ON THE MECHANISM OF OXAZOLINE-DIRECTED METALATIONS - EVIDENCE FOR NITROGEN-DIRECTED REACTIONS

We recently described a method for the synthesis of ferrocene complexe s possessing planar chirality which relies on the asymmetric deprotona tion of chiral ferrocenyloxazolines. The unexpected stereochemical out come of these reactions led us to examine whether the metalation is di rected by the oxygen or the nitrogen of the oxazoline. In this paper, we describe the synthesis of a constrained ferrocenyloxazoline (compou nd 13) in which oxygen- and nitrogen-directed metalations provide diff erent stereochemical outcomes. Our results show that nitrogen is respo nsible for the directive effects of the oxazoline when alkyllithium re agents are used to deprotonate the ferrocene. The implications of this result on the origin of asymmetric induction in the metalation of the unconstrained ferrocenyloxazolines 19 and 20 are discussed.