Mechanistic
Study of Chemoselectivity in Ni-Catalyzed
Coupling Reactions between Azoles and Aryl Carboxylates
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Abstract
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)