A hexameric resorcinarene capsule as a hydrogen bonding catalyst in the conjugate addition of pyrroles and indoles to nitroalkenes

Abstract

A hexameric resorcinarene capsule acts as a hydrogen bonding catalyst for the activation of encapsulated nitroalkenes toward the addition of pyrroles and indoles. Once inside the capsule (C), β-nitrostyrene 3a establishes H-bonding interactions with the bridging water molecules having free H-bond-donating valences. Thus, the activation of nitroalkenes favors the conjugate addition of pyrroles and indoles. It is demonstrated that the inherent acidity of capsule C is sufficient to promote the Michael-type Friedel–Crafts (MTFC) reaction under the mild reaction conditions described herein. In silico calculations point out the catalytically relevant role of the bridging water molecules of C and confirm the supramolecular control exerted by the capsule, which protects the nitronic acid intermediate with respect to the degradation pathway that commonly occurs in the bulk medium.