Samarium diiodide limitations

Allylic halides are not as readily accessible as allylic alcohols or their ester derivatives. Thus, the requirement that allylic halides must be used as precursors for carbonyl addition reactions in conjunction with magnesium and other similar reductants is a severe restriction limiting the convenience of these routes to homoallylic alcohols. In this regard, samarium diiodide can be used to great advantage, because substrates other than allylic halides are suitable precursors for such transformations. For example, allylic phosphate esters have been reported to couple with carbonyl substrates in the presence of Smh (equation 15). Since esters and nitriles are unreactive under these conditions, the Smh-mediated process is likely to be more chemoselective than those promoted by magnesium or lithium. 

Under all these conditions, a dark blue solution of samarium diiodide in THF is formed. The limiting concentration of samarium diiodide in THF is approximately 0.1 M. It is a stable solution and can be stored for a long period of time under an inert atmosphere. One of the key features of SmU is that it can be used as an intermediate to other Sm(II)-based reductants. Scheme 1 shows a schematic representation of different approaches to prepare several Sm(II) based reductants from Sml2. 

While it is recognized that samarium diiodide (Sml2) is by far the most utilized reductant among Sm(II) halides, samarium dichloride (SmCh) and samarium dibromide (SmBr2) have been used in a number of applications. The main limitation of these two reductants is their... 

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