Pd-catalyzed site-specific heteroaromatic C–H/peri-C–H annulative coupling for synthesis of cyclopenta-fused polycyclic heteroarenes†

Abstract

Cyclopenta-fused polycyclic heteroarenes (CP-PHAs) composed of acenaphthylene and heteroaromatic units have emerged as intriguing photoelectronic materials in view of the electron-accepting feature of the fused CP rings and the tunable electronic properties of the heteroaromatics. To date, Pd-catalyzed aromatic C–H/peri-C–halogen coupling has been used as one of the most general methods to construct such CP-rings. In contrast, Pd-catalyzed direct aromatic C–H/peri-C–H coupling may provide a shortcut to this CP-ring formation, which is applicable to a wide range of polycyclic arenes bearing peri-C–H bonds, but remains unexploited so far. We herein describe a Pd-catalyzed site-specific C–H/peri-C–H annulative coupling between (benzo)thiophenes or indoles with various polycyclic arenes to access a variety of π-extended CP-PHAs. Benzo[b]thiophenes or thiophenes with a 1-naphthalenyl group at the C3 position gave C2-naphthylated CP-PHAs in the Pd(OPiv)₂/AgOPiv system, whereas indoles with a 1-naphthalenyl group at the C2 position gave C3-naphthylated CP-PHAs in the Pd(OPiv)₂/AgSbF₆ system. Notably, deuteration experiments demonstrated that silver salts play a crucial role in the formation of heteroaryl-Ag species via site-specific C–H metalation of heteroaromatics, which subsequently undergo transmetalation with Pd(ii) to form heteroaryl-Pd species and activate the peri-C–H bond of the naphthalene group. Furthermore, this method enables the synthesis of structurally diverse CP-PHAs via activation of peri-C–H bonds in large polycyclic aromatic hydrocarbons such as anthracene, phenanthrene, and pyrene.