Samarium reagents reductive cleavage

The samarium(II) reagent Cpf Sm(THF)2 affords a reductive homologenation of CO to a ketene carboxylate (Eq. 44 [284]. The reaction sequence formally involves two one-electron reductions, CO coupling and a remarkable cleavage of a CO bond. The 02CCC0 unit forms one Sm-O-tr-bond (Table 18). 

A number of low-valent metal ions have been shown to reduce a-halocarbonyl compounds. The most commonly used species for this purpose have been chromium(II) and low-valent titanium " salts, although vanadium(II), samarium(II), iron(II) and tin(II) salts have also been used. 7 222 chloro, bromo and iodo ketones can all be reduced by chromium(II) and titanium(III) salts. Selective reductions are possible axial halides are reduced in preference to equatorial, and a,a-dihalo ketones can be selectively reduced to the corresponding monohalides (equation 10). 7 The use of samarium(II) iodide has recently been advocated for such a-cleavages.72 a-Halo esters and ketones are reduced instantaneously at -78 °C in excellent yields. a-Acetoxy esters are stable to this reagent. 

---