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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.

A biomimetic approach to the surface modification of nanocellulose is proposed. This strategy was inspired by plant cell wall lignification, in which lignin, a hydrophobic biopolymer, tightly assem...

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.

Cellulose nanofibers (CNFs) are finding a wide range of applications in the forthcoming sustainable society because of their carbon-neutral renewability and superior physicochemical properties. Here, we first show a cooperative organocatalysis by combining TEMPO-oxidized cellulose nanofiber (TOCN) and proline to enhance the catalytic efficiency in a direct aldol reaction. The yields of proline-catalyzed aldol products drastically increased in the presence of catalytically-inactive TOCN. This effect was also achieved by simply adding the TOCN to the reaction conditions where various proline analogues including structurally simple pyrrolidine and piperidine were used instead of proline. TOCN was superior to physically-pulverized CNF in the organocatalytic efficiency, and the nanofibrillation of cellulose microfibrils in reaction media was essential to induce the drastic enhancement in catalytic activity. The present finding will bring a new entry in the applications of CNFs, and open up a new phase in developing highly efficient molecular transformations in green chemical industries.

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.

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.