Preliminary Theoretical Insights into SmI2-Mediated Reactions: Activation of Ketones in THF

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Abstract

The structure of samarium iodide in THF solution and its reactivity towards aromatic ketones has been studied by means of DFT calculations. A comparison between the X-ray and DFT optimised structure of [SmI2(THF)32-MeOCH2H4OMe)] and [SmI2(THF)5]+ shows that the use of DFT-B3PW91 in conjunction with the appropriate relativistic electron core potential is able to reproduce the structures of these complexes. In THF solution, the simulation of SmI2(THF)n complexes reveals that an equilibrium should exist between SmI2(THF)5, SmI2(THF)4, SmI2(THF)3 and dimetallic [SmI(μ-I)(THF)2]2. The formation of SmI2(THF)2 and the dissociation of an iodide to yield [SmI (THF)n]+ is less likely to occur. A first estimation of the thermodynamics of ketone reduction by SmI2(THF)n is given and qualitative values compared with experiment are presented. Concerning the pinacolic coupling of ketones, a “key intermediate” based on experimental interpretation has been considered. Formation of this so-called “Streitwiser dimer” is strongly endothermic with respect to the monomeric SmIII–OCPh2 radical anion adduct. A doubly reduced ketone sandwiched between two Sm centres is more likely to occur and is consistent with the overall pinacol coupling reaction.

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