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Abstract The reductive catalytic fractionation (RCF) of lignocellulose, considering lignin valorization at design time, has demonstrated the entire utilization of all lignocellulose components; however, such processes always require catalysts based on precious metals or high‐loaded nonprecious metals. Herein, the study develops an ultra‐low loaded, atomically dispersed cobalt catalyst, which displays an exceptional performance in the RCF of lignocellulose. An approximately theoretical maximum yield of phenolic monomers (48.3 wt.%) from lignin is realized, rivaling precious metal catalysts. High selectivity toward 4‐propyl‐substituted guaiacol/syringol facilitates their purification and follows syntheses of highly adhesive polyesters. Lignin nanoparticles (LNPs) are generated by simple treatment of the obtained phenolic dimers and oligomers. RCF‐resulted carbohydrate pulp are more obedient to enzymatic hydrolysis. Experimental studies on lignin model compounds reveal the concerted cleavage of Cα–O and Cβ–O pathway for the rupture of β‐O‐4 structure. Overall, the approach involves valorizing products derived from lignin biopolymer, providing the opportunity for the comprehensive utilization of all components within lignocellulose.

Abstract The conversion of lignocellulosic feedstocks to fermentable sugar for biofuel production is inefficient, and most strategies to enhance efficiency directly target lignin biosynthesis, with associated negative growth impacts. Here we demonstrate, for both laboratory- and field-grown plants, that expression of Pag-miR408 in poplar (Populus alba × P. glandulosa) significantly enhances saccharification, with no requirement for acid-pretreatment, while promoting plant growth. The overexpression plants show increased accessibility of cell walls to cellulase and scaffoldin cellulose-binding modules. Conversely, Pag-miR408 loss-of-function poplar shows decreased cell wall accessibility. Overexpression of Pag-miR408 targets three Pag-LACCASES, delays lignification, and modestly reduces lignin content, S/G ratio and degree of lignin polymerization. Meanwhile, the LACCASE loss of function mutants exhibit significantly increased growth and cell wall accessibility in xylem. Our study shows how Pag-miR408 regulates lignification and secondary growth, and suggest an effective approach towards enhancing biomass yield and saccharification efficiency in a major bioenergy crop.

Lignin valorization becomes the spotlight on the full utilization of biomass. Here, the authors report a highly dispersed Ru catalyst for reductive catalytic fractionation of lignocellulose, which affords monophenols in theoretical maximum yields, along with high preservation of carbohydrate.

C-lignin represents an ideal feedstock for producing catechol derivatives. Here, the authors engineered an atomically dispersed Ru catalyst, which can cleave C−O bonds efficiently and circumvent C=C bonds hydrogenation selectively, thus leading to propenylcatechol in high yields with high TONs.

Abstract Background Lignocellulosic biomass, which is composed of cellulose, hemicellulose and lignin, represents the most abundant renewable carbon source with significant potential for the production of sustainable chemicals and fuels. Current biorefineries focus on cellulose and hemicellulose valorization, whereas lignin is treated as a waste product and is burned to supply energy to the biorefineries. The depolymerization of lignin into well-defined mono-aromatic chemicals suitable for downstream processing is recognized increasingly as an important starting point for lignin valorization. In this study, conversion of all three components of Eucalyptus grandis into the corresponding monomeric chemicals was investigated using solid and acidic catalyst in sequence. Results Lignin was depolymerized into well-defined monomeric phenols in the first step using a Pd/C catalyst. The maximum phenolic monomers yield of 49.8 wt% was achieved at 240 °C for 4 h under 30 atm H2. In the monomers, 4-propanol guaiacol (12.9 wt%) and 4-propanol syringol (31.9 wt%) were identified as the two major phenolic products with 90% selectivity. High retention of cellulose and hemicellulose pulp was also obtained, which was treated with FeCl3 catalyst to attain 5-hydroxymethylfurfural, levulinic acid and furfural simultaneously. The optimal reaction condition for the co-conversion of hemicellulose and cellulose was established as 190 °C and 100 min, from which furfural and levulinic acid were obtained in 55.9% and 73.6% yields, respectively. Ultimately, 54% of Eucalyptus sawdust can be converted into well-defined chemicals under such an integrated biorefinery method. Conclusions A two-step process (reductive catalytic fractionation followed by FeCl3 catalysis) allows the fractionation of all the three biopolymers (cellulose, hemicellulose, and lignin) in Eucalyptus biomass, which provides a promising strategy to make high-value chemicals from sustainable biomass.

Deep eutectic solvents (DESs) can efficiently promote the efficiency of cellulose enzymatic hydrolysis through the removal of lignin component in lignocellulosic biomass pretreatment. Unraveling the fundamental structural variant of lignin during DES treatment would facilitate to understand the DES-based biomass pretreatment in a clear perspective. Herein, an enzymatic mild acidolysis lignin (EMAL) and a series of β-O-4 lignin model compounds were employed to be treated with choline chloride (ChCl)/lactic acid (LA) (1:2) DES, from which the structural variant of all lignin fractions can be realized in a detailed version. The β-O-4 linkages, existing in either realistic lignin or model compounds, could be cleaved by ChCl/LA, thus leading to the decrease of molecular weight and the rise of hydroxyl groups. The influence of reaction temperature and time was also examined in view of some key structural parameters. Experimental evidences from model compounds confirmed that the repolymerization occurs with the depolymerization of lignin, which may account for the low production of monomeric products during DES treatment.

A method is developed for the high yield and facile synthesis of isoquinolones from benzamides and alkynes through the oxidative ortho C-H activation of benzamides. The reactivity of the structurally related 1-hydroxyisoquinoline is shown, where both N- and O-containing rhodacyclic intermediates is generated, leading to the construction of different O- or N-containing heterocycles.

Silver transmetallation is used for preparing a new type of quinoline-functionalized palladium N-heterocyclic carbene (NHC) complexes. NHCs with a methylene linker have formed trans bicarbene complexes with pendent quinoline groups in the reaction of Pd(COD)[Cl.sub.2], which NHCs without C[H.sub.2] ;inker has formed chelating palladium NHC-quinoline complexes.

Conspectus: The possibility of developing new methods for the efficient construction of organic molecules via disconnections other than traditional functional group transformations has driven the interest in direct functionalization of C-H bonds. The ubiquity of C-H bonds makes such transformations attractive, but they also pose several challenges. The first is the reactivity and selectivity of C-H bonds. To achieve this, directing groups (DGs) are often installed that can enhance the effective concentration of the catalyst, leading to thermodynamically stable metallacyclic intermediates. However, the presence of a pendant directing group in the product is often undesirable and unnecessary. This may account for the limitation of applications of C-H functionalization reactions in more common and general uses. Thus, the development of removable or functionalizable directing groups is desirable. Another key problem is that the reactivity of the resulting M-C bond can be low, which may limit the scope of the coupling partners and hence limit the reaction patterns of C-H activation reactions. While the first Cp*Rh(III)-catalyzed C-H activation of arenes was reported only 7 years ago, significant progress has been made in this area in the past few years. We began our studies in this area in 2010, and we and others have demonstrated that diversified catalytic functionalization of arenes can be realized using Cp*Rh(III) complexes with high reactivity, stability, and functional group compatibility. This Account describes our efforts to solve some of these challenges using Rh(III) catalysis. We fulfilled our design and activation of the arene substrates by taking advantage of the nucleophilicity, electrophilicity, oxidizing potential, and properties of a participating ligand of the directing groups when the arenes are coupled with relatively reactive unsaturated partners such as alkenes and alkynes. These in situ funtionalizable roles of the DG allowed extensive chemical manipulation of the initial coupled product, especially in the construction of a diverse array of heterocycles. In the coupling of arenes with polar coupling partners, the polar Rh(III)-C(aryl) bond showed higher reactivity as both an organometallic reagent and a nucleophilic aryl source. The polar coupling partners were accordingly activated by virtue of umpolung, ring strain, and rearomatization. All of these transformations have been made possible by integration of the higher reactivity, stability, and compatibility of Rh(III)-C bonds into catalytic systems. We have demonstrated that to date some of these transformations can be achieved only under rhodium catalysis. In addition, by means of stoichiometric reactions, we have gained mechanistic insights into the interactions between the Rh-C bond and the other coupling partners, which have opened new avenues in future direct C-H functionalization reactions. [ABSTRACT FROM AUTHOR]

Intramolecular oxygen transfer of nitrone-- and sulfoxide--alkynes was achieved using a catalytic amount of Au(I) and a stoichiometric amount of iodine. The Au(I)-catalyzed cyclization of a nitrone--terminal alkyne afforded a cyclic iminoester, while cyclization of analogous nitrone--internal alkynes yielded aldehyde--enones. The I2-mediated cyclization of nitrone--alkynes afforded iodinated γ-lactams and the I2-mediated internal redox of the closely related sulfoxide--alkynes gave diketones functionalized with a thoiether. [ABSTRACT FROM AUTHOR]

Selective synthesis of quinolines has been achieved via oxidative annulation of functionalized pyridines with two alkyne molecules under Rh(III)-catalyzed cascade C-H activation of pyridines using Cu(OAc)2 as an oxidant. The selectivity of this reaction is oxidant-dependent, particularly on the anion of the oxidant. [ABSTRACT FROM AUTHOR]

Rh(III)-catalyzed oxidative coupling reactions between isoquinolones with 3-aryl groups and activated olefins have been achieved using anhydrous Cu(OAc)2 as an oxidant to give tetracyclic products. The nitrogen atom acts as a directing group to facilitate ortho C--H activation. This reaction can be one-pot starting from methyl benzohydroxamates, without the necessity of the isolation of isoquinolone products. A broad scope of substrates has been demonstrated, and both terminal and internal activated olefins can be applied. In the coupling of N-methylmaleimide, a Wacker-like mechanism was proposed, where backside attack of the NH group in isoquinolones is suggested as a key step. Selective C--H activation has also been achieved at the 8-position of 1-naphthol, leading to an olefination product. [ABSTRACT FROM AUTHOR]

A highly efficient and selective palladium-catalyzed diarylation of (2-azaaryl)methanes at the methyl group is described. Aryl chlorides proved reactive enough. A palladium η3-azaallyl intermediate has been identified on the basis of DFT studies. [ABSTRACT FROM AUTHOR]

A Pd(II)-catalyzed oxidative coupling between pyridine N-oxides and N-substituted indoles via 2-fold C−H bond activation was achieved with high selectivity using Ag2CO3as an oxidant. [ABSTRACT FROM AUTHOR]

[RhCp*Cl2]2(1−2 mol %) can catalyze the oxidative coupling of N-aryl-2-aminopyridines with alkynes and arylates to give N-(2-pyridyl)indoles and N-(2-pyridyl)quinolones, respectively, using Cu(OAc)2as an oxidant. Coupling with styrenes gave mono- and/or disubstituted olefination products. [ABSTRACT FROM AUTHOR]

Rh(III)-catalyzed oxidative coupling reactions between benzamides or heteroaryl carboxamides and olefins have been developed. The vinylation product can further undergo a Michael reaction leading to γ-lactam in the case of electron-withdrawing olefins. [ABSTRACT FROM AUTHOR]

Neutral and cationic Pd(II) and Ni(II) OCO-type pincer complexes have been synthesized from the metalation of 1,3-dinitrones via C−H or C−Br bond activation using Pd(0), Pd(II), and Ni(0) starting materials. In each complex, the two six-membered metallacycles are non-coplanar and atropisomerization proceeds with a low barrier. Palladium dinitrone complexes have been shown as active catalysts for Heck couplings and the related Ni complex for Kumada reactions with turnover numbers up to 950 000. [ABSTRACT FROM AUTHOR]

A series of iridium(III) hydride complexes of chelating pyridine-based N-heterocyclic carbenes (py-NHCs) and remote N-heterocyclic carbenes (remote py-NHCs) were synthesized as stable products from the C−H oxidative addition reactions of pyridine-tethered pyridinium halides and [Ir(COD)2]+. X-ray crystal structures of representative iridium py-NHC and remote py-NHC complexes were determined and in all cases mutual cisorientations of the halide and hydride ligands were observed in the solid state. In the solution phase, the cisisomer was observed as the only product for each iridium(III) py-NHC complex, whereas both cisand transisomers have been observed as equilibration mixtures for iridium(III) remote py-NHC complexes. No β-insertion of the hydride into a CC bond of the COD unit of iridium(III) py-NHC hydride complexes was observed, and theoretical studies indicate that this step is thermodynamically unfavorable. Iridium(III) py-NHC hydride complexes have shown high catalytic activity in transfer hydrogenation of ketone and enones. Computational studies of the donor capacity of those ligands indicate that py-NHCs are more donating than imidazole-derived NHCs. [ABSTRACT FROM AUTHOR]

Rhodium and iridium complexes of a new type of abnormal N-heterocyclic carbenes (NHCs) derived from imidazo[1,2- a]pyridiniums have been prepared via silver transmetalation, where metalation can be directed to either the C-2 or the C-3 position of imidazo[1,2- a]pyridine ring provided that the other position is appropriately blocked. The donating abilities of these new NHC ligands have been assessed from the average CO stretching frequencies of their corresponding iridium dicarbonyl complexes. By varying the N-alkyl groups or fusing the pyridine moiety with an aromatic ring, electronic effects of abnormal NHCs of this type can be readily tuned to match the extremely donating imidazole-derived abnormal NHCs or the relatively less donating normal NHCs (imidazolin-2-ylidenes). Both VT NMR studies and CO stretching frequency values strongly support the π-accepting ability of an abnormal NHC derived from imidazo[1,2- a]quinoline. [ABSTRACT FROM AUTHOR]

Imidazoliums with proximal phosphines undergo C−H oxidative addition on [Ir(COD)Cl] 2to give iridium(III) abnormal carbene hydrides. The effects of the length of the linker between the imidazolium and the phosphine are systematically studied. These C−H activation products can undergo base-promoted H−Cl reductive elimination to afford the corresponding Ir(I) abnormal NHC complexes. [ABSTRACT FROM AUTHOR]

An efficient one‐pot four‐component coupling process for the synthesis of polyhydroquinoline derivatives catalyzed by montmorillonite K10 clay is described. This procedure has such advantages as short reaction time, high yields, and simple workup. The catalyst could be reused several times and keeps its initial activity in the recycle reactions. [ABSTRACT FROM AUTHOR]

Ir(i)-induced tautomerization of a pyridine moiety to a carbene in 2,3-bipyridyls has been successfully achieved where an amide group plays a key role as a hydrogen-bonding acceptor and the carbene tautomer is stabilized by both chelation effect and by hydrogen-bonding. [ABSTRACT FROM AUTHOR]