Arene chromium tricarbonyl complexes are widely used in organic synthesis because they are synthetically equivalent to the free arene, yet possess different chemical properties.
Besides electronic effects, (due to the electron withdrawing character of the tricarbonyl chromium unit), the metal carbonyl moiety imparts a third dimension to the molecule: ortho and meta unsymmetrically disubstituted chromium tricarbonyl complexes are chiral and in many cases they can be resolved into the corresponding enantiomers.
Chiral tricarbonyl (ortho-substituted benzaldehyde)chromium complexes are the most widely applied examples in asymmetric synthesis.
The accepted model for stereoselection is based on the fact that the metal spatially occupies one face of the ring and attack of any reagent at the benzylic position is constrained to approach only from the side anti to the metal; in addition the carbonyl group lies preferentially in the anti conformation with o respect to the ortho substituent.
These two factors afford a high degree of stereoselectivity.