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The excessive use of petroleum-based non-biodegradable plastic products has resulted in severe environmental pollution and ecological problems, which has stimulated the development of biodegradable and renewable alternative materials. Lignin, as the most abundant aromatic polymer with great biodegradability and biocompatibility, exhibits enormous potential for preparing various functional and sustainable materials as alternatives to plastics. Recently, lignin-derived biodegradable films have received extensive attention in both fundamental research and practical applications, and many significant achievements have been made in this field. Herein, the latest progress in the preparation and advanced applications of lignin-derived biodegradable film materials is summarized from a sustainability point of view. An introduction to the structural and chemical characteristics of lignin is presented first, and then the designs and advances of construction systems are reviewed based on the building matrixes, including lignocellulose, natural macromolecules, biodegradable synthetic copolymers, and other cutting-edge materials. In this review, the tailored applications of these film materials are mainly focused on sensors and responsive materials, energy storage systems, packaging, and biomedical materials. Finally, the main challenges of lignin-derived biodegradable film materials are presented, and potential development directions for sustainable and eco-friendly lignin-derived film materials are also proposed.

The search for efficient and selective routes for the disassembly of C-lignin and G/S lignin coexisting in Euphorbiaceae seed coats is of great interest and importance, because a plentiful supply of C-lignin can serve as a green carbon feedstock for the production of catechols in a selective manner. Herein, we investigated a series of pretreatment methods for the disassembly of C and G/S lignins in Jatropha seed coats, and extraction by dilute HCl in dioxane was established as the optimized process. This protocol enabled the almost complete segregation of the two types of lignins in Jatropha seeds, and can be generalized to other Euphorbiaceae plants (candlenut, tung, and castor) containing both C and G/S lignins. Characterization by NMR spectroscopic analyses revealed that the isolated lignin samples were mostly composed of benzodioxane units. The disassembled C-lignin samples can produce a single catechylpropanol product (selectivity up to 97%) through Pd/C-catalyzed hydrogenolysis, and are superior to the feedstocks containing C and G/S lignins in terms of catalytic activity and selectivity.

Gaining more insight into the lignin hydrogenolysis mechanism is of great importance for developing next-generation catalysts and regulating product distribution because lignin is the only renewabl...

Remote C-H functionalization at C5 is the most sparingly observed selectivity in the functionalization of indole templates. Herein, we reported that the combination of a AgSbF6 catalyst and phenyliodine diacetate oxidation enabled the C-H selenylation at the C5 position of indole scaffolds in a selective version, thus leading to the formation of a wide scope of 5-selenylated indole derivatives, which are otherwise difficult to prepare. Mechanistic studies indicated that current transformation follows a radical process, and the tethered C3 pivaloyl group on indole scaffolds plays roles in both blocking the active C3 position and manipulating the electronic affinity of the arenes.

Biodegradable materials are increasingly imperative in modern society owing to their eco-friendly character, but their high cost and limited properties restrict their applications. In this study, lignin-based (30–50 wt%) biodegradable composites with superior performances were successfully prepared by incorporating lignin into the poly(butylene adipate-co-terephthalate) (PBAT) matrix. In order to improve the compatibility, lignin was efficiently modified by a green esterification reaction under microwave-assisted solvent-free conditions. Comprehensive characterization revealed that modified lignin had fewer inter-unit linkages, increased molecular weights, lower Tg and enhanced hydrophobicity. Moreover, the prepared modified lignin/PBAT composites reinforced with maleic anhydride (MAH) exhibited controllable improvements in mechanical properties and exhibited excellent UV-shielding properties, and their elongation at break, even with 40 wt% lignin, increased by 500% compared to that of unmodified counterparts. The ideal dispersion and compatibility of the modified lignin in the matrix facilitated the mechanical properties of composites, as revealed by morphological and thermal analysis. Importantly, molecular dynamics (MD) simulation unprecedentedly confirmed that the modification of lignin distinctly strengthened molecular interaction and compatibility of the matrices, enhancing the mechanical performance of the composites. This work presents a green and feasible route to produce cost-efficient biodegradable materials with controlled mechanical and UV-shielding properties for packaging application.

Substantial attention has been given to depolymerization of lignin into monomeric phenols in recent years because lignin is a renewable and CO2-netural aromatic resource. Recent results indicated t...

Lignin, which is biosynthesized through oxidative radical polymerization from primary monolignols during plant growth, represents the most abundant source of renewable aromatic resources. The searc...

C-lignin is a homo-biopolymer, being made up of caffeyl alcohol exclusively. There is significant interest in developing efficient and selective catalyst for depolymerization of C-lignin, as it represents an ideal feedstock for producing catechol derivatives. Here we report an atomically dispersed Ru catalyst, which can serve as an efficient catalyst for the hydrogenolysis of C-lignin via the cleavage of C−O bonds in benzodioxane linkages, giving catechols in high yields with TONs up to 345. A unique selectivity to propenylcatechol (77%) is obtained, which is otherwise hard to achieve, because this catalyst is capable of hydrogenolysis rather than hydrogenation. This catalyst also demonstrates good reusability in C-lignin depolymerization. Detailed investigations by model compounds concluded that the pathways involving dehydration and/or dehydrogenation reactions are incompatible routes; we deduced that caffeyl alcohol generated via concurrent C−O bonds cleavage of benzodioxane unit may act as an intermediate in the C-lignin hydrogenolysis. Current demonstration validates that atomically dispersed metals can not only catalyze small molecules reactions, but also drive the transformation of abundant and renewable biopolymer., 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.

Lignin, which can be recycled from bioethanol and paper production wastes, is the second largest renewable resource. However, studies and applications on sustainable upgrading of lignin are extremely limited and most of them only focus on its combustion to obtain energy. The abundance of reactive groups in lignin makes it a potential feedstock for the production of sustainable low carbon footprint value-added materials. Among them, the preparation of flocculants from lignin has attracted much attention. This paper systematically reviews recent progress of low carbon footprint lignin-based flocculants. The modification methods for the production of lignin-based flocculants, Mannich, sulfonation, carboxylation, crosslinking and graft copolymerization, and the flocculation mechanisms in wastewater treatment are discussed emphatically. Moreover, the factors affecting the flocculation performance of lignin-based flocculants, including internal factors, such as charge density, molecular weight, conformation, and external factors, such as dosage, treatment time, wastewater pH, and temperature are discussed in detail, which are conducive to design, production and application of lignin-based flocculants for lower carbon footprint. Furthermore, we critically point out the current deficiencies of the lignin-based flocculants and the challenges facing commercialization, then we propose the corresponding possible solutions. The production and application of high-efficiency lignin-based flocculants are of great significance for resource conservation, low carbon footprint, and wastewater reuse.

Reductive catalytic fractionation (RCF) of lignocellulosic biomass, that is depolymerization of the native lignin component into well-defined monomeric phenols in the first step, offers an opportunity to utilize entire biomass components. Herein, we report that Ni@ZIF-8 can serve as a chemodivergent catalyst in RCF of eucalyptus sawdust, thus selectively producing phenolic compounds having either a propyl or propanol end-chain under different reaction conditions. In both cases, high yields of lignin monomers and a high degree of delignification were achieved, next to well-preserved carbohydrate pulp suitable for further processing. A mechanistic study using model compounds indicated that the dehydroxylation at the γ-position of the β-O-4 structure may be involved in the selectivity-controlling step.

Lignin is the most abundant renewable resource for production of bio-aromatic chemicals. However, the catalytic depolymerisation of lignin into phenolic monomers is still challenging due to its intrinsic heterogeneity. Herein, we report that Ni/AC can serve as an efficient and stable catalyst in the hydrogenolysis of biorefinery corncob lignin into low-molecular-weight aromatics. This catalyst showed high activity towards reductive fragmentation of hydroxycinnamic esters and β-O-4 linkages through scission by reaction with hydrogen to cleave C–O bonds. Under optimal conditions, this catalytic system produced 31% selectivity towards unsaturated substituents containing coumarate and ferulate derivatives, which afforded mono-aromatic phenols (up to 12.1 wt%) derived from hydroxycinnamic acid moieties (8.1 wt%) and β-O-4 units (2.7 wt%), respectively. The use of non-precious metals to develop highly efficient heterogeneous catalysts makes this approach of great interest in the production of aromatic phenols for the valorisation of industrial lignin.

The lignin-first strategy has emerged as one of the most powerful approaches for generating novel platform chemicals from lignin by efficient depolymerization of native lignin. Because of the emergence of this novel depolymerization method and the definition of viable platform chemicals, future focus will soon shift towards innovative downstream processing strategies. Very recently, many interesting approaches have emerged that describe the production of valuable products across the whole value chain, including bulk and fine chemical building blocks, and several concrete examples have been developed for the production of polymers, pharmaceutically relevant compounds, or fuels. This Minireview provides an overview of these recent advances. After a short summary of catalytic systems for obtaining aromatic monomers, a comprehensive discussion on their separation and applications is given. This Minireview will fill the gap in biorefinery between deriving high yields of lignin monomers and tapping into their potential for making valuable consumer products.

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.

Lignin is the largest renewable resource of bioaromatics, and the catalytic fragmentation of lignin into phenolic monomers is increasingly recognized as an important starting point for lignin valorization. Herein, we report that ZnMoO4 supported on MCM-41 can catalyze the fragmentation of biorefinery technical lignin, enzymatic mild acidolysis lignin, and native lignin derived from corncob to yield lignin oily products that contain 15-37.8 wt % phenolic monomers, in which the high selectivities towards methyl coumarate (1) and methyl ferulate (2) were obtained (up to 78 %). The effects of some key parameters such as the influence of the solvent, reaction temperature, time, H2 pressure, and catalyst dosage were examined in view of activity and selectivity. The loss of Zn from the catalyst is discussed as the primary cause of deactivation, and the catalytic activity and selectivity can be well preserved in at least six runs by thermal calcination. The high selectivity to 1 and 2 leads to their easy separation and purification from lignin oily product to provide sustainable monomers for the preparation of functional polyether esters and polyesters.

The search for nonprecious-metal-based catalysts for the synthesis of γ-valerolactone (GVL) through hydrogenation of levulinic acid and its derivatives in an efficient fashion is of great interest and importance, as GVL is an important a sustainable liquid. We herein report a pincer iron complex that can efficiently catalyze the hydrogenation of levulinic acid and methyl levulinate into GVL, achieving a turnover number of up to 23 000 and a turnover frequency of 1917 h-1 . This iron-based catalyst also enabled the formation of GVL from various biomass-derived carbohydrates in aqueous solution, thus paving a new way toward a renewable chemical industry.

The recently discovered catechyl lignin (C-lignin) represents an ideal lignin platform for depolymerization because of its exclusive benzodioxane linkages between caffeyl alcohol monolignols. In this report, we described catalytic hydrogenolysis of castor seed coats by using deep eutectic solvents, where naturally grown C-lignin was extracted from endocarp matrix and depolymerized into monomeric catechols. The production efficiency of catechols from present one-pot manner was comparable to that from a two-step approach. High selectivity towards 4-propyl catechol was observed in DESs reactions, which was complementary to previous reports. This study offered a step-efficient protocol for producing a single, valuable catechol compound from bulk sustainable castor seed coats.

Lignin represents the most abundant source of renewable aromatic resources, and the depolymerization of lignin has been identified as a prominent challenge to produce low-molecular-mass aromatic chemicals. Herein, we report a nanostructured MoOx/CNT, which can serve as an efficient catalyst in hydrogenolysis of enzymatic mild acidolysis lignins (EMALs) derived from various lignocellulosic biomass, thus giving monomeric phenols in high yields (up to 47 wt %). This catalyst showed high selectivity toward phenolic compounds having an unsaturated substituent, because the cleavage of C–O bonds in β-O-4 units is prior to reduction of double bonds by MoOx/CNT under a H2 atmosphere, which was confirmed by examination of lignin model compound reactions. The effects of some key parameters such as the influence of solvent, temperature, reaction time, and catalyst recyclability were also examined in view of monomer yields and average molecular weight. This method constitutes an economically responsible pathway for li...

The asymmetric C–H annulation of O-pivaloyl 1-indolehydroxamic acid with donor/acceptor diazo compounds has been achieved for the first time, to the best of our knowledge, by using a rhodium catalyst embedded in a chiral binaphthyl backbone. This protocol constitutes a straightforward route for the synthesis of a new family of 1,2-dihydro-3H-imidazo[1,5-a]indol-3-one derivatives having a quaternary carbon stereocenter in high yields and excellent enantioselectivity (up to 98:2 er).

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.

A fundamental understanding of lignin solubilization offers structural information that would benefit a variety of value added applications. Small angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) spectroscopy were used to study correlations between the functional groups/substructures and solution structures of lignin in DMSO-d6 and 0.1 N NaOD. Three types of alkaline lignins (Sigma-Aldrich kraft lignin, poplar wood kraft lignin, and corncob soda lignin), exhibiting different degrees of aggregation in 0.4 wt% solutions, were investigated to identify the major intermolecular interactions that cause lignin aggregation. Intermolecular hydrogen bonding, non-covalent π–π interactions between phenyl rings, lignin chain conformation and the degree of branching were discussed. Different operating forces for lignin solubilization and aggregation were found in DMSO-d6 and NaOD solutions.

A new family of protic NHC Ru complexes ligated with a phosphine-tethered imidazole moiety were prepared, which can act as excellent catalysts for acceptorless dehydrogenation of secondary alcohols and dehydrogenative coupling of primary and secondary alcohols, thus leading to the formation of a variety of carbonyl compounds with release of H2.

The high value utilization of lignin is facing sheer challenge, and the use of lignin as a raw material to prepare flocculants is of tremendous significance for wastewater treatment. In this study, lignin-based flocculants (LBFs) with tunable performances were prepared by mild copolymerization of lignosulfonate and [2-(Methacryloyloxy) ethyl] trimethylammonium chloride solution (METAC) under short-wavelength ultraviolet light. The effect of preparation conditions on the product performances was investigated, and two classes of LBFs were obtained in the same system by adjusting the preparation conditions. As compared with the preparation of LBFs suitable for simulated dye wastewater, higher monomer dose and lower pH were required for the preparation of LBFs suitable for kaolin and E. coli suspensions. Among them, LBF-1 could remove more than 95 % dye, and LBF-2 had 99.2 % and 97.5 % turbidity and bacterial removal rates, respectively. Moreover, the effects of external factors such as the LBF-1 dose, initial concentration of dye and wastewater pH on the decolorization efficiency were also explored. In addition, the sterilization function of LBF was studied, and LBF-2 experienced a process of first flocculation followed by sterilization. Furthermore, the flocculation mechanisms of LBFs in treating three kinds of wastewaters were discussed in detail.

The efficient and selective ortho-alkylation of N,N-dimethyl anilines via C–H addition to alkenes was achieved for the first time using a cationic half-sandwich yttrium catalyst. This protocol constitutes a straightforward and atom-economical route for the synthesis of a new family of tertiary aniline derivatives with branched alkyl substituents, which are otherwise difficult to obtain. DFT calculation studies suggest that the interaction between the yttrium atom and the NMe2 group plays an important role and the intramolecular C–H activation through a σ-bond metathesis pathway is the rate-determining step, which is consistent with the experimental KIE observations.

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.

A Rh(III)-catalyzed site-selective C–H activation of C(3)-functionalized indoles in a coupling with diazo esters has been realized with carbonyl as a weakly coordinating group. The coupling selectivity is dictated by the temperature and additives, affording either C4-alkylated indoles or C2-annulated lactones in moderate to excellent efficiency.

Heteroatoms enhance polymerization and copolymerization of functional olefins., Heteroatom-functionalized polyolefins are of fundamental interest and practical importance. This has spurred investigations of the copolymerization of polar and nonpolar olefins. We report the first syndiospecific polymerization of a series of heteroatom-containing α-olefins and their copolymerization with ethylene catalyzed by half-sandwich rare-earth complexes. We have found that the interaction between a heteroatom in a functional α-olefin monomer and a rare-earth metal catalyst can significantly raise the olefin polymerization activity and thereby promote its copolymerization with ethylene. By using this heteroatom-assisted olefin polymerization (HOP) strategy, we have successfully synthesized a series of heteroatom (O, S, Se, N, and P)–functionalized polyolefins with high molecular weights and controllable functional monomer contents. The mechanistic aspect of the HOP process has been elucidated by computational studies. We expect that our findings will guide the design of new catalyst systems for the synthesis of various desired functional polyolefins.

Cationic half-sandwich scandium alkyl complexes bearing monocyclopentadienyl ligands embedded in chiral binaphthyl backbones act as excellent catalysts for the enantioselective C-H bond addition of pyridines to various 1-alkenes, leading to formation of a variety of enantioenriched alkylated pyridine derivatives in high yields and excellent enantioselectivity (up to 98:2 er).

Reductive catalytic fractionation (RCF) has emerged as a new biorefinery paradigm for the fractionation and sequential utilization of entire components of biomass. Herein, we investigated the RCF of bamboo, a highly abundant herbaceous feedstock, in the presence of Pd/C catalyst. The lignin fraction in bamboo was preferentially depolymerized into well-defined low-molecular-weight phenols, with leaving carbohydrates pulp as a solid residue. In the soluble fraction, four major phenolic compounds, e.g., methyl coumarate/ferulate derived from hydroxycinnamic units and propanol guaiacol/syringol derived from β-O-4 units, were generated up to 41.7 wt% yield based on original lignin content. In the insoluble fraction, the carbohydrates of bamboo were recovered with high retentions of cellulose (68%) and hemicellulose (49%), which upon treatment with enzyme gave glucose (90%) and xylose (85%). Overall, the three major components of bamboo could efficient to be fractionated and converted into useful platform chemicals on the basis of this study.

Hydrothermal process is a potential technology to convert lignocellulosic resources into biofuels and value-added chemicals in a green fashion. Fractionation of woody biomass allows the exploration of the concept of integrated forestry biorefinery. The hydrolysis of hemicellulose produces oligosaccharides, pentose (xylose and arabinose), hexose (glucose, mannose, and galactose), acids (acetic acid, formic acid, and levulinic acid), and furans (furfural and 5-hydroxymethylfurfural), as well as resulting insoluble humins as by-products under harsh conditions. This chapter highlights recent results on hydrothermal processing of various lignocellulosic biomasses, including fundamentals of hydrothermal pretreatment and mechanistic and kinetic studies in such process, especially in analysis and characterization of hemicelluloses before and after hydrothermal processing.

The efficient and selective ortho-alkylation of N,N-dimethyl anilines via C–H addition to alkenes was achieved for the first time using a cationic half-sandwich yttrium catalyst. This protocol constitutes a straightforward and atom-economical route for the synthesis of a new family of tertiary aniline derivatives with branched alkyl substituents, which are otherwise difficult to obtain. DFT calculation studies suggest that the interaction between the yttrium atom and the NMe2 group plays an important role and the intramolecular C–H activation through a σ-bond metathesis pathway is the rate-determining step, which is consistent with the experimental KIE observations.

Depending on the reaction conditions, the oxidative Rh-catalyzed and Ag(I)-mediated process provides pyridine or sterically defined cyclopentanone scaffolds.

Palladium(II) can catalyze the oxidative coupling of tert-butyl 2-alkynylbenzoates with olefins such as acrylates and styrenes, leading to isocoumarines. The reaction was carried out under simple aerobic conditions, and in most cases, high selectivity has been attained.

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.

Room temperature reaction of [Ir(COD)2]BF4 (COD = 1,5-cyclooctadiene) and amide-tethered or simple 2,3′-bipyridyls gave iridium(I) complexes bearing chelating protic pyridylidenes. This protic pyridylidene tautomer is stabilized by both chelation effect and by hydrogen bonding. The mechanistic details of this tautomerization of N-heterocycles to N-heterocyclic carbenes (NHCs) were investigated using the density functional theory (DFT). DFT studies suggested that cyclometalation of 2,3′-bipyridyls took place to give an iridium(III) hydride, which subsequently undergoes formal 1,3-hydrogen shift from the iridium to the pyridyl nitrogen atom. Two possible mechanisms of this formal 1,3-hydrogen shift process have been examined: the β-insertion of the hydride into an olefin followed by proton abstraction and the water-assisted proton transfer via a cyclic transition state. The latter mechanism is strongly favored in the presence of a catalytic amount of water, and this mechanism is applicable to the tautomerization of both amide-tethered and amide-free 2,3′-bipyridyls.

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.

[RhCp*Cl(2)](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)(2) as an oxidant. Coupling with styrenes gave mono- and/or disubstituted olefination products.

Catalytic oxidative coupling between acrylamides and alkynes was achieved using 0.5 mol % loading of [RhCp*Cl(2)](2) with Cu(OAc)(2) as an oxidant. 2-Pyridones, iminoesters, and substituted indoles could be obtained as a result of the electronic and steric effects of the substituents in the acrylamides.

A methodology for the high yield and facile synthesis of isoquinolones from benzamides and alkynes via the oxidative ortho C-H activation of benzamides has been developed. Ag(2)CO(3) proved to be an optimal oxidant when MeCN was used as a solvent, and [RhCp*Cl(2)](2) was utilized as an efficient catalyst. Both N-alkyl and N-aryl secondary benzamides can be applied as effective substrates. Furthermore, primary benzamides react with two alkyne units, leading to tricyclic products via double C-H activation and oxidative coupling. The reactivity of the structurally related 1-hydroxyisoquinoline was also demonstrated, where both N- and O-containing rhodacyclic intermediates can be generated, leading to the construction of different O- or N-containing heterocycles.

Palladium, iridium, and rhodium complexes of 2-methyleneimidazolines have been synthesized by selective phosphine-assisted activation of the 2-methyl C-H bonds in 2-methylimidazolium compounds. Metallacycles of various sizes were obtained in the reaction of phosphine-tethered 2-methylimidazolium compounds and [{M(cod)X}(2)] (M = Rh or Ir; cod = 1,5-cyclooctadiene; X = alkoxyl or Cl). Representative complexes were characterized by X-ray crystallography. The selectivity for aliphatic C(sp(3))H versus aromatic C(sp(2))H activation could be adjusted by means of the steric bulk of the OR ligand, whereby a bulky OR group favors activation of the 2-methyl C(sp(3))-H bond. Experimental results confirmed that a methyl C-H activation product (a seven-membered iridacycle) is the kinetic product, while the aryl C-H activation product (a six-membered iridacycle) is the thermodynamic product.

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.

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 cis orientations of the halide and hydride ligands were observed in the solid state. In the solution phase, the cis isomer was observed as the only product for each iridium(III) py-NHC complex, whereas both cis and trans isomers have been observed as equilibration mixtures for iridium(III) remote py-NHC complexes. No beta-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.

A series of air stable rhodium(III) hydride complexes are synthesized via cyclometalation of functionalized arenes, and are active catalysts for regioselective H/D exchange in various arenes via chelation-assisted C–H activation in acetone-d6.

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.