Resorcinarene-Functionalised Imidazolium Salts as Ligand Precursors for Palladium-Catalysed SuzukiMiyaura Cross-Couplings
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Three imidazolium salts based on a rigid resorcinarene platform (13) were synthesised and used as catalyst precursors in the SuzukiMiyaura cross-coupling of aryl halides with phenylboronic acid. In these pro-carbene ligands, the heterocyclic moiety has one N atom connected to a C2 atom of a resorcinolic ring, and the other is substituted by an alkyl group (R=n-propyl, iso-propyl, benzyl). The methinic C atoms of the macrocyclic core are all substituted by a pentyl group. The best catalytic performances were obtained by using an imidazolium/Pd ratio of 1:1. The catalytic systems displayed high activities, which increased in the order R=n-propyl(1)<i-propyl(2)<benzyl(3). For example, the use of 3 resulted in activities up to 30100mol(converted ArX)mol(Pd)1h1 in the arylation of bromotoluene at 100 degrees C in dioxane. Comparative studies showed that the performance of imidazolium salt 3 is significantly superior to that of related salts devoid of a cavity-shaped substituent. These experiments illustrate the role of the bulky resorcinarene unit, which facilitates the reductive elimination step. Modification of the cavitand structure by replacement of the pentyl substituents with phenyl groups further revealed that the catalytic outcome is influenced by the nature of the lower belt-substituents. The results strongly suggest that the Pd catalysts obtained from 13 have the metal centre preferentially located outside the cavity.