Visible-light-mediated Diels-Alder reactions under single-chain polymer confinement: investigating the role of the crosslinking moiety on catalyst activity.

Macromolecular scaffolds are rapidly emerging in catalysis owing to the ability to control catalyst placement at precise locations. This spatial proximity allows for enhanced catalyst activity that may not be observed using small molecules. Herein, we describe a triphenylpyrylium (TPT)-based visible-light active single-chain polymer nanoparticle (SCNP) that facilitates the radical cation [4 + 2]-cycloaddition. We find that the catalytic activity is highly dependent on the styrylarene comonomer used, wherein it can act as a redox mediator under confinement, increasing the catalytic turnover (TON) by up to 30 times in comparison to free TPT in solution. Mechanistic studies indicate that TPT excited states are quenched by the acene, with the resultant radical cation formed from naphthalene-based SCNPs able to proceed in oxidizing the dienophile in the elementary step of the reaction, while leading to near quantitative yields of the cycloadduct. The TPT-SCNP demonstrates enhanced photocatalyst efficiency compared to molecular TPT, and is able to be recycled and reused in three iterations of the reaction prior to decreased performance from photobleaching. Our results overall suggest that the confined nature of the SCNP and spatial proximity of acene-based pendants enforces their participation as cocatalytic redox mediators that impart enhanced photoredox catalysis under confinement.

Enhanced basicity of an electron donor-acceptor complex.

An electron donor-acceptor (EDA) complex forms between 1,4-dicyanobenzene and N-phenylpyrrolidine, which are coupling partners for the α-aminoarylation photoredox reaction. Calculations and experiments demonstrate the EDA complex is a better base than N-phenylpyrroline. A re-analysis of the α-aminoarylation reaction suggests that the EDA complex is a proton acceptor in the reaction.

Alginate Encapsulation Stabilizes Xylanase Toward the Laccase Mediator System.

Xylanase, a hydrolytic enzyme, is susceptible to inactivation by the oxidative conditions generated by the laccase mediator system (LMS). Given the impetus to develop a mixed enzyme system for application in biomass processing industries, xylanase was encapsulated with either Cu2+- or Ca2+-alginate and then exposed to the LMS with variations such as mediator type, mediator concentration, and treatment pH. Results demonstrate that alginate-encapsulated xylanase retains substantial activity (> 80%) when exposed to the LMS relative to non-encapsulated xylanase. Cu2+-alginate generally provided better protection than Ca2+-alginate for all mediators, and protection was observed even at a low pH, where the LMS is most potent. Despite encapsulation, xylanase was still capable of hydrolyzing its polymeric substrate xylan, given kcat/Km values within an order of magnitude of that for non-encapsulated xylanase. The alginate matrix does not impede the function of the oxidized mediator, since comparable Vmax values were observed for the conversion of veratryl alcohol to veratraldehyde by free and Cu2+-alginate encapsulated laccase. Overall, these results support development of a mixed enzyme system for biomass delignification and, more broadly, show potential for protecting protein function in an oxidative environment.