Factors Controlling the Reactivity of Heteroarenes in Direct Arylation with Arylpalladium Acetate Complexes

The palladium-catalyzed direct arylation of heteroarenes with aryl halides has emerged as a viable alternative to conventional cross-coupling reactions. This paper reports a detailed mechanistic study on factors controlling the reactivity of heteroarenes in direct arylation with well-defined models of the presumed intermediate [PdAr(O2CMe-kappa O-2)L] (1). Although recent theoretical studies have provided a reasonable description of the mechanism of C-H bond cleavage by 1, its model compounds so far tested have been evidently less reactive than that expected. We found that [PdPh(O2CMe-kappa O-2)(PPh3)] (1a) and [Pd(2,6-Me2C6H3)(O2CMe-kappa O-2)(PPh3)] (1c), generated in situ from isolated [PdPh(mu-O2CMe)(PPh3))(2) (4a) and [Pd(2,6-Me2C6H3)(mu-O2CMe)(PPh3)](4) (4c), respectively, react with a variety of heteroarenes in almost quantitative yields. The reactivity order of heteroarenes was evaluated by competitive reactions, showing that benzothiazole (8) is significantly less reactive than 2-methylthiophene (6), despite the acidity of 8 (pK(a) = 27) being much higher than that of 6 (pK(a) = 42). This reason was examined by kinetic experiments using 1c as well as DFT calculations using the model compound [PdPh(O2CMe-kappa O-2)(PH3)] (1d). Both heteroarenes reacted with 1 via a sequence of three elementary processes (i.e., substrate coordination, C-H bond cleavage, and C-C reductive elimination), but their energy profiles were significantly different from each other. The reaction of 6 obeyed simple second-order kinetics, and the deuterium-labeling experiments and DFT calculations indicated the occurrence of rate-determining reductive elimination. On the other hand, the reaction of 8 displayed saturation kinetics due to the occurrence of relatively stable coordination of 8 prior to C-H bond cleavage. This coordination stability enhances the activation barrier for C-H bond cleavage, thereby causing the modest reactivity of 8. Thus, although the previous mechanistic studies on direct arylation have been focused largely on the C-H bond cleavage process, not only the C-H bond cleavage but also the substrate coordination and C-C reductive elimination must be considered.