Isbut-Box: A new chiral C2 symmetric bis-oxazoline for catalytic enantioselective synthesis

Abstract

A series of chiral non-racemic isobutylene bis-oxazoline (Isbut-Box) ligands have been synthesised in satisfactory overall yields. The utility of such ligands for catalytic asymmetric synthesis has been demonstrated using the Cu(I) catalysed enantioselective cyclopropanation of styrene and derivatives with ethyl diazoacetate as a test reaction. The Cu(I) moiety is generated in situ using ethyl diazoacetate prior to the addition of the olefin. In these preliminary studies, the aforementioned cyclopropanation reaction gave yields, enantioselectivities and diastereoselectivities of up to 69%, 70% ee and 72% de. The reaction results were shown to be solvent dependent, with dichloromethane demonstrating good yields and toluene showing good stereoselectivities. DFT studies and NMR spectroscopic studies were undertaken to probe the structure and behaviour of the Cu(I)-Isbut-Box ligands.