VI. Radical Addition and Hydrogen-Atom Abstraction

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The reaction shown in Scheme 11 describes the formation of the C‑glycoside 25 by addition of the oxygen-stabilized radical 21 or the organosamarium compound 22 (or both) to a molecule of acetone.⁹⁵ There is evidence for participation of both of these intermediates at some stage in this reaction. Conducting the reaction in the presence of t-BuSH quenches the addition process and dramatically increases the yield of the reduction product 23. Such a change would be expected from hydrogen-atom abstraction by the radical 21. In the absence of t-BuSH, formation of 23 and the elimination product 24 provide evidence for the organosamarium compound 22 also being present in the reaction mixture. Since conducting the reaction in the presence of D₂O decreases the yield of the C-glycoside 25 in favor of the reduction and elimination products 23 and 24, respectively, the organosamarium compound 22 appears to be a likely intermediate in the addition process, but since a large excess of D₂O only modestly reduces the yield of 25, radical addition remains a possible (perhaps major) pathway to C-glycoside formation.

The radical-addition pathway shown in Scheme 11 involves the nucleophilic, carbon-centered radical 21 adding to the carbonyl carbon atom in acetone. The carbonyl carbon atom is rendered quite electron deficient by complexation of acetone with SmI₂. This combination of a reactive radical adding to a double bond with a decidedly electron-deficient atom is found in other reactions promoted by SmI₂.^96,97