Cu-Catalyzed Sequential Dehydrogenation–Conjugate Addition for β-Functionalization of Saturated Ketones: Scope and Mechanism

An Approach for the Synthesis of Pyrazolo[1,5-a]pyrimidines via Cu(II)-Catalyzed [3+3] Annulation of Saturated Ketones with Aminopyrazoles

A one-step synthesis of diversely substituted pyrazolo[1,5-a]pyrimidines from saturated ketones and 3-aminopyrazoles is presented. This transformation involves the in situ formation of α,β-unsaturated ketones via a radical process, followed by [3+3] annulation with 3-aminopyrazoles in one pot. Mechanistic studies have shown that the dual C(sp³)-H bond functionalization of inactive ketones is required for the formation of the title compounds. Notably, this dehydrogenative coupling process provides access to a host of functionalized pyrazolo[1,5-a]pyrimidines with antitumor potential from commercially available substrates.

Bioinspired Network Analysis Enabled Divergent Syntheses and Structure Revision of Pentacyclic Cytochalasans

We accomplished the divergent total syntheses of ten pentacyclic cytochalasans (aspergillin PZ, trichodermone, trichoderones, flavipesines, and flavichalasines) from a common precursor aspochalasin D and revised the structures of trichoderone B, spicochalasin A, flavichalasine C, aspergilluchalasin based on structure network analysis of the cytochalasans biosynthetic pathways and DFT calculations. The key steps of the syntheses include transannular alkene/epoxyalkene and carbonyl-ene cyclizations to establish the C/D ring of pentacyclic aspochalasans. Our bioinspired approach to these pentacyclic cytochalasans validate the proposed biosynthetic speculation from a chemical view and provide a platform for the synthesis of more than 400 valuable cytochalasans bearing different macrocycles and amino-acid residues.

Copper-catalyzed domino synthesis of multi-substituted benzo[b]thiophene through radical cyclization using xanthate as a sulfur surrogate

The Cu-catalyzed domino synthesis of multi-substituted benzo[b]thiophene through radical cyclization of 2-iodophenyl ketones was developed using xanthate as a sulfur surrogate. This method was extended to obtain tetracyclic Lupinalbin analogues through double C-S/C-O bond formation by changing the substituents. The products were converted to a HTI photoswitch, benzothiophene-fused flavone.

Copper-catalyzed cyclization reaction: synthesis of trifluoromethylated indolinyl ketones

A novel, simple, effective and rapid synthetic method to construct the C-2 trifluoromethylated indolinyl ketones via a copper-catalyzed cyclization reaction between N-alkylaniline and β-(trifluoromethyl)-α,β-unsaturated enones was developed. The results of the control experiments show that the reaction may involve a radical mechanism by a single-electron transfer process. Moreover, a broad substrate scope and good functional groups, high diastereoselectivities (dr, up to >20 : 1) as well as gram-scale synthesis make this approach highly attractive.

1,2-Aryl Migration Induced by Amide C-N Bond-Formation: Reaction of Alkyl Aryl Ketones with Primary Amines Towards α,α-Diaryl β,γ-Unsaturated γ-Lactams

Rearrangement reactions incorporated into cascade reactions play an important role in rapidly increasing molecular complexity from readily available starting materials. Reported here is a Cu-catalyzed cascade reaction of α-(hetero)aryl-substituted alkyl (hetero)aryl ketones with primary amines that incorporates an unusual 1,2-aryl migration induced by amide C-N bond formation to produce a class of structurally novel α,α-diaryl β,γ-unsaturated γ-lactams in generally good-to-excellent yields. This cascade reaction has a broad substrate scope with respect to primary amines, allows a wide spectrum of (hetero)aryl groups to smoothly undergo 1,2-migration, and tolerates electronically diverse α-substituents on the (hetero)aryl ring of the ketones. Mechanistically, this 1,2-aryl migration may stem from the intramolecular amide C-N bond formation which induces nucleophilic migration of the aryl group from the acyl carbon center to the electrophilic carbon center that is conjugated with the resulting iminium moiety.

Platinum-Catalyzed α,β-Desaturation of Cyclic Ketones through Direct Metal-Enolate Formation

The development of a platinum-catalyzed desaturation of cyclic ketones to their conjugated α,β-unsaturated counterparts is reported in this full article. A unique diene-platinum complex was identified to be an efficient catalyst, which enables direct metal-enolate formation. The reaction operates under mild conditions without using strong bases or acids. Good to excellent yields can be achieved for diverse and complex scaffolds. A wide range of functional groups, including those sensitive to acids, bases/nucleophiles, or palladium species, are tolerated, which represents a distinct feature from other known desaturation methods. Mechanistically, this platinum catalysis exhibits a fast and reversible α-deprotonation followed by a rate-determining β-hydrogen elimination process, which is different from the prior Pd-catalyzed desaturation method. Promising preliminary enantioselective desaturation using a chiral-diene-platinum complex has also been obtained.

Palladium-catalyzed sequential acylation/annulation of indoles with acyl chlorides using primary amine as the directing group

An efficient, palladium(ii)-catalyzed, C(sp²)–H acylation/annulation of indoles with acyl chlorides for the synthesis of substituted indolo[1,2-a]quinazolines is reported.

Microwave-Assisted Copper Catalysis of α-Difluorinated gem-Diol toward Difluoroalkyl Radical for Hydrodifluoroalkylation of para-Quinone Methides

Reported herein is a unified strategy to generate difluoroalkyl radicals from readily prepared α-difluorinated gem-diols by single electron oxidation. Under microwave irradiation, a catalytic amount of oxidant Cu(OAc)₂ succeeds in the formation of transient difluoroalkyl radicals in situ, for the first time. The reaction features a simple protocol, short reaction time, scalability, and high yield. The synthetic utility of this new methodology was also explored for the synthesis of difluoroalkylated spiro-cyclohexadienones, which is an important core structure in natural products and pharmaceuticals.

Copper(I)/DDQ-Mediated Double-Dehydrogenative Diels-Alder Reaction of Aryl Butenes with 1,4-Diketones and Indolones

A copper(I)/DDQ-mediated double-dehydrogenative Diels-Alder (DDDA) reaction of simple butenes with 1,4-diketones and indolones has been established for the first time. This strategy is based on a tandem double-dehydrogenation/Diels-Alder reaction from nonprefunctionalized starting materials, in which both a diene and dienophile were in situ generated via activation of fourfold inert C(sp³)-H bonds in one catalytic system.

Intramolecular β-Alkenylation of Cyclohexanones via Pd-Catalyzed Desaturation-Mediated C(sp3)-H/Alkyne Coupling

Site-selective C-C bond formation through the direct coupling of C(sp³)-H bonds with unsaturated hydrocarbons represents an atom-economical and redox-neutral way to functionalize chemically inert positions, such as those β to a carbonyl group. While most existing β-functionalization methods utilize a directing group (DG) strategy, here we report a Pd-catalyzed intramolecular β-alkenylation of ketones using alkynes as the coupling partner without the aid of DGs. Mediated by a ketone desaturation process, the reaction is redox-neutral and avoids using strong acids or bases. The resulting cis-5,6-fused bicycles can be diversely derivatized with excellent selectivity. Mechanistic studies imply an unusual "hydride-transfer" chain-like pathway, which involves the cyclometalation of an enyne intermediate and protonation of the resulting Pd enolate followed by an intermolecular hydride transfer through the desaturation of another substrate.

Cu/TEMPO catalyzed dehydrogenative 1,3-dipolar cycloaddition in the synthesis of spirooxindoles as potential antidiabetic agents

A series of spiro-[indoline-3,3'-pyrrolizin/pyrrolidin]-2-ones, 4, 5 and 6 were synthesized in a sequential manner from Cu-TEMPO catalyzed dehydrogenation of alkylated ketones, 1 followed by 1,3-dipolar cycloaddition of azomethine ylides via decarboxylative condensation of isatin, 2 and l-proline/sarcosine, 3 in high regioselectivities and yields. The detailed mechanistic studies were performed to identify the reaction intermediates, which revealed that the reaction proceeds via dehydrogenative cycloaddition. Additionally, the regio and stereochemistry of the synthesized derivatives were affirmed by 2D NMR spectroscopic studies. The synthesized derivatives were explored further with molecular docking, in vitro antioxidant, and anti-diabetic activities.

Oxidation of Enones for Regioselective [3+2] Cycloaddition through γ-Enone Radical Intermediates

Herein, an oxidization reaction of enones with a Cu^II complex that leads to a new type of regioselective [3+2] cycloaddition is reported. Highly functionalized cyclopentenes and spirocyclic compounds are obtained in moderate-to-good yields. This cycloaddition reaction occurred through the formation of γ-enone radicals, providing a rarely explored reactivity pattern for enones.

Synthesis of (E,E)-Dienones and (E,E)-Dienals via Palladium-Catalyzed γ,δ-Dehydrogenation of Enones and Enals

A new strategy for the synthesis of conjugated (E,E)-dienones and (E,E)-dienals via a palladium-catalyzed aerobic γ,δ-dehydrogenation of enones and enals has been developed. The method can be employed in the direct and efficient synthesis of various (E,E)-dienones and (E,E)-dienals, including non-substituted α-, β-, and γ- and/or δ-substituted (E,E)-dienones and (E,E)-dienals. The protocol is featured by the ready accessibility and elaboration of the starting materials, good functional group compatibility, and mild reaction conditions. Furthermore, the reaction is of complete E,E-stereoselectivity and uses molecular oxygen as the sole clean oxidant.

Dehalogenative Deuteration of Unactivated Alkyl Halides Using D2O as the Deuterium Source

The general dehalogenation of alkyl halides with zinc using D₂O or H₂O as a deuterium or hydrogen donor has been developed. The method provides an efficient and economic protocol for deuterium-labeled derivatives with a wide substrate scope under mild reaction conditions. Mechanistic studies indicated that a radical process is involved for the formation of organozinc intermediates. The facile hydrolysis of the organozinc intermediates provides the driving force for this transformation.

Copper-catalyzed dehydrogenative γ-C(sp3)-H amination of saturated ketones for synthesis of polysubstituted anilines

Metal-catalyzed β-C-H functionalization of saturated carbonyls via dehydrogenative desaturation proved to be a powerful tool for simplifying synthesis of valuable β-substituted carbonyls. Here, we report a copper-catalyzed dehydrogenative γ-C(sp3)-H amination of saturated ketones that initiates the three-component coupling of saturated ketones, amines and N-substituted maleimides to construct polysubstituted anilines. The protocol presented herein enables both linear and α-branched butanones to couple a wide spectrum of amines and various N-substituted maleimides to produce diverse tetra- or penta-substituted anilines in fair-to-excellent yields with good functional group tolerance. The mechanism studies support that this ketone dehydrogenative γ-C(sp3)-H amination was triggered by the ketone α,β-dehydrogenation desaturation that activates the adjacent γ-C(sp3)-H bond towards functionalization. This α,β-dehydrogenation desaturation-triggered cascade sequence opens up a new avenue to the remote C(sp3)-H functionalization of saturated ketones and has the potential to enable the rapid syntheses of complex compounds from simple starting materials.

Lactone Synthesis by Enantioselective Orthogonal Tandem Catalysis

In this work, we report enantioselective orthogonal tandem catalysis for the one pot conversion of Meldrum's acid derivatives and alkynes into δ-lactones. This new transformation, which resembles a formal [4+2] cycloaddition with concomitant decarboxylation and loss of acetone, proceeds in high yields and excellent enantioselectivity (up to 99 % ee) over a broad substrate scope. The products are densely functionalized and ripe for further transformations, as demonstrated here by both ring-opening reactions and reduction to saturated lactones. It was discovered that a new and serendipitously formed Ag^I -Me-StackPhos complex efficiently catalyzes the highly selective 6-endo-dig cyclization, completely reversing the regiochemistry that has been previously reported in related systems. More generally, in this study we identify a pair of compatible catalysts for alkyne difunctionalization that operate concurrently, which enable the alkyne to act as both a nucleophile and an electrophile in sequential one-pot transformations.

Allyl-Nickel Catalysis Enables Carbonyl Dehydrogenation and Oxidative Cycloalkenylation of Ketones

We herein disclose the first report of a first-row transition metal-catalyzed α,β-dehydrogenation of carbonyl compounds using allyl-nickel catalysis. This development overcomes several limitations of previously reported allyl-palladium-catalyzed oxidation, and is further leveraged for the development of an oxidative cycloalkenylation reaction that provides access to bicycloalkenones with fused, bridged, and spirocyclic ring systems using unactivated ketone and alkene precursors.

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

Visible light-induced direct α C-H functionalization of alcohols

Considering the synthetic value of introducing active alcoholic hydroxyl group, developing C-H functionalization of alcohols is of significance. Herein, we present a photochemical method that under visible light irradiation, selectfluor can effectively promote the oxidative cross-coupling between alcohols and heteroarenes without the external photocatalysis, achieving the selective α sp³ C-H arylation of alcohol, even in the presence of ether. The N-F activation of selectfluor under blue LEDs irradiation is evidenced by electron paramagnetic resonance (EPR) study, which is the key process for the oxidative activation of α sp³ C-H alcohols. The observed reactivity may have significant implications for chemical transformations.