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 (M06L/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(acyl)-O 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-O 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-O coupling mechanism, the release of the carboxylates occurs before the rate-determining 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-O electrophiles (such as aryl ester), while C-H/C-O coupling product is predominant for bulky group substituted C-O electrophiles (such as phenyl pivalate).
Department of Chemistry, University of Science and Technology of China , Hefei 230026, PR China.
Qianqian
Q
Lu
Haizhu
H
Yu
Yao
Y
Fu
D001393:Azoles / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D002384:Catalysis; D004952:Esters; D008958:Models, Molecular; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D058451:Oxidative Coupling; D010421:Pentanoic Acids / Q000737:chemistry
D001393:Azoles; D002264:Carboxylic Acids; D004952:Esters; D009942:Organometallic Compounds; D010421:Pentanoic Acids; D009532:Nickel; C005566:pivalic acid