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Organics, Volume 2, Issue 1 (March 2021) – 6 articles

Cover Story (view full-size image): Newly discovered protecting group 2-(trimethylsilyl)ethyl 2,2,2-trichloroacetimidate has been shown to be an effective reagent for the formation of 2-trimethylsilylethyl esters without the need for an exogenous promoter or catalyst. A deuterium labeling study indicated that a β-silyl carbocation intermediate is involved in the transformation. The cover was designed by Francesco Barletta. View this paper.
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Article
The Role of the Catalyst on the Reactivity and Mechanism in the Diels–Alder Cycloaddition Step of the Povarov Reaction for the Synthesis of a Biological Active Quinoline Derivative: Experimental and Theoretical Investigations
Organics 2021, 2(1), 57-71; https://0-doi-org.brum.beds.ac.uk/10.3390/org2010006 - 18 Mar 2021
Viewed by 755
Abstract
An experimental and theoretical study of the reactivity and mechanism of the non-catalyzed and catalyzed Povarov reaction for the preparation of a 4-ethoxy-2,3,4,4a-tetrahydro-2-phenylquinoline as a biological active quinoline derivative has been performed. The optimization of experimental conditions indicate that the use of a [...] Read more.
An experimental and theoretical study of the reactivity and mechanism of the non-catalyzed and catalyzed Povarov reaction for the preparation of a 4-ethoxy-2,3,4,4a-tetrahydro-2-phenylquinoline as a biological active quinoline derivative has been performed. The optimization of experimental conditions indicate that the use of a catalyst, namely Lewis acid with an electron-releasing group, creates the best experimental conditions for this kind of reaction. The chemical structure was characterized by the usual spectroscopic methods. The prepared quinoline derivative has been also tested in vitro for antibacterial activity, which displays moderate inhibitory activity against both Escherichia coli and Staphylococcus aureus. The antioxidant activity was investigated in vitro by evaluating their reaction with 1,1-diphenyl-2-picrylhydrazyl DPPH radical, which reveals high reactivity. The computational study was performed on the Diels–Alder step of the Povarov reaction using a B3LYP/6-31G(d,p) level of theory. The conceptual DFT reactivity indices explain well the reactivity and the meta regioselectivity experimentally observed. Both catalysts enhance the reactivity of the imine, favoring the formation of the meta regioisomers with a low activation energy, and they change the mechanism to highly synchronous for the Lewis acid and to stepwise for the Brønsted acid. The reaction of imine with allyl alcohol does not give any product, which requires high activation energy. Full article
(This article belongs to the Special Issue Cycloaddition Reaction in Organic Synthesis)
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Article
Pd-Based Polysaccharide Hydrogels as Heterogeneous Catalysts for Oxidation of Aromatic Alcohols
Organics 2021, 2(1), 50-56; https://0-doi-org.brum.beds.ac.uk/10.3390/org2010005 - 17 Mar 2021
Viewed by 6151
Abstract
Immobilization of Pd(OAc)2(TPPTS)2 in various renewable polysaccharides hydrogels, yielded heterogeneous catalysts that were successfully used, for the first time, in the aerobic oxidation of benzylic alcohol. The new catalysts were easily removed from the reaction mixture and recycled with some [...] Read more.
Immobilization of Pd(OAc)2(TPPTS)2 in various renewable polysaccharides hydrogels, yielded heterogeneous catalysts that were successfully used, for the first time, in the aerobic oxidation of benzylic alcohol. The new catalysts were easily removed from the reaction mixture and recycled with some loss of activity. Among all tested polysaccharides, iota-carrageenan was found to be the most suitable support, using calcium chloride as a gelation agent. Full article
(This article belongs to the Special Issue Feature Papers in Organics)
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Article
Theoretical Insight into the Reversal of Chemoselectivity in Diels-Alder Reactions of α,β-Unsaturated Aldehydes and Ketones Catalyzed by Brønsted and Lewis Acids
Organics 2021, 2(1), 38-49; https://0-doi-org.brum.beds.ac.uk/10.3390/org2010004 - 05 Mar 2021
Viewed by 669
Abstract
Experimentally, a reversal of chemoselectivity has been observed in catalyzed Diels–Alder reactions of α,β-unsaturated aldehydes (e.g., (2E)-but-2-enal) and ketones (e.g., 2-hexen-4-one) with cyclopentadiene. Indeed, using the triflimidic Brønsted acid Tf2NH as catalyst, the reaction gave a Diels–Alder adduct derived from [...] Read more.
Experimentally, a reversal of chemoselectivity has been observed in catalyzed Diels–Alder reactions of α,β-unsaturated aldehydes (e.g., (2E)-but-2-enal) and ketones (e.g., 2-hexen-4-one) with cyclopentadiene. Indeed, using the triflimidic Brønsted acid Tf2NH as catalyst, the reaction gave a Diels–Alder adduct derived from α,β-unsaturated ketone as a major product. On the other hand, the use of tris(pentafluorophenyl)borane B(C6F5)3 bulky Lewis acid as catalyst gave mainly the cycloadduct of α,β-unsaturated aldehyde as a major product. Our aim in the present work is to put in evidence the role of the catalyst in the reversal of the chemoselectivity of the catalyzed Diels–Alder reactions of (2E)-but-2-enal and 2-Hexen-4-one with cyclopentadiene. The calculations were performed at the ωB97XD/6-311G(d,p) level of theory and the solvent effects of dichloromethane were taken into account using the PCM solvation model. The obtained results are in good agreement with experimental outcomes. Full article
(This article belongs to the Special Issue Cycloaddition Reaction in Organic Synthesis)
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Article
Application of β-Phosphorylated Nitroethenes in [3+2] Cycloaddition Reactions Involving Benzonitrile N-Oxide in the Light of a DFT Computational Study
Organics 2021, 2(1), 26-37; https://0-doi-org.brum.beds.ac.uk/10.3390/org2010003 - 16 Feb 2021
Cited by 2 | Viewed by 3839
Abstract
The regiochemistry of [3+2] cycloaddition (32CA) processes between benzonitrile N-oxide 1 and β-phosphorylated analogues of nitroethenes 2a–c has been studied using the Density Functional Theory (DFT) at the M062X/6-31+G(d) theory level. The obtained results of reactivity indices show that benzonitrile N-oxide 1 can [...] Read more.
The regiochemistry of [3+2] cycloaddition (32CA) processes between benzonitrile N-oxide 1 and β-phosphorylated analogues of nitroethenes 2a–c has been studied using the Density Functional Theory (DFT) at the M062X/6-31+G(d) theory level. The obtained results of reactivity indices show that benzonitrile N-oxide 1 can be classified both as a moderate electrophile and moderate nucleophile, while β-phosphorylated analogues of nitroethenes 2a–c can be classified as strong electrophiles and marginal nucleophiles. Moreover, the analysis of CDFT shows that for [3+2] cycloadditions with the participation of β-phosphorylatednitroethene 2a and β-phosphorylated α-cyanonitroethene 2b, the more favored reaction path forms 4-nitro-substituted Δ2-isoxazolines 3a–b, while for a reaction with β-phosphorylated β-cyanonitroethene 2c, the more favored path forms 5-nitro-substituted Δ2-isoxazoline 4c. This is due to the presence of a cyano group in the alkene. The CDFT study correlates well with the analysis of the kinetic description of the considered reaction channels. Moreover, DFT calculations have proven the clearly polar nature of all analyzed [3+2] cycloaddition reactions according to the polar one-step mechanism. Full article
(This article belongs to the Special Issue Cycloaddition Reaction in Organic Synthesis)
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Article
Esterifications with 2-(Trimethylsilyl)ethyl 2,2,2-Trichloroacetimidate
Organics 2021, 2(1), 17-25; https://0-doi-org.brum.beds.ac.uk/10.3390/org2010002 - 04 Feb 2021
Viewed by 758
Abstract
2-(Trimethylsilyl)ethyl 2,2,2-trichloroacetimidate is readily synthesized from 2-trimethylsilylethanol in high yield. This imidate is an effective reagent for the formation of 2-trimethylsilylethyl esters without the need for an exogenous promoter or catalyst, as the carboxylic acid substrate is acidic enough to promote ester formation [...] Read more.
2-(Trimethylsilyl)ethyl 2,2,2-trichloroacetimidate is readily synthesized from 2-trimethylsilylethanol in high yield. This imidate is an effective reagent for the formation of 2-trimethylsilylethyl esters without the need for an exogenous promoter or catalyst, as the carboxylic acid substrate is acidic enough to promote ester formation without an additive. A deuterium labeling study indicated that a β-silyl carbocation intermediate is involved in the transformation. Full article
(This article belongs to the Special Issue Feature Papers in Organics)
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Article
Mpro-SARS-CoV-2 Inhibitors and Various Chemical Reactivity of 1-Bromo- and 1-Chloro-4-vinylbenzene in [3 + 2] Cycloaddition Reactions
Organics 2021, 2(1), 1-16; https://0-doi-org.brum.beds.ac.uk/10.3390/org2010001 - 04 Jan 2021
Cited by 2 | Viewed by 1014
Abstract
The regioselectvity and the mechanism of the (32CA) cycloadditions reactions of 1-bromo-4-vinylbenzene 1 and 1-chloro-4-vinylbenzene 2 with benzonitrile oxide 3 were investigated under the molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p) computational level. Evaluation of the ELF reveals that these zwitterionic type [...] Read more.
The regioselectvity and the mechanism of the (32CA) cycloadditions reactions of 1-bromo-4-vinylbenzene 1 and 1-chloro-4-vinylbenzene 2 with benzonitrile oxide 3 were investigated under the molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p) computational level. Evaluation of the ELF reveals that these zwitterionic type (zw-type) 32CA reactions take place in a two-stage one-step mechanism. This MEDT study shows that the meta isoxazolines are kinetically and thermodynamically favored over the ortho ones, these 32CA reactions being completely regioselective, in agreement with experimental outcomes. In addition, the efficiency of isoxazolines against SARS-CoV-2 have been also investigated. According to the docking analysis, the present study concludes that 5-(p-bromophenyl)-3-phenyl-2-isoxazoline (B-m) shows better interactions for the inhibition of SARS-CoV-2 in comparison to chloroquine. Full article
(This article belongs to the Special Issue Cycloaddition Reaction in Organic Synthesis)
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