Isotopic labeling study of the formation of calix[5]pyrroles catalyzed by Bi(NO3)3

Abstract

One of the more elusive classes of macrocycles includes the calix[5]pyrroles, which can be obtained from pyrrole and acetone in the presence of low concentrations of Bi(NO3)3-5H2O. An isotopic labeling experiment aided the proposal of a mechanism to explain the formation of calix[5]pyrrole at low acid concentrations and the exclusive formation of calix[4]pyrrole at high acid concentrations. We assume that the mechanism involves HNO3, which is released from the Bi salt. © 2013 by the authors; licensee MDPI, Basel, Switzerland.