Aza-Michael Mono-addition Using Acidic Alumina under Solventless Conditions

Thia-Michael Reaction under Heterogeneous Catalysis

Thia-Michael reactions between aliphatic and aromatic thiols and various Michael acceptors were performed under environmentally-friendly solvent-free conditions using Amberlyst® A21 as a recyclable heterogeneous catalyst to efficiently obtain the corresponding adducts in high yields. Ethyl acrylate was the main acceptor used, although others such as acrylamide, linear, and cyclic enones were also utilized successfully. Bifunctional Michael donor, 3-mercaptopropanoic acid, positively furnished the product, albeit in a lower yield and after leaving the reaction to take place for a longer time. The catalyst was easy and safe to handle and successfully recycled for five consecutive cycles.

NIPAm-Based Modification of Poly(L-lysine): A pH-Dependent LCST-Type Thermo-Responsive Biodegradable Polymer

Polylysine is a biocompatible, biodegradable, water soluble polypeptide. Thanks to the pendant primary amines it bears, it is susceptible to modification reactions. In this work Poly(L-lysine) (PLL) was partially modified via the effortless free-catalysed aza-Michael addition reaction at room temperature by grafting N-isopropylacrylamide (NIPAm) moieties onto the amines. The resulting PLL-g-NIPAm exhibited LCST-type thermosensitivity. The LCST can be tuned by the NIPAm content incorporated in the macromolecules. Importantly, depending on the NIPAm content, LCST is highly dependent on pH and ionic strength due to ionization capability of the remaining free lysine residues. PLL-g-NIPAm constitutes a novel biodegradable LCST polymer that could be used as “smart” block in block copolymers and/or terpolymers, of any macromolecular architecture, to design pH/Temperature-responsive self-assemblies (nanocarriers and/or networks) for potential bio-applications.

Rational design of carborane-based Cu2-paddle wheel coordination polymers for increased hydrolytic stability

A new unsymmetric carborane-based dicarboxylic linker provided a 1D Cu₂-paddle wheel coordination polymer (2) with much higher hydrolytic stability than the corresponding 2D Cu₂-paddle wheel polymer (1), obtained from a related more symmetrical carborane-based linker. Both 1 and 2 were used as efficient heterogeneous catalysts for a model aza-Michael reaction but only 2 can be reused several times without significant degradation in catalytic activity.

Direct Conjugation of Streptavidin to Encoded Hydrogel Microparticles for Multiplex Biomolecule Detection with Rapid Probe-Set Modification

Encoded hydrogel microparticles synthesized via flow lithography have drawn attention for multiplex biomarker detection due to their high multiplex capability and solution-like hybridization kinetics. However, the current methods for preparing particles cannot achieve a flexible, rapid probe-set modification, which is necessary for the production of various combinations of target panels in clinical diagnosis. In order to accomplish the unmet needs, streptavidin was incorporated into the encoded hydrogel microparticles to take advantage of the rapid streptavidin–biotin interactions that can be used in probe-set modification. However, the existing methods suffer from low efficiency of streptavidin conjugation, cause undesirable deformation of particles, and impair the assay capability. Here, we present a simple and powerful method to conjugate streptavidin to the encoded hydrogel microparticles for better assay performance and rapid probe-set modification. Streptavidin was directly conjugated to the encoded hydrogel microparticles using the aza-Michael addition click reaction, which can proceed in mild, aqueous condition without catalysts. A highly flexible and sensitive assay was developed to quantify DNA and proteins using streptavidin-conjugated encoded hydrogel microparticles. We also validated the potential applications of our particles conducting multiplex detection of cancer-related miRNAs.

Michael Addition Reaction Catalyzed by Imidazolium Chloride to Protect Amino Groups and Construct Medium Ring Heterocycles

An effective approach for amino protection and construction of a seven-membered ring has been developed. The method uses imidazolium chloride to carry out the Michael addition reaction at low temperatures and perform amino deprotection or construction of a seven-membered ring at high temperatures.

Improving the Post-polymerization Modification of Bio-Based Itaconate Unsaturated Polyesters: Catalyzing Aza-Michael Additions With Reusable Iodine on Acidic Alumina

Bio-based platform molecules such as itaconic, fumaric, and muconic acid offer much promise in the formation of sustainable unsaturated polyester resins upon reaction with suitable diols and polyols. The C=C bonds present in these polyester chains allows for post-polymerization modification and such moieties are conventionally utilized in curing processes during the manufacture of coatings. The C=C modification sites can also act as points to add useful pendants which can alter the polymers final properties such as glass transition temperature, biodegradability, hardness, polarity, and strength. A commonly observed modification is the addition of secondary amines via an aza-Michael addition. Conventional procedures for the addition of amines onto itaconate polyesters require reaction times of several days as a result of undesired side reactions, in particular, the formation of the less reactive mesaconate regioisomer. The slow reversion of the mesaconate back to itaconate, followed by subsequent amine addition, is the primary reason for such extended reaction times. Herein we report our efforts toward finding a suitable catalyst for the aza-Michael addition of diethylamine onto a model substrate, dimethyl itaconate, with the aim of being able to add amine onto the itaconate units without excessive regioisomerization to the inactive mesaconate. A catalyst screen showed that iodine on acidic alumina results in an effective, heterogeneous, reusable catalyst for the investigated aza-Michael addition. Extending the study further, itaconate polyester was prepared by Candida Antartica Lipase B (CaL-B) via enzymatic polytranesterification and subsequently modified with diethylamine using the iodine on acidic alumina catalyst, dramatically reducing the required length of reaction (>70% addition after 4 h). The approach represents a multidisciplinary example whereby biocatalytic polymerization is combined with chemocatalytic modification of the resultant polyester for the formation of useful bio-based polyesters.

Infection-responsive electrospun nanofiber mat for antibacterial guided tissue regeneration membrane

The release of anti-infection drugs in a targeted and efficient manner in response to the attack time and degree of severity of infection is a requirement of new generation implants. Herein, we design an infection-responsive guided tissue regeneration (GTR)/guided bone regeneration (GBR) membrane based on electrospun nanofibers. Polycaprolactone (PCL) nanofiber mats are coated with polydopamine to endow hydroxyl groups on the surface and then functionalized with siloxane to introduce amino groups. Metronidazole (MNA), an antibiotic drug, is esterified and then grafted onto the surface of the modified PCL nanofiber mats via ester linkages. The ester bonds can be selectively hydrolyzed by cholesterol esterase (CE), an enzyme secreted by macrophagocytes accumulated at the site of infection, whose concentration is positively related to the severity of the infection. The drug can be triggered to release from the nanofiber membranes in responsive to the CE. With the increase of the CE concentration, a higher amount of MNA is released from the nanofiber mat, resulting in the enhancement of the antibacterial capability of the MNA-grafted nanofiber mat. The nanofiber mat has good cytocompatibility. This CE-responsive drug delivery system based on the electrospun nanofiber mat is promising as an optimal choice for antibacterial GTR/GBR membrane.

High-Performing Biodegradable Waterborne Polyester/Functionalized Graphene Oxide Nanocomposites as an Eco-Friendly Material

The development of high-performing nanocomposites of homogeneously dispersed graphene oxide in a waterborne polyester matrix with controlled interfacial interactions is a daunting challenge owing to the presence of strong cohesive energy in both. Thus, in this study, graphene oxide was functionalized with toluene diisocyanate and butane diol through a simple method and incorporated into the waterborne polyester matrix through a facile in situ bulk polymerization technique without using any compatibilizing agent or organic solvent for the first time. The thermoset of the nanocomposite was formed by curing it with hyperbranched epoxy of glycerol and poly(amido amine). The resultant thermosetting nanocomposites with 0.1-1 wt % functionalized graphene oxide exhibited significant enhancement in mechanical properties such as elongation at break (245-360%), tensile strength (7.8-39.4 MPa), scratch hardness (4 to >10 kg), toughness (17.18-86.35 MJ/m³), Young's modulus (243-358 MPa), impact resistance (8.3 to >9.3 kJ/m), and thermostability. Further, the Halpin-Tsai model was used to predict the alignment of graphene oxide. The nanocomposite was also biodegradable against the Pseudomonas aeruginosa bacterial strain. Furthermore, this nanocomposite exhibited strong catalytic activity for the aza-Michael addition reaction. Thus, the nanocomposite can be utilized as a high-performing sustainable material in different potential applications including as heterogeneous catalysts.

Solvent-Free Henry and Michael Reactions with Nitroalkanes Promoted by Potassium Carbonate as a Versatile Heterogeneous Catalyst

The use of a simple weak inorganic base such as potassium carbonate facilitated the formation of carbon-carbon bonds through both the Henry and the Michael reactions with nitrocompounds. The application of this catalyst under environmentally friendly solventless heterogeneous conditions gave satisfactory to good yields of β-nitroalcohols, involving aliphatic and aromatic starting materials, as well as high to excellent yields in the formation of Michael adducts using several different Michael acceptors and nitroalkanes.

Special Issue "Recent Synthetic Aspects on the Chemistry of Nitro, Nitroso and Amino Compounds"

Nitrogen-containing molecules are key scaffolds that are widely applied in organic synthesis as precursors of highly functionalized materials, and are also investigated for their biological activities. This Special Issue collects seven innovative contributions which expand our knowledge of the chemistry of nitro compounds, amines, diazonium salts, and peptides, and that provide a good overview about their main reactivities.