Salen as a Chiral Activator:anti versussyn Switchable Diastereoselection in the Enantioselective Addition of Crotyl Bromide to Aromatic Aldehydes

To be syn or to be anti? This is an “open” matter! A good degree of syn simple diastereoselection (up to 83:17) and high enantioselectivity (ee values up to 90 %) were obtained with the addition of crotyl bromide to aromatic aldehydes promoted by the [Cr(salen)]/Mn/Me3SiCl catalytic system (see scheme, path B). The remarkable degree of syn simple diastereoselection obtained with organochromium reagents can be explained by an open transition state in a chromium-mediated process.