Abstract
The use of organic photoredox catalysts provides new ways to perform metal-free reactions controlled by light. While these reactions are usually performed in organic media, the application of these catalysts at ambient temperatures in aqueous media is of considerable interest. We here compare the activity of two established organic photoredox catalysts, one based on 10-phenylphenothiazine (PTH) and one based on an acridinium dye (ACR), in the light-activated dehalogenation of aromatic halides in pure water. Both PTH and ACR were covalently attached to amphiphilic polymers that are designed to form polymeric nanoparticles with hydrodynamic diameter DH ranging between 5 and 11 nm in aqueous solution. Due to the hydrophobic side groups that furnish the interior of these nanoparticles after hydrophobic collapse, water-insoluble reagents can gather within the nanoparticles at high local catalyst and substrate concentrations. We evaluated six different amphiphilic polymeric nanoparticles to assess the effect of polymer length, catalyst loading and nature of the catalyst (PTH or ACR) in the dechlorination of a range of aromatic chlorides. In addition, we investigate the selectivity of both catalysts for reducing different types of aryl-halogen bonds present in one molecule, as well as the activity of the catalysts for C-C cross-coupling reactions. We find that all polymer-based catalysts show high activity for the reduction of electron-poor aromatic compounds. For electron-rich compounds, the ACR-based catalyst is more effective than PTH. In the selective dehalogenation reactions, the order of bond stability is C-Cl > C-Br > C-I irrespective of the catalyst applied. All in all, both water-compatible systems show good activity in water, with ACR-based catalysts being slightly more efficient for more resilient substrates.
Highlights
Photoredox catalysis has emerged in the last decade as a powerful tool for chemical bond transformations and late-stage functionalization of pharmaceutically relevant compounds [1,2,3,4,5]
We evaluated six different amphiphilic polymeric nanoparticles to assess the effect of polymer length, catalyst loading and nature of the catalyst (PTH or acridinium dye (ACR)) in the dechlorination of a range of aromatic chlorides
Since reactions only occur when the catalyst is in its excited state, which can be accurately controlled by light, photoredox-catalyzed reactions show an unprecedentedly high degree of spatial and temporal control
Summary
Photoredox catalysis has emerged in the last decade as a powerful tool for chemical bond transformations and late-stage functionalization of pharmaceutically relevant compounds [1,2,3,4,5]. Organocatalytic alternatives for photoredox catalysts have been developed to avoi2dof 12 the use of expensive Ir- and Ru-based systems and to tune the redox potential of the catalysts [5,11,12,13] Their synthetic potential has been illustrated by their applicability for a broad spectrum of substrates and in a range of reactions such as polymerizations, substitutiobnrso,abdosnpdecctlreuamvaogfesruebascttrioatness aanndd cinycaloraadndgeitioofnrsea[2ct,1io4n–s16s]u.cUhsausaplloyl,ytmheesreizoartgioannso, csautbas-titulyststiaornese, vboalnudatceledaivnagoergraenaicctimonesdaian,dmcaykclionagdsdeiptiaornasti[o2n,1o4f–1p6r]o.dUuscutsa,llsyo,ltvheensteaonrdgacnaotacalytastlysts laboarrioeuesv.aAludadteitdioinnaollryg,ahneicrem, tehdeiau,sme aokfiwngatseerpaasrastoiolvneonft pbryoidnutrcotsd,uscoilnvgenmt aicnedllacratsaylsytsetmlasbori[17–o2u1]s.oAr bdydiutisoinnagllwy,ahteerre-s,othluebulesecaotfawlyattiecrcaosmspollvexenest b[2y2i,n23tr]ohdauvceinsghomwicnelglarreastypstoetmenst[i1a7l.–21] Miceolrlabrysuyssitnemg ws aarteere-ssopleucbialellycaitnatleyrteicstcionmg psilnexceesth[2e2h,2y3d] rhoapvheosbhiocwdnomgraeiantspporteesnetniatli.nMthiceellar aquesoyustsemmsedairuemesaplelociwalluysintoteirnecsrteinagsesitnhceelothcealhcyadtarloypsht ocboinccdenotmraatiinosn,parensdenwthinenthheyadqruoe- ous phobmicedsiuubmstaralltoews aursetouisnecdretahsies trheesulolctsalicnatfaalsytsetrccoonncvenertrsaiotinona,taenqduwalhecnathalyydsrtolpohaodbinicgsub[24,2s5tr]a. Molecular Design, Synthesis, and Characterization of Photocatalyst Loaded PNs We designed and synthesized six different amphiphilic polymers, PN1–6, with the aim of assessing the influence of the polymer chain length, catalyst loading and catalyst type on catalyst activity in aqueous solutions (Scheme 2) To this end, we first synthesized two polyacrylates (P100 and P200) with degrees of polymerization (DP) of 100 and 200, using. This shows that both catalysts have been successfully attached to the PNs and that the optical properties are not affected by the presence of the amphiphilic polymer
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