Mechanistically, the reaction pathway was proposed to start from the
photoexcitation of 4CzIPN photocatalyst, followed by reductive quenching
with the saccharide-derived DHP 35 (Scheme 4-B). This results
in the formation of a radical cation 36 , giving rise to a
saccharyl radical 38 and an aromatized pyridine derivative from
a rapid radical fragmentation. The saccharyl radical 38 then
adds to the Ni(0) catalyst 43 , forming a saccharyl-Ni(I)
complex 39 . Subsequently, oxidative addition of this complex
with an aryl bromide takes place, leading to the Ni(III) species41 . This species then undergoes a reductive elimination,
yielding a non-anomeric aryl C-glycoside product 34 and a Ni(I)
species 42 that is in turn reduced to Ni(0) with the reduced
4CzIPN, thus regenerating the active catalysts. It is worth noting that
the diastereomeric ratios (d.r.) of the product can be influenced by
both the saccharide backbone and the aromatic partner. Previous
mechanistic studies indicated high valence Ni(III) 41ultimately dictates the observed diastereoselectivity after the
irreversible reductive elimination. [14] Thus, to
improve the d.r. values, the authors tested the use of bidentate and
tridentate ligands with both Ni(0) and Ni(II) species. Modifying the
bipyridine backbone by replacing electron-donating methoxy groups with
bulkier, less electron-rich tert-butyl substituents proved to be
successful in achieving excellent diastereoselectivity (Scheme 4-C).
Likewise, phenanthroline yielded >20:1 d.r. for34l , whereas dtbbpy (4,4‘-di-tert-butyl-2,2‘-bipyridine) did
not show any improvement. A general approach for the coupling of
saccharyl radicals with aryl and heteroaryl bromides has not yet been
established.
Scheme 5 Synthesis of aryl/heteroaryl-C-glycosides via the
nickel-catalyzed cross-coupling of glycosyl ester with aryl bromide