| Samarium diiodide (SmI{dollar}sb2{dollar}) has become an extremely useful reagent for effecting a variety of organic reactions, several of which result in the production of cyclic molecules in a highly stereoselective fashion. SmI{dollar}sb2{dollar} is an extremely efficient one-electron reducing species, and is among the most easily oxidized of the lanthanide species. This reagent can be prepared, is soluble and is stable in tetrahydrofuran, a commonly used organic solvent. In addition, Sm{dollar}sp{lcub}+3{rcub}{dollar} species are extremely oxophilic, a property which has been exploited in several areas of the research described herein.; The three areas research to be discussed include the use of SmI{dollar}sb2{dollar} in effecting the cyclopropanation of allylic alcohols, intramolecular Reformatsky reactions of {dollar}beta{dollar}-bromoacetoxy and {dollar}gamma{dollar}-bromoacetoxy ketones and aldehydes, and a sequential reductive radical cyclization/electrophile trapping reaction. The first study involved the use of SmI{dollar}sb2{dollar} as a cyclopropanating agent in the presence of variously substituted allylic alcohols. The high diastereoselectivity achieved in many cases was attributed to coordination of the hydroxyl group of the substrate with the incoming samarium-carbenoid species, as well as the minimization of other steric interactions.; The second area of study examined the ability of SmI{dollar}sb2{dollar} to induce 1,2- 1,3- and 1,4-asymmetric induction in the intramolecular Reformatsky reaction of a variety of {dollar}beta{dollar}-bromoacetoxy and {dollar}gamma{dollar}-bromoacetoxy ketones and -aldehydes. The effect of a Sm{dollar}sp{lcub}+3{rcub}{dollar} chelated intermediate was very apparent in several cases in which single diastereomeric products were isolated. In general, 1,2 and 1,3 asymmetric induction were much more efficient than the corresponding 1,4 process, unless some inherent geometrical restraints were built into the substrate.; The final area of study focussed on a sequential reductive radical cyclization/electrophile trapping reaction which was efficiently promoted by SmI{dollar}sb2{dollar}. Several radical precursors were subjected to reduction with SmI{dollar}sb2{dollar}/HMPA in the presence of a carbonyl electrophile. While aldehydes were not as efficiently trapped as ketones, a range of functional groups could be tolerated in the ketone electrophile. Nitrogen-containing precursors were also shown to undergo this cyclization/trapping reaction. Highly functionalized furanylmethyl-, benzofuranylmethyl, and indoline species were produced in this process. |
