화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.136, No.23, 8252-8260, 2014
Mechanistic Study of Chemoselectivity in Ni-Catalyzed Coupling Reactions between Azoles and Aryl Carboxylates
Itami et al. recently reported the C-O electrophile-controlled chemoselectivity of Ni-catalyzed coupling reactions between azoles and esters: the decarbonylative C H coupling product was generated with the aryl ester substrates, while C H/C-O coupling product was generated with the phenol derivative substrates (such as phenyl pivalate). With the aid of DFT calculations (MO6L/6-311+G(2d,p)SDD//B3LYP/6-31G(d)-LANL2DZ), the present study systematically investigated the mechanism of the aforementioned chemoselective reactions. The decarbonylative C H coupling mechanism involves oxidative addition of C(acy1)-0 bond, base-promoted C H activation of azole, CO migration, and reductive elimination steps (C H/Decar mechanism). This mechanism is partially different from Itami's previous proposal (Decar/C H mechanism) because the C H activation step is unlikely to occur after the CO migration step. Meanwhile, C H/ C-O coupling reaction proceeds through oxidative addition of C(phenyl)-O bond, base-promoted C H activation, and reductive elimination steps. It was found that the C-0 electrophile significantly influences the overall energy demand of the decarbonylative C H coupling mechanism, because the rate-determining step (i.e., CO migration) is sensitive to the steric effect of the acyl substituent. In contrast, in the C H/C-0 coupling mechanism, the release of the carboxylates occurs before the ratedetermining step (i.e., base-promoted C H activation), and thus the overall energy demand is almost independent of the acyl substituent. Accordingly, the decarbonylative C H coupling product is favored for less-bulky group substituted C-0 electrophiles (such as aryl ester), while C H/C-0 coupling product is predominant for bulky group substituted C-0 electrophiles (such as phenyl pivalate).