Chemical hermaphroditism: The potential of the Cr(CO)(3) moiety to stabilize transition states and intermediates with anionic, cationic, or radical character at the benzylic position

It is known that both benzylic cations and anions are stabilized by Cr(CO)( 3) complexation. This unusual characteristic of chromium arenes has been th e subject of many synthetic, spectroscopic, and physical organic studies ov er the last four decades. The effect of Cr(CO)(3) on benzylic radicals has received comparatively little attention, however. In this report, cycloprop ylcarbinyl anions, cations, and radicals substituted with both phenyl and C r(CO)(3)-phenyl groups are shown to rearrange via ring-opening to produce, selectively, Cr(CO)(3)-stabilized benzylic anions. cations, and radicals, i mplying that the Cr(CO)(3) moiety is capable of stabilizing transition stat es with ionic or radical character at the benzylic position. The highest se lectivity (>99:<1) was observed in the anionic reaction, slightly lesser se lectivity (95:5) was observed in the cationic reaction, and only modest sel ectivity (2.5:1) was observed in the radical reaction. A parallel trend in ground-state stabilities of Cr(CO)(3)-complexed benzyl anion, cation, and r adical is predicted by density functional theory calculations. These calcul ations reveal that considerable structural distortions of both benzyl anion and cation occur upon complexation, but that little distortion occurs for benzyl radical. The connections between Cr(CO)(3) complexation and the stab ility of the complexed species are explained in terms of interactions betwe en frontier molecular orbitals of the Cr(CO)(3) and benzyl fragments.