Ease to Challenges in Achieving Successful Synthesized Schiff Base, Chirality, and Application as Antibacterial Agent

This study reports how to overcome the challenges experienced in achieving successful synthesized Schiff base via types of Schiff base (chiral and achiral), synthesis, nature of products, and its antibacterial applications. Schiff base is a versatile ligand which is useful in asymmetric reactions to prepare chiral catalysts. It is also used in symmetric reactions to prepare achiral compounds. In line with the achiral compounds, conventional (room temperature and refluxing) and microwave irradiation methods are the two main types of methods to synthesize achiral Schiff base as reported in this review. Among various experimental approaches, this study supports the green chemistry microwave approach to synthesize Schiff base because of its benefits environmental sustainability. Problems relating to the nature of products formed from the synthesized Schiff bases were examined and resolved. Herein, the products could either be solid (crystals, powder, and precipitate), oily, or viscous (sticky) products. Some familiar characterization techniques used to identify and confirm the successful syntheses of Schiff bases, such as solubility test, melting point (MP), Fourier transform infrared (FTIR), ultraviolet-visible (UV-Vis), and nuclear magnetic resonance (NMR, 1H NMR, and 13C NMR), were discussed. In addition, the antibacterial studies on Schiff base and corresponding metal complexes confirmed their biological relevance to the human.

Recent Research Progress on Lignin-Derived Resins for Natural Fiber Composite Applications

By increasing the environmental concerns and depletion of petroleum resources, bio-based resins have gained interest. Recently, lignin, vanillin (4-hydroxy-3-methoxybenzaldehyde), and divanillin (6,6'-dihydroxy-5,5'-dimethoxybiphenyl-3,3'-dicarbaldehyde)-based resins have attracted attention due to the low cost, environmental benefits, good thermal stability, excellent mechanical properties, and suitability for high-performance natural fiber composite applications. This review highlights the recent use of lignin, vanillin, and divanillin-based resins with natural fiber composites and their synthesized processes. Finally, discussions are made on the curing kinetics, mechanical properties, flame retardancy, and bio-based resins' adhesion property.

Electrochemical synthesis of biobased polymers and polymer building blocks from vanillin

Vanillin, one of the few biobased aromatic compounds available on an industrial level, is an attractive candidate for the synthesis of biobased polymers and polymer building blocks. This study presents a detailed investigation of the reductive electrochemical coupling process by pinacolization of vanillin and divanillin in an electrochemical H-type cell setup to the polymer building block hydrovanilloin and to polyvanillin, respectively. Therein, different cathode materials are screened by linear sweep voltammetry for their capability and activity of hydrodimerization of phenolic aromatic aldehydes in alkaline aqueous media. Product distributions and faradaic efficiencies of the electrochemical vanillin reduction are investigated in bulk electrolysis experiments. Dependencies on electrochemical parameters such as current densities, applied charges and cathode materials are studied. Furthermore, the polyvanillin synthesis from divanillin is also investigated by bulk electrolysis experiments. The effects of selected electrochemical parameters (current density, applied charge and electrode material) on yield and structural features (weight-average molecular weight (M _W), number-average molecular weight (M _N), polydispersity (M _W/M _N)) measured by size exclusion chromatography of the obtained polyvanillin were evaluated. Structural features of isolated polyvanillin were determined by 2D-NMR (HSQC, ¹³C/¹H) analyses and by ³¹P-NMR analyses after in situ labeling with Cl-TMDP and possible pathways for their generation are discussed. These two promising electro-synthetic processes studied are free of hazardous materials and reagents and highlight the contributions of preparative electrochemistry to green chemistry and further pave the way toward the application of electrochemistry in the synthesis of biobased building blocks and polymers.

Recent Advances in Renewable Polymer Production from Lignin-Derived Aldehydes

Lignin directly derived from lignocellulosic biomass has been named a promising source of platform chemicals for the production of bio-based polymers. This review discusses potentially relevant routes to produce renewable aromatic aldehydes (e.g., syringaldehyde and vanillin) from lignin feedstocks (pre-isolated lignin or lignocellulose) that are used to synthesize a range of bio-based polymers. To do this, the processes to make aromatic aldehydes from lignin with their highest available yields are first presented. After that, the routes from such aldehydes to different polymers are explored. Challenges and perspectives of the production the lignin-derived renewable chemicals and polymers are also highlighted.

Adsorption and anti-corrosion characteristics of vanillin Schiff bases on mild steel in 1 M HCl: experimental and theoretical study

Herein, two Schiff base derivatives of vanillin and divanillin with 2-picolylamine, namely, 2-methoxy-4-((pyridin-2-ylmethylimino)methyl)phenol (compound A) and 3,3′-dimethoxy-5,5′-bis-((pyridin-2-ylmethylimino)methyl)-[1,1′-biphenyl]-2,2′-diol (compound B), respectively, were synthesized. Additionally, their adsorption characteristics and corrosion inhibition behavior were compared for mild steel in 1 M HCl using electrochemical impedance spectroscopy, potentiodynamic polarization and weight loss methods. Compound B was found to impart a better anti-corrosive effect (around 95% inhibition efficiency at 313 K) than compound A. The inhibitors act as effective mixed-type inhibitors and exhibit Langmuir-type adsorption behaviour. The kinetic–thermodynamic parameters together with the data obtained from density functional theory (DFT) and molecular dynamics (MD) simulations illustrate the mechanism of corrosion and mode of adsorption of both inhibitors on the metal surface. The better corrosion mitigation propensity of the dimeric form of the inhibitor (compound B) over the monomeric form (compound A) was tested experimentally and explained according to the theoretical data.

Two Schiff base derivatives of vanillin and divanillin with 2-picolylamine are synthesized and their anti-corrosive propensity for mild steel in aqueous HCl are compared.

Fabrication of chitosan-bis (4-formyl-2 methoxy phenyl carbonate) Schiff base nanoparticles and evaluation of their antioxidant and anticancer properties

The present study details on the mechanism of synthesis of bis (4-formyl-2 methoxy phenyl carbonate), using two green reagents dimethyl carbonate and vanillin for application as therapeutic agent. The synthesized FMPC was identified from the ¹³C nuclear magnetic resonance spectra. The novel modified Schiff base nanoparticles resulted from the crosslinking of FMPC with chitosan were confirmed by cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance spectroscopy. The incorporation of the FMPC was identified from the amorphous X-ray diffraction patterns of C-FMPC-Nps. The thermal stability of the formed nanoparticles was predicted using thermogravimetric analysis. The morphology of the nanoparticles as observed from HRTEM was found to be smooth and spherical in nature. Both FMPC and C-FMPC-Nps showed significant radical scavenging potential and anticancer property. The carbonate ester backbone and the moiety present in chitosan-FMPC-nanoparticles, underwent hydrolysis at the targeted cancer causing microenvironment to release vanillin and chitosan and enhance the anticancer activity. Both FMPC and C-FMPC-Nps exhibits a dose dependent cytotoxicity towards the different cell lines and it was tested with a commercial drug for application studies. Effective synthesis of FMPC, successful incorporation onto chitosan nanoparticles for the formation of C-FMPC-Nps. The formed Schiff base compound proves to have enhanced antioxidant and anticancer efficacy.

Poly(N-isopropylacrylamide-co-2-((diethylamino)methyl)-4-formyl-6-methoxyphenyl acrylate) Environmental Functional Copolymers: Synthesis, Characterizations, and Grafting with Amino Acids

Vanillin was used to synthesize a new derivative with an active aldehyde group and response to pH. It is named 2-((diethylamino) methyl)-4-formyl-6-methoxyphenyl acrylate, abbreviated to DEAMVA. The chemical structures were evaluated by ¹H, 13C nuclear magnetic resonance (NMR), infrared (IR), and UV-Vis-spectroscopy, and all results demonstrated good statement. In order to achieve the dual responsive behavior thermo-pH with functionality, free radical polymerization of N-isopropylacrylamide with DEAMVA in different molar ratios (5, 10, 15 mol%) has been used, with azobisisobutyronitrile (AIBN) as the initiator. The chemical structure of the polymers was investigated by ¹H NMR and IR. The dual responsive functional copolymer was exposed to a grafted process with tryptophan and tyrosine, both of which were also evaluated by ¹HNMR and IR. Copolymers before and after grafting were physically investigated by size exclusion chromatography (SEC) for estimation of the molecular weight, the glass transition temperature by differential scanning calorimeter (DSC) and scanning electron microscope (SEM) for the surface morphology. The phase separation or lower critical solution temperature (LCST) (Tc) of the polymer solution was determined not only by a turbidity method using the change in the transmittance with temperature, but also by micro-DSC. The conversion to an amino acid-grafted polymer was detected through Beer's law for the absorption of the ⁻CH=N- imine group by UV-Vis-Spectroscopy.

Bright Side of Lignin Depolymerization: Toward New Platform Chemicals

Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the future considered as potential lignin-derived platform chemicals.

Creating Oxidase-Peroxidase Fusion Enzymes as a Toolbox for Cascade Reactions

A set of bifunctional oxidase-peroxidases has been prepared by fusing four distinct oxidases to a peroxidase. Although such fusion enzymes have not been observed in nature, they could be expressed and purified in good yields. Characterization revealed that the artificial enzymes retained the capability to bind the two required cofactors and were catalytically active as oxidase and peroxidase. Peroxidase fusions of alditol oxidase and chitooligosaccharide oxidase could be used for the selective detection of xylitol and cellobiose with a detection limit in the low-micromolar range. The peroxidase fusions of eugenol oxidase and 5-hydroxymethylfurfural oxidase could be used for dioxygen-driven, one-pot, two-step cascade reactions to convert vanillyl alcohol into divanillin and eugenol into lignin oligomers. The designed oxidase-peroxidase fusions represent attractive biocatalysts that allow efficient biocatalytic cascade oxidations that only require molecular oxygen as an oxidant.

Naturally occurring phenolic sources: monomers and polymers

Phenolic compounds sourced from agro-based feedstock, viz. cashew nut shell liquid, lignin, tannin, palm oil, and coconut shell tar, have come up as sustainable alternatives to petro-based feedstock. This review explores their utility as green polymer feedstock with citation of ~ 600 references.