Photocatalyzed Perfluoroalkylation of Endoglycals

The visible light-induced perfluoroalkyl (R_F) radical reactions on peracetylglycals derived from hexoses and pentoses (galactal, glucal, arabinal, and xylal derivatives) were investigated. Various photocatalysts and perfluoroalkyl iodides (R_F-I) were employed as sources of R_F radicals with LEDs as the irradiation source. Particularly noteworthy was the use of an Iridium photocatalyst, Ir[dF(CF₃)ppy]₂(dtbpy))PF₆, which yielded two distinct product types when applied to glucal. On the one hand, the 2-R_F-substituted glucal was formed, a trend observed even when utilizing organic dyes as photocatalysts. On the other hand, the unexpected addition product, namely the 1-R_F-2-iodo-α-manno-configured C-glycosyl derivative, was also obtained, as a result of a highly regioselective addition reaction of the R_F moiety into the anomeric carbon, followed by attachment of the iodine atom on C-2 in axial disposition. This result contrasted with other radical reactions carried out on 2-unsubstituted glycals, where the incipient radical adds to C-2, generating a stabilized 1-glycosyl radical. The photocatalyzed radical perfluoroalkylations of peracetyl glycals derived from galactose, arabinose, and xylose all afforded the 2-R_F-substituted glycals in good yields as a result of the expected vinylic substitution reaction. Mechanistic studies revealed that the 1-R_F-2-iodo-α-manno-configured C-glycosyl derivatives arise from a radical chain reaction, whereas the 2-R_F-substituted glycals proceed from inefficient chain processes.