Cobalt(II)-Catalyzed Synthesis of γ-Diketones from Aryl Alkenes and Its Utilization in the Synthesis of Various Heterocyclic Compounds

An earth-abundant Co(II) salt-catalyzed mild and affordable synthetic route has been developed for the synthesis of industrially relevant 1,4-dicarbonyl compounds (or γ-diketones) via oxidative coupling between aryl alkenes and ketones (both cyclic and acyclic) using TBHP and DBU as the oxidant and base, respectively. 1,4-Dicarbonyl compounds are known to be synthesized using expensive metal catalysts, dual catalysts, or low-cost metal complexes combined with an additive or ligand template, which further needs to be synthesized. Herein, we report the synthesis of 1,4-dicarbonyl compounds using cobalt(II) acetate as a catalyst without any expensive co-catalyst or ligand templates. This methodology has a broad substrate scope with significant yields and good functional group tolerance. Generation of unsymmetrical 1,4-dicarbonyls at room temperature and its versatile synthetic expansion to produce synthetically and biologically valuable heterocyclic compounds are salient features of this novel methodology. In addition, various controlled experiments such as primary kinetic isotope effect study, Hammett analysis with variation of the nature of the substituents on the styrene ring, and theoretical calculations (density functional theory) unravel the mechanistic intricacies involved in this new, simple, and atom-economic methodology.

Cu(II) Promoted C(sp3 )-H Activation in Unactivated Cycloalkanes: Oxo-Alkylation of Styrenes to Synthesize β-Disubstituted Ketones

We report the Cu(II) catalyzed synthesis of β-disubstituted ketones from styrene via oxo-alkylation with unactivated cycloalkanes as the alkylating agent in presence of tert-butylhydroperoxide (TBHP) and 1-methylimidazole as oxidant and base respectively. β-disubstituted ketones are known to be synthesized by using either expensive Ru/Ir complexes, or low-cost metal complexes (e. g., Fe, Mn) with activated species like aldehyde, acid, alcohol, or phthalimide derivatives as the alkylating agent, however, use of unactivated cycloalkanes directly as the alkylating agent remains challenging. A wide range of aliphatic C-H substrates as well as various olefinic arenes and heteroarene (35 substrates including 14 new substrates) are well-tolerated in this method. Hammett analysis shed more light on the substitution effect in the olefinic part on the overall mechanism. Furthermore, the controlled experiments, kinetic isotope effect study, and theoretical calculations (DFT) enable us to gain deeper insight of mechanistic intricacies of this new simple and atom-economic methodology.

Radical selenation of C(sp3)-H bonds to asymmetric selenides and mechanistic study

Selenides are important structural motifs with a broad range of biological activities and versatile transformational abilities. In this study, a novel and mild method was developed for the facile synthesis of asymmetric selenides under metal-free conditions. The key features of this reaction include good functional-group tolerance, the use of readily available reagents and cheap, low-toxicity solvent, and amenability to gram-scale synthesis. The results of preliminary radical-trapping experiments and a kinetic isotope effect study support a radical process.

Decatungstate-Photocatalyzed Direct Coupling of Inert Alkanes and Quinoxalin-2(1H)-ones with H2 Evolution

A tetrabutylammonium decatungstate (TBADT)-photocatalyzed direct coupling of inert alkanes and quinoxalin-2(1H)-ones with H2 evolution was developed at room temperature. The present transformation achieved the direct C(sp3)-H/C(sp2)-H coupling under noble metal-free,...

Transition-Metal-Free C(sp3)-H Coupling of Cycloalkanes Enabled by Single-Electron Transfer and Hydrogen Atom Transfer

Here we report a unique transition-metal-free C(sp³)-H/C(sp³)-H coupling of cycloalkanes at room temperature. Unactivated cycloalkanes and 2-azaallyls underwent the combination process of single-electron transfer (SET) and hydrogen atom transfer (HAT) to deliver a wide variety of cycloalkane-functionalized products. This expedient approach enables C(sp³)-H/C(sp³)-H coupling of cycloalkanes under mild conditions without transition metals, initiators, and oxidants.

Copper-catalyzed oxidative decarboxylative alkylation of cinnamic acids with 4-alkyl-1,4-dihydropyridines

We have developed a new oxidative decarboxylation of cinnamic acids with 4-alkyl-1,4-dihydropyridines to construct C(sp3)-C(sp2) bonds in the presence of copper catalyst and dicumyl peroxide (DCP). A variety of internal alkenes have been obtained with mild conditions, broad substrate scope and excellent functional group tolerance. This method has significant potential for application by using inexpensive and stable cinnamic acids instead of alkenyl halides and nitro-olefins.

Photoinduced site-selective alkenylation of alkanes and aldehydes with aryl alkenes

The dehydrogenative alkenylation of C-H bonds with alkenes represents an atom- and step-economical approach for olefin synthesis and molecular editing. Site-selective alkenylation of alkanes and aldehydes with the C-H substrate as the limiting reagent holds significant synthetic value. We herein report a photocatalytic method for the direct alkenylation of alkanes and aldehydes with aryl alkenes in the absence of any external oxidant. A diverse range of commodity feedstocks and pharmaceutical compounds are smoothly alkenylated in useful yields with the C-H partner as the limiting reagent. The late-stage alkenylation of complex molecules occurs with high levels of site selectivity for sterically accessible and electron-rich C-H bonds. This strategy relies on the synergistic combination of direct hydrogen atom transfer photocatalysis with cobaloxime-mediated hydrogen-evolution cross-coupling, which promises to inspire additional perspectives for selective C-H functionalizations in a green manner.

Iron catalyzed ketoalkylation and ketoalkylation/etherification of styrenes initiated by selective C–C bond cleavage

A mild and efficient iron-catalyzed ketoalkyl-Heck-type coupling initiated by radical C–C bond cleavage is described. Furthermore, this concise catalytic system was also applicable for the three-component ketoalkylation/etherification of styrenes.

Iron-catalyzed regioselective alkylation of 1,4-quinones and coumarins with functionalized alkyl bromides

A simple and efficient Fe-catalyzed regioselective alkylation of 1,4-quinones and coumarins, using functionalized alkyl bromides as alkylating reagents, has been developed.

Alternative pathways to α,β-unsaturated ketones via direct oxidative coupling transformation using Sr-doped LaCoO3 perovskite catalyst

A strontium-doped lanthanum cobaltite perovskite material was prepared, and used as a recyclable and effective heterogeneous catalyst for the direct oxidative coupling of alkenes with aromatic aldehydes to produce α,β-unsaturated ketones. The reaction afforded high yields in the presence of di-tert-butylperoxide as oxidant. Single oxides or salts of strontium, lanthanum and cobalt, and the undoped perovskite offered a lower catalytic activity than the strontium-doped perovskite. Benzaldehyde could be replaced by benzyl alcohol, dibenzyl ether, 2-oxo-2-phenylacetaldehyde, 2-bromoacetophenone or (dimethoxymethyl) benzene in the oxidative coupling reaction with alkenes. To our best knowledge, reactions between these starting materials with alkenes are new and unknown in the literature.

Diacetyl as a "traceless" visible light photosensitizer in metal-free cross-dehydrogenative coupling reactions

Minisci alkylation is of prime importance for its applicability in functionalizing diverse heteroarenes, which are core structures in many bioactive compounds. In alkyl radical generation processes, precious metal catalysts, high temperatures and excessive oxidants are generally involved, which lead to sustainability and safety concerns. Herein we report a new strategy using diacetyl (2,3-butanedione) as an abundant, visible light-sensitive and "traceless" hydrogen atom abstractor to achieve metal-free cross-dehydrogenative Minisci alkylation under mild conditions. Mechanistic studies supported hydrogen atom transfer (HAT) between an activated C(sp3)-H substrate and diacetyl. Moreover, with the assistance of di-tert-butyl peroxide (DTBP), the scope of the reaction could be extended to strong aliphatic C-H bonds via diacetyl-mediated energy transfer. The robustness of this strategy was demonstrated by functionalizing complex molecules such as quinine, fasudil, nicotine, menthol and alanine derivatives.

Copper-Catalyzed Oxidative C-H Bond Functionalization of N-Allylbenzamide for Regioselective C-N and C-O Bond Formation

Copper-catalyzed oxidative couplings of N-allylbenzamides for C-N and C-O bond formations have been developed through C-H bond functionalization. To demonstrate the utility of this approach, it was applied to the synthesis of β-aminoimides and imides. To the best of our knowledge, these are the first examples in which different classes of N-containing compounds have been directly prepared from the readily available N-allylbenzamides using an inexpensive catalyst/oxidant/base (CuSO₄ /TBHP/Cs₂ CO₃ ) system.

Direct dehydrogenative alkyl Heck-couplings of vinylarenes with umpolung aldehydes catalyzed by nickel

Alkenes are fundamental functionalities in nature and highly useful intermediates in organic synthesis, medicinal chemistry and material sciences. Transition-metal-catalyzed Heck couplings with organic halides as electrophiles have been established as a powerful protocol for the synthesis of this valuable building block. However, the requirement of organic halides and the generation of stoichiometric hazardous halide wastes may cause significant sustainable concerns. The halide-free oxidative Heck alkenylations involving organometallics or arenes as the coupling partners provide a facile and alternative pathway. Nonetheless, stoichiometric amounts of extra oxidant are essential in most cases. Herein, we present a direct dehydrogenative alkyl Heck-coupling reaction under oxidant-free conditions, liberating hydrogen, nitrogen and water as the side products. Excellent regioselectivity is achieved via neighboring oxygen atom coordination. Broad substrate scope, great functional group (ketone, ester, phenol, free amine, amide etc) tolerance and modification of pharmaceutical candidates and biological molecules exemplified its generality and practicability.

Regioselective Copper-Catalyzed Oxidative Coupling of α-Alkylated Styrenes with Tertiary Alkyl Radicals

A radical-mediated oxidative cross-coupling of readily accessible α-alkylated styrenes with 1,3-dicarbonyl compounds utilizing a combination of Cu(OAc)₂ and air as a catalytic system is described. Rather than requiring α-halocarbonyl compounds, this efficient approach enables direct installation of tertiary functionalized alkyl motifs to olefins with simple carbonyl derivatives. The novel protocol is characterized with high allylic selectivities via a competing β-H elimination. Both radical-clock and -trapping experiments provided clear-cut evidence for the intermediacy of an α-keto carbon-centered radical.

Nickel-Catalyzed Alkylarylation of Activated Alkenes with Benzyl-amines via C-N Bond Activation

A nickel-catalyzed alkylarylation of active alkenes with tertiary benzylamines was achieved by charge-transfer-complex promoted C-N bond activation. The reaction proceeded through initial Ni-catalyzed C-N bond activation, followed by sequential radical addition, redox and proton abstraction with cleaved amine moiety in the absence of oxidant, and provides an efficient method to prepare various alkyl-substituted oxindoles and dihydroquinolinones in good yields.

C-H functionalisation of cycloalkanes

Among organic compounds hydrocarbons are inexpensive and possibly the most abundant among natural resources. Developing strategies for selective functionalisation of inert hydrocarbon C-H bonds is one of the most ideal synthetic paths that a synthetic chemist could think of. This critical review focuses on the recent development of various directed and non-directed cycloalkylations leading to the formation of carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds. Apart from various transition metal catalysed cycloalkylations, this review also covers various metal-free cycloalkylation processes.

Peroxy mediated Csp2–Csp3 dehydrogenative coupling: regioselective functionalization of coumarins and coumarin-3-carboxylic acids

Regioselective functionalization of coumarins/coumarin carboxylic acids at C-3 via activation of Csp³–H bonds of ethers under metal-free conditions is developed.

Fe-Catalyzed three-component carboazidation of alkenes with alkanes and trimethylsilyl azide

Iron out the difference. Carboazidation of alkenes using cycloalkanes, CH₂Cl₂, CHCl₃ and CCl₄ as alkylating reagents proceeded smoothly in the presence of TMSN₃, DTBP and a catalytic amount of Fe(acac)₃ to afford chain extended alkyl azides and γ-azido chloroalkanes in good to high yields.

Iodine-mediated cross-dehydrogenative coupling of pyrazolones and alkenes

An iodine-mediated alkenylation of pyrazolones with simple alkenes under air has been developed, leading directly to alkenylated pyrazolone derivatives in good yield.

Transforming Olefins into γ,δ-Unsaturated Nitriles through Copper Catalysis

We have developed a strategy to transform olefins into homoallylic nitriles through a mechanism that combines copper catalysis with alkyl nitrile radicals. The radicals are easily generated from alkyl nitriles in the presence of the mild oxidant di-tert-butyl peroxide. This cross-dehydrogenative coupling between simple olefins and alkylnitriles bears advantages over the conventional use of halides and toxic cyanide reagents. With this method, we showcase the facile synthesis of a flavoring agent, a natural product, and a polymer precursor from simple olefins.

Copper-catalyzed radical cascade oxyalkylation of olefinic amides with simple alkanes: highly efficient access to benzoxazines

A copper-catalyzed C(sp³)-H bond functionalization of simple alkanes with olefinic amides was developed for the efficient synthesis of important benzoxazine derivatives. It involves new C-C and C-O bond formation in one step via a radical cascade process.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Photocatalytic Dehydrogenative Cross-Coupling of Alkenes with Alcohols or Azoles without External Oxidant

Direct cross-coupling between alkenes/R-H or alkenes/RXH is a dream reaction, especially without external oxidants. Inputting energy by photocatalysis and employing a cobalt catalyst as a two-electron acceptor, a direct C-H/X-H cross-coupling with H₂ evolution has been achieved for C-O and C-N bond formation. A new radical alkenylation using alkene as the redox compound is presented. A wide range of aliphatic alcohols-even long chain alcohols-are tolerated well in this system, providing a new route to multi-substituted enol ether derivatives using simple alkenes. Additionally, this protocol can also be used for N-vinylazole synthesis. Mechanistic insights reveal that the cobalt catalyst oxidizes the photocatalyst to revive the photocatalytic cycle.