Abstract
Conclusion: Using an electron-rich, 16e− metal fragment, aromatic molecules may be coordinated through a single π-bond. Such coordination acts to disrupt the π-system and activates the aromatic toward electrophilic addition reactions. The resulting arenium, allyl, and 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 the activation of unsaturated molecules. This approach is complementary to the more established chemistry of nucleophilic addition to metal-coordinated π-systems. Until recently, this methodology has been limited to the pentaammineosmium(II) system, a reagent which, though remarkably versatile, has practical limitations such as cost and toxicity. On the horizon, however, is the next generation of dearomatization agents of the type {ReTp(CO)(L)}.24., 66. These materials, in addition to being substantially less expensive and more safe than osmium, are chiral at metal, and electronically and sterically tunable. Thus, they can be tailored to optimize a particular reaction sequence.
