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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.

Layered serpentine Ni3 Ge2 O5 (OH)4 is compositionally active and structurally favorable for adsorption and diffusion of reactants in oxygen evolution reactions (OER). However, one of the major problems for these materials is limited active sites and low efficiency for OER. In this regard, a new catalyst consisting of layered serpentine Ni3 Ge2 O5 (OH)4 nanosheets is introduced via a controlled one-step synthetic process where the morphology, size, and layers are well tailored. The theoretical calculations indicate that decreased layers and increased exposure of (100) facets in serpentine Ni3 Ge2 O5 (OH)4 lead to much lower Gibbs free energy in adsorption of reactive intermediates. Experimentally, it is found that the reduction in number of layers with minimized particle size exhibits plenty of highly surface-active sites of (100) facets and demonstrates a much enhanced performance in OER than the corresponding multilayered nanosheets. Such a strategy of tailoring active sites of serpentine Ni3 Ge2 O5 (OH)4 nanosheets offers an effective method to design highly efficient electrocatalysts.

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.

Imidazoliums with proximal phosphines undergo C−H oxidative addition on [Ir(COD)Cl]2 to 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.

Fe3+-montmorillonite was first used as a solid acidic catalyst for the synthesis of xanthenedione derivatives through condensation reactions between aromatic aldehydes and dimedone. This process provides several advantages such as good yields of products, simple operation, convenient separation, and inexpensive and recyclable catalyst.

ConspectusThe 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 co...

Sulfamic acid (H2NSO3H) has been used as an efficient and recyclable catalyst for acetalization and ketalization between carbonyl compounds and diols. This system is reasonably general and can be applied to converting several carbonyl compounds to the corresponding acetals and ketals using neopentyl glycol and other diols as protection reagents. The use of an inexpensive, green and stable catalyst, convenient reaction operation and high selectivity to desired products are the features contained in this reaction. Sulfamic acid can be reused five times in the reaction between benzaldehyde and neopentyl glycol without loss of activity.

Fe3+‐montmorillonite clay catalyst was used for the first time for pyrroles preparation by Paal–Knorr condensation. The features of this catalytic system are improved yields, mild conditions, a simple procedure, and high catalytic efficiency. The catalyst could be easily recycled and reused for three times without visible losing of activity.

The catalytic C-H addition of pyridines to allenes has been achieved for the first time by using a half-sandwich scandium catalyst, thus constituting a straightforward and atom-economical route for the synthesis of alkenylated pyridine derivatives. The reaction proceeded regio- and stereoselectively, affording a new family of alkenylated pyridine compounds which are otherwise difficult to synthesize. A cationic Sc-η(2) -pyridyl species was isolated and confirmed to be a key catalyst species in this transformation.

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

Rhodium(III)-catalyzed oxidative olefination of N-(1-naphthyl)sulfonamides has been achieved at the peri position. Three categories of olefins have been successfully applied. Activated olefins reacted to afford five-membered azacycles as a result of oxidative olefination-hydroamination. Unactivated olefins reacted to give the olefination product. 2-fold oxidative C-C and C-N coupling was achieved for allylbenzenes.

Treatment of nitrones of type (I) and (IV) with iodine results in selective formation of isoindolones.

Treatment of 4-functionalized pyridines with acetylenes in the presence of Cu(O-Ac)2 and Rh(III) catalyst allows an effective access to quinoline structures.

Internal redox cyclizations of nitrone-functionalized terminal alkynes (I) are mediated by simple iridium complexes to give the cyclized azomethine ylides (II), which undergo [3+2] dipolar cycloaddition reactions with several alkynes and alkenes.

Rhodium(III)-catalyzed oxidative olefination of N-(1-naphthyl)sulfonamides has been achieved at the peri position. Three categories of olefins have been successfully applied. Activated olefins reacted to afford five-membered azacycles as a result of oxidative olefination-hydroamination. Unactivated olefins reacted to give the olefination product. 2-fold oxidative C-C and C-N coupling was achieved for allylbenzenes.

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 I(2)-mediated cyclization of nitrone-alkynes afforded iodinated γ-lactams and the I(2)-mediated internal redox of the closely related sulfoxide-alkynes gave diketones functionalized with a thoiether.

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.

In most cases, a selective diarylation takes place at the C-2 methyl group of the heterocyclic substrate.

A variety of pyridine-oxides undergoes a Pd-catalyzed oxidative coupling with substituted indoles at the 3-position.

Isolation of Azomethine Ylides and Their Complexes: Iridium(III)-Mediated Cyclization of Nitrone Substrates Containing Alkynes

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.

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 η(3)-azaallyl intermediate has been identified on the basis of DFT studies.

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 Ag(2)CO(3) as an oxidant.

Synthesis of γ-lactams via the oxidative coupling of benzamides with electron-poor olefins is demonstrated.

Catalytic oxidative coupling between acrylamides and alkynes was achieved using 0.5 mol % loading of [RhCp*Cl2]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.

The aminopyridine substrates undergo oxidative coupling with alkynes and acrylates under conditions A) to generate N-pyridyl-substituted indoles or N-(2-pyridyl)-quinolones, respectively.

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. Ag2CO3 proved to be an optimal oxidant when MeCN was used as a solvent, and [RhCp*Cl2]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.