ceverelst

Building blocks: This critical review discusses recent developments on the value addition of biomass-derived levulinic acid (LA), focusing on the reactivity patterns of the functionalities present in LA. The effect of catalysts, reagents, and reaction conditions on the selectivity and yield of various crucial derivatives of LA have been exemplified. This work will enkindle ideas in designing novel preparative routes and expanding the derivative chemistry of LA.

Abstract

Levulinic acid (LA) is one of the most prominent biomass-derived chemical building blocks that can be transformed into specialty chemicals like fuels, solvents, monomers for polymers, plasticizers, surfactants, agrochemicals, and pharmaceuticals. Over the past three decades, an enormous amount of research data have been acquired on the preparation and downstream value addition of LA, and these works have been reviewed. However, considering the astonishing number of publications appearing every year on LA derivatives, the periodical review of recent works focusing on unique aspects of chemistry must be undertaken to critically evaluate the achievements to date, reassess the challenges, and recognize new opportunities. This review discusses the chemical-catalytic synthesis of various derivatives of LA by focusing on its functionalities and reactivity patterns. Recent literature on some crucial derivatives such as γ-valerolactone, 4,4’-diphenolic acid, and ethyl levulinate have been tabulated and discussed. The synthetic interconversion between various derivatives, mechanistic insights, critical analysis of the reaction parameters toward selective preparation of various derivatives, and their potential commercial applications have been elaborated using predominantly heterogeneous catalysts. A critical assessment of the relative advantages and shortcomings of the existing synthetic strategies for various derivatives of LA has been presented to enkindle fresh ideas.

Synlett
DOI: 10.1055/s-0040-1706042

Reduction-and-oxidation (redox) reactions are one of the most utilized approaches for the synthesis of value-added compounds. With the growing awareness of green chemistry, researchers have searched for new and sustainable pathways for performing redox reactions. From this, a new field has gained tremendous attention, namely photoredox catalysis. Here, molecules can be easily oxidized or reduced with the use of one of Nature’s biggest resources: visible light. This tutorial paper gives the basics of photoredox catalysis along with limited examples to encourage further research in this blooming research area.1 Introduction2 Redox Chemistry3 Photochemistry3.1 Laws of Photochemistry3.2 Principles3.3 Examples4 Photoredox Catalysis4.1 General Principles4.2 Classification of Redox Processes4.3 Other Mechanistic Considerations4.4 Stern–Volmer Plots4.5 Photophysical Properties4.6 Redox Potentials5 Photocatalysts5.1 Metal-Based Photocatalysts5.2 Organic Dyes5.3 Semiconductors6 Dual Catalysis7 Conclusions
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
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Abstract

Pyrazoleamides are a class of compounds which have received significant attention in the last years in asymmetric catalysis as more reactive and practical surrogates of esters or amides. These readily available reagents have served in a variety of diastereo- and enantioselective metal- and organocatalytic transformations, spanning from simple carbon-carbon or carbon-heteroatom bond formation to cascade and rearrangement reactions, to produce valuable classes of heterocyclic compounds. More recently, pyrazoleamides have proved to be pertinent reagents in asymmetric catalysis merged with photoredox catalysis, and in more challenging stereoselective bond-forming reactions, occurring directly from a visible-light activated substrate/catalyst complex without any charge separation. Both strategies enable new activation concepts useful for the production of difficult-to-access intermediates and pharmaceuticals. In this review, an overview of the progress achieved in asymmetric catalytic reactions of pyrazoleamides from 2015 up to middle 2020, is illustrated.

Sweet F2A: A protocol for the transition from bioderived furanic platform chemicals to functionalized aromatics by sequential cycloaddition and aromatization is described. The furan–yne cycloaddition is performed in solvent-free conditions. Structure–activity relationship for different HMF derivatives is established. The reaction space of the furanics-to-aromatics (F2A) process is explored both experimentally and by quantum chemistry methods.

Abstract

The reaction space of the furanics-to-aromatics (F2A) conversion process for 5-hydroxymethylfurfural (HMF)-based platform chemicals has been explored both experimentally and by quantum chemistry methods. For the first time, a structure-activity relationship was established in furan-yne cycloaddition for a number of different HMF derivatives. Correlations between the activation energy of the cycloaddition stage and the structure of the substrates were established by molecular modeling methods. Analysis of the concerted and stepwise mechanisms of cycloaddition in the singlet and triplet electronic states of the molecular system was carried out. A series of biobased 7-oxanorbornadienes was obtained in the reaction with dimethyl acetylenedicarboxylate. Various methods of aromatization of the obtained [4+2] adducts have been examined. Rearrangement catalyzed by a Lewis acid leads to the formation of a phenol derivative, whereas reduction by diiron nonacarbonyl leads to the formation of functionalized benzene. Systematic study of the cycloaddition process has revealed a simple way to analyze and predict the relative reactivity of furanic substrates.

Ta-DA! The Diels-Alder (DA) reaction of electron-rich furans with electron-poor alkenes has been a benchmark for the preparation of complex scaffolds such as 7-oxanorbornenes. Recently this technology has been used to prepare aromatics from bio-based furanic platforms. The recently described direct DA reaction of bio-based furfurals expands the scope of bioaromatic preparation to afford key biomass derivatives.

Abstract

The preparation of high value-added chemicals from renewable resources is a crucial approach towards a sustainable economy. One prominent alternative to the production of petroleum-based chemicals from fossil resources is through the sequential Diels-Alder/aromatization reactions of biomass-derived furan platforms. This Concept is focused on the recent boom in bio-based furan DA strategies for aromatization of bio-based platform chemicals, particularly that of furfurals, ranging from indirect use and activation strategies to recent examples of direct DA reaction of these electron-withdrawing biomass-derived furans.

Abstract

The oxidative cleavage of indoles (Witkop oxidation) involving the use of H₂O₂ or urea hydrogen peroxide in combination with a polar solvent has been described. Among these solvents, 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) stands out as the one affording the corresponding 2-ketoacetanilides generally in higher yields The protocol described has also enabled the oxidation of different pyrroles and furans derivatives. Furthermore, the procedure was implemented in a larger-scale and HFIP was distilled from the reaction mixture and reused (up to 4 cycles) without a significant detriment in the reaction outcome, which remarks its sustainability and applicability.

Abstract

Over the last several years, radical-mediated decarboxylative cross-coupling reactions employing alkyl carboxylic acids have emerged as a powerful tool for the regiospecific construction of carbon−carbon bonds. Under thermal or photocatalytic conditions, a wide variety of C(sp ³)-carboxylic acids and their redox-active esters undergo decarboxylative C−C bond formation with suitable reactant partners, leading to complex chemical scaffolds with wide-ranging applications. This synthetic strategy has several advantages over the more conventional organometallic reagents, including abundant starting material availability and high functional group tolerance associated with the mild reaction conditions. This review article highlights recent developments in the functionalization of α-heteroatom-substituted carboxylic acids as well as the more challenging unactivated acids, with representative examples discussed against the backdrop of insightful comments on reaction mechanisms. In addition, examples of the synthesis of natural products, drug molecules, and the late-stage modification of bioactive molecules employing this non-traditional C−C bond formation strategy are included. This review has been categorized into three main sections that are organized around the type of C−C bond being forged: C(sp ³)−C(sp ²), C(sp ³)−C(sp ³), and C(sp ³)−C(sp). Further, the reactions of carboxylic acids and their redox-active esters have been organized separately in each section.

Fossil-based platform molecules such as ethylene and ethylene oxide currently serve as the primary feedstock for the C₂-based chemical industry. This Review compares and critically discusses several production routes toward bio-based structural analogues of ethylene oxide, for example glycolaldehyde, and the required adaptations for their implementation in state-of-the-art C₂-based chemical processes.

Abstract

Fossil-based platform molecules such as ethylene and ethylene oxide currently serve as the primary feedstock for the C₂-based chemical industry. However, in the search for a more sustainable chemical industry, fossil-based resources may preferentially be replaced by renewable alternatives, provided there is realistic economic feasibility. This Review compares and critically discusses several production routes toward bio-based structural analogues of ethylene oxide and the required adaptations for their implementation in state-of-the-art C₂-based chemical processes. For example, glycolaldehyde, a structural analogue obtainable from carbohydrates by atom-economic retro-aldol reactions, may replace ethylene oxide's leading role. This alternative chemical route may not only allow the carbon footprint of conventional chemicals production to be lowered, but the introduction of a bio-based pathway may also contribute to safer production processes. Where possible, challenges, drawbacks, and prospects are highlighted.

Building blocks: This critical review discusses recent developments on the value addition of biomass-derived levulinic acid (LA), focusing on the reactivity patterns of the functionalities present in LA. The effect of catalysts, reagents, and reaction conditions on the selectivity and yield of various crucial derivatives of LA have been exemplified. This work will enkindle ideas in designing novel preparative routes and expanding the derivative chemistry of LA.

Abstract

Levulinic acid (LA) is one of the most prominent biomass-derived chemical building blocks that can be transformed into specialty chemicals like fuels, solvents, monomers for polymers, plasticizers, surfactants, agrochemicals, and pharmaceuticals. Over the past three decades, an enormous amount of research data have been acquired on the preparation and downstream value addition of LA, and these works have been reviewed. However, considering the astonishing number of publications appearing every year on LA derivatives, the periodical review of recent works focusing on unique aspects of chemistry must be undertaken to critically evaluate the achievements to date, reassess the challenges, and recognize new opportunities. This review discusses the chemical-catalytic synthesis of various derivatives of LA by focusing on its functionalities and reactivity patterns. Recent literature on some crucial derivatives such as γ-valerolactone, 4,4’-diphenolic acid, and ethyl levulinate have been tabulated and discussed. The synthetic interconversion between various derivatives, mechanistic insights, critical analysis of the reaction parameters toward selective preparation of various derivatives, and their potential commercial applications have been elaborated using predominantly heterogeneous catalysts. A critical assessment of the relative advantages and shortcomings of the existing synthetic strategies for various derivatives of LA has been presented to enkindle fresh ideas.

TBN‐catalyzed green esterification of carboxylic acids has been developed, which features a broad range of substrates and excellent functional groups tolerance. The mechanistic study confirmed that the nitrous acid formed in situ in the system is the actual catalyst for this transformation.

Abstract

In this work, the green esterification of carboxylic acids promoted by tert‐butyl nitrite has been well developed. This transformation is compatible with a broad range of substrates and exhibits excellent functional group tolerance. Various drugs and substituted amino acids are applicable to this reaction under near neutral conditions, with good to excellent yields.

Bio-based methyl vinyl glycolate (MVG) has been oxidized to methyl 2-oxobut-3-enoate with the Dess-Martin periodinane. Densely functionalized methyl 2-oxobut-3-enoate is stable at low temperatures but slowly dimerizes by a hetero-Diels-Alder reaction at room temperature. Accordingly, methyl 2-oxobut-3-enoate has been developed as a new dienophile in the classical Diels-Alder reaction with 1,3-dienes

Abstract

Methyl vinyl glycolate (MVG, methyl 2-hydroxybut-3-enoate) is a new biobased platform chemical, which is available by degradation of carbohydrates. In the present work, the oxidation of MVG to methyl 2-oxobut-3-enoate has been investigated and a procedure developed with the Dess-Martin periodinane. Methyl 2-oxobut-3-enoate has not been characterized before and is stable for days at −18 °C, but slowly dimerizes by an unusual hetero-Diels-Alder reaction at room temperature. Therefore, the reactivity of methyl 2-oxobut-3-enoate in the normal Diels-Alder reaction with 1,3-dienes has been investigated and a one-pot procedure developed where MVG is first oxidized with the Dess-Martin periodinane followed by the addition of the 1,3-diene. A number of different 1,3-dienes can take part in the cycloaddition to afford the functionalized cyclohexene products in moderate-to-good yields.

Aryl carboxylic acids are valuable, stable, and abundant functional handles in organic synthesis. Historically, their activation with established two-electron methods requires forcing conditions, and such protocols are limited in scope. In contrast, we envisioned that copper’s ability to generate open-shell species through ligand-to-metal charge transfer (LMCT), combined with its unique capacity to act as a potential aroyloxy and aryl radical reservoir, could mediate facile light- and copper-enabled aromatic decarboxylative functionalization by mitigating undesired reactivity of radical intermediates formed during aromatic decarboxylation. We report herein a general copper-LMCT open-shell activation platform for aromatic halodecarboxylation. Catalytic decarboxylative chlorination, bromination, and iodination of diverse (hetero)aryl carboxylic acids have been achieved to provide broadly used electrophilic cross-coupling handles from widely available aromatic acid precursors. Notably, decarboxylative fluorination of aryl carboxylic acids ­– a long-standing challenge in the field of organic synthesis – is readily accessible over a wide breadth of (hetero)aryl substrates. Ultrafast transient absorption (TA) spectroscopy experiments in combination with steady-state UV-vis spectroscopy studies are consistent with the proposed copper-LMCT mechanism, supporting the mechanistic basis of this activation platform.