Preliminary mechanistic studies provided evidence of the formation of
the EDA complex between HE and redox-active ester and the generation of
the alkyl radical during the reaction. An EDA complex is proposed to
form between HE and redox-active ester (Scheme 24-B). This EDA complex180 triggers a single electron transfer event upon
photoirradiation, which produces a dihydropyridine radical cation and
the corresponding alkyl radical 176 . Concurrently, saccharide
acid 171 would be activated by DMDC (dimethyl dicarbonate),
affording an activated carbonic anhydride 174 in situ.
Subsequently, the oxidative addition of this anhydride with Ni(0)
species forms an acyl-Ni(II) intermediate 175 , which is trapped
with alkyl radical 176 , affording a high valent Ni(III) complex177 . Subsequent reductive elimination of 177 yields
the desired non-anomeric C-acyl glycoside 173 and the
corresponding Ni(I) complex 178 . The resulting Ni(I) complex is
then reduced by the photoexcited HE
[Ered(HE*/HE+ ●) = -2.28 V vs SCE]
via SET to regenerate the active Ni(0) catalyst. The other plausible
reaction pathway is proposed to begin with the initial combination of an
alkyl radical 176 with a Ni(0) complex, furnishing an
alkyl-Ni(I) species 179 . Subsequent oxidative addition of the
resulting alkyl-Ni(I) species 179 to the anhydride 174generates the high valent Ni(III) species 177 , which continues
through the reaction pathway described above.
Scheme 24 Nickel-catalyzed synthesis of non-anomeric acyl
C-glycosides via EDA complex photoactivation