Rational perturbation of the fluorescence quantum yield in emission-tunable and predictable fluorophores (Seoul-Fluors) by a facile synthetic method involving C-H activation.
Bottom Line: Herein, we report a facile synthesis of emission-tunable and predictable Seoul-Fluors, 9-aryl-1,2-dihydrolopyrrolo[3,4-b]indolizin-3-ones, with various R(1) and R(2) substituents by coinage-metal-catalyzed intramolecular 1,3-dipolar cycloaddition and subsequent palladium-mediated CH activation.We also showed that the quantum yields of Seoul-Fluors are controlled by the electronic nature of the substituents, which influences the extent of photoinduced electron transfer.On the basis of this understanding, we demonstrated our design strategy by the development of a Seoul-Fluor-based chemosensor 20 for reactive oxygen species that was not accessible by a previous synthetic route.
Affiliation: Department of Chemistry/Bio-MAX Institute, Seoul National University, Seoul 151-747 (Korea).Show MeSH
Mentions: After the formation of the indolizine core, we aimed to use a transition‐metal‐catalyzed site‐specific C—H activation7 at the C9 position for the late‐stage diversification of the R1 group. Metal‐catalyzed C—H activation methods are powerful, straightforward, and atom‐economical synthetic tools for carbon–carbon and carbon–heteroatom bond formation.7a,b Palladium(II)‐catalyzed C—H activation has emerged as an important catalytic transformation because of its versatility for the installation of many different types of bonds and its exceptional practicality under ambient conditions in the presence of moisture.7c,d Therefore, we envisioned the efficient synthesis of Seoul‐Fluor analogues containing diverse R1 groups that were unobtainable by our previously reported route by the cross‐coupling of the lactam‐embedded indolizine cores with aryl iodides. We selected indolizine 2, which contains an acetyl group at the C7 position, as a model substrate for C—H activation owing to its excellent photophysical properties. By screening various catalysts, ligands, oxidants, solvents, and temperatures (results not shown), we found that the optimal cross‐coupling conditions for the site‐specific C—H activation of indolizine at the C9 position were conventional heating in the presence of palladium(II) acetate and silver acetate.8 Under these optimized conditions, we successfully synthesized a series of Seoul‐Fluor analogues with various R1 substituents, ranging from electron‐rich 4‐diethylaminophenyl to electron‐deficient phthalonitrile (Scheme 3, products 10–19), in moderate to good yield, as well as other R1 substituents, including heterocycles (see Table S1 in the Supporting Information). This palladium‐mediated cross‐coupling reaction also enabled the introduction of R1 substituents onto various indolizine cores (compounds 3–6) containing R2 substituents with a range of electronic characteristics (see Table S2), and even onto indolizine 9 with an electron‐rich isoquinoline moiety (see Table S3). Notably, the yield of the palladium‐catalyzed cross‐coupling reaction increased as the electron‐donating ability of the R1 substituent decreased.
Affiliation: Department of Chemistry/Bio-MAX Institute, Seoul National University, Seoul 151-747 (Korea).