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Starting from readily available tertiary alcohols, four different reactions (a 1,3-migration of a hydroxy group, kinetic resolution, racemization, and an intramolecular Diels–Alder reaction) took place under co-catalysis by lipase and oxovanadium compounds in a one-pot process to produce polysubstituted tricyclic carbon frameworks in high yields and with high enantioselectivities. The key to the success of this process was the discovery that a silyl group attached to the terminal carbon of the vinyl moiety completely controls the direction of hydroxy group migration

The precise mechanism of the chiral phosphoric acid-catalyzed aldol-type reaction of azlactones with vinyl ethers was investigated. DFT calculations suggested that the reaction proceeds through a Conia-ene-type transition state consisting of the vinyl ether and the enol tautomer of the azlactone, in which the catalyst protonates the nitrogen atom of the azlactone to promote enol tautomerization. In addition, the phosphoryl oxygen of the catalyst interacts with the vinyl proton of the vinyl ether. The favorable transition structure features dicoordinating hydrogen bonds. However, these hydrogen bonds are not involved in the bond recombination sequence and hence the catalyst functions as a template for binding substrates. From the results of theoretical studies and experimental supports, the high enantioselectivity is induced by the steric repulsion between the azlactone substituent and the binaphthyl backbone of the catalyst under the catalyst template effect.

Cellulose nanofibers obtained from wood pulp by TEMPO-mediated oxidation acted as a chiral enhancer in direct aldol reactions of 4-nitrobenzaldehyde and cyclopentanone with (S)-proline as an organocatalyst. Surprisingly, catalytically inactive TEMPO-oxidized cellulose nanofibers enriched the (R,R)-enantiomer in this reaction, affording 89% ee in the syn form with a very high yield (99%). Conversely, nanocellulose-free (S)-proline catalysis resulted in poor selectivity (64% ee, syn form) with a low yield (18%). Green organocatalysis occurring on nanocellulose solid surfaces bearing regularly aligned chiral carbons on hydrophobic crystalline facets will provide new insight into asymmetric synthesis strategies for interfacial catalysis.

Highly crystalline cellulose nanofibers with a high density of carboxylate groups only on the surfaces were prepared from both softwood and non-wood cellulose pulp. The preparation method used 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation of native cellulose fibrils in an aqueous TEMPO/NaBr/NaClO system and subsequent postoxidation with NaClO2 in acetate buffer (pH 4.8). The TEMPO-oxidized cellulose nanofibers (TOCNs) possessed a carboxylate content of 1.7 mmol g−1 and a crystallinity of 67–69% with a crystallite size of ∼3 nm. The TOCNs were used to produce highly crystalline TOCN-graft-poly(lactic acid) (PLA) nanocomposites via ring-opening polymerization of l-lactide in a polar aprotic solvent. Effects of the reaction temperature and the molar ratio of l-lactide to carboxylate surface groups, on the efficiency of surface grafting were investigated to potentially improve the crystallinity and thermal properties of the nanocomposites. The crystallinity of TOCN-g-PLA products was 59–66% greater than the crystallinity of neat PLA.

A chitosan nanofiber (CsNF)-catalyzed Knoevenagel reaction in green solvent, namely aqueous methanol, was investigated. CsNFs solely catalyzed the desired C–C bond formations in high yield with high selectivity, while conventional small-molecule amines, such as n-hexylamine and triethylamine, inevitably promoted transesterification to produce a large amount of solvolysis byproducts. Structural and chemical analyses of CsNFs suggested that the unique nanoarchitecture, in which chitosan molecules were bundled to ensure the high accessibility of substrates to catalytic sites, was critical to the highly efficient Knoevenagel condensation. The products were obtained in high purity without solvent-consuming purification, and the CsNF catalyst was easily removed and recycled. This study highlights a novel and promising function of CsNFs in green catalysis as emerging polysaccharide-based nanofibers.

Cellulose nanocrystals (CNCs) were successfully isolated by hydrochloric acid hydrolysis followed by ultrasonic homogenization of oil palm empty fruit bunches (OPEFB), which are a major form of agricultural waste in Southeast Asia. Currently sulfuric acid is mainly used in CNC preparation to achieve high dispersibility; however, we demonstrated that CNC suspensions prepared from OPEFB by hydrochloric acid hydrolysis remained stable without any sedimentation over 6 months. The obtained CNCs were fully characterized by elemental analysis, electron microscopic observation, X-ray diffraction measurement, and thermal analysis. The OPEFB-derived CNCs exhibited higher aspect ratios of 23–29 and higher thermal stability of 347–359 °C as maximum degradation temperature, as compared with those of woody CNCs prepared by sulfuric acid hydrolysis (15 and 311 °C, respectively). Although as-prepared CNCs showed comparable morphological and physicochemical properties to those prepared from oil palm biomass by other methods including sulfuric acid hydrolysis, use of hydrochloric acid and ultrasonication for hydrolysis of OPEFB was effective to yield crystalline CNCs with long-term nanodispersibility showing clear birefringence.

Paper-like porous composites of ceramic fibers and ZnO whiskers were prepared using a papermaking technique, followed by the in situ synthesis of a Pd catalyst on the ZnO whiskers using a facile impregnation method. The flexible Pd@ZnO papers had micrometer-sized pores of average diameter ca. 25 μm, which promoted the effective diffusion of reactants passing through an assembly of vertically stacked papers in a flow reactor. The catalytic efficiency of the stacked Pd@ZnO papers in a flow Suzuki–Miyaura cross-coupling (SMC) reaction to synthesize 4-methylbiphenyl from phenylboronic acid and 4-iodotoluene was higher than that of a bead-type Pd particulate catalyst in a reactor. Microchannels originating from the porous fiber-network microstructures in the stacked papers contributed to effective heterogeneous catalysis, possibly by enabling smooth diffusion of substrates to the surfaces of the Pd catalysts, as in a microreactor system. K2CO3, which was used as the base in the SMC reaction, was also immobilized in the paper-structured fibrous composites. Stacks of two types of paper, i.e., containing either Pd catalysts or K2CO3, significantly affected the SMC catalytic activity in a continuous microflow reaction. A combination of K2CO3 papers upstream and Pd@ZnO papers downstream in the flow system provided higher catalytic efficiency via on-site K2CO3-mediated borate formation of phenylboronic acid in the initial stage in the reactor. Tailoring of the stacking patterns of the paper-structured composites is expected to be effective for sequential SMC reaction and to improve catalytic process engineering.

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs), which have a high-density of exposed carboxylic acid groups on their crystalline surfaces, effectively act as acid catalysts in acetal hydrolysis. Carboxy-free cellulose nanofibers, polymeric carboxylic acids, and homogeneous acetic acid do not show significant catalytic activity under the same reaction conditions. Mercerized TOCNs differing from the original TOCNs in a crystalline structure were also ineffective, which suggests that the unique nanoarchitectural features of TOCNs, such as regularly aligned carboxylic acid groups, large specific surface areas, and structural rigidity, must be major factors in the acceleration of acetal hydrolysis. Kinetic analysis suggested that substrates and/or acid catalyst species were concentrated on the TOCN crystalline surfaces, which significantly enhanced the catalytic activity.

DFT studies were carried out to clarify the reaction mechanism and the stereocontrolling elements in the addition reaction of vinylindoles with azlactones catalyzed by a chiral phosphoric acid. The results suggest that the reaction proceeds via a six-membered transition state that is composed of the vinyl group of vinylindole and the enol moiety of azlactone.

An enantioselective Friedel–Crafts reaction with aliphatic ketimines generated in situ from hemiaminal ethers afforded products with high enantioselectivity under the influence of a chiral phosphoric acid catalyst., An enantioselective Friedel–Crafts reaction with aliphatic ketimines generated in situ from hemiaminal ethers catalyzed by a chiral Brønsted acid was investigated. The reaction of 2-methoxyfuran with (thio)hydantoin-derived hemiaminal methyl ether proceeded under the influence of a chiral phosphoric acid catalyst to afford the corresponding adduct possessing a quaternary stereogenic center in high yield with high enantioselectivity. Theoretical studies were also conducted to clarify the mechanism of the stereochemical outcome and the major factors contributing to the efficient enantioselection.

Chiral phosphoric acids have emerged as promising asymmetric Bronsted acid catalysts that harness hydrogen bonding interactions as key stereocontrolling elements. A new approach to chiral phosphoric acid catalysis through ion-pairing interactions between the anionic conjugate base of the catalyst and a cationic electrophile has recently attracted attention. However, the mechanism of stereocontrol through ion-pairing interactions is still elusive. As a probe reaction for studying the stereocontrolling element involved in such catalytic reactions, we investigated the Petasis–Ferrier-type rearrangement of a 7-membered cyclic vinyl acetal catalyzed by chiral phosphoric acids. DFT calculations suggested that non-classical C–H⋯O hydrogen bonds between the catalyst and the substrate play an important role in determining the stereoselectivity. In addition, π–π stacking interactions were found to be a key factor for stereocontrol when using a 9-anthryl group-bearing catalyst.

Highly enantioselective Michael-type addition (MTA) reactions between N-protected alkenyl benzimidazoles and either pyrazoles or indazoles as nitrogen nucleophiles are accomplished for the first time using chiral phosphoric acid catalyst. Theoretical studies elucidated the reaction pathway and the origin of the stereochemical outcomes, where the catalyst substituent and the N-protecting group of benzimidazole contributed to the resulting high enantioselectivity.

Syn-gled out: The syn diastereo- and enantioselective addition of azlactones to 3-vinylindoles was accomplished by using a chiral, binapthol-derived, Bronsted acid catalyst (see scheme). This method enables facile access to tryptophan derivatives with adjacent quaternary and tertiary stereogenic centers, which are potentially useful for the synthesis of peptidomimetics.