Lewis Acid-Promoted Atom-Transfer Free Radical Additions

Abstract

The reactions of α-bromo oxazolidinone imides of acetic and propionic acid and terminal and internal alkenes were investigated in the presence of Lewis acids. Thus, the primary bromide, bromoacetyl-2-oxazolidinone amide (1), undegoes clean atom-transfer addition to 1-hexene as well as cis- or trans-3-hexene at room temperature or below. The best Lewis acids for this conversion are Sc(Otf)3 and Yb(Otf)3. Quantitative yields are obtained with Yb(Otf)3 for addtion of the bromide to both 1-hexene and cis-3-hexene, while the yield with trans-3-hexene is 63%. Yields obtained with Sc(Otf)3 are somewhat lower. The effects of solvent, temperature, and Lewis acid loading have been investigated. The secondary bromide, α-bromoproprionyl-2-oxazolidinone amide (7), was also investigated in atom-transfer addition to 1-hexene. Yields are comparable to those in the reaction of 1 with 1-hexene, but internal alkenes fail to react with this substrate. Tertiary bromides do not react with any of the alkenes studied. Control of stereochemistry in the atom-transfer addition is possible by the use of chiral auxiliary oxazolidinones. Thus, the benzyl oxazolidinone and isopropyl analogue give excellent control of configuration in the new stereogenic center generated in the addition of the propionate to 1-hexene. Attempts to achieve enantioselective atom-transfer addition fail to give product in high yield or stereoselectivity.