Solar Photochemical Oxidation of Alcohols using Catalytic Hydroquinone and Copper Nanoparticles under Oxygen: Oxidative Cleavage of Lignin Models

Oxygenolytic cleavage of 1,2-diols with dioxygen by a mononuclear nonheme iron complex: Mimicking the reaction of myo-inositol oxygenase

A mononuclear iron(II) complex, [(TpPh2)FeII(OTf)(CH3CN)] (1) (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, OTf = triflate) has been isolated and its efficiency toward the aliphatic CC bond cleavage reaction of 1,2-diols with dioxygen has been investigated. Separate reactions between 1 and different 1,2-diolates form the corresponding iron(II)-diolate complexes in solution. While the iron(II) complex of the tetradentate TPA (tris(2-pyridylmethyl)amine) ligand is not efficient in affecting the CC cleavage of 1,2-diol with dioxygen, complex 1 displays catalytic activity to afford carboxylic acid and aldehyde. Isotope labeling studies with 18O2 reveal that one oxygen atom from dioxygen is incorporated into the carboxylic acid product. The oxygenative CC cleavage reactions occur on the 1,2-diols containing at least one α-H atom. The kinetic isotope effect value of 5.7 supports the abstraction of an α-H by an iron(III)-superoxo species to propagate the CC cleavage reactions. The oxidative cleavage of 1,2-diolates by the iron(II) complex mimics the reaction catalyzed by the nonheme diiron enzyme, myo-inositol oxygenase.

Photocatalytic, Oxidative Cleavage of C-C Bond in Lignin Models and Native Lignin

It is challenging to realize the selective C-C bond cleavage of lignin β-O-4 linkages for production of high-value aromatic chemicals due to its intrinsic inertness and complex structure. Here we report a light-driven, chlorine-radical-based protocol to realize the oxidative C-C bond cleavage in various lignin model compounds catalyzed by commercially available TPT and CaCl2, achieving high conversion and good to high product yields at room temperature. Mechanistic studies reveal that the preferential activation of Cβ-H bond facilitates the oxidation and C-C bond cleavage of lignin β-O-4 model via chlorine radical. Furthermore, this method is also applicable to the depolymerization of natural lignin extracts, furnishing the aromatic oxygenates from the cleavage of Cα-Cβ bonds. This study provides experimental foundations to the depolymerization and valorization of lignin into high value-added aromatic compounds.

A Mild Heteroatom (O-, N-, and S-) Methylation Protocol Using Trimethyl Phosphate (TMP)–Ca(OH)2Combination

A mild heteroatom methylation protocol using trimethyl phosphate (TMP)–Ca(OH)2 combination has been developed, which proceeds in DMF, or water, or under neat conditions, at 80 °C or at room temperature. A series of O-, N-, and S-nucleophiles, including phenols, sulfonamides, N-heterocycles, such as 9H-carbazole, indole derivatives, and 1,8-naphthalimide, and aryl/alkyl thiols, are suitable substrates for this protocol. The high efficiency, operational simplicity, scalability, cost-efficiency, and environmentally friendly nature of this protocol make it an attractive alternative to the conventional base-promoted heteroatom methylation procedures.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

Depolymerization of Lignin into Monophenolics by Ferrous/Persulfate Reagent under Mild Conditions

This study aimed to use a persulfate together with transition metal ions as the reagent to effectively depolymerize lignin into monophenolic compounds under mild conditions (ambient pressure, temperature <100 °C). The Box-Behnken experimental design in combination with the response surface methodology was applied to obtain optimized reaction conditions. The results showed that this reagent could depolymerize up to 99 % of lignin dimers to mainly veratraldehyde. This reaction also successfully depolymerized industrial lignins with a high yield of phenolic oils and monophenolic compounds. Quantum chemistry calculations using the density functional theory level indicated that the persulfate free radical attacks C_β to break the β-O-4 bond of lignin through a five-membered ring mechanism. This mechanism using persulfate free radicals has a lower activation barrier than that using hydroxyl radicals. Gel permeation chromatography and 2D-NMR spectroscopy demonstrated the effective cleavage of the β-O-4 bonds of lignin after depolymerization.

Copper-Catalyzed Oxidative Cleavage of the C-C Bonds of β-Alkoxy Alcohols and β-1 Compounds

Copper-catalyzed aerobic oxidation conditions were employed to promote the C-C bond cleavage of β-alkoxy alcohols and β-1 compounds (lignin model compounds). Besides these compounds, various 1,2 and 1,3-diols were successfully converted to aldehydes. We propose the Cu(I)-catalyzed mechanism explaining the C-C cleavage of these 1,2 and 1,3-dihydroxy compounds and β-alkoxy alcohols based on XPS data. Although our reaction conditions do not include large excess of bases and elaborated ligand-modified catalysts, copper salts with/without Me-TBD show good catalytic activities for C-C bond cleavage of various lignin model compounds.

Electrochemical oxidation mechanisms for selective products due to C-O and C-C cleavages of β-O-4 linkages in lignin model compounds

Electrochemical oxidation is a promising and effective method for lignin depolymerization owing to its selective oxidation capacity and environmental friendliness. Herein, the electrooxidation of non-phenolic alkyl aryl ether monomers and β-O-4 dimers was experimentally (by cyclic voltammetry, in situ spectroelectrochemistry, and gas chromatography-mass spectroscopy) and theoretically (by DFT calculations) explored in detail. Compared to the reported literature (T. Shiraishi, T. Takano, H. Kamitakahara and F. Nakatsubo, Holzforschung, 2012, 66(3), 303-309), 1-(4-ethoxyphenyl)ethanol showed a distinguishable oxidation pathway, where the resulting carbonyl product surprisingly underwent a bond cleavage on alkyl-aryl ether to ultimately produce a quinoid like compound. In contrast, β-O-4 dimers, like 2-phenoxy-1-phenethanol and 2-phenoxyacetophenone also demonstrated electrochemical oxidation induced by C_β-O and C_α-C_β bond cleavages. For the oxidation products, the presence of the C_α-hydroxyl group in dimers was the key to selectively generate aldehyde-containing species under mild electrochemical conditions, otherwise it produces alcohol-containing products following a different mechanism compared to the C_α[double bond, length as m-dash]O containing dimers.

Photocatalytic transformations of lignocellulosic biomass into chemicals

This review highlights recent advances in photocatalytic transformations of lignocellulosic biomass (polysaccharides and lignin) into chemicals (in particular organic oxygenates).

Catalytic Scissoring of Lignin into Aryl Monomers

Lignin is an aromatic polymer, which is the biggest and most sustainable reservoir for aromatics. The selective conversion of lignin polymers into aryl monomers is a promising route to provide aromatics, but it is also a challenging task. Compared to cellulose, lignin remains the most poorly utilized biopolymer due to its complex structure. Although harsh conditions can degrade lignin, the aromatic rings are usually destroyed. This article comprehensively analyzes the challenges facing the scissoring of lignin into aryl monomers and summarizes the recent progress, focusing on the strategies and the catalysts to address the problems. Finally, emphasis is given to the outlook and future directions of this research.

Catalytic Redox Chain Ring Opening of Lactones with Quinones To Synthesize Quinone-Containing Carboxylic Acids

Catalytic ring opening of five- to eight-membered lactones with quinones is achieved through a redox chain mechanism. With low loading of a simple metal triflate Lewis acid catalyst and a chain initiator, C-H bonds of many quinones were efficiently functionalized with carboxylic acid-containing side chains. This method also features 100% atom economy and wide substrate scope. A novel route to the anti-asthma drug Seratrodast was developed. Mechanism study suggests that the redox chain reaction likely undergoes a carbocation intermediate.

Visible-light mediated C-C bond cleavage of 1,2-diols to carbonyls by cerium-photocatalysis

We describe a photocatalytic method for the cleavage of vicinal diols to aldehydes and ketones. The reaction is catalyzed by blue light and a cerium-catalyst and the scope includes aryl as well as alkyl substituted diols. The simple protocol which works under air and at room temperature enables the valorization of abundant diols.

Cleavage of lignin model compounds and ligninox using aqueous oxalic acid

Aqueous oxalic acid cleaves oxidised β-O-4 lignin model compounds by two distinct mechanisms that are dependent on the presence of the hydroxymethyl substituent.

Conversion of Lignin Models by Photoredox Catalysis

One prominent goal of 21st century research is to develop a sustainable carbon-neutral biorefinery. Lignin is an important component of lignocellulosic biomass; however, it is currently underutilized owing to its highly cross-linked, complex, and randomly polymerized composition, which poses a significant challenge to its depolymerization and valorization. Chemical catalytic approaches based on transition metals represent the primary research area to drive degradation reactions. Recently, alternative photocatalytic strategies that employ sustainable solar energy to initiate the transformation of lignin have started to emerge. This Concept article examines new developments of photocatalyzed reactions and provides insight into C-O and C-C bond-cleavage reactions of lignin models in both homogeneous and heterogeneous systems.

Palladium-catalyzed synthesis of fluoreones from bis(2-bromophenyl)methanols

Palladium plays a dual role in synthesizing fluorenones from bis(2-bromophenyl)methanols.

Unravelling the enigma of ligninOX: can the oxidation of lignin be controlled?

As societal challenges go, the development of efficient biorefineries as a means of reducing our dependence on petroleum refineries is high on the list.

As societal challenges go, the development of efficient biorefineries as a means of reducing our dependence on petroleum refineries is high on the list. One of the core strengths of the petroleum refinery is its ability to produce a huge range of different products using all of the components of the starting material. In contrast, the target of using all the biopolymers present in lignocellulosic biomass is far from realised. Even though our ability to use the carbohydrate-based components has advanced, our plans for lignin lag behind (with the notable exception of vanillin production). One approach to lignin usage is its controlled depolymerisation. This study focuses on an increasingly popular approach to this challenge which involves highly selective lignin oxidation to give a material often referred to as lignin^OX. But what do we mean by lignin^OX? In this study we show that it is possible to form many different types of lignin^OX depending on the oxidation conditions that are used. We show that variations in the levels of processing of the β–O-4, the β–β and a third linkage occur. Through use of this information, we can form a well-defined lignin^OX from six different hardwood lignins. This process is reproducible and can be carried out on a large scale. With a source of well-defined lignin^OX in hand, we show that it can be converted to simple aromatic monomers and that any remaining lignin^OX is sufficiently soluble for further processing to be carried out.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

Two new tetracyclic triterpenoids from the barks of Melia azedarach

Two new tetracyclic triterpenoids, together with 21 known compounds, were isolated from the barks of Melia azedarach. The structures of new compounds were elucidated by the means of HRESIMS, 1D NMR, 2D NMR, and X-ray crystallography analysis.

Chelation-assisted Pd-catalysed ortho-selective oxidative C–H/C–H cross-coupling of aromatic carboxylic acids with arenes and intramolecular Friedel–Crafts acylation: one-pot formation of fluorenones

Starting from the readily available substrates, an efficient method for the synthesis of fluorenone derivatives has been developed.

Fragmentation of structural units of lignin promoted by persulfate through selective C–C cleavage under mild conditions

A high selectivity and activity for the fragmentation of β-O-4 and β-1 lignin models to high-value chemicals were achieved by using persulfate.

Towards lignin valorisation: comparing homogeneous catalysts for the aerobic oxidation and depolymerisation of organosolv lignin

Lignin is a unique potential source of aromatic chemicals derived from renewable resources.