Carbonic Acid Diester Activation by Polymer-Bound DBU and Its Relevance to Catalytic N‑Carbonylation of N‑Heteroaromatics: Direct Evidence for an Elusive N‑Carboxy-Substituted Amidinium Cation Intermediate

Polymer-bound DBU (PS-DBU, DBU = 1,8-diazabicyclo[5.4.0]­undec-7-ene) is an effective and selective catalyst for solventless N-phenoxycarbonylation of N-heteroaromatics (pyrrole (<b>1</b>), indole (<b>2</b>), and carbazole (<b>3</b>)) with diphenyl carbonate (DPC), used as an eco-friendly active carbonyl species in place of phosgene-derivatives. The immobilized catalyst is less active than unsupported DBU but can be recovered easily at the end of catalytic run and recycled effectively. Dedicated studies have demonstrated that DBU can react as a nucleophile with DPC even when it is anchored on the polymeric matrix, and provided the first direct evidence (FTIR and solid-state ¹³C NMR) of formation of a 8-carboxy-substituted 1,8-diazabicyclo[5.4.0]­undec-7-enium cation in DBU-promoted nucleophilic activation of carbonic acid diesters