The Prins/Friedel-Crafts cascade reactions of the terpenoid trans-4-hydroxymethyl-2-carene (synthesized from 3-carene) with aromatic aldehydes were systematically studied for the first time on acidic ...mesoporous clays (halloysite, illite, montmorillonites). Both the reaction rate and selectivity to the desired polycyclic product with tetrahydrofuran moiety increased with an increase in the catalyst acidity and their drying temperature, indicating that relatively strong Brønsted and Lewis acid sites favored their formation. The best activity and selectivity (up to 97%) was demonstrated over commercial montmorillonite K-10 with acidity of ca. 100 μmol/g. In contrast, on strongest acids (resin Amberlyst-15), dehydration/aromatization of the substrate was observed. It was shown, that mesoporosity of the catalyst is one of the key factors governing catalytic behavior. The presence of at least one an electron-donor substituent at the meta-position of benzaldehyde is critical for the Prins-Friedel-Crafts reaction. Overall, available montmorillonites are an effective replacement for homogeneous catalysts for the Prins/Friedel-Crafts cascade reactions.
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•Mesoporous clays as catalysts for cascade Prins/Friedel-Crafts reactions.•Montmorillonites are most effective ones.•Moderate acidity (100 μmol/g) provides high activity and selectivity.•The reaction mechanism was analyzed at the DFT level.•Mild reaction conditions and catalyst reusability.
A number of SO3H-functionalized solids (biochar, montmorillonites, carbon and halloysite nanotubes) has been studied as catalysts in the cascade Prins-Ritter reaction of (-)-isopulegol with ...benzaldehyde and acetonitrile for synthesis of octahydro-2H-chromene amides (as 4R- and 4S-isomers). A high selectivity to these products at 30 °C in the presence of H2O was observed on catalysts modified with chlorosulfonic acid (CSA) reaching 84% (4R/4S of 5.7) in the case of biochar, while a relatively large amount of octahydro-2H-chromenols (up to 31%), products of Prins condensation, was formed on the materials functionalized by 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSP). Although Prins condensation proceeds efficiently on weak acid sites, the Prins-Ritter reaction requires sulfated materials with strong (0.33 – 5.8 mmol/g) Brønsted acidity. Catalysts functionalized by CSP were stable, while for the materials modified with chlorosulfonic acid, leaching of -SO3H groups was observed. Nonetheless, on resistant Bioсhar-CSP, selectivity to the amides at 30 °C (67%) was higher than that with the commercial Amberlyst-15 (47%), and triflic acid at − 25 °C (62%). Similar selectivity to the desired products on Biochar-CSA (-SO3H groups) and H2SO4 (81–84%) as well as on Biochar-CSP (-PhSO3H) and with p-toluenesulfonic acid (67–70%) was observed. DFT calculations and experimental results showed that at 30 °C formation of 4S-amide thermodynamically is more beneficial than of alcohols and dehydration products. However, addition of water results in a sharp increase in the reaction rate and 4R-amide selectivity due to a change to the kinetic control, leading eventually to both high yields and stereoselectivity. The proposed reaction pathways also were confirmed by kinetic modelling.
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•Heterogeneous and homogeneous catalysts for the Prins-Ritter reaction at ambient conditions.•The reaction requires sulfated materials with strong Brønsted acidity.•Selectivity to amides up to 84% (4 R/4 S of 5.7) on Biochar-CSA or sulfuric acid.•Formation of 4S-amide is thermodynamically more favorable.•Addition of water results in a kinetic control with high stereoselectivity.
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•First systematic study of catalytic terpenoid diol condensation with an aldehydes to bioactive hexahydro-2H-chromene-4,8-diols.•Halloysite nanocatalyst increases both the yields of ...hexahydro-2H-chromene-4,8-diols and 4S/4R isomers ratio.•Inversion of stereoselectivity with increasing acidity or catalyst drying temperature.•The reaction mechanism is discussed with DFT calculations and kinetic modeling.
Condensation of α-pinene derived p-menta-1,8-diene-5,6-diol (diol) with decanal was studied for the first time over modified halloysite nanotubes (HNT). The yield of the desired hexahydro-2H-chromene-4,8-diol with analgesic activity was 76–80 % practically not depending on the catalyst type, while selectivity to 4S-isomer decreased, and to 4R-isomer increased with increasing acidity. The highest selectivity to 4S-diastereomer (48.1 %) on halloysite is a result of weak acidity of this catalyst. DFT optimization of the key intermediate structure shows that the nucleophile attack proceeds at the equatorial position with the 4S-diastereomer formation, which was preferred on halloysite. On strong Brønsted (Amberlyst-15) and Lewis (scandium triflate) acids the target product yield did not exceed 37 % because of dehydration. Halloysite nanocatalysts displayed a stable performance. In the case of diol reaction with a set of carbonyl compounds, the yields of hexahydro-2H-chromene-4,8-diols (up to 88.0%) and the ratio of its 4S/4R isomers (up to 21.0) were significantly higher than on other catalysts.
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•Condensation of (–)-α-pinene with formaldehyde into 8-acetoxy-6-hydroxymethyllimonene.•Selectivity of 24 % with traditional Lewis and Brønsted acids (ZnCl2, FeCl3, H3PO4).•Comparable ...selectivity for strongly acidic H-Beta-25 zeolite and phosphoric acid.•Reaction mechanism elucidated using kinetic and quantum chemical approaches.•Successful scaling up 8-acetoxy-6-hydroxymethyllimonene synthesis.
While synthesis of hydroxymethyl derivatives of common terpenes by a reaction with formaldehyde (FA) has been commercialized, no open data are available on the preparation of such α-pinene derivatives. Here, an approach for the renewable (–)-α-pinene catalytic utilization by its condensation with FA into a novel terpenoid 8-acetoxy-6-hydroxymethyllimonene is proposed. Both common homogeneous acids and aluminosilicates (halloysite nanotubes pretreated with H3PO4 and HCl, montmorillonite K-10, zeolite H-Beta-25) have been investigated. The solids were characterized by SEM, EDX, 27Al and 29Si MAS NMR, as well as FTIR with pyridine and N2 adsorption–desorption methods. Traditional Lewis and Brønsted acids (ZnCl2, FeCl3, H3PO4 etc.) catalyzed the desired condensation although selectivity did not exceed 24 % in the case of phosphoric acid due to the side reactions, including opening of the α-pinene cyclobutane ring, as well as skeletal rearrangements of its bicyclic structure. On aluminosilicates with a weak to moderate Lewis and Brønsted acidity (45–104 µmol/g), the products of a direct substrate protonation (up to 52.8 %) were predominantly formed. In the presence of strongly acidic H-Beta-25 (301 µmol/g) and phosphoric acid, selectivity to α-pinene addition products with formaldehyde (32.3–35.3 %) and substrate direct protonation (30.0–36.8 %) were comparable. However, with H3PO4 the content of borneol derivatives formed via the Wagner-Meerwein rearrangement was the smallest (13.5 %) giving subsequently the largest selectivity to the desired 8-acetoxy-6-hydroxymethyllimonene. Higher selectivity to the target product was observed at lower reaction temperature and a larger formaldehyde amount. The reaction mechanism has been proposed and elucidated using kinetic and quantum chemical approaches. The developed kinetic model adequately describes the experimental results. The DFT calculations showed that in the presence of H3PO4 there are no significant differences between the energy of the intermediates formed due to formaldehyde addition to α-pinene or its direct protonation. Synthesis of 8-acetoxy-6-hydroxymethyllimonene was performed on 22 g scale. This compound can be considered as a chiral platform for further utilization, including synthesis of heterocyclic compounds.
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•Halloysite, montmorillonite and illite as effective catalysts for the production of tetrahydropyranols.•Synthesis without solvent at a relatively low temperature.•Increased ...selectivity for the target product with an increase in the clays acidity.•The reaction pathways depending on the catalyst have been discussed.
A number of acid-modified clays (halloysite, montmorillonite, illite) were studied as catalysts for isoprenol cyclization with isovaleraldehyde to tetrahydropyranols used in perfumery as a mixture of cis- and trans- isomers. The main attention was paid to properties of halloysite nanotubes after treating them with 5% HCl at temperatures from 30 to 100 °C. The catalysts were characterized by XRD, EDX, MAS NMR, SEM, and N2 adsorption-desorption methods. The acid modification of halloysite allows an efficient increase of its acidity and specific surface area, while preserving morphology of nanotubes. On relatively weakly acidic clays, the formation of the target product occurs through a hemiacetal as a precursor. On the contrary, over strongly acidic resin Amberlyst-15, the yield of tetrahydropyranol was significantly lower because of a direct formation of tetrahydropyranols and dehydration by-products. The yield of tetrahydropyranol in the presence of clays was 65–72 %, slightly increasing with an increase in their acidity. The selectivity towards the cis- isomer did not practically depend on the acid sites concentration in clays, decreasing slightly for the trans-isomer with decreasing acidity due to formation of the dehydration by-products. In general, acid-modified clays are promising catalysts for the preparation of tetrahydropyranols under mild conditions and in the absence of any solvent.
Net ecosystem exchange (NEE) is an important indicator of
carbon cycling in terrestrial ecosystems. Many previous studies have
combined flux observations and meteorological, biophysical, and ...ancillary
predictors using machine learning to simulate the site-scale NEE. However,
systematic evaluation of the performance of such models is limited.
Therefore, we performed a meta-analysis of these NEE simulations. A total of
40 such studies and 178 model records were included. The impacts of various
features throughout the modeling process on the accuracy of the model were
evaluated. Random forests and support vector machines performed better than
other algorithms. Models with larger timescales have lower average
R2 values, especially when the timescale exceeds the monthly scale.
Half-hourly models (average R2 = 0.73) were significantly more
accurate than daily models (average R2 = 0.5). There are
significant differences in the predictors used and their impacts on model
accuracy for different plant functional types (PFTs). Studies at continental
and global scales (average R2 = 0.37) with multiple PFTs, more
sites, and a large span of years correspond to lower R2 values than studies
at local (average R2 = 0.69) and regional (average R2 = 0.7) scales. Also, the site-scale NEE predictions need more focus on the
internal heterogeneity of the NEE dataset and the matching of the training
set and validation set.
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