Cyclo- and hydrodimerization of α,β-unsaturated ketones promoted by samarium diiodide

Samarium(II) iodide is a strong one-electron transfer reducing agent, and is effective for the cyclo- and hydrodimerization of cyclic and non-cyclic alpha,beta-unsaturated ketones. The title dimers can easily be prepared in good yields at room temperature under neutral conditions, using two-mole equivalent of SmI2 per mole of starting substrate. The reaction is stereocontrolled. The absence of an alcohol as a proton source is essential in the process and the use of HMPA as a copromoter improves the yield of dimeric products, making the reaction regioselective over the competitive C=C double bond reduction. The crystal structures of some of the dimeric derivatives are reported. When eta(2)- or eta(4)-iron-coordinated alpha,beta-unsaturated ketones are used as substrates, the reaction gives mainly the 1,4-reduced products.