Nonbonded, Attractive Cation−π Interactions in Azide-Mediated Asymmetric Ring Expansion Reactions

The influence of attractive, nonbonded interactions on the reactions of 1,2- and 1,3-hydroxyalkyl azides with ketones has been investigated through experimental and computational means. A series of 1,3-hydroxyalkyl azides bearing electronically tuned aromatic groups at the 2 position were prepared and reacted along with several derivatives designed to conformationally restrict the rotational orientation of the aromatic substituent. These studies showed that a cation−π interaction between an aryl moiety and an N2+ leaving group plays a role in determining the stereoselectivity of these reactions. A series of ab initio calculations supported this hypothesis. A computational and experimental analysis suggested a primarily steric model for the analogous reactions of substituted 2-azido-1-ethanol analogues.