This paper presents the effect of additives on the mechanism and selectivity of the SmI2-mediated coupling of alkyl halides and ketones. The reaction of 1-iodobutane and 2-octanone was carried out with SmI2 in the absence of cosolvent and in the presence of HMPA, LiBr, and LiCl. The experiments using cosolvent free SmI2 and SmI2−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 SmI2-mediated coupling of 1-iodododecane and 2-octanone in the presence of LiBr, LiCl, and HMPA was also performed. The combination of SmI2 and LiBr again produced the pinacol coupling product exclusively and left the 1-iodododecane unreduced. In contrast, the SmI2−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 SmI2-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 SmI2 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.

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The measured value of E°(SmI 2 + −HMPA − SmI 2 −HMPA) was used in combination with literature values for the E°of primary radicals to estimate the bimolecular rate constant for an outer-sphere electron transfer (ET). Comparison with the experimentally measured rate constant is consistent with an outer-sphere ET.Samarium diiodide (SmI 2 ) is one of the most important reducing reagents utilized by organic chemists. It is useful in functional group reductions, the coupling of halides with π bonds, and the coupling of two π bonds. 1 The most important feature of SmI 2 is its ability to promote one-pot, sequential reactions including both one-and two-electron processes. 2 The success of many SmI 2 -mediated reactions is often dependent on the addition of a ligating cosolvent such as HMPA, 3 which accelerates reactions of SmI 2 . 4 In addition to accelerating reductions by SmI 2 , HMPA also enhances the stereochemical outcome and diastereoselectivity of many reactions. 3,5 Although the combination of SmI 2 -HMPA in THF produces an extremely versatile reductant, there is a paucity of detail on its interaction with organic substrates.Recent results in our laboratory suggest that SmI 2 (HMPA) 4 is the species responsible for the unique reactivity of the SmI 2 -HMPA combination in THF. 6 Careful examination of the recently published X-ray crystal structure of the SmI 2 -(HMPA) 4 reductant shows that the complex is sterically crowded. 7 There are two potential ways that a reducible functional group (alkyl halide, carbonyl, or radical) can be reduced by the sterically crowded reductant (Scheme 1). The first would involve displacement of an SmI 2 (HMPA) 4 ligand by substrate which could lead to an inner-sphere electron transfer (ET). The second potential mechanistic pathway would have the ligands remain intact and the ET take place through an outer-sphere process. Since radicals are formed during numerous reductive coupling processes mediated by SmI 2 , we carried out the experiments described below to determine the mode of electron transfer from SmI 2 (HMPA) 4 to primary radicals.One of the most important values that needs to be defined in order to determine the mode of electron transfer is the standard potential of the SmI 2 (HMPA) 4 -SmI 2 + -(HMPA) 4 redox couple in THF. We recently utilized linear sweep voltammetry (LSV) to examine the influence of HMPA concentration on the reducing power of SmI 2 . 8 While these experiments showed unequivocally that HMPA increases the reducing power of SmI 2 , the reported values were not standard potentials. Figure 1 contains the cyclic voltammogram of SmI 2 and SmI 2 (HMPA) 4 in THF. The CV of SmI 2 containing HMPA did not change with the addition of more than 4 equiv of HMPA. Both voltammograms contained in Figure 1 are quasireversible, so we employed the model recently described by Skrydstrup for the SmI 2 + -SmI 2 redox (1) Curran, D. P.; Fevig, T. L.; Jasperse, C. P.; Totleben, M. J. Synlett 1992, 943-961.(2) Molander, G. A.; Harris, C. R.