Organo-Mediated Ring-Opening Polymerization of Ethylene Brassylate from Cellulose Nanofibrils in Reactive Extrusion.

Ethylene brassylate is a renewable macrolactone from castor oil that can be polymerized via ring-opening polymerization (ROP) to obtain a fully biosourced biodegradable polyester. ROP mediated by organometallic catalysts leads to high molar mass poly(ethylene brassylate) (PEB). However, the use of metal-free organocatalysis has several advantages, such as the reduction of toxic and expensive metals. In this work, a novel cellulose nanofibril (CNF)/PEB nanocomposite fabrication process by organocatalysis and reactive extrusion (REx) is disclosed. Here, ROP was carried out via solvent-free REx in the presence of CNFs using organic 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a catalyst. Neat or lactate-esterified CNFs (LACNF) were used as initiators to investigate the effect of surface topochemistry on the in situ polymerization and the properties of the nanocomposites. A molar mass of 9 kDa was achieved in the presence of both unmodified and LACNFs with high monomer conversion (>98%) after 30 min reaction in a microcompounder at 130 °C. Tensile analysis showed that both nanofibril types reinforce the matrix and increase its elasticity due to the efficient dispersion obtained through the grafting from polymerization achieved during the REx. Mechanical recycling of the neat polymer and the nanocomposites was proven as a circular solution for the materials' end-of-life and showed that lactate moieties induced some degradation.

Heterogeneous Copper-Catalyzed Cross-Coupling for Sustainable Synthesis of Chiral Allenes: Application to the Synthesis of Allenic Natural Products.

Classical Crabbé type SN 2' substitutions of propargylic substrates has served as one of the standard methods for the synthesis of allenes. However, the stereospecific version of this transformation often requires either stoichiometric amounts of organocopper reagents or special functional groups on the substrates, and the chirality transfer efficiency is also capricious. Herein, we report a sustainable methodology for the synthesis of diverse 1,3-di and tri-substituted allenes by using a simple and cheap cellulose supported heterogeneous nanocopper catalyst (MCC-Amp-Cu(I/II)). This approach represents the first example of heterogeneous catalysis for the synthesis of chiral allenes. High yields and excellent enantiospecificity (up to 97 % yield, 99 % ee) were achieved for a wide range of di- and tri-substituted allenes bearing various functional groups. It is worth noting that the applied heterogeneous catalyst could be recycled at least 5 times without any reduced reactivity. To demonstrate the synthetic utility of the developed protocol, we have applied it to the total synthesis of several chiral allenic natural products.

A facile route for concurrent fabrication and surface selective functionalization of cellulose nanofibers by lactic acid mediated catalysis.

Celulose nanofibers are lightweight, recycable, biodegradable, and renewable. Hence, there is a great interest of using them instead of fossil-based components in new materials and biocomposites. In this study, we disclose an environmentally benign (green) one-step reaction approach to fabricate lactic acid ester functionalized cellulose nanofibrils from wood-derived pulp fibers in high yields. This was accomplished by converting wood-derived pulp fibers to nanofibrillated "cellulose lactate" under mild conditions using lactic acid as both the reaction media and catalyst. Thus, in parallel to the cellulose nanofibril production, concurrent lactic acid-catalyzed esterification of lactic acid to the cellulose nanofibers surface occured. The direct lactic acid esterification, which is a surface selective functionalization and reversible (de-attaching the ester groups by cleavage of the ester bonds), of the cellulose nanofibrils was confirmed by low numbers of degree of substitution, and FT-IR analyses. Thus, autocatalytic esterification and cellulose hydrolysis occurred without the need of metal based or a harsh mineral acid catalysts, which has disadvantages such as acid corrosiveness and high recovery cost of acid. Moreover, adding a mineral acid as a co-catalyst significantly decreased the yield of the nanocellulose. The lactic acid media is successfully recycled in multiple reaction cycles producing the corresponding nanocellulose fibers in high yields. The disclosed green cellulose nanofibril production route is industrial relevant and gives direct access to nanocellulose for use in variety of applications such as sustainable filaments, composites, packaging and strengthening of recycled fibers.

Cellulose-Supported Heterogeneous Gold-Catalyzed Cycloisomerization Reactions of Alkynoic Acids and Allenynamides.

Herein, we describe efficient nanogold-catalyzed cycloisomerization reactions of alkynoic acids and allenynamides to enol lactones and dihydropyrroles, respectively (the latter via an Alder-ene reaction). The gold nanoparticles were immobilized on thiol-functionalized microcrystalline cellulose and characterized by electron microscopy (HAADF-STEM) and by XPS. The thiol-stabilized gold nanoparticles (Au0) were obtained in the size range 1.5-6 nm at the cellulose surface. The robust and sustainable cellulose-supported gold nanocatalyst can be recycled for multiple cycles without losing activity.

Subtilisin integrated artificial plant cell walls as heterogeneous catalysts for asymmetric synthesis of (S)-amides.

Subtilisin integrated artificial plant-cell walls (APCWs) were fabricated by self-assembly using cellulose or nanocellulose as the main component. The resulting APCW catalysts are excellent heterogeneous catalysts for the asymmetric synthesis of (S)-amides. This was demonstrated by the APCW-catalyzed kinetic resolution of several racemic primary amines to give the corresponding (S)-amides in high yields with excellent enantioselectivity. The APCW catalyst can be recycled for multiple reaction cycles without loss of enantioselectivity. The assembled APCW catalyst was also able to cooperate with a homogeneous organoruthenium complex, which allowed for the co-catalytic dynamic kinetic resolution (DKR) of a racemic primary amine to give the corresponding (S)-amide in high yield. The APCW/Ru co-catalysis constitutes the first examples of DKR of chiral primary amines when subtilisin is used as a co-catalyst.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives.

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

Design and fabrication of superhydrophobic cellulose nanocrystal films by combination of self-assembly and organocatalysis.

Cellulose nanocrystals, which have unique properties of high aspect ratio, high surface area, high mechanical strength, and a liquid crystalline nature, constitute a renewable nanomaterial with great potential for several uses (e.g., composites, films and barriers). However, their intrinsic hydrophilicity results in materials that are moisture sensitive and exhibit poor water stability. This limits their use and competitiveness as a sustainable alternative against fossil-based materials/plastics in packaging, food storage, construction and materials application, which cause contamination in our oceans and environment. To make cellulose nanocrystal films superhydrophobic, toxic chemicals such as fluorocarbons are typically attached to their surfaces. Hence, there is a pressing need for environmentally friendly alternatives for their modification and acquiring this important surface property. Herein, we describe the novel creation of superhydrophobic, fluorocarbon-free and transparent cellulose nanocrystal films with functional groups by a bioinspired combination of self-assembly and organocatalytic surface modification at the nanoscale using food approved organic acid catalysts. The resulting film-surface is superhydrophobic (water contact angle > 150°) and has self-cleaning properties (the lotus effect). In addition, the superhydrophobic cellulose nanocrystal films have excellent water stability and significantly decreased oxygen permeability at high relative humidity with oxygen transmission rates better than those of commonly used plastics.

Regio- and Stereoselective Carbon-Boron Bond Formation via Heterogeneous Palladium-Catalyzed Hydroboration of Enallenes.

A highly efficient regio- and stereoselective heterogeneous palladium-catalyzed hydroboration reaction of enallenes was developed. Nanopalladium immobilized on microcrystalline cellulose (MCC) was successfully employed as an efficient catalyst for the enallene hydroboration reaction. The nanopalladium particles were shown by HAADF-STEM to have an average size of 2.4 nm. The cellulose-supported palladium catalyst exhibits high stability and provides vinyl boron products in good to high isolated yields (up to 90 %). The nanopalladium catalyst can be efficiently recycled and it was demonstrated that the catalyst can be used in 7 runs with a maintained high yield (>80 %). The vinylboron compounds prepared from enallenes are important synthetic intermediates that can be used in various organic synthetic transformations.

Efficient Heterogeneous Copper-Catalyzed Alder-Ene Reaction of Allenynamides to Pyrrolines.

Herein, we describe an efficient nanocopper-catalyzed Alder-ene reaction of allenynamides. The copper nanoparticles were immobilized on amino-functionalized microcrystalline cellulose. A solvent-controlled chemoselectivity of the reaction was observed, leading to the chemodivergent synthesis of pyrrolines (2,5-dihydropyrroles) and pyrroles. The heterogeneous copper catalyst exhibits high efficiency and good recyclability in the Alder-ene reaction, constituting a highly attractive catalytic system from an economical and environmental point of view.

Artificial plant cell walls as multi-catalyst systems for enzymatic cooperative asymmetric catalysis in non-aqueous media.

The assembly of cellulose-based artificial plant cell wall (APCW) structures that contain different types of catalysts is a powerful strategy for the development of cascade reactions. Here we disclose an APCW catalytic system containing a lipase enzyme and nanopalladium particles that transform a racemic amine into the corresponding enantiomerically pure amide in high yield via a dynamic kinetic resolution.

Copper nanoparticles on controlled pore glass (CPG) as highly efficient heterogeneous catalysts for "click reactions".

We herein report that supported copper nanoparticles (CuNPs) on commercially available controlled pore glass (CPG), which exhibit high mechanical, thermal and chemical stability as compared to other silica-based materials, serve as a useful heterogeneous catalyst system for 1,3-dipolar cycloadditions ("click" reactions) between terminal alkynes and organic azides under green chemistry conditions. The supported CuNPs-CPG catalyst exhibited a broad substrate scope and gave the corresponding triazole products in high yields. The CuNPs-CPG catalyst exhibit recyclability and could be reuced multiple times without contaminating the products with Cu.

Mild and Versatile Functionalization of Nacre-Mimetic Cellulose Nanofibrils/Clay Nanocomposites by Organocatalytic Surface Engineering.

Development of surface-engineering strategies, which are facile, versatile, and mild, are highly desirable in tailor-made functionalization of high-performance bioinspired nanocomposites. We herein disclose for the first time a general organocatalytic strategy for the functionalization and hydrophobization of nacre-mimetic nanocomposites, which includes vide supra key aspects of surface engineering. The merging of metal-free catalysis and the design of nacre-mimetic nanocomposite materials were demonstrated by the organocatalytic surface engineering of cellulose nanofibrils/clay nanocomposites providing the corresponding bioinspired nanocomposites with good mechanical properties, hydrophobicity, and useful thia-, amino, and olefinic functionalities.

Sustainable and recyclable heterogenous palladium catalysts from rice husk-derived biosilicates for Suzuki-Miyaura cross-couplings, aerobic oxidations and stereoselective cascade carbocyclizations.

A new eco-friendly approach for the preparation of sustainable heterogeneous palladium catalysts from rice husk-derived biogenic silica (RHP-Si and RHU-Si). The designed heterogeneously supported palladium species (RHP-Si-NH2-Pd and RHU-Si-NH2-Pd) were fully characterized and successfully employed as catalysts for various chemical transformations (C-C bond-forming reactions, aerobic oxidations and carbocyclizations). Suzuki-Miyaura transformations were highly efficient in a green solvent system (H2O:EtOH (1:1) with excellent recyclability, providing the cross-coupling products with a wide range of functionalities in high isolated yields (up to 99%). Palladium species (Pd(0)-nanoparticles or Pd(II)) were also efficient catalysts in the green aerobic oxidation of an allylic alcohol and a co-catalytic stereoselective cascade carbocyclization transformation. In the latter case, a quaternary stereocenter was formed with excellent stereoselectivity (up to 27:1 dr).

Silver-Triggered Activity of a Heterogeneous Palladium Catalyst in Oxidative Carbonylation Reactions.

A silver-triggered heterogeneous Pd-catalyzed oxidative carbonylation has been developed. This heterogeneous process exhibits high efficiency and good recyclability, and was utilized for the one-pot construction of polycyclic compounds with multiple chiral centers. AgOTf was used to remove chloride ions in the heterogeneous catalyst Pd-AmP-CNC, thereby generating highly active PdII , which results in high efficiency of the heterogeneous catalytic system.

Enamine/Transition Metal Combined Catalysis: Catalytic Transformations Involving Organometallic Electrophilic Intermediates.

The concept of merging enamine activation catalysis with transition metal catalysis is an important strategy, which allows for selective chemical transformations not accessible without this combination. The amine catalyst activates the carbonyl compounds through the formation of a reactive nucleophilic enamine intermediate and, in parallel, the transition metal activates a wide range of functionalities such as allylic substrates through the formation of reactive electrophilic π-allyl-metal complex. Since the first report of this strategy in 2006, considerable effort has been devoted to the successful advancement of this technology. In this chapter, these findings are highlighted and discussed.

Sustainable Surface Engineering of Lignocellulose and Cellulose by Synergistic Combination of Metal-Free Catalysis and Polyelectrolyte Complexes.

A sustainable strategy for synergistic surface engineering of lignocellulose and cellulose fibers derived from wood by synergistic combination of metal-free catalysis and renewable polyelectrolyte (PE) complexes is disclosed. The strategy allows for improvement and introduction of important properties such as strength, water resistance, and fluorescence to the renewable fibers and cellulosic materials. For example, the "green" surface engineering significantly increases the strength properties (up to 100% in Z-strength) of chemi-thermomechanical pulp (CTMP) and bleached sulphite pulp (BSP)-derived sheets. Next, performing an organocatalytic silylation with a nontoxic organic acid makes the corresponding lignocellulose and cellulose sheets hydrophobic. A selective color modification of polysaccharides is developed by combining metal-free catalysis and thiol-ene click chemistry. Next, fluorescent PE complexes based on cationic starch (CS) and carboxymethylcellulose (CMC) are prepared and used for modification of CTMP or BSP in the presence of a metal-free catalyst. Laser-scanning confocal microscopy reveals that the PE-strength additive is evenly distributed on the CTMP and heterogeneously on the BSP. The fluorescent CS distribution on the CTMP follows the lignin distribution of the lignocellulosic fibers.

The Chemical Synthesis and Applications of Tropane Alkaloids.

Tropanes are an important class of alkaloid natural products that are found in plants all over the world. These compounds can exhibit significant biological activity and are among the oldest known medicines. In the early 19th century, tropanes were isolated, characterized, and synthesized by notable chemical researchers. Their significant biological activities have inspired tremendous research efforts toward their synthesis and the elucidation of their pharmacological activity both in academia and in industry. In this chapter, which addresses the developments in this field since 1994, the focus is on the synthesis of these compounds, and several examples of sophisticated synthetic protocols involving both asymmetric and catalytic approaches are described. In addition, the structures of more than 100 new alkaloids are included as well as the applications and pharmacological properties of some tropane alkaloids.

Sustainable Design for the Direct Fabrication and Highly Versatile Functionalization of Nanocelluloses.

This study describes a novel sustainable concept for the scalable direct fabrication and functionalization of nanocellulose from wood pulp with reduced energy consumption. A central concept is the use of metal-free small organic molecules as mediators and catalysts for the production and subsequent versatile surface engineering of the cellulosic nanomaterials via organocatalysis and click chemistry. Here, "organoclick" chemistry enables the selective functionalization of nanocelluloses with different organic molecules as well as the binding of palladium ions or nanoparticles. The nanocellulosic material is also shown to function as a sustainable support for heterogeneous catalysis in modern organic synthesis (e.g., Suzuki cross-coupling transformations in water). The reported strategy not only addresses obstacles and challenges for the future utilization of nanocellulose (e.g., low moisture resistance, the need for green chemistry, and energy-intensive production) but also enables new applications for nanocellulosic materials in different areas.

Combinations of Aminocatalysts and Metal Catalysts: A Powerful Cooperative Approach in Selective Organic Synthesis.

The cooperation and interplay between organic and metal catalyst systems is of utmost importance in nature and chemical synthesis. Here innovative and selective cooperative catalyst systems can be designed by combining two catalysts that complement rather than inhibit one another. This refined strategy can permit chemical transformations unmanageable by either of the catalysts alone. This review summarizes innovations and developments in selective organic synthesis that have used cooperative dual catalysis by combining simple aminocatalysts with metal catalysts. Considerable efforts have been devoted to this fruitful field. This emerging area employs the different activation modes of amine and metal catalysts as a platform to address challenging reactions. Here, aminocatalysis (e.g., enamine activation catalysis, iminium activation catalysis, single occupied molecular orbital (SOMO) activation catalysis, and photoredox activation catalysis) is employed to activate unreactive carbonyl substrates. The transition metal catalyst complements by activating a variety of substrates through a range of interactions (e.g., electrophilic π-allyl complex formation, Lewis acid activation, allenylidene complex formation, photoredox activation, C-H activation, etc.), and thereby novel concepts within catalysis are created. The inclusion of heterogeneous catalysis strategies allows for "green" chemistry development, catalyst recyclability, and the more eco-friendly synthesis of valuable compounds.

Abiotic synthesis of amino acids and self-crystallization under prebiotic conditions.

Building on previous research on the origin and homochirality of life, this study focuses on analyses profiling important building blocks of life: the natural amino acids. The spark discharge variation of the iconic Miller experiment was performed with a reducing gas mixture of ammonia, methane, water and hydrogen. Amino acid analysis using liquid chromatography coupled with tandem mass spectrometry after pre-column derivatizaiton revealed the generation of several amino acids including those essential for life. Re-crystallization of the synthetic products and enantiomeric ratio analysis were subsequently performed. Results from liquid chromatography coupled with either fluorescent detector or tandem mass spectrometry after pre-column derivatization with chiral reagent revealed spontaneous and effective asymmetric resolution of serine and alanine. This work describes a useful analytical platform for investigation of hypotheses regarding the origin and homochirality of amino acids under prebiotic conditions. The formation of numerous amino acids in the electric discharge experiment and the occurrence of high enantiomeric ratios of amino acids in re-crystallization experiment give valuable implications for future studies in unraveling fundamental questions regarding origins and evolution of life.