text: The majority of benzylic functionalization reactions of tricarbonyl(arene)chromium complexes proceed by mechanisms analogous to those followed by the free arenes. The aromatic ring and benzylic position are activated towards solvolysis, deprotonation, and nucleophilic attack (at the ortho and para positions of the arene) upon complexation to chromium, which is able to stabilize developing charges in the arene ligand. As a result, these reactions of chromium arene complexes are often faster than analogous reactions of free arenes.
  Second, in benzylic cations and anions of chromium arene complexes, rotation about the bond connecting the benzylic carbon and aromatic ring is severely restricted. This bond possesses a significant amount of double bond character due to the delocalization of charge into the aromatic ring (and the stabilization of that charge by chromium).
  Finally, the chromium tri(carbonyl) moiety serves as a sterically bulky group in reactions of arene chromium complexes, preventing the approach of a reagent endo to chromium. In addition, ortho-substituted aromatic aldehydes and styrenes prefer to adopt a conformation in which the doubly bound oxygen or carbon is pointed away from the  ortho substituent. As a result, only one face of the double bond is exposed on the exo face of the aromatic ring. If this were not the case, addition to styrenes and aromatic aldehydes would not be diastereoselective, despite the presence of the chromium tri(carbonyl) group. The ortho substituent is necessary for high stereoselectivity; meta-substituted arenes exhibit very low diastereoselectivity.