Bimacrocyclic concave N-heterocyclic carbenes (NHCs): Synthesis, structure and application in catalyses

Abstract

Imidazolinium moieties have been incorporated into bimacrocycles to generate precursors for concave N-heterocyclic carbenes (NHCs). Starting from 2-nitroresorcinol and alkenols, symmetric concave imidazolinium salts 1 were obtained. Bimacrocyclisation was achieved via ring-closing metathesis (RCM). In an analogous fashion, axially chiral concave imidazolinium salts 2 were obtained using a naphthalene bridgehead devoid of local C2-symmetry. The concave NHCs derived from the symmetric precursors 1 were used as nucleophilic catalysts for the synthesis of γ-butyrolactones and as ligands for transition metal catalysts. Copper complex 18 was used as a catalyst for the cyclopropanation of styrene and indene with ethyl diazoacetate (EDA). Palladium complex 24 was used as a catalyst for the Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions.