Insight in the activity and diastereoselectivity of various Lewis acid catalysts for the citronellal cyclization

Citronellal: a natural aldehyde with important properties

Among the several terpenes existing in nature, Citronellal, a monoterpene aldehyde, deserves to be highlighted for its biological properties that have been pointed out in numerous studies. This work aimed to conduct a literature review on its biological properties. Citronellal is a prominent compound in the essential oils of Cymbopogon genus plants. Apart from being employed as a fragrance ingredient in aromas, fragrances, and cosmetics, it is also used as an intermediate in synthesising (-)-menthol. Various studies have demonstrated Citronellal's potential as an antibacterial compound, particularly anti-Staphylococcus and Escherichia bacteria. Citronellal also has antifungal properties against several fungi, especially fungi of the genus Candida. The studies found showed that Citronellal also has insecticidal, acaricidal, antiparasitic, anaesthetic, antiviral, antioxidant, antinociceptive, cardioprotective, antihypertensive, anti-inflammatory, antidiabetic, and anticancer properties.

One-Pot Transformation of Citronellal to Menthol Over H-Beta Zeolite Supported Ni Catalyst: Effect of Catalyst Support Acidity and Ni Loading

Citronellal was converted to menthol in a one-pot approach using H-Beta zeolite-based Ni catalyst in a batch reactor at 80 °C, under 20 bar of total pressure. The effects of H-Beta acidity (H-Beta-25 with the molar ratio SiO2/Al2O3 = 25 and H-Beta-300 with SiO2/Al2O3 = 300) and Ni loading (5, 10 and 15 wt %) on the catalytic performance were investigated. Ni was impregnated on H-Beta support using the evaporation-impregnation method. The physico-chemical properties of the catalysts were characterized by XRD, SEM, TEM, ICP-OES, N2 physisorption, TPR, and pyridine adsorption–desorption FTIR techniques. Activity and selectivity of catalysts were strongly affected by the Brønsted and Lewis acid sites concentration and strength, Ni loading, its particle size and dispersion. A synergetic effect of appropriate acidity and suitable Ni loading in 15 wt.% Ni/H-Beta-25 catalyst led to the best performance giving 36% yield of menthols and 77% stereoselectivity to ( ±)-menthol isomer at 93% citronellal conversion. Moreover, the catalyst was successfully regenerated and reused giving similar activity, selectivity and stereoselectivity to the desired ( ±)-menthol isomer as the fresh one.

Graphical Abstract

A resorcin[4]arene hexameric capsule as a supramolecular catalyst in elimination and isomerization reactions

The hexameric resorcin[4]arene capsule as a self-assembled organocatalyst promotes a series of reactions like the carbonyl-ene cyclization of (S)-citronellal preferentially to isopulegol, the water elimination from 1,1-diphenylethanol, the isomerization of α-pinene and β-pinene preferentially to limonene and minor amounts of camphene. The role of the supramolecular catalyst consists in promoting the protonation of the substrates leading to the formation of cationic intermediates that are stabilized within the cavity with consequent peculiar features in terms of acceleration and product selectivity. In all cases the catalytic activity displayed by the hexameric capsule is remarkable if compared to many other strong Brønsted or Lewis acids.

α,β-Enone Borylation by Bis(Pinacolato)Diboron Catalyzed by Cu3(BTC)2 Using Cesium Carbonate as a Base

Cu₃(BTC)₂ (BTC: 1,3,5-benzenetricarboxylate) as a heterogeneous catalyst in the presence of cesium carbonate as a base is reported for the borylation of α,β-conjugated enones by bis(pinacolato)diboron (B₂pin₂). According to the hot-filtration test, Cu₃(BTC)₂ is acting as a heterogeneous catalyst. Further, Cu₃(BTC)₂ exhibits a wide substrate scope and can be reused in consecutive runs, maintaining a crystal structure as evidenced by powder X-ray diffraction (XRD). A suitable mechanism is also proposed for this transformation using Cu₃(BTC)₂ as catalyst.

Metal-organic frameworks as catalytic selectivity regulators for organic transformations

Selective organic transformations using metal-organic frameworks (MOFs) and MOF-based heterogeneous catalysts have been an intriguing but challenging research topic in both the chemistry and materials communities. Analogous to the reaction specificity achieved in enzyme pockets, MOFs are also powerful platforms for regulating the catalytic selectivity via engineering their catalytic microenvironments, such as metal node alternation, ligand functionalization, pore decoration, topology variation and others. In this review, we provide a comprehensive introduction and discussion about the role of MOFs played in regulating and even boosting the size-, shape-, chemo-, regio- and more appealing stereo-selectivity in organic transformations. We hope that it will be instructive for researchers in this field to rationally design, conveniently prepare and elaborately functionalize MOFs or MOF-based composites for the synthesis of high value-added organic chemicals with significantly improved selectivity.

UiO‐66 and hcp UiO‐66 Catalysts Synthesized from Ionic Liquids as Linker Precursors

Using ionic liquids (ILs) as linker precursors, the well‐known metal‐organic framework (MOF) UiO‐66 (Universitetet i Oslo) and the recently reported MOF hcp UiO‐66 (hexagonal closed packed) have been successfully synthesized and characterized. The advantage of the applied novel synthesis approach is an economically and environmentally benign work‐up procedure, due to the better solubility of the IL. Additionally, the reactivity of the terephthalate anions is increased compared to terephthalic acid, resulting in faster MOF formation with an increased amount of defects in the MOF structure. In order to explore to the influence of defects on the catalytic performance, the cyclisation of citronellal to isopulegol was employed as test reaction. The activity of hcp UiO‐66 and fcc UiO‐66 (face centered cubic) is improved compared to other MOF or zeolite based catalysts, while the selectivity is similar.

Tuning the Catalytic Properties of UiO-66 Metal-Organic Frameworks: From Lewis to Defect-Induced Brønsted Acidity

The Lewis/Brønsted acidity and catalytic properties of UiO-66-type metal-organic frameworks are studied in the context of tunable acid catalysts based on the presence of linker defects that create coordinatively unsaturated Zr⁴⁺ centers. Fourier transform infrared spectroscopy of adsorbed CO and direct pH measurements are employed to characterize hydrated and dehydrated UiO-66 containing different number of Zr⁴⁺ sites associated with defects. These sites can strongly polarize coordinated water molecules, which induces Brønsted acidity in the hydrated material. Upon dehydration of the solid, the coordinated water molecules are removed, and the underlying coordinatively unsaturated Zr⁴⁺ cations become exposed and available as Lewis acid sites. Herein we show, for various acid-catalyzed reactions, how it is possible to shift from a Brønsted acid to a Lewis acid catalyst by simply controlling the hydration degree of the solid. This control adds a new dimension to the design and engineering of MOFs for catalytic applications.

Synthesis of p-menthane-3,8-diol from citronellal over lignin-derived carbon acid catalysts

Sustainable carbon acid catalysts exhibited high-performance in the conversion of citronellal to p-menthane-3,8-diol via a carbocation-hydration pathway.

Unveiling reactive metal sites in a Pd pincer MOF: insights into Lewis acid and pore selective catalysis

NOBF₄ enables efficient oxidative ligand exchange in a Pd pincer MOF, generating a highly active and recyclable Lewis acid catalyst.

From Conventional Lewis Acids to Heterogeneous Montmorillonite K10: Eco-Friendly Plant-Based Catalysts Used as Green Lewis Acids

The concept of green chemistry began in the USA in the 1990s. Since the publication of the 12 principles of this concept, many reactions in organic chemistry have been developed, and chemical products have been synthesized under environmentally friendly conditions. Lewis acid mediated synthetic transformations are by far the most numerous and best studied. However, the use of certain Lewis acids may cause risks to environmental and human health. This Review discusses the evolution of Lewis acid catalyzed reactions from a homogeneous liquid phase to the solid phase to yield the expected organic molecules under green, safe conditions. In particular, recent developments and applications of biosourced catalysts from plants are highlighted.

Modifying the reactivity of a solid Lewis acid: niobium and antimony doped nanoscopic aluminum fluoride

A series of acid catalysts were prepared, using niobium and antimony as dopants in the fluorolytic sol–gel synthesis of high surface aluminium fluoride.

Construction of two Zn(ii)/Cd(ii) multifunctional coordination polymers with mixed ligands for catalytic and sensing properties

Two Zn(ii)/Cd(ii) coordination polymers with different networks were constructed with mixed ligands. The former compound shows high catalytic activity for Knoevenagel condensation, and the latter compound can selectively recognize l-cysteine through the luminescence quenching effect.

A novel fluoride-doped aluminium oxide catalyst with tunable Brønsted and Lewis acidity

The Graphical Abstract image shows the influence of fluoride doping and temperature on the catalytic activity.

Metal-organic and covalent organic frameworks as single-site catalysts

Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.

Flow Pickering Emulsion Interfaces Enhance Catalysis Efficiency and Selectivity for Cyclization of Citronellal

Cyclization of citronellal is a necessary intermediate step to produce the important flavor chemical (-)-menthol. Here, a continuous-flow Pickering emulsion (FPE) strategy for selective cyclization of citronellal to (-)-isopulegol by using water droplets hosting a heteropolyacid (HPA) catalyst to fill a column reactor is demonstrated. Owing to the large liquid-liquid interface and the excellent confinement ability of droplets toward HPA, the FPE system exhibited a much higher catalysis efficiency than its batch counterpart (2-5-fold) and an excellent durability (two months). Moreover, a remarkably enhanced selectivity was observed from 34.8 % for batch reactions to 64 % for the FPE reactions. It was found that the water droplet size and the flow rate significantly impact the catalysis selectivity and efficiency. This study not only represents an unprecedented and sustainable process for the selective cyclization of citronellal but also demonstrates a new flow-interface catalysis effect that can be useful for designing innovative catalysis systems in the future.

The Remarkable Amphoteric Nature of Defective UiO-66 in Catalytic Reactions

One of the major requirements in solid acid and base catalyzed reactions is that the reactants, intermediates or activated complexes cooperate with several functions of catalyst support. In this work the remarkable bifunctional behavior of the defective UiO-66(Zr) metal organic framework is shown for acid-base pair catalysis. The active site relies on the presence of coordinatively unsaturated zirconium sites, which may be tuned by removing framework linkers and by removal of water from the inorganic bricks using a dehydration treatment. To elucidate the amphoteric nature of defective UiO-66, the Oppenauer oxidation of primary alcohols has been theoretically investigated using density functional theory (DFT) and the periodic approach. The presence of acid and basic centers within molecular distances is shown to be crucial for determining the catalytic activity of the material. Hydrated and dehydrated bricks have a distinct influence on the acidity and basicity of the active sites. In any case both functions need to cooperate in a concerted way to enable the chemical transformation. Experimental results on UiO-66 materials of different defectivity support the theoretical observations made in this work.

Synthesis, characterization and performance of bifunctional catalysts for the synthesis of menthol from citronellal

TUD-1 based catalysts containing WO₃ and Pt were synthesized and assessed in the conversion of citronellal to menthol. The hydrogenation on Pt is highest for small WO₃ particles, while their agglomeration leads to a decreased interaction and catalytic performance.

MOFs vs. zeolites: carbonyl activation with M(iv) catalytic sites

Comparative review of the catalytic performance of isolated tetravalent metals as Lewis acid sites in zeolite and MOF materials.

Synthesis of bimetallic Zr(Ti)-naphthalendicarboxylate MOFs and their properties as Lewis acid catalysis

Bimetallic Zr(Ti)-NDC based metal–organic frameworks (MOFs) have been prepared by incorporation of titanium(iv) into zirconium(iv)-NDC-MOFs (UiO family).

BINOL-Al catalysed asymmetric cyclization and amplification: preparation of optically active menthol analogs

We report a highly selective asymmetric ring-closing ene reaction catalysed by aluminum complexes with chiral BINOL. This reaction yields optically active 6-membered cyclized alcohols from unsaturated aldehydes, with good diastereo- and enantioselectivities. Asymmetric amplification of this reaction was investigated by varying the ee of the BINOL employed in the catalyst.

Active site engineering in UiO-66 type metal–organic frameworks by intentional creation of defects: a theoretical rationalization

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 increases by using synthesis modulators as trifluoroacetate (TFA) or hydrochloric acid (HCl), which can be removed post-synthetically.

Effect of Sulfation on Zirconia-Pillared Montmorillonite to the Catalytic Activity in Microwave-Assisted Citronellal Conversion

Preparation of sulfated zirconia-pillared montmorillonite was carried out in two steps; zirconia pillarization and sulfation to zirconia-pillared montmorillonite. The prepared materials were characterized by using X-ray diffraction (XRD), measurement of the specific surface area, total pore volume and pore size distribution by the N2adsorption method, scanning electron microscopy-energy dispersive X-ray (SEM-EDX), and surface acidity determination by using pyridine adsorption-FTIR analysis. The activity of the materials as catalysts was evaluated for a microwave-assisted conversion of citronellal. The results showed that the prepared materials had a physicochemical character that promoted high catalytic activity to convert citronellal. From varied Zr content and study of the effect of sulfation on the activity, it was found that Zr content and sulfation increase the surface acidity of the material as shown by the higher total conversion and tendency to produce menthol as a product of the tandem cyclization-hydrogenation mechanism.

Highly selective aluminium-catalysed intramolecular Prins reaction for l-menthol synthesis

The perfect production of l-menthol.