The Activation of Aromatic Molecules with Pentaammineosmium(II)

Conclusion: The true significance of this work lies not so much in the demonstration of a transition metal localizing aromatic π systems, but rather in the ability of an η²-coordinated metal to activate π systems toward electrophilic addition often at a site away from metal coordination even if this causes disruption of an aromatic system. Thus, arenium, allyl, and even vinyl cations are stabilized by the metal to the point that nucleophilic addition becomes competitive with electrophilic substitution. In this regard, the chemistry exhibited by the osmium system provides a new method for activating unsaturated molecules that is complementary to the more established chemistry of nucleophilic addition to metal-coordinated π systems. The true potential of this approach, however, will not be realized until alternative transition metal systems become available. The successful system, in addition to having the appropriate steric requirements, must contain ancillary ligands that tolerate moderate electrophiles and a metal center that resists oxidative addition, either by direct attack of electrophile or by insertion of a C−H bond. The author hopes that a convincing case has been made to search for new transition metal systems that can function as this type of π base. ⁹²