Progress in the synthesis of aldehydes from Pd-catalyzed Wacker-type reactions of terminal olefins

Palladium-free Wacker-inspired oxidation: challenges and opportunities in catalysis

Palladium-catalysed Wacker-type oxidation of olefins to ketones or aldehydes is one of the most prominent homogeneous reactions globally, even performed at a multi-tonne scale. From a fundamental and an applied point of view, it is extremely appealing to replace palladium catalysts by other metal-based catalysts to increase the efficiency, selectivity and sustainability, particularly considering the reactivity of well-defined first-row transition metal complexes as catalysts. In this case, the ligand(s) coordinating to the metal(s) play a major role in controlling selectivity and activity, thanks to unique mechanistic considerations. This mechanistically-driven feature article emphasizes the advantages and disadvantages of currently existing approaches. Besides the main efforts devoted to homogeneous catalysis, heterogenous systems and biocatalysis have also been studied as they offer complementary strategies. Overall, this review presents an up-to-date analysis of key contributions while highlighting existing gaps for future developments in this important and exciting field.

Pd anchored to layered double hydroxide modified with chitosan and Echinophora platyloba extract as a nanocatalyst for the formylation of aryl iodides with formic acid

The novel Zn-Cu-Al layered double hydroxide (LDH) encapsulated within a chitosan/glutaraldehyde matrix, designated as LDH@Cs/G@Pd, was synthesized through simplified methodologies for the preparation of aromatic aldehyde derivatives. Formic acid served as the carbon monoxide source and hydrogen donor, while chitosan/glutaraldehyde acted as the linking agent between the substrate and palladium nanoparticles, with Echinophora platyloba extract functioning as the reducing agent for palladium. The characterization of LDH@Cs/G@Pd was conducted using a variety of analytical techniques, including Fourier-transform infrared spectroscopy (FT-IR), energy-dispersive X-ray spectroscopy (EDS), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area analysis, and inductively coupled plasma optical emission spectroscopy (ICP-OES). The results indicate that the catalyst has been successfully synthesized and Showed promising characteristics for its intended application. Afterward, the catalyst was utilized to Synthesize aromatic aldehydes. The catalyst developed in this study demonstrated a synthesis yield of approximately 95 % for aromatic aldehydes, confirming its potential as an effective candidate for industrial applications.

Enantioselective Total Synthesis of (-)-Cyathin B2: A Desymmetric Double-Allylboration Approach

A powerful Pt-catalyzed asymmetric diboration/desymmetric double-allylboration cascade reaction has been developed for the construction of synthetically useful, densely functionalized hydrindanes with five stereocenters, including three quaternary ones, in good yields and excellent enantiomeric excess (ee) values within a single synthetic operation. A unified strategy utilizing this key tandem methodology enabled the concise asymmetric total synthesis of cyathane diterpene (-)-Cyathin B2 in 14 steps from commercially available starting materials, thereby demonstrating its remarkable potential in the synthesis of hydrindane-containing natural products and pharmaceuticals.

Preparation and isolation of mono-Nb substituted Keggin-type phosphomolybdic acid and its application as an oxidation catalyst for isobutylaldehyde and Wacker-type oxidation

The potassium and proton mixed salt of mono-Nb substituted Keggin-type phosphomolybdate, KH3[PMo11NbO40], was isolated in a pure form by reacting Keggin-type phosphomolybdic acid (H3[PMo12O40]) and potassium hexaniobate (K8Nb6O19) in water, followed by freeze-drying. The all protonic form, H4[PMo11NbO40], was isolated via proton exchange with H-resin and subsequent freeze-drying. The most crucial factor to isolate KH3[PMo11NbO40] and H4[PMo11NbO40] in pure forms is the evaporation of water using the freeze-drying method. Using a similar procedure, the potassium salt of the di-Nb substituted compound K5[PMo10Nb2O40] was isolated. H4[PMo11NbO40] exhibited high catalytic activity for oxidizing isobutylaldehyde to methacrolein and moderate catalytic activity for the Wacker-type oxidation of allyl phenyl ether when combined with Pd(OAc)2.

Palladium/Iron-Catalyzed Wacker-Type Oxidation of Aliphatic Terminal and Internal Alkenes Using O2

The Wacker-type oxidation of aliphatic terminal alkenes proceeds using a Pd/Fe catalyst system under mild reaction conditions using 1 atm O2 without other additives. The use of 1,2-dimethoxyethane/H2O as a mixed solvent was effective. The slow addition of alkenes is also important for improving product yields. Fe(III) citrate was the most efficient cocatalyst among the iron complexes examined, whereas other complexes such as FeSO4, Fe2(SO4)3, Fe(NO3)3, and Fe2O3 were also operative. This method is also applicable to aliphatic internal alkenes, which are generally difficult to oxidize using conventional Pd/Cu catalyst systems. The gram-scale synthesis and reuse of the Pd catalysts were also demonstrated.

Palladium-catalyzed intramolecular aza-Wacker-type cyclization of vinyl cyclopropanecarboxamides to access conformationally restricted aza[3.1.0]bicycles

A palladium(ii)-catalyzed intramolecular oxidative aza-Wacker-type reaction of vinyl cyclopropanecarboxamides to access a series of conformationally restricted highly substituted aza[3.1.0]bicycles is reported. The transformation proceeded through a typical aza-Wacker reaction mechanism to forge a new C-N bond with oxygen as the terminal oxidant. The desired fused heterocycles were obtained in moderate yields. The process is tolerant of a range of functional aryl groups under mild conditions.

Multi-site programmable functionalization of alkenes via controllable alkene isomerization

Direct and selective functionalization of hydrocarbon chains is a fundamental problem in synthetic chemistry. Conventional functionalization of C=C double bonds and C(sp3)-H bonds provides some solutions, but site diversity remains an issue. The merging of alkene isomerization with (oxidative) functionalization provides an ideal method for remote functionalization, which would provide more opportunities for site diversity. However, the reported functionalized sites are still limited and focus on a specific terminal position and internal site; new site-selective functionalization, including multi-functionalization, remains a largely unmet challenge. Here we describe a palladium-catalysed aerobic oxidative method for the multi-site programmable functionalization, involving the C=C double bond and multiple C(sp3)-H bonds, of terminal olefins via a strategy that controls the reaction sequence between alkene isomerization and oxidative functionalization. Specifically, 1-acetoxylation (anti-Markovnikov), 2-acetoxylation, 1,2-diacetoxylation and 1,2,3-triacetoxylation have been realized, accompanied by controllable remote alkenylation. This method enables available terminal olefins from petrochemical feedstocks to be readily converted into unsaturated alcohols and polyalcohols and particularly into different monosaccharides and C-glycosides.

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.

Synthesis of 2-hydroxytetrahydrofurans by Wacker-type oxidation of 1,1-disubstituted alkenes

1,1-Disubstituted alkenes feature high steric hindrance, which renders their Wacker-type oxidation difficult. We demonstrate the stereoselective synthesis of 2-hydroxytetrahydrofurans via the Wacker-type oxidation of 3-methyl-3-buten-1-ols by using a PdCl₂(MeCN)₂/NO/BQ catalyst system under 1 atm O₂ in H₂O or H₂O/DMF.

Realization of Anti-Markovnikov Selectivity in Pd-Catalyzed Oxidative Acetalization and Wacker-Type Oxidation of Terminal Alkenes

Catalytic oxidative acetalization and Wacker-type oxidation of terminal alkenes normally proceed with Markovnikov selectivity to afford internally oxyfunctionalized compounds, such as internal acetals and ketones. Thus, the realization of anti-Markovnikov (AM) selectivity in these reactions is challenging. This account focuses on our recent development of Pd-catalyzed AM oxidation of terminal alkenes (mainly styrenes and aliphatic alkenes), that is, oxidative acetalization (oxidation to terminal acetals) and Wacker-type oxidation (oxidation to aldehydes). The key factors that enhance the yield and AM selectivity of the products found in our studies are: 1) the steric bulkiness of the oxygen nucleophiles that attack on the coordinated alkenes, 2) the electron-deficient cyclic alkenes as additives that withdraw electrons from Pd, 3) the slow addition of substrates in the case of the aliphatic alkenes, which suppresses the isomerization of the terminal alkenes into internal alkenes, and 4) the halogen directing groups in the case of aliphatic alkenes.