Reactivity of (eta(6)-arene)tricarbonylchromium complexes toward additionsof anions, cations, and radicals

A computational and experimental study of additions of electrophiles, nucle ophiles, and radicals to tricarbonylchromium-complexed arenes is reported. Competition between addition to a complexed arene and addition to a noncomp lexed arene was tested using 1,1-dideuterio-1-iodo-2-((phenyl)tricarbonylch romium)2-phenylethane. Reactions under anionic and cationic conditions give exclusive formation of 1,1-dideuterio-1-((phenyl)tricarbonylchromium)-2-ph enylethane arising from addition to the complexed arene. Radical conditions (SmI2) afford two isomeric products, reflecting a 2:1 preference for radic al addition to the noncomplexed arene. In contrast, intermolecular radical addition competition experiments employing ketyl radical addition to benzen e and (benzene)tricarbonylchromium show that addition to the complexed arom atic ring is faster than attack on the noncomplexed species by a factor of at least 100 000. Density functional theory calculations using the B3LYP me thod, employing a LANL2DZ basis set for geometry optimizations and a DZVP2 basis set for energy calculations, for all three reactive intermediates sh owed that tricarbonylchromium stabilizes all three types of intermediates. The computational results for anionic addition agree well with established chemistry and provide structural and energetic details as reference points for comparison with the other reactive intermediates. Intermolecular radica l addition leads to exclusive reaction on the complexed arene ring as predi cted by the computations. The intramolecular radical reaction involves init ial addition to the complexed arene ring followed by an equilibrium leading to the observed product distribution due to a high-energy barrier for homo lytic cleavage of an exo bond in the intermediate cyclohexadienyl radical c omplex. Mechanisms are explored for electrophilic addition to complexed are nes. The calculations strongly favor a pathway in which the cation initiall y adds to the metal center rather than to the arene ring.