Reactions and photochemistry of samarium(II) complexes
Knettle, Brian Wesley
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Over the previous twenty years, divalent lanthanide reagents have become reagents of choice for organic functional group transformations. Samarium diiodide has made a particularly impressive impact on the way synthetic chemists perform reductions, reductive couplings of multiple 7t-bonds, and coupling of alkyl halides to ;i-bonds. It has been shown that the rate of reduction and the reducing ability of samarium(II) complexes can be influenced by the coordinating ligands and solvent medium. The most common additive is HMPA, which accelerates many reactions, and can also alter the stereoselectivity of products. This is due to the electron donating ability of HMPA to the divalent cation (increasing the reducing power) and the increased steric bulk about the samarium reductant. The first portion of this research focused on the behavior of samarium(II) complexes towards imines. It was found that substitution of Sml2 (which does not mediate imine reductions) with SmBr2, Sm[N(SiMe3)2]2, or a mixture of Sml2-Et3N-H20 allowed for imine reduction. However, the study showed that profound differences in reactivity could be related to the choice of ligand. SmBr2 and Sm[N(Si(CH3)3)2]2 were both effective at reduction of ketimines to amines. Sm[N(Si(CH3)3)2]2 was also able to reductively couple certain aldimines in a stereoselective manner. The Sml2-Et3N-H20 mixture was found to be effective at coupling both aldimines and ketimines. It had been previously shown that illumination of Sml2 increased its reducing power. To further examine this phenomenon, photochemical quenching experiments were performed upon Sml2 solutions containing a quencher molecule. Experimental rate constants were calculated for quenching by the N-benzyl imine of acetophenone, styrene, 1-chlorobutane, 2-butanone, and 4-toludine, and were found to be in good agreement with theoretical rates derived from Marcus theory. This indicates that the electron transfer is an outer sphere process. Lastly, a spectroscopic study of several samarium(II) reagents was performed. Relative quantum yields for Sml2 and SmBr2 were found to be 0.13 and 0.011, respectively. Molar extinction coefficients were also found for these complexes and clearly showed that Sml2 is more efficient in the photon absorption process.