The Influence of Imidazolylidene Ligands with Bulky Resorcinarenyl Substituents on Catalysts for Suzuki–Miyaura Coupling
Résumé
PEPPSI-type imidazolylidene palladium complexes having their carbenic ring N-substituted with an aryl ring and a cavity-shaped unit [25,26,27,28-tetrapropyloxycalix[4]aren-5-yl or 6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]aren-5-yl (TPR)] have been prepared and assessed in Suzuki–Miyaura cross-couplings. Remarkable efficiency in the coupling of aryl chlorides with sterically hindered arylboronic acids was observed for the carbene ligand having its N atoms (N1, N2) substituted by a mesityl and a TPR group, respectively. This good performance possibly arises from strong steric interactions between the pentyl-substituted cavitand unit and the catalytic centre, which favours reductive elimination. Two of the imidazolium salts used for complex synthesis were characterised by X-ray diffraction analysis