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16 pages, 4526 KiB

Open AccessArticle

Iridium-Catalyzed Synthesis of Chiral 1,2,3-Triazoles Units and Precise Construction of Stereocontrolled Oligomers

by Xueyan Zhang, Tian Yu and Shengtao Ding

Molecules 2023, 28(9), 3726; https://doi.org/10.3390/molecules28093726 - 26 Apr 2023

Cited by 1 | Viewed by 1545

Abstract

Iridium-catalyzed azide-thioalkyne cycloaddition reaction (IrAAC) has proved to be a powerful tool for the synthesis of fully substituted 1,2,3-triazole compounds with exclusive regioselectivity. Here we report its successful use in the precise construction of stereocontrolled oligomers that have great potential in diverse applications. [...] Read more.

Iridium-catalyzed azide-thioalkyne cycloaddition reaction (IrAAC) has proved to be a powerful tool for the synthesis of fully substituted 1,2,3-triazole compounds with exclusive regioselectivity. Here we report its successful use in the precise construction of stereocontrolled oligomers that have great potential in diverse applications. Starting with the azide derived from L-prolinol and different functionalized thioalkynes, chiral 1,2,3-triazole units were fabricated with high efficiency under the IrAAC condition, which were further assembled into stereocontrolled oligotriazoles through metal-free exponential growth strategies. The structure and uniformity of these oligomers were well identified by ¹H NMR, size-exclusion chromatography, and mass spectrometry, the stereoregularity of which were studied through circular dichroism and circular polarized luminescence analysis. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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17 pages, 15803 KiB

Open AccessArticle

Electro-Oxidative C3-Selenylation of Pyrido[1,2-a]pyrimidin-4-ones

by Jianwei Shi, Zhichuan Wang, Xiaoxu Teng, Bing Zhang, Kai Sun and Xin Wang

Molecules 2023, 28(5), 2206; https://doi.org/10.3390/molecules28052206 - 27 Feb 2023

Cited by 5 | Viewed by 1741

Abstract

In this work, we achieved a C3-selenylation of pyrido[1,2-a]pyrimidin-4-ones using an electrochemically driven external oxidant-free strategy. Various structurally diverse seleno-substituted N-heterocycles were obtained in moderate to excellent yields. Through radical trapping experiments, GC-MS analysis and cyclic voltammetry study, a plausible [...] Read more.

In this work, we achieved a C3-selenylation of pyrido[1,2-a]pyrimidin-4-ones using an electrochemically driven external oxidant-free strategy. Various structurally diverse seleno-substituted N-heterocycles were obtained in moderate to excellent yields. Through radical trapping experiments, GC-MS analysis and cyclic voltammetry study, a plausible mechanism for this selenylation was proposed. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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16 pages, 2033 KiB

Open AccessArticle

A Lewis Acid-Promoted Michael Addition and Ring-Expansion Cascade for the Construction of Nitrogen-Containing Medium-Sized Rings

by Jiaming Wang, Jia Li and Changgui Zhao

Molecules 2023, 28(4), 1650; https://doi.org/10.3390/molecules28041650 - 8 Feb 2023

Cited by 5 | Viewed by 2823

Abstract

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also [...] Read more.

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also been explored using a chiral base. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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15 pages, 3814 KiB

Open AccessArticle

Two Novel Schiff Base Manganese Complexes as Bifunctional Electrocatalysts for CO₂ Reduction and Water Oxidation

by Xin Zhao, Jingjing Li, Hengxin Jian, Mengyu Lu and Mei Wang

Molecules 2023, 28(3), 1074; https://doi.org/10.3390/molecules28031074 - 20 Jan 2023

Cited by 4 | Viewed by 2250

Abstract

One mononuclear Mn(III) complex [Mn^IIIL(H₂O)(MeCN)](ClO₄) (1) and one hetero-binuclear complex [(Cu^IILMn^II(H₂O)₃)(Cu^IIL)₂](ClO₄)₂·CH₃OH (2) [...] Read more.

One mononuclear Mn(III) complex [Mn^IIIL(H₂O)(MeCN)](ClO₄) (1) and one hetero-binuclear complex [(Cu^IILMn^II(H₂O)₃)(Cu^IIL)₂](ClO₄)₂·CH₃OH (2) have been synthesized with the Schiff base ligand (H₂L = N,N′-bis(3-methoxysalicylidene)-1,2-phenylenediamine). Single crystal X-ray structural analysis manifests that the Mn(III) ion in 1 has an octahedral coordination structure, whereas the Mn(II) ion in 2 possesses a trigonal bipyramidal configuration and the Cu(II) ion in 2 is four-coordinated with a square-planar geometry. Electrochimerical catalytic investigation demonstrates that the two complexes can electrochemically catalyze water oxidation and CO₂ reduction simultaneously. The coordination environments of the Mn(III), Mn(II), and Cu(II) ions in 1 and 2 were provided by the Schiff base ligand (L) and labile solvent molecules. The coordinately unsaturated environment of the Cu(II) center in 2 can perfectly facilitate the catalytic performance of 2. Complexes 1 and 2 display that the over potentials for water oxidation are 728 mV and 216 mV, faradaic efficiencies (FEs) are 88% and 92%, respectively, as well as the turnover frequency (TOF) values for the catalytic reduction of CO₂ to CO are 0.38 s⁻¹ at −1.65 V and 15.97 s⁻¹ at −1.60 V, respectively. Complex 2 shows much better catalytic performance for both water oxidation and CO₂ reduction than that of complex 1, which could be owing to a structural reason which is attributed to the synergistic catalytic action of the neighboring Mn(III) and Cu(II) active sites in 2. Complexes 1 and 2 are the first two compounds coordinated with Schiff base ligand for both water oxidation and CO₂ reduction. The finding in this work can offer significant inspiration for the future development of electrocatalysis in this area. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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8 pages, 1468 KiB

Open AccessArticle

A New Regioselective Synthesis of the Cysteine-Rich Peptide Linaclotide

by Zhonghao Qiu, Xiandong Dai, Chongxu Fan, Ying Cao, Zirui Lv, Xingyong Liang and Fanhua Meng

Molecules 2023, 28(3), 1007; https://doi.org/10.3390/molecules28031007 - 19 Jan 2023

Cited by 2 | Viewed by 2412

Abstract

Linaclotide is a 14-amino acid residue peptide approved by the FDA for the treatment of irritable bowel syndrome with constipation (IBS-C), which activates guanylate cyclase C to accelerate intestinal transit. Here we show a new method for the synthesis of linaclotide through the [...] Read more.

Linaclotide is a 14-amino acid residue peptide approved by the FDA for the treatment of irritable bowel syndrome with constipation (IBS-C), which activates guanylate cyclase C to accelerate intestinal transit. Here we show a new method for the synthesis of linaclotide through the completely selective formation of three disulfide bonds in satisfactory overall yields via mild oxidation reactions of the solid phase and liquid phase, using 4-methoxytrityl (Mmt), diphenylmethyl (Dpm) and 2-nitrobenzyl (O-NBn) protecting groups of cysteine as substrate, respectively. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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17 pages, 2400 KiB

Open AccessArticle

Direct Regioselective C-H Cyanation of Purines

by Luyong Li, Jie Hu, Yuqing Fu, Xiaolin Shi, Hongguang Du, Jiaxi Xu and Ning Chen

Molecules 2023, 28(3), 914; https://doi.org/10.3390/molecules28030914 - 17 Jan 2023

Viewed by 2382

Abstract

A direct regioselective C-H cyanation of purines was developed through a sequential triflic anhydride activation, nucleophilic cyanation with TMSCN, followed by a process of base-mediated elimination of triflous acid (CF₃SO₂H). In most cases, the direct C-H cyanation occurred on [...] Read more.

A direct regioselective C-H cyanation of purines was developed through a sequential triflic anhydride activation, nucleophilic cyanation with TMSCN, followed by a process of base-mediated elimination of triflous acid (CF₃SO₂H). In most cases, the direct C-H cyanation occurred on the electron-rich imidazole motif of purines, affording 8-cyanated purine derivatives in moderate to excellent yields. Various functional groups, including allyl, alkynyl, ketone, ester, nitro et al. were tolerated and acted as a C8 directing group. The electron-donating 6-diethylamino, as C2-directing group substituent, can switch the regioselectivity of purine from 8- to 2-position, enabling the synthesis of 8- and 2-cyano 6-dialkylaminopurines from corresponding 6-chloropurine in different reaction order. Further functional manipulations of the cyano group allow the conversions of 8-cyanopurines to corresponding purine amides, imidates, imidothioates, imidamides, oxazolines, and isothiazoles. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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9 pages, 1589 KiB

Open AccessArticle

The Origin of Stereoselectivity in the Hydrogenation of Oximes Catalyzed by Iridium Complexes: A DFT Mechanistic Study

by Qaim Ali, Yongyong Chen, Ruixue Zhang, Zhewei Li, Yanhui Tang, Min Pu and Ming Lei

Molecules 2022, 27(23), 8349; https://doi.org/10.3390/molecules27238349 - 30 Nov 2022

Cited by 3 | Viewed by 1864

Abstract

Herein the reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium (III) complexes with chiral substituted single cyclopentadienyl ligands (Ir catalysts A1 and B1) under acidic condition were unveiled using DFT calculations. [...] Read more.

Herein the reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium (III) complexes with chiral substituted single cyclopentadienyl ligands (Ir catalysts A1 and B1) under acidic condition were unveiled using DFT calculations. The catalytic cycle for this reaction consists of the dihydrogen activation step and the hydride transfer step. The calculated results indicate that the hydride transfer step is the chirality-determining step and the involvement of methanesulfonate anion (MsO⁻) in this reaction is of importance in the asymmetric hydrogenation of oximes catalyzed by A1 and B1. The calculated energy barriers for the hydride transfer steps without an MsO⁻ anion are higher than those with an MsO⁻ anion. The differences in Gibbs free energies between TSA5−1fR/TSA5−1fS and TSB5−1fR/TSB5−1fS are 13.8/13.2 (ΔΔG^‡ = 0.6 kcal/mol) and 7.5/5.6 (ΔΔG^‡ = 1.9 kcal/mol) kcal/mol for the hydride transfer step of substrate protonated oximes with E configuration (E−2a−H⁺) with MsO⁻ anion to chiral hydroxylamines product R−3a/S−3a catalyzed by A1 and B1, respectively. According to the Curtin–Hammet principle, the major products are hydroxylamines S−3a for the reaction catalyzed by A1 and B1, which agrees well with the experimental results. This is due to the non-covalent interactions among the protonated substrate, MsO⁻ anion and catalytic species. The hydrogen bond could not only stabilize the catalytic species, but also change the preference of stereoselectivity of this reaction. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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13 pages, 6084 KiB

Open AccessArticle

Iridium-Catalyzed and pH-Dependent Reductions of Nitroalkenes to Ketones

by Tingting Wang, Changmeng Liu, Dong Xu, Jiaxi Xu and Zhanhui Yang

Molecules 2022, 27(22), 7822; https://doi.org/10.3390/molecules27227822 - 13 Nov 2022

Cited by 3 | Viewed by 1969

Abstract

A highly chemoselective conversion of α,β-disubstituted nitroalkenes to ketones is developed. An acid-compatible iridium catalyst serves as the key to the conversion. At a 2500 S/C ratio, nitroalkenes were readily converted to ketones in up to 72% isolated yields. A new mechanistic [...] Read more.

A highly chemoselective conversion of α,β-disubstituted nitroalkenes to ketones is developed. An acid-compatible iridium catalyst serves as the key to the conversion. At a 2500 S/C ratio, nitroalkenes were readily converted to ketones in up to 72% isolated yields. A new mechanistic mode involving the reduction of nitroalkene to nitrosoalkene and N-alkenyl hydroxylamine is proposed. This conversion is ready to amplify to a gram-scale synthesis. The pH value plays an indispensable role in controlling the chemoselectivity. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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18 pages, 4094 KiB

Open AccessArticle

Ru-Catalyzed Asymmetric Addition of Arylboronic Acids to Aliphatic Aldehydes via P-Chiral Monophosphorous Ligands

by Rui Miao, Yanping Xia, Yifei Wei, Lu Ouyang and Renshi Luo

Molecules 2022, 27(12), 3898; https://doi.org/10.3390/molecules27123898 - 17 Jun 2022

Cited by 5 | Viewed by 2798

Abstract

Chiral alcohols are among the most widely applied in fine chemicals, pharmaceuticals and agrochemicals. Herein, the Ru-monophosphine catalyst formed in situ was found to promote an enantioselective addition of aliphatic aldehydes with arylboronic acids, delivering the chiral alcohols in excellent yields and enantioselectivities [...] Read more.

Chiral alcohols are among the most widely applied in fine chemicals, pharmaceuticals and agrochemicals. Herein, the Ru-monophosphine catalyst formed in situ was found to promote an enantioselective addition of aliphatic aldehydes with arylboronic acids, delivering the chiral alcohols in excellent yields and enantioselectivities and exhibiting a broad scope of aliphatic aldehydes and arylboronic acids. The enantioselectivities are highly dependent on the monophosphorous ligands. The utility of this asymmetric synthetic method was showcased by a large-scale transformation. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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15 pages, 1463 KiB

Open AccessArticle

Visible-Light-Induced Decarboxylation of Dioxazolones to Phosphinimidic Amides and Ureas

by Jie Pan, Haocong Li, Kai Sun, Shi Tang and Bing Yu

Molecules 2022, 27(12), 3648; https://doi.org/10.3390/molecules27123648 - 7 Jun 2022

Cited by 8 | Viewed by 3221

Abstract

A visible-light-induced external catalyst-free decarboxylation of dioxazolones was realized for the bond formation of N=P and N–C bonds to access phosphinimidic amides and ureas. Various phosphinimidic amides and ureas (47 examples) were synthesized with high yields (up to 98%) by this practical strategy [...] Read more.

A visible-light-induced external catalyst-free decarboxylation of dioxazolones was realized for the bond formation of N=P and N–C bonds to access phosphinimidic amides and ureas. Various phosphinimidic amides and ureas (47 examples) were synthesized with high yields (up to 98%) by this practical strategy in the presence of the system’s ppm Fe. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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Review

Jump to: Research

15 pages, 4626 KiB

Open AccessReview

A Review on Generation and Reactivity of the N-Heterocyclic Carbene-Bound Alkynyl Acyl Azolium Intermediates

by Ziyang Dong, Chengming Jiang and Changgui Zhao

Molecules 2022, 27(22), 7990; https://doi.org/10.3390/molecules27227990 - 17 Nov 2022

Cited by 11 | Viewed by 2294

Abstract

N-heterocyclic carbene (NHC) has been widely used as an organocatalyst for both umpolung and non-umpolung chemistry. Previous works mainly focus on species including Breslow intermediate, azolium enolate intermediate, homoenolate intermediate, alkenyl acyl azolium intermediate, etc. Notably, the NHC-bound alkynyl acyl azolium has [...] Read more.

N-heterocyclic carbene (NHC) has been widely used as an organocatalyst for both umpolung and non-umpolung chemistry. Previous works mainly focus on species including Breslow intermediate, azolium enolate intermediate, homoenolate intermediate, alkenyl acyl azolium intermediate, etc. Notably, the NHC-bound alkynyl acyl azolium has emerged as an effective intermediate to access functionalized cyclic molecular skeleton until very recently. In this review, we summarized the generation and reactivity of the NHC-bound alkynyl acyl azolium intermediates, which covers the efforts and advances in the synthesis of achiral and axially chiral cyclic scaffolds via the NHC-bound alkynyl acyl azolium intermediates. In particular, the mechanism related to this intermediate is discussed in detail. Full article

(This article belongs to the Special Issue Catalytic Green Reductions and Oxidations)

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