Pushing SmI₂ Reactions towards the Limit- Entropy Driven Reduction of a Benzene Ring

In order to reduce a benzene ring, alkali metals or similar strong reducing agents are needed. We have found that SmI₂, which is considered to be a relatively mild reagent, can also perform this task. This is achieved by attaching a side arm of either ethanolamine or ethylenediamine to a benzene ring. These side arms strongly coordinate to SmI₂, holding it in close vicinity to the benzene ring. As a result, the probability of electron transfer from the SmI₂ to the benzene ring is significantly increased. However, at the same time, this proximity also enhances the rate of the back electron transfer. In order to slow the rate of this step, small amounts of water and Et₃N were added to the reaction mixture. The two typical products of the classical Birch reduction of benzene were obtained with a yield as high as 95 %. This method was also applied to the corresponding chlorobenzene and methylbenzoate where the reactions were much enhanced but functionality reduction prevailed over ring reduction. It is remarkable that going from a termolecular to a unimolecular reaction is more effective than excitation of the SmI₂ around 600 nm which is equivalent to ca. 47 kcal/mol.