Sustainable Aerobic Allylic C-H Bond Oxidation with Heterogeneous Iron Catalyst

Advancing Propylene Epoxidation: the Role of Ethyl Acetate Autoxidation via Cobalt-Nickel Catalyzed C(acyl)─O Bond Scission

The selective autoxidation for the synthesis of valuable oxygenates has provoked keen interest from both academic and industrial sectors. Although the generation of reactive oxygen species via oxygen attack on C─H bonds near ester linkages is well-established, research into aliphatic ester oxidation has primarily focused on combustion, neglecting their potential utility in oxidation processes. Herein, a protocol for producing propylene oxide through the autoxidation of ethyl acetate in tandem with propylene epoxidation is demonstrated. The ethoxy radical, generated by ester C(acyl)─O bond cleavage in situ, subsequently underwent proton-coupled electron transfer with the Co(OAc)(μ-H2O)2Ni, followed by the formation of the peracetic acid optimally suited for the epoxidation reaction. The research not only eliminates the need for co-substrates in the epoxidation process but also fills the application gap in bulk-ester autoxidation, offering insights into the effective utilization of oxy-intermediates in autoxidation reactions.

Flow Electroreductive Nickel-Catalyzed Cyclopropanation of Alkenes Using gem-Dichloroalkanes

Cyclopropanes are valuable motifs in organic synthesis, widely featured in pharmaceuticals and functional materials. Herein, we report an efficient electrochemical methodology for the cyclopropanation of alkenes, leveraging a nickel-catalyzed process in continuous-flow. The developed protocol demonstrates broad substrate scope, accommodating both electron-rich and electron-poor alkenes with high functional group tolerance. Beyond dichloromethane as a feedstock methylene source, the methodology enables the synthesis of methylated, deuterated, and chloro-substituted cyclopropanes. Mechanistic investigations suggest the electro-generation of a nickel carbene as key intermediate. Notably, the reaction operates under ambient conditions, tolerates air and moisture, and achieves scalability through continuous-flow technology, offering a straightforward route to multi-gram quantities with enhanced throughput.

Palladium-catalyzed allylic C-H alkylation of terminal olefins with 3-carboxamide oxindoles

A novel palladium-catalyzed allylic C-H alkylation of terminal olefins with 3-carboxamide oxindoles is described. A variety of new 3-carboxamide-3-allylation oxindoles with an all-carbon quaternary center were obtained in moderate to good yields (up to 99%). In addition, the asymmetric version of this reaction was also explored, providing moderate enantioselectivity.

Bioinspired Synthesis of Cucurbalsaminones B and C

Cucurbalsaminones B (1) and C (2) are two abeo-cucurbitane triterpenoids with a unique 5/6/3/6/5-fused ring system and exhibit potent multidrug resistance (MDR)-reversing activity. Herein, we report the first synthesis of these two natural products, both of them were accomplished in 14 steps from commercially available inexpensive resource compound lanosterol. Key features of this synthesis include a biomimetic tandem Wagner-Meerwein type lanostane-to-cucurbitane rearrangement followed by a bioinspired photochemical oxa-di-π-methane (ODPM) rearrangement to complete the skeleton construction and an Eosin Y photoinduced Barton-McCombie deoxygenation to realize the challenging oxidation state adjustment of the sterically hindered C11 position.

Metal-Free Electrochemical Allylic C-H Aerobic Oxidation

A scalable and sustainable electrochemical protocol for allylic C-H aerobic oxidation has been developed, enabling the formation of enones without the use of stoichiometric toxic oxidants or metal catalysts and offering an environmentally benign alternative to traditional chemical oxidation techniques. The process has been successfully applied to selectively oxidize a series of natural products and drug molecules, underscoring its potential for widespread adoption in both academic and industrial contexts.

Cobalt-Catalyzed Synthesis of Alkenes and Vinyl Sulfones Via Dehydrogenative Coupling of Alcohols and Sulfones

A novel protocol to access vinyl sulfones and internal/terminal olefins via cobalt-catalyzed acceptorless dehydrogenation coupling (ADC) has been established. This system enables the divergent synthesis of three kinds of olefin compounds through the coupling of alcohols and sulfones under oxidant-free conditions. The broad applicability of this procedure is demonstrated by over forty olefin products, including pharmaceutical-related compounds and complex substrates, in a one-pot process. Preliminary mechanistic studies were conducted, and a proposed reaction pathway was presented.

Na-Promoted Bimetallic Hydroxide Nanoparticles for Aerobic C-H Activation: Catalyst Design Principles and Insights into Reaction Mechanism

A precious metal-free bimetallic FexMn1-x(OH)y hydroxide catalyst was developed that is capable of catalyzing aerobic C-H oxidation reactions at low temperatures, without the need for an initiator, relying sustainably on molecular oxygen. Through a systematic synthetic effort, we scanned a wide nanoparticle synthesis parameter space to lay out a detailed set of catalyst design principles unraveling how the Fe/Mn cation ratio, NaOH(aq) concentration used in the synthesis, catalyst washing procedures, extent of residual Na+ promoters on the catalyst surface, reaction temperature, and catalyst loading influence catalytic C-H activation performance as a function of the electronic, surface chemical, and crystal structure of FexMn1-x(OH)y bimetallic hydroxide nanostructures. Our comprehensive XRD, XPS, BET, ICP-MS, 1H NMR, and XANES structural/product characterization results as well as mechanistic kinetic isotope effect (KIE) studies provided the following valuable insights into the molecular level origins of the catalytic performance of the bimetallic FexMn1-x(OH)y hydroxide nanostructures: (i) catalytic reactivity is due to the coexistence and synergistic operation of Fe3+ and Mn3+ cationic sites (with minor contributions from Fe2+ and Mn2+ sites) on the catalyst surface, where in the absence of one of these synergistic sites (i.e., in the presence of monometallic hydroxides), catalytic activity almost entirely vanishes, (ii) residual Na+ species on the catalyst surface act as efficient electronic promoters by increasing the electron density on the Fe3+ and Mn3+ cationic sites, which in turn, presumably enhance the electrophilic adsorption of organic reactants and strengthen the interaction between molecular oxygen and the catalyst surface, (iii) in the fluorene oxidation reaction the step dictating the reaction rate likely involved the breaking of a C-H bond (kH/kD = 2.4), (iv) reactivity patterns of a variety of alkylarene substrates indicate that the C-H bond cleavage follows a stepwise PT-ET (proton transfer-electron transfer) pathway.

Direct oxygen insertion into C-C bond of styrenes with air

Skeletal editing of single-atom insertion to basic chemicals has been demonstrated as an efficient strategy for the discovery of structurally diversified compounds. Previous endeavors in skeletal editing have successfully facilitated the insertion of boron, nitrogen, and carbon atoms. Given the prevalence of oxygen atoms in biologically active molecules, the direct oxygenation of C-C bonds through single-oxygen-atom insertion like Baeyer-Villiger reaction is of particular significance. Herein, we present an approach for the skeletal modification of styrenes using O2 via oxygen insertion, resulting in the formation of aryl ether frameworks under mild reaction conditions. The broad functional-group tolerance and the excellent chemo- and regioselectivity are demonstrated in this protocol. A preliminary mechanistic study indicates the potential involvement of 1,2-aryl radical migration in this reaction.

Tosylhydrazide-Induced 1,6-Enyne Radical Cyclization under Copper Catalysis: Access to 3,4-Dihydronaphthalen-1(2H)-one Derivatives

We describe an approach to access 4-aroyl-3-aryl-3,4-dihydronaphthalen-1(2H)-one derivatives in 41-79% yields through the Cu-catalyzed radical cyclization/desulfonylation of 1,6-enynes with tosylhydrazide under air conditions. This alternative desulfonylation strategy combines mild conditions, external oxidant-free processes, and sustainability, contributing to more environmentally friendly organic synthesis. The mechanistic studies showed that the CuCl/O2 combination serves as the source of the oxygen atom needed to form the C═O bond. The existence of tosylhydrazide is crucial for this conversion.

Single-Atom Fe-Catalyzed Acceptorless Dehydrogenative Coupling to Quinolines

A single-atom iron catalyst was found to exhibit exceptional reactivity in acceptorless dehydrogenative coupling for quinoline synthesis, outperforming known homogeneous and nanocatalyst systems. Detailed characterizations, including aberration-corrected HAADF-STEM, XANES, and EXAFS, jointly confirmed the presence of atomically dispersed iron centers. Various functionalized quinolines were efficiently synthesized from different amino alcohols and a range of ketones or alcohols. The iron single-atom catalyst achieved a turnover number (TON) of up to 105, far exceeding the results of current homogeneous and nanocatalyst systems. Detailed mechanistic studies verified the significance of single-atom Fe sites in the dehydrogenation process.

Cobalt-Catalyzed Acceptorless Dehydrogenation of Primary Amines to Nitriles

The direct double dehydrogenation from primary amines to nitriles without an oxidant or hydrogen acceptor is both intriguing and challenging. In this paper, we describe a non-noble metal catalyst capable of realizing such a transformation with high efficiency. A cobalt-centered N,N-bidentate complex was designed and employed as a metal-ligand cooperative dehydrogenation catalyst. Detailed kinetic studies, control experiments, and DFT calculations revealed the crucial hydride transfer, proton transfer, and hydrogen evolution processes. Finally, a tandem outer-sphere/inner-sphere mechanism was proposed for the dehydrogenation of amines to nitriles through an imine intermediate.