A New Generation of π-Basic Dearomatization Agents

A new generation of dearomatizing π-bases having the general form TpM (L) (π-acid) (where Tp = hydridotris (pyrazolyl) borate, M = rhenium, molybdenum, tungsten, L = variable ligand, and π-acid = CO, NO+) has been developed. These fragments show reactivity patterns similar to those of their pentaammineosmium (II) predecessor, binding aromatic molecules across two carbons and subsequently activating these ligands toward electrophilic addition and cycloaddition reactions. In many cases, these second-generation dearomatization agents show a greater degree of π-back-bonding than was observed for the analogous pentaammineosmium (II) complexes. A number of unprecedented reaction pathways have been discovered for these second-generation systems, most notably a broad array of cyclization reactions. In addition, these chiral-at-metal dearomatization agents are readily resolved and have been utilized in reaction sequences to give highly enantioenriched polycyclic molecules from common aromatic precursors.