Palladium-Catalyzed Oxidation Reactions of Alkenes with Green Oxidants

Palladium-catalyzed C-C bond cleavage of N-cyclopropyl acylhydrazones

Despite their utility as directing groups, the C-C bond cleavage of cyclopropanes utilizing hydrazones has not been explored. Herein, Pd-catalyzed C-C bond cleavage reaction of N-cyclopropyl acylhydrazones, followed by cycloisomerization to yield pyrazoles, has been developed. The protocol enables the synthesis of various α-pyrazole carbonyl compounds, which have a potential of biological activity. Control experiments and DFT calculations suggest that β-carbon elimination of a stable 6-membered chelate palladium complex occurs, generating a conjugated azine as a reaction intermediate for the following cycloisomerization.

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

Emerging techniques are revolutionizing the realm of chemical synthesis by introducing new avenues for C-H bond functionalization, which have been exploited for the synthesis of pharmaceuticals, natural compounds, and functional materials. Allylic C-H bond oxidation of alkenes serves as possibly the most employed C-H bond functionalization reaction. However, sustainable and selective approaches remain scarce, and the majority of the existing conditions still hinge on hazardous oxidants or costly metal catalysts. In this context, we introduce a heterogeneous iron catalyst that addresses the above-mentioned concerns by showcasing the aerobic oxidation of steroids, terpenes, and simple olefins to the corresponding enone products. This novel method provides a powerful tool for the arsenal of allylic C-H bond oxidation while minimizing the environmental concerns.

Markovnikov-Selective Cobalt-Catalyzed Wacker-Type Oxidation of Styrenes into Ketones under Ambient Conditions Enabled by Hydrogen Bonding

The replacement of palladium catalysts for Wacker-type oxidation of olefins into ketones by first-row transition metals is a relevant approach for searching more sustainable protocols. Besides highly sophisticated iron catalysts, all the other first-row transition metal complexes have only led to poor activities and selectivities. Herein, we show that the cobalt-tetraphenylporphyrin complex is a competent catalyst for the aerobic oxidation of styrenes into ketones with silanes as the hydrogen sources. Remarkably, under room temperature and air atmosphere, the reactions were exceedingly fast (up to 10 minutes) with a low catalyst loading (1 mol %) while keeping an excellent chemo- and Markovnikov-selectivity (up to 99 % of ketone). Unprecedently high TOF (864 h-1 ) and TON (5,800) were reached for the oxidation of aromatic olefins under these benign conditions. Mechanistic studies suggest a reaction mechanism similar to the Mukaiyama-type hydration of olefins with a change in the last fundamental step, which controls the chemoselectivity, thanks to a unique hydrogen bonding network between the ethanol solvent and the cobalt peroxo intermediate.

Rapid assembly of structurally diverse cyanamides and disulfanes via base-mediated aminoalkylation of aryl thiourea

A general method for the preparation of cyanamides and disulfanes from aryl thiourea and halide through a base-mediated strategy is described. Mercaptan and N-aryl cyanamide are the key intermediates in the reaction. The current method is convenient, eco-friendly, and has high yields for the synthesis of substituted cyanamide and functional disulfanes in a one-pot procedure from readily available starting materials.

Sustainable Wacker-Type Oxidations

The Wacker reaction is the oxidation of olefins to ketones and typically requires expensive and scarce palladium catalysts in the presence of an additional copper co-catalyst under harsh conditions (acidic media, high pressure of air/dioxygen, elevated temperatures). Such a transformation is relevant for industry, as shown by the synthesis of acetaldehyde from ethylene as well as for fine-chemicals, because of the versatility of a carbonyl group placed at specific positions. In this regard, many contributions have focused on controlling the chemo- and regioselectivity of the olefin oxidation by means of well-defined palladium catalysts under different sets of reaction conditions. However, the development of Wacker-type processes that avoid the use of palladium catalysts has just emerged in the last few years, thereby paving the way for the generation of more sustainable procedures, including milder reaction conditions and green chemistry technologies. In this Minireview, we discuss the development of new catalytic processes that utilize more benign catalysts and sustainable reaction conditions.

Facile synthesis of isoquinolines and isoquinoline N-oxides via a copper-catalyzed intramolecular cyclization in water

A highly efficient method for the facile access of isoquinolines and isoquinoline N-oxides via a Cu(i)-catalyzed intramolecular cyclization of (E)-2-alkynylaryl oxime derivatives in water has been developed. This protocol was performed under simple and mild conditions without organic solvent, additives or ligands. By switching on/off a hydroxyl protecting group of oximes, the selective N-O/O-H cleavage could be triggered, delivering a series of isoquinolines and isoquinoline N-oxides, respectively, in moderate to high yields with good functional group tolerance and high atom economy. Moreover, the practicality of this method was further demonstrated by the total synthesis of moxaverine in five steps.

Oxidation of Styrene to Benzaldehyde Catalyzed by Schiff Base Functionalized Triazolylidene Ni(II) Complexes

Four new Schiff base functionalized 1,2,3-triazolylidene nickel complexes, [Ni-(L1NHC)2](PF6)2; 3, [Ni-(L2NHC)2](PF6)2; 4, [Ni-(L3NHC)](PF6)2; 7 and [Ni-(L4NHC)](PF6)2; 8, (where L1NHC = (E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 1, L2NHC = (E)-3-methyl-4-(2-((phenethylimino)methyl)phenyl)-1-propyl-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 2, L3NHC = 4,4′-(((1E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2,1-phenylene))bis(3-methyl-1-propyl-1H-1,2,3-triazol-3-ium) hexafluorophosphate(V), 5, and L4NHC = 4,4′-(((1E)-(butane-1,4-diylbis(azanylylidene))bis(methanylylidene))bis(2,1-phenylene))bis(3-methyl-1-propyl-1H-1,2,3-triazol-3-ium) hexafluorophosphate(V), 6), were synthesised and characterised by a variety of spectroscopic methods. Square planar geometry was proposed for all the nickel complexes. The catalytic potential of the complexes was explored in the oxidation of styrene to benzaldehyde, using hydrogen peroxide as a green oxidant in the presence of acetonitrile at 80 °C. All complexes showed good catalytic activity with high selectivity to benzaldehyde. Complex 3 gave a conversion of 88% and a selectivity of 70% to benzaldehyde in 6 h. However, complexes 4 and 7–8 gave lower conversions of 48–74% but with higher (up to 90%) selectivity to benzaldehyde. Results from kinetics studies determined the activation energy for the catalytic oxidation reaction as 65 ± 3 kJ/mol, first order in catalyst and fractional order in the oxidant. Results from UV-visible and CV studies of the catalytic activity of the Ni-triazolylidene complexes on styrene oxidation did not indicate any clear possibility of generation of a Ni(II) to Ni(III) catalytic cycle.

Palladium(II)-catalyzed enantioselective intermolecular oxidative diarylation of internal enamides

The construction of vicinal stereogenic centers via the simultaneous formation of two C-C bonds across alkenes under oxidative conditions is a stubborn challenge. Herein, we report a Pd(II)-catalyzed highly enantioselective intermolecular oxidative 1,2-diarylation reaction of internal enamides with aryl boronic acids, enabling the expedient construction of two vicinal stereocenters with excellent diastereo-, and enantioselectivities.

Scalable Electrochemical Aerobic Oxygenation of Indoles to Isatins without Electron Transfer Mediators by Merging with an Oxygen Reduction Reaction

An approach to electrochemical oxygenation of indoles leading to isatins was developed by merging with a complementary cathode oxygen reduction reaction. The features of this green protocol include the use of molecular oxygen as the sole oxidant, it being free of an electron transfer mediator, and gram-scale preparation. Mechanistic studies suggested a radical process, and the two oxygen atoms in the isatins were both most likely from molecular oxygen. A detailed mechanism of the reaction utilizing density functional theory calculations was elucidated.

Unconventional mechanism and selectivity of the Pd-catalyzed C-H bond lactonization in aromatic carboxylic acid

The search for more effective and highly selective C-H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C-H oxidation reactions. Here, we reveal the stepwise intramolecular S_N2 nucleophilic substitution mechanism with the rate-limiting C-O bond formation step for the Pd(II)-catalyzed C(sp³)-H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C-O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O₂ oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η³-(π-benzylic)-Pd and K⁺-O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η³-(π-benzylic)-Pd and η²-(π-phenyl)-Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K⁺ cation in the ortho-C-H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp³)-H bond lactonization in the presence of C(sp²)-H.

Highly Effective and Chemoselective Hydrodeoxygenation of Aromatic Alcohols

Effective hydrodeoxygenation (HDO) of aromatic alcohols is very attractive in both conventional organic synthesis and upgrading of biomass-derived molecules, but the selectivity of this reaction is usually low because of...

Enantioselective Synthesis of Fused Isocoumarins via Palladium-Catalyzed Annulation of Alkyne-Tethered Malononitriles

An enantioselective palladium-catalyzed annulation of alkyne-tethered malononitriles for the synthesis of 3,4-ring-fused isocoumarins is described. This cascade strategy involves oxypalladation of ortho-alkynylbenzoates and desymmetrizing addition onto one cyano group of the pendant malononitriles, which enables the concurrent construction of two rings and an all-carbon quaternary stereocenter in a single operation.

Efficient Aerobic Oxidation of Organic Molecules by Multistep Electron Transfer

This Minireview presents recent important homogenous aerobic oxidative reactions which are assisted by electron transfer mediators (ETMs). Compared with direct oxidation by molecular oxygen (O2 ), the use of a coupled catalyst system with ETMs leads to a lower overall energy barrier via stepwise electron transfer. This cooperative catalytic process significantly facilitates the transport of electrons from the reduced form of the substrate-selective redox catalyst (SSRCred ) to O2 , thereby increasing the efficiency of the aerobic oxidation. In this Minireview, we have summarized the advances accomplished in recent years in transition-metal-catalyzed as well as metal-free aerobic oxidations of organic molecules in the presence of ETMs. In addition, the recent progress of photochemical and electrochemical oxidative functionalization using ETMs and O2 as the terminal oxidant is also highlighted. Furthermore, the mechanisms of these transformations are showcased.

Visible-Light-Driven Selective Air-Oxygenation of C-H Bond via CeCl3 Catalysis in Water

Visible-light-induced C-H aerobic oxidation is an important chemical transformation that can be applied for the synthesis of aromatic ketones. High-cost catalysts and toxic solvents were generally needed in the present methodologies. Here, an efficient aqueous C-H aerobic oxidation protocol was reported. Through CeCl₃ -mediated photocatalysis, a series of aromatic ketones were produced in moderate to excellent yields. With air as the oxidant, this reaction could be performed under mild conditions in water and demonstrated high activity and functional group tolerance. This method is economical, highly efficient, and environmentally friendly, and it will provide inspiration for the development of aqueous photochemical synthesis reactions.

Thiol-initiated photocatalytic oxidative cleavage of the CC bond in olefins and its extension to direct production of acetals from olefins

Oxidative cleavage of a broad scope of olefins is realized over ZnIn₂S₄ under visible light, using air as oxidant and thiol as initiator. Coupled with the condensation between aldehydes/ketones and alcohols, this strategy can be used to yield acetals directly from olefins.

Efficient Palladium-Catalyzed Aerobic Oxidative Carbocyclization to Seven-Membered Heterocycles

The use of molecular oxygen in palladium-catalyzed oxidation reactions is highly widespread in organic chemistry. However, the direct reoxidation of palladium by O2 is often kinetically unfavored, thus leading the deactivation of the palladium catalyst during the catalytic cycle. In the present work, we report a highly selective palladium-catalyzed carbocyclization of bisallenes to seven-membered heterocycles under atmospheric pressure of O2 . The use of a homogenous hybrid catalyst (Co(salophen)-HQ, HQ=hydroquinone) significantly promotes efficient electron transfer between the palladium catalyst and O2 through a low-energy pathway. This aerobic oxidative transformation shows broad substrate scope and functional group compatibility and allowed the preparation of O-containing seven-membered rings in good yields in most cases.

Selective Hydroboration-Oxidation of Terminal Alkenes under Flow Conditions

An efficient flow process for the selective hydroboration and oxidation of different alkenes using 9-borabicyclo(3.3.1)nonane (9-BBN) allows facile conversion in high productivity (1.4 g h-1 ) of amorpha-4,11-diene to the corresponding alcohol, which is an advanced intermediate in the synthesis of the antimalarial drug artemisinin. The in situ reaction of borane and 1,5-cyclooctadiene using a simple flow generator proved to be a cost efficient solution for the generation of 9-BBN.

Palladium-catalyzed three-component cascade arylthiolation with aryldiazonium salts as S-arylation sources

A novel palladium-catalyzed three-component cascade arylthiolation for the assembly of 3-sulfenylindoles and 3-sulfenylbenzofurans is described, with aryldiazonium salts as the ideal S-arylation sources.