful intermediates for organic synthesis. CuCl acted as both a one-electron reductant and a transition-metal catalyst in this transformation. NFSI served as a one-electron oxidant and generated a N-centered radical as a H-abstractor. The reaction displayed broad substrate scope and mild reaction conditions.

Visible-light promoted α-alkylation of glycine derivatives with alkyl boronic acids

A visible-light-mediated aerobic α-alkylation reaction of glycine derivatives with alkyl boronic acids has been established in the presence of a Ru/Cu catalyst system, giving the desired radical coupling products efficiently.

Copper-promoted cyanoalkylation/ring-expansion of vinylcyclopropanes with α-C-H bonds in alkylnitriles toward 3,4-dihydronaphthalenes

A copper-promoted oxidative cyanomethylation/ring-expansion of vinylcyclopropanes with α-C(sp3)-H bonds in alkyl nitriles is established for the generation of 1-cyanoethylated 3,4-dihydronaphthalenes. This cyanomethylation/ring-expansion involves a radical pathway and proceeds via cyanomethyl radical formation, radical addition and ring-expansion. This ring-expansion strategy offers a highly atom-economical route for the construction of nitrile-containing 3,4-dihydronaphthalenes, which can be transformed into other useful products under simple conditions.

On the Mechanism of Cross-Dehydrogenative Couplings between N-aryl Glycinates and Indoles: A Computational Study

Despite the widespread use of cross-dehydrogenative couplings in modern organic synthesis, mechanistic studies are still rare in the literature and those applied to α-amino carbonyl compounds remain virtually unexplored. Herein, the mechanism of Co-catalyzed cross-dehydrogenative couplings of N-aryl glycinates with indoles is described. Density functional theory studies supported the formation of an imine-type intermediate as the more plausible transient electrophilic species. Likewise, key information regarding the role of the N-aryl group and free NH motif within the reaction outcome has been gained, which may set the stage for further developments in this field of expertise.

Revealing the Iron-Catalyzed β-Methyl Scission of tert-Butoxyl Radicals via the Mechanistic Studies of Carboazidation of Alkenes

We describe here a mechanistic study of the iron-catalyzed carboazidation of alkenes involving an intriguing metal-assisted β-methyl scission process. Although t-BuO radical has frequently been observed in experiments, the β-methyl scission from a t-BuO radical into a methyl radical and acetone is still broadly believed to be thermodynamically spontaneous and difficult to control. An iron-catalyzed β-methyl scission of t-BuO is investigated in this work. Compared to a free t-BuO radical, the coordination at the iron atom reduces the activation energy for the scission from 9.3 to 3.9 ~ 5.2 kcal/mol. The low activation energy makes the iron-catalyzed β-methyl scission of t-BuO radicals almost an incomparably facile process and explains the selective formation of methyl radicals at low temperature in the presence of some iron catalysts. In addition, a radical relay process and an outer-sphere radical azidation process in the iron-catalyzed carboazidation of alkenes are suggested by density functional theory (DFT) calculations.

Ketoxime peptide ligations: oxidative couplings of alkoxyamines to N-aryl peptides

Chemoselective ligation methods that preserve or introduce side chain diversity are critical for chemical synthesis of peptides and proteins. Starting from ketone substrates instead of aldehydes in oxime ligation reactions would allow substitution at the site of ligation; however, synthetic challenges to readily access ketone derivatives from common amino acid building blocks have precluded the widespread use of ketoxime peptide ligation reactions thus far. Moreover, ketones are typically much slower to react in condensation reactions compared to aldehydes. Here, one-pot catalyst-free oxidative couplings of α-substituted N-aryl peptides with alkoxyamines provide access to oxime linkages with diverse side chains. Electron-rich N-(p-Me2N-phenyl)-amino acids possessing substituents at the α-carbon were found to be uniquely capable of undergoing site-selective α-C-H oxidations in situ under an O2 atmosphere at neutral pH. Comparative studies with N-arylglycinyl peptides revealed that substitution at the α-carbon caused notable changes in reactivity, with greater sensitivity to solvent and buffer salt composition.

Base-promoted domino radical cyclization of 1,6-enynes

A good regioselective, high atom-economical and transition-metal-free method for the synthesis of α-functionalized ether derivatives via the domino radical cyclization of 1,6-enynes is described. A series of α-functionalized ether derivatives could be easily obtained in good yields with wide functional group tolerance by using less toxic and inexpensive Cs₂CO₃ as the base. The control experiment results show that the reaction involves a radical process. This strategy provides a regioselective way toward the formation of dual C-C bonds in one step.

TBHP promoted demethylation of α-amino carbonyl compounds: a concise approach to substituted γ-lactams

A novel tert-butyl hydroperoxide (TBHP) promoted CH₂-extrusion reaction of α-amino carbonyl compounds has been developed, which is driven by a demethylenation process to give various ring contraction products γ-lactams under radical conditions.

Palladium-catalyzed oxidative cross-coupling for the synthesis of α-amino ketones

A novel oxidative cross-coupling reaction for the synthesis of α-aryl α-amino ketones in the presence of palladium catalysts using T⁺BF₄⁻ as an oxidant has been developed. This transformation was achieved by direct C–H oxidation of α-aminocarbonyl compounds and arylation. The mild reaction has a broad reaction scope and gives desired α-aryl α-amino ketones in moderate to excellent yields.

A novel oxidative cross-coupling reaction for the synthesis of α-aryl α-amino ketones in the presence of palladium catalysts using T⁺BF₄⁻ as an oxidant has been developed.

Site-Selective Cu-Catalyzed Alkylation of α-Amino Acids and Peptides toward the Assembly of Quaternary Centers

The Cu^I -catalyzed selective α-alkylation of α-amino acid and peptide derivatives with 2-alkyl-1,3-dioxolanes is reported. This oxidative coupling is distinguished by its site-specificity, high diastereoselectivity, and chirality preservation and exhibits absolute chemoselectivity for N-aryl glycine motifs over other amino acid units. Collectively, the method allows for the assembly of challenging quaternary centers, as well as compounds derived from natural products of high structural complexity, which may provide ample opportunities for late-stage functionalization of peptides.

Direct C-4 alkylation of quinazoline N-oxides with ethers via an oxidative cross-coupling reaction under metal-free conditions

A novel and efficient protocol for the direct C-4 alkylation of quinazoline-3-oxides via radical oxidative coupling between quinazoline 3-oxides and ethers in the presence of TBHP was developed. The reaction proceeded smoothly under metal-free conditions, which provided quinazoline-containing heterocyclic molecules in moderate to good yields.

Site-Specific Functionalization of 1,3-Dioxolane with Imines: A Radical Chain Approach to Masked α-Amino Aldehydes

A thiol-promoted site-specific addition of 1,3-dioxolane to imines through a radical chain process is described. This process represents a metal-free and redox-neutral way to convert inexpensive materials to a broad range of protected α-amino aldehydes in good to excellent yields using only a catalytic amount of radical precursor. Control experiments revealed that both the thiol and a small amount of oxygen from air are indispensable to the success of this reaction.

Direct oxidative C-H alkynylation of N-carbamoyl tetrahydroisoquinolines and dihydroisoquinolines

An efficient oxidative C-H alkynylation of N-carbamoyl tetrahydroisoquinolines mediated by a TEMPO oxoammonium salt has been established. A variety of electronically varied N-carbamoyl tetrahydroisoquinolines reacted with a range of alkynyl potassium trifluoroborates smoothly under mild metal-free conditions. Dihydroisoquinolines were also suitable components for the reaction. The synthetic applicability of the method for facile access to structurally diverse bioactive molecules was further demonstrated.

FeCl3-Catalyzed synthesis of pyrrolo[1,2-a]quinoxaline derivatives from 1-(2-aminophenyl)pyrroles through annulation and cleavage of cyclic ethers

A straightforward Fe-catalyzed method for the synthesis of pyrrolo[1,2-a]quinoxalines from 1-(2-aminophenyl)pyrroles and cyclic ethers, which includes functionalization of C(sp³)-H bonds and the construction of C-C and C-N bonds, has been developed. The features of this reaction are Fe catalysis, low-cost and readily accessible starting materials. Moreover, this procedure exhibits good functional group tolerance and a series of pyrrolo[1,2-a]quinoxaline derivatives are obtained in moderate to good yields.

A copper/O2-mediated direct sp3 C–H/N–H cross-dehydrogen coupling reaction of acylated amines and N-aryl glycine esters

We described a mild and efficient CDC reaction between N-aryl glycine esters and various acylated amines for the synthesis of α-amino α-amide acid esters with an excellent regioselectivity in the presence of copper salts.