Calix[4]pyrroles as macrocyclic organocatalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide

Calix[4]pyrrole 1, calix[4]arene 2, or porphyrin 3 together with tetrabutylammonium iodide (TBAI) catalyzed the conversion of epoxides and CO₂ into cyclic carbonates. Among them, meso-octamethylcalix[4]pyrrole 1a, which was synthesized easily in one step by the acid-catalyzed condensation of pyrrole with acetone, was found to be a very active and robust organocatalyst. DFT calculations suggested that 1a adopts a 1,3-alternate conformation to stabilize the anionic species generated during the catalysis. In the key transition state of the epoxide ring-opening step, one pyrrolic NH group and the tetrabutylammonium (TBA) cation activate the epoxide by hydrogen bonding while another NH group on the opposite side guides the I^- anion, which is located away from the TBA cation, to the backside attack on the epoxide.