Later, Liang and co-workers developed a regioselective and
stereoselective method for the preparation of aryl C-glycosides through
nickel-catalyzed ortho -C-H glycosylation of 8-aminoquinoline
benzamides (Scheme 18). [27] The reaction exhibits
excellent α-selectivity and regioselectivity. Notably, the scope of
glycosyl bromide is somewhat limited. For example, the pyranosides, such
as Bn-protected L-rhamnose (133f )/D-mannose (133g ),
showed poor reactivity under the developed conditions, though exclusive
α-selectivity was observed. Additionally, only a trace amount of aryl
C-glycosides were formed during the reactions using Me-protected
D-mannosyl chloride (133h ), Bn-protected D-galactosyl chloride
(133i ), and Bn-protected D-glucosyl chloride (133j ).
Preliminary mechanistic experiments indicated that the ortho-C-H
activation could be the rate-determining step, and C-H activation was
reversible. A plausible mechanism was proposed, as described in Scheme
18-B. The reaction starts with the coordination of amide and Ni(II)
catalyst, followed by ligand exchange and reversible cleavage
of the ortho C-H bond. The resulting Ni(II) intermediate 134gets engaged in the oxidative addition of the C-Cl bond in α-glycosyl
chloride 131 prior to reductive elimination to afford product133 after proton-metal exchange. Another possible pathway
invoking oxocarbenium ion being attacked by Ni(II) species was not ruled
out.
Scheme 18 Nickel-catalyzed ortho-C-H glycosylation of
8-aminoquinoline benzamides