Experimental and theoretical studies on radical trifluoromethylation of titanium ate and lithium enolates

摘要

The radical trifluoromethylation of Ti ate and Li enolates has been investigated by both experiments and density functional (UB3LYP/6-311+G*//UB3LYP/6-31+G*) calculations. Radical CF3 addition to the enolates proceeds in a highly exothermic manner without significant reaction barriers in both Ti ate and Li enolates. There are two possible reaction paths after the addition of CF3 radical in the case of Ti ate enolate; one is the elimination of Ti(III) from the ketyl radical intermediate and the other is the direct reaction of the ketyl radical intermediate with CF3I. However, in the case of Li enolate, only the latter path is possible due to the high energy barrier of the elimination of the Li radical. This analysis provides an explanation of the experimental observation that the Li enolate could form the radical cycle efficiently but the Ti ate enolate could not. To make the radical cycle complete, I- has to be extracted from CF3I itself or the radical anion of CF3I. In the case of Li, formation of Li-I bond could be the driving force for the extraction of I- and regeneration of CF3 radical. However, Ti does not give exothermic Ti-I formation and thus regeneration of CF3 radical is less likely.