Cooperative catalysis of cellulose nanofiber and organocatalyst in direct aldol reactions

Emerging Nanocellulose Technologies: Recent Developments

Nanocelluloses have unique morphologies, characteristics, and surface nanostructures, and are prepared from abundant and renewable plant biomass resources. Therefore, expansion of the use of CO₂ -accumulating nanocelluloses is expected to partly contribute to the establishment of a sustainable society and help overcome current global environmental issues. Nanocelluloses can be categorized into cellulose nanonetworks, cellulose nanofibrils, and cellulose nanocrystals, depending on their morphologies. All of these materials are first obtained as aqueous dispersions. In particular, cellulose nanofibrils have homogeneous ≈3 nm widths and average lengths of >500 nm, and significant amounts of charged groups are present on their surfaces. Such charged groups are formed by carboxymethylation, C6-carboxylation, phosphorylation, phosphite esterification, xanthation, sulfate esterification, and C2/C3 dicarboxylation during the pretreatment of plant cellulose fibers before their conversion into cellulose nanofibrils via mechanical disintegration in water. These surface-charged groups in nanocelluloses can be stoichiometrically counterion-exchanged into diverse metal and alkylammonium ions, resulting in surface-modified nanocelluloses with various new functions including hydrophobic, water-resistant, catalytic, superdeodorant, and gas-separation properties. However, many fundamental and application-related issues facing nanocelluloses must first be overcome to enable their further expansion.

Chronicle of Nanocelluloses (NCs) for Catalytic Applications: Key Advances

Nanocellulose (NC) is a biomaterial with growing interest in the field of nanocomposites and sustainable materials. NC has various applications including biodegradable materials, reinforcing agents, packaging films, transpiring membranes and medical devices. Among the many applications, the use of NC functionalized with organic and inorganic groups has found wide use as a catalyst in chemical transformations. The goal of this review is to collect the current knowledge on its catalytic applications for chemical groups conversion. We have chosen to organize the manuscript according to subdivision of NC into Bacterial Nanocellulose (BNC), Cellulose Nanocrystals (CNCs), and Cellulose Nanofibers (CNFs) and their role as inorganic- and organic-functionalized NC-catalysts in organic synthesis. However, in consideration of the fact that the literature on this field is very extensive, we have decided to focus our attention on the scientific productions of the last five years.

Aerogels from Cellulose Phosphates of Low Degree of Substitution: A TBAF·H2O/DMSO Based Approach

Biopolymer aerogels of appropriate open-porous morphology, nanotopology, surface chemistry, and mechanical properties can be promising cell scaffolding materials. Here, we report a facile approach towards the preparation of cellulose phosphate aerogels from two types of cellulosic source materials. Since high degrees of phosphorylation would afford water-soluble products inappropriate for cell scaffolding, products of low DS_P (ca. 0.2) were prepared by a heterogeneous approach. Aiming at both i) full preservation of chemical integrity of cellulose during dissolution and ii) utilization of specific phase separation mechanisms upon coagulation of cellulose, TBAF·H₂O/DMSO was employed as a non-derivatizing solvent. Sequential dissolution of cellulose phosphates, casting, coagulation, solvent exchange, and scCO₂ drying afforded lightweight, nano-porous aerogels. Compared to their non-derivatized counterparts, cellulose phosphate aerogels are less sensitive towards shrinking during solvent exchange. This is presumably due to electrostatic repulsion and translates into faster scCO₂ drying. The low DS_P values have no negative impact on pore size distribution, specific surface (S_BET ≤ 310 m² g⁻¹), porosity (Π 95.5–97 vol.%), or stiffness (Eρ ≤ 211 MPa cm³ g⁻¹). Considering the sterilization capabilities of scCO₂, existing templating opportunities to afford dual-porous scaffolds and the good hemocompatibility of phosphorylated cellulose, TBAF·H₂O/DMSO can be regarded a promising solvent system for the manufacture of cell scaffolding materials.

Functionalized cellulose nanofibril aerogels as cooperative acid-base organocatalysts for liquid flow reactions

Cellulose nanomaterial aerogels are macroscopic porous solids with relatively high surface areas and are thus an interesting basis for renewable catalyst materials. Cross-linked acid-base bifunctional catalyst aerogels are produced here from TEMPO-oxidized cellulose nanofibrils (TOCNF) and demonstrated in both batch and flow catalysis. Recently established acid-base modification for catalysis is expanded upon for chemical or physical cross-linking with small molecules and polymers. Low density and relatively high surface area (up to 74 m² g^-1) aerogel catalysts are produced with a variety of processing approaches and then freeze-dried from water or tert-butyl alcohol/water mixtures. Finer pore structure and increased surface area are achieved with tert-butyl alcohol as co-solvent. Chemical cross-linking improved aerogel stability to solvents. Homogeneous and aerogel TOCNF catalysts are shown to be effective acid-base cooperative catalysts for aldol condensation reactions in batch reactions. Continuous flow reactions are performed with glass column reactors packed with aerogel catalysts that showed improved rates relative to batch experiments, while also demonstrating physical stability. Catalyst deactivation in flow reactions is observed and observations of deactivation support previously reported mechanisms of site poisoning by competitive chemisorption of reactants in analogous acid-base catalysts. This report is a key demonstration of cellulose nanofibril aerogels for catalysis in continuous liquid flow reactions.

Valorization of Waste: Sustainable Organocatalysts from Renewable Resources

One of the greatest challenges facing our society is to reconcile our need to develop efficient and sophisticated chemical processes with the limited resources of our planet and its restricted ability to adsorb pollution. Organocatalysis has allowed many issues to be addressed in the development of sophisticated, but less polluting, processes. However, minimizing waste also means an efficient utilization of raw and renewable materials. Waste biomass represents an alternative to conventional petroleum-based chemical manufacturing and is a highly attractive renewable resource for the production of chemicals and high-value-added organocatalysts. Recent achievements in the use of renewable biomass feedstocks for the synthesis of organocatalysts are presented. Their application in synthetic methodologies, including multicomponent reactions, which are performed under solvent-free conditions or in eco-friendly reaction media, as well as recycling and reusing the organocatalysts, is illustrated. A few pioneering examples that demonstrate the potential of these promoters in asymmetric synthesis have also been documented. In particular, this review covers examples on the use of hetero- and homogeneous organocatalysts derived from 1) waste biopolymers, such as chitosan, alginic acid, and cellulose; ii) renewable platform molecules, such as levoglucosenone, isosorbide, mannose, d-glucosamine, and lecithin; 3) terpenes and rosin, such as pinane, isosteviol, and abietic acid; and iv) natural proteins (gelatin, bovine tendons, silk fibroin proteins).

Chitosan nanofiber-catalyzed highly selective Knoevenagel condensation in aqueous methanol

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.

Chitosan nanofibers bearing abundant and accessible amines exposed on the solid surface catalyze a highly selective Knoevenagel condensation in green solvent, which completely avoids the formation of solvolysis byproducts.

Nanocellulose enriches enantiomers in asymmetric aldol reactions

Catalytically inactive cellulose nanofibers with crystalline solid surfaces enhance highly enantioselective organocatalysis at the interface in proline-mediated aldol reactions.

Concerted Catalysis by Nanocellulose and Proline in Organocatalytic Michael Additions

Cellulose nanofibers (CNFs) have recently attracted much attention as catalysts in various reactions. Organocatalysts have emerged as sustainable alternatives to metal-based catalysts in green organic synthesis, with concerted systems containing CNFs that are expected to provide next-generation catalysis. Herein, for the first time, we report that a representative organocatalyst comprising an unexpected combination of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-oxidized CNFs and proline shows significantly enhanced catalytic activity in an asymmetric Michael addition.

Theoretical Study of the Histidine-catalyzed Asymmetric Aldol Reaction of Acetone and Benzaldehyde

The mechanism of histidine-catalyzed asymmetrical aldol reaction of acetone with benzaldehyde was studied by using B3LYP method of density functional theory at the levels of 6-31G(d,p) and cc-pvdz basis sets. The calculation results showed that the reaction mechanism included four steps: (I) nucleophilic attack of histidine on acetone to form alcohol intermediate Inter-A through the transition state TS1 (considered a rate control step because the activation energy (49.95 kcal/mol) was relatively high); (II) dehydration of the alcohol intermediate to form the cis- or trans-enamine through the transition states TS3 and TS4 with the energy barriers of 36.12 and 38.15 kcal/mol; (III) electrophilic addition of cis-enamine or trans-enamine with benzaldehyde to form imine Inter-C or Inter-E through the transition states TS8, TS9, TS10, and TS11 (energy barriers 18.43, 22.34, 13.24, and 13.24 kcal/mol, respectively); (IV) after combination of the imine intermediate with water through the transition states TS12, TS13, TS14, and TS15 (energy barriers 22.79, 34.6, 28.2, 25.12 kcal/mol, respectively), removal of the histidine catalys to obtain the final S or R aldol product. Through analyzing the potential energy profile of reaction, we found that the histidine-catalyzed reaction of acetone with benzaldehyde was more energetically favorable to obtain the R-product (ee value >99%). Solvent effects computed with a polarizable continuum model (PCM) indicated that the DMSO and water can reduce the reaction energy barrier.