Thermodynamics and kinetic studies of single electron transfer reactions mediated by samarium(II) iodide in tetrahydrofuran

One of the most important and exciting recent developments in organic chemistry has been the evolution and use of divalent lanthanide reagents in synthesis. The unique reactivity of divalent lanthanides (particularly SMI ₂) makes them important reducing reagents for highly selective synthetic transformations. There are essentially three transformations mediated by SMI ₂; functional group reductions, reductive coupling of π-bonds, and reductive coupling of halides and π-bonds. Of particular interest is the fact that the addition of hexamethylphosphoramide (HMPA) and other cosolvents (TMP, TMU, DMPU, DBU, LiBr and LiCl) to SMI₂-mediated reactions, effectively increases the reducing power Of SMI₂, and has a drastic effect on the rates and outcome of these reactions. In this dissertation, we have developed methods and techniques that shed more light on the mechanisms of reactions mediated by SMI₂ and on the roles played by cosolvents in these reactions.; Although there are numerous synthetic examples of the ability of HMPA (and other cosolvents used in synthetic applications) to increase the redox potential Of SMI₂ in THF, there have been no attempts to quantify (in term of reducing power) the effect of these additives on the oxidation potential Of SMI₂ (or other reducing species). In addition, the experimental descriptions on the use of cosolvents as reported in literature offer no rationale as to why specific cosolvents and concentrations were selected over the others. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were employed to investigate the effect of cosolvents on the reducing power Of SmI₂ in THE Factors that influence this behavior (cosolvent concentrations, basicity and steric bulk) are discussed.; The thermodynamic and kinetic evaluation of a single electron transfer in bimolecular reactions Of SMI₂ reducing species and organic substrates (alkyl halides, alkyl radical and selected carbonyl compounds) was investigated using the Marcus theory. This theory was essential in differentiating between the two potential types of electron transfer mechanisms (inner-sphere ET and outer-sphere ET). Our results clearly indicate that reduction of alkyl halides by SMI₂, and the reduction of alkyl radicals by SmI ₂ containing HMPA are an outer-sphere ET mechanism. But the reduction of carbonyl compounds by SmI₂ has the characteristic of an inner-sphere ET mechanism.