Reactions mediated by the coenzyme B12 are not only important for the maintenance of human health, they are some of the most unusual chemical transformations known. Previous work has suggested that B12 catalyzed reactions are initiated by homolysis of a cobalt-carbon bond. However, in the absence of protein and substrate the cobalt-carbon bond in B12 is reasonably stable. Our goal is to prepare systems which mimic this substrate-induced homolysis of a metal-carbon bond. The effect of the substrate in B12 systems is one example of an allosteric effect, so that our proposed work should shed light on the general problem of the mechanisms, at the molecular level, of allosteric effects. The particular system on which attention will be focused will be orthometalled derivatives of 2-phenyl-1,10-phenanthroline. Transition metals react with this ligand (and its derivatives) to form tridentate N,N,C chelates. In these complexes the phenyl ring is kept in the plane of the 1,10-phenanthroline ring by the metal carbon bond. Metal-binding groups will be placed on the ortho position of the metallated phenyl ring so that upon addition of a metal ion (such as Mg++) a torque will be induced, breaking the metal-carbon bond. Thus, this system will model how substrate binding can promote M-C homolysis. The chemistry of these M-C diradicals will be studied to see if they can carry out the same rearrangements as B12 based enzymes.
