Steric effect of substituents in haloarenes on the rate of cross-coupling reactions

The relative reactivity of ortho- and para-methyl-substituted iodoarenes in the Sonogashira reaction and palladium-catalyzed methoxycarbonylation, as well as of similarly substituted bromoarenes in the Suzuki reactions and cobalt-catalyzed methoxycarbonylation, was studied. Introduction of a methyl group into the para position of aryl halide slows down the cross-coupling. o-Methylhaloarenes are less reactive in palladiumcatalyzed reactions as compared to both unsubstituted haloarene and para-substituted analog. The presence of a methyl group in the ortho position with respect to the reaction center accelerates cobalt-catalyzed methoxycarbonylation.