Samarium diiodide

Abstract

(A) By transferring an electron to aldehydes and ketones, SmI2 generates ketyl radical anions, which can undergo pinacolic-coupling reactions. The addition of benzaldehyde 1 to a solution of SmI2 in THF leads to its rapid reductive coupling affording 2 and 3 in high yield, but with poor diastereoselectivity (2:3 = 1:1.3). However, addition of polyethylene glycols such as tetraglyme increases the diastereoselectivity up to 6:1 in favour of the erythro-isomer 2. (B) SmI2-promoted intramolecular ketone-olefin coupling reactions offer a highly efficient way to cyclic and bicyclic systems. The utility of this method is not only due to the variety of ring sizes, which can be prepared, but also to the capacity of SmI2 to promote sequential reactions. SmI2-mediated cyclisation of 4 and 5 in the presence of HMPA, which is necessary for effireac efficient ketone-olefin coupling,9 proceeds smoothly to afford cycloalkanols 6 and 7 in good yield and high diastereoselectivity favouring a cis relationship between the two methyl groups. As an example for a novel 6-trig cyclisation the addition of 8 to a solution of SmI2 and HMPA in THF results in the formation of hexahydronaphthalene derivative 9 in 52% yield as a single diastereomer. (C) SmI2 can ano be used for the reductive cleavage of carbon-heteroatom bonds or of N-O bonds in hydroxylamine derivatives and heterocycles. Subjecting 10 to a solution of SmI2 in THF provides the amino alcohol 11 in excellent yield.