Subtle Steric Effects in Nickel-Catalysed Kumada-Tamao-Corriu Cross-Coupling Using Resorcinarenyl-Imidazolium Salts

Three resorcinarene-cavitands bearing a 3-R-1-imidazolylium substituent (R = alkyl) grafted to the wider rim of the cavitand (1-3) were assessed in the Kumada-Tamao-Corriu cross-coupling of aryl halides with arylmagnesium halides. Their combination with [Ni(cod)(2)] (cod = 1,5-cyclooctadiene; 1:1 stoichiometry) resulted in highly efficient catalysts, the activities of which varied in the following order: R = n-propyl (1) < isopropyl (2) approximate to benzyl (3). A remarkable turnover frequency of 60400 mol(ArX) mol(Ni)(-1) h(-1) was obtained in the coupling of 2-bromo-6-methoxynaphthalene with PhMgBr (100 degrees C in dioxane, with precursor 2). The high activities of the cavitand derivatives were attributed to steric effects that facilitate the reductive-elimination/product decoordination step. Comparative experiments carried out with a structurally modified resorcinarene, as well as cavity-free imidazolium salts bearing 2-methoxyaryl substituents, suggest that the efficiency of the catalysts mainly relies on steric interactions between the metal and the flexible substituents attached to two methine carbon atoms. These steric interactions are probably reinforced each time the metal binds one of the two oxygen atoms located next to the catalytic centre.