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 selectivit y 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 usin g 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-tetradecanedi ol, exclusively. A careful product analysis of the SmI2-mediated coupling o f 1-iodododccane and 2-octanone in the presence of LiBr, LiCI, and HMPA was also performed. The combination of SmI2 and LiBr again produced the pinaco l coupling product exclusively and left the 1-iodododecane unreduced. In co ntrast, the SmI2-HMPA combination gave only the Barbier product. Analysis o f the Sm(II) reductants employing cyclic voltammetry and UV-vis spectroscop y 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 e lectron-transfer process, while the reaction utilizing SmI2. with LiBr or L iCl gives the pinacol product through an inner-sphere reductive coupling of ketones. The results presented herein show that it is possible to alter th e reactivity and selectivity of Sm(I) reagents through the choice of additi ves or cosolvents, primarily by changing the steric bulk around the reducta nt.