Reactions of SmI₂ with Alkyl Halides and Ketones:  Inner-Sphere vs Outer-Sphere Electron Transfer in Reactions of Sm(II) Reductants

This paper presents the effect of additives on the mechanism and selectivity of the SmI 2 -mediated coupling of alkyl halides and ketones. The reaction of 1-iodobutane and 2-octanone was carried out with SmI 2 in the absence of cosolvent and in the presence of HMPA, LiBr, and LiCl. The experiments using cosolvent free SmI 2 and SmI 2 −HMPA reductants gave the Barbier product, 5-methyl-5-undecanol predominantly. The same procedure carried out with LiBr as an additive produced the pinacol product, 7,8-dimethyl-7,8-tetradecanediol, exclusively. A careful product analysis of the SmI 2 -mediated coupling of 1-iodododecane and 2-octanone in the presence of LiBr, LiCl, and HMPA was also performed. The combination of SmI 2 and LiBr again produced the pinacol coupling product exclusively and left the 1-iodododecane unreduced. In contrast, the SmI 2 −HMPA combination gave only the Barbier product. Analysis of the Sm(II) reductants employing cyclic voltammetry and UV−vis spectroscopy coupled with reaction protocol changes and mechanistic studies led to the conclusion that the SmI 2 -mediated coupling of alkyl halides and carbonyls in the presence of HMPA gives the Barbier product through an outer-sphere electron-transfer process, while the reaction utilizing SmI 2 with LiBr or LiCl gives the pinacol product through an inner-sphere reductive coupling of ketones. The results presented herein show that it is possible to alter the reactivity and selectivity of Sm(II) reagents through the choice of additives or cosolvents, primarily by changing the steric bulk around the reductant.