De Novo Synthesis of Long-Wavelength Absorbing Chlorin-13,15-dicarboximides

Chlorins bearing a six-membered imide ring spanning positions 13−15, commonly referred to as purpurinimides, exhibit long-wavelength absorption yet have heretofore only been available via semisynthesis from naturally occurring chlorophylls. A concise route to synthetic chlorins, which bear a geminal dimethyl group in the pyrroline ring, has been extended to provide access to chlorin-13,15-dicarboximides. The new route entails (i) synthesis of a 13-bromochlorin, (ii) palladium-catalyzed carbamoylation at the 13-position, (iii) regioselective 15-bromination under acidic conditions, and (iv) one-flask palladium-mediated carbonylation and ring closure to form the imide. In some cases the ring closure reaction afforded the isomeric (and readily separable) chlorin−isoimide in addition to the chlorin−imide. The resulting chlorin−imides and chlorin−isoimides exhibit long-wavelength absorption (679−715 nm) and emission (683−720 nm) in the far-red and near-infrared spectral region. The absorption of the chlorin−(iso)imides fills the spectral window between that of analogous synthetic chlorins and 131-oxophorbines (603−687 nm) and bacteriochlorins (707−792 nm). The synthetic versatility of the de novo route complements the existing semisynthetic route from chlorophylls and should enable fundamental spectroscopic studies and photochemical applications.