Radical Thiol-ene Reaction Research Articles

UV-Induced [2+2] Grafting-To Reactions for Polymer Modification of Cellulose.

Benzaldehyde-functional cellulose paper sheets have been synthesized via tosylation of cellulose (Whatman No 5) followed by addition of p-hydroxy benzaldehyde. Via UV-induced Paterno-Büchi [2+2] cycloaddition reactions, these aldehyde functional surfaces are grafted with triallylcyanurate, trimethylolpropane allyl ether, and vinyl chloroacetate. In the following, allyl-functional polymers (poly(butyl acrylate), pBA, Mn = 6990 g mol(-1) , Đ = 1.12 and poly(N-isopropyl acrylamide), pNIPAAm, Mn = 9500 g mol(-1) , Đ = 1.16) synthesized via reversible addition fragmentation chain transfer polymerization are conjugated to the celloluse surface in a UV-induced grafting-to approach. With pBA, hydrophobic cellulose sheets are obtained (water contact angle 116°), while grafting of pNIPAAm allows for generation of "smart" surfaces, which are hydrophilic at room temperature, but that become hydrophobic when heated above the characteristic lower critical solution temperature (93° contact angle). The Paterno-Büchi reaction has been shown to be a versatile synthetic tool that also performs well in grafting-to approaches whereby its overall performance seems to be close to that of radical thiol-ene reactions.

Ring-opening reactions of backbone epoxidized polyoxanorbornene

In this article, we report the synthesis of poly(oxanorbornene imide) (PONB) with internal epoxy groups (epoxidized-PONB30) and its ring-opening reactions with various nucleophiles, such as amine, azide, and thiols. The ring-opening reactions with amines yielded the amine–hydroxyl PONBs in the range of 36–95% of functionalization depending upon the amine content per epoxy. An allylamine–hydroxyl functionalized PONB was further functionalized efficiently with 1-octanethiol by radical thiol-ene reaction. The ring-opening reaction of the main chain epoxy using thiols resulted in a lower functionalization than amines with a similar functional group (e.g., allyl). In addition, sodium azide together with NH4Cl, was employed efficiently in the ring-opening reaction of the main chain epoxy, resulting in an azide/hydroxyl-functional PONB30 with 53% efficiency, which was determined after a model copper-catalyzed azide–alkyne cycloaddition (CuAAC) experiment was carried out.

Multifunctional Poly[N-(2-hydroxypropyl)methacrylamide] Copolymers via Postpolymerization Modification and Sequential Thiol–Ene Chemistry

Poly[N-(2-hydroxypropyl)methacrylamide] is a promising candidate material for biomedical applications. However, synthesis of functional pHPMA via compolymerization results can lead to variations in monomer composition, molar mass, and dispersity making comparison difficult. Postpolymerization modification routes, most commonly aminolysis of poly[active ester methacrylates], have alleviated some of these problems, but ester hydrolysis can lead to other problems. Here we report the synthesis of multifunctional pHPMA via a simple two-step derivatization of pHPMA homopolymer using readily available standard reagents and atom-efficient procedures. First, treatment with allyl isocyanate yields the corresponding carbamate with predictable incorporation of side-chain functionality. Allyl-pHPMA can then be derivatized further via radical thiol–ene reactions to generate pHPMA with multiple diverse functionalities but without adverse effects on the molecular weight and dispersity of the polymer. The applicability of...

Ready access to end-functional polystyrenes via a combination of ARGET ATRP and thiol–ene chemistry

Bromine end-groups of ATRP polystyrenes were converted into terminal alkene groupsviaa facile synthetic strategy. Subsequently, a light-induced radical thiol–ene reaction was employed for the functionalization of the terminal double bonds.

Modular modification of xylan with UV-initiated thiol-ene reaction

Birch xylan was functionalized with various thiols through UV initiated radical thiol-ene reaction under mild conditions. Xylan was allylated through etherification with allyl glycidyl ether under alkaline conditions. The allylated xylan was then reacted with thiols containing varying functional groups: trimethylbenzyl mercaptan, dodecanethiol, thioglycolic acid, l-cysteine and cysteamine hydrochloride. The reactions were conducted under homogeneous conditions at room temperature, either in water (hydrophilic thiols) or in DMF (hydrophobic thiols). The effect of reaction parameters to the functionalization efficiency was studied, including, for example, thiol excess, thiol character, initiator amount and reaction mixture concentration. The reactions were fast and 100% conversion of allyl groups was reached in most cases, sometimes already within 10min. Water as solvent resulted generally in faster reactions when compared to DMF, and it was possible to conduct the aqueous reaction even without added UV initiator. It was also possible to incorporate two functionalities simultaneously during one reaction into the xylan structure.

Bioinspired Anchorable Thiol-Reactive Polymers: Synthesis and Applications Toward Surface Functionalization of Magnetic Nanoparticles

In this study, we report on the synthesis of novel thiol-reactive polymers containing a catechol moiety at their chain ends and demonstrate their application toward surface modification of magnetic nanoparticles. Poly(ethylene glycol) methyl ether acrylate (PEGMEA) based hydrophilic polymers incorporating maleimide group containing side-chains were synthesized using reversible addition–fragmentation chain transfer (RAFT) polymerization. A novel furan-protected maleimide-containing acrylate monomer was synthesized and copolymerized with hydrophilic monomers in the presence of a catechol moiety containing chain transfer unit. Removal of the furan protecting group after polymerization yields thiol-reactive polymers possessing an intact catechol unit at their chain-end for anchoring onto various surfaces. Polymers with varying composition of the masked maleimide monomer were obtained with narrow molecular weight distributions as inferred from size exclusion chromatography. Obtained polymers were characterized using proton nuclear magnetic resonance (1HNMR), infrared (IR) spectroscopy, and electrochemistry to establish their chemical composition. Oleic acid coated organo-dispersible magnetic nanoparticles were modified with the hydrophilic catechol terminated polymers to obtain water dispersible polymer-coated magnetic nanoparticles. The reactive maleimide units on the polymer-coated nanoparticles were utilized to conjugate thiol-containing hydrophobic dye using the nucleophilic Michael addition reaction to obtain water dispersible magnetic and fluorescent nanoparticles. Alternatively, it was demonstrated that radical thiol–ene reaction can also be utilized to functionalize nanoparticles coated with polymers where the maleimide group is in masked as a furan-cycloadduct. Efficient anchoring of the polymer onto nanoparticle surfaces and their subsequent functionalization was verified using various techniques such as infrared spectroscopy (IR), thermogravimetric analysis (TGA), and phase transfer studies, whereas the subsequent functionalization was evident from the organic to aqueous phase transfer of the hydrophobic fluorescent dye.

ChemInform Abstract: Redox Mediators in Visible Light Photocatalysis: Photocatalytic Radical Thiol—Ene Additions.

Synthetically useful radical thiol–ene reactions can be initiated by visible light irradiation in the presence of transition metal polypyridyl photocatalysts. The success of this method relies upon the use of p-toluidine as an essential additive. Using these conditions, high-yielding thiol–ene reactions of cysteine-containing biomolecules can be accomplished using biocompatibile wavelengths of visible light, under aqueous conditions, and with the thiol component as the limiting reagent. We present evidence that p-toluidine serves as a redox mediator that is capable of catalyzing the otherwise inefficient photooxidation of thiols to the key thiyl radical intermediate. Thus, we show that co-catalytic oxidants can be important in the design of synthetic reactions involving visible light photoredox catalysis.

Direct Covalent Attachment of DNA Microarrays by Rapid Thiol–Ene “Click” Chemistry

A rapid strategy for the covalent immobilization of DNA onto silicon-based materials using the UV-initiated radical thiol-ene reaction is presented in this study. Following this approach, thiol- and alkene-modified oligonucleotide probes were covalently attached in microarray format, reaching immobilization densities around 6 pmol·cm(-2). The developed methodology presents the advantages of spatially controlled probe anchoring (using a photomask), direct attachment without using cross-linkers (one-pot fashion), and short irradiation times (20 min). Using the described strategy, hybridization efficiencies up to 65% with full complementary strands were reached. The approach was evaluated by scoring single-base pair mismatches with discrimination ratios around 15. Moreover, the efficacy of the proposed DNA detection scheme is further demonstrated through the assay on a genomic target of bacterial Escherichia coli.

Redox mediators in visible light photocatalysis: photocatalytic radical thiol-ene additions.

Synthetically useful radical thiol–ene reactions can be initiated by visible light irradiation in the presence of transition metal polypyridyl photocatalysts. The success of this method relies upon the use of p-toluidine as an essential additive. Using these conditions, high-yielding thiol–ene reactions of cysteine-containing biomolecules can be accomplished using biocompatibile wavelengths of visible light, under aqueous conditions, and with the thiol component as the limiting reagent. We present evidence that p-toluidine serves as a redox mediator that is capable of catalyzing the otherwise inefficient photooxidation of thiols to the key thiyl radical intermediate. Thus, we show that co-catalytic oxidants can be important in the design of synthetic reactions involving visible light photoredox catalysis.

Transition Metal Photoredox Catalysis of Radical Thiol-Ene Reactions

We describe the anti-Markovnikov hydrothiolation of olefins using visible-light-absorbing transition metal photocatalysts. The key thiyl radical intermediates are generated upon quenching of photoexcited Ru*(bpz)3(2) with a variety of thiols. The adducts of a wide variety of olefins and thiols are formed in excellent yield (73-99%).

One-Pot Multistep Reactions Based on Thiolactones: Extending the Realm of Thiol−Ene Chemistry in Polymer Synthesis

The in situ generation of thiols by nucleophilic ring-opening of a thiolactone with amines, followed by a UV-initiated radical thiol-ene reaction in a one-pot fashion, has been evaluated as an accelerated and versatile protocol for the synthesis of several types of polymeric architectures. After elaboration of a model amine-thiol-ene conjugation reaction, a number of routes based on readily available thiolactone-containing structures have been developed to successfully assemble functional, linear polymers and networks via a mild and facile radical photopolymerization process.