Based on literature precedents and a previous radical trapping
experiment, [15] the authors proposed a mechanism
shown in Scheme 26-B. The key step involves the subsequent fragmentation
of the DHP ester to deliver glycosyl radicals upon the release of
Hantzsch pyridine and dioxide. Another important step is the activation
of the carboxylic acid by DEDC (diethyl decarbonate). The oxidative
addition of the resulting mixed decarbonates 192 by Ni(0)
species 191 delivers Ni(II) intermediate 193 , followed
by capturing glycosyl radical 189 to form acyl-Ni(III)-glycosyl
complex 194 . Subsequently, the reductive elimination of194 produces the desired acyl C-glycosides 183 and
Ni(I)Br 190 . Ni(I)Br is reduced by reduced photocatalyst to
regenerate active Ni(0) catalyst 191 .
Scheme 26 Synthetic application and plausible mechanism