Benzylic activation and stereocontrol in tricarbonyl(arene)chromium complexes

Uemura, Motokazu


This chapter covers reactions of (η6-arene) chromium complexes that occur in the side chain of the arene, and reactions that exhibit stereocontrol as a result of complexation. Reactions that focus on nucleophilic additions to the chromium-coordinated aromatic ring and on directed lithiation of the arene ring are not covered.

The tricarbonyl(η6-arene) complexes are readily prepared by several convenient methods, and these complexes are relatively stable to air and moisture. Most arenes will coordinate to the tricarbonyl fragment. Certain functional groups are incompatible and electron withdrawing groups, such as CHO and CO2H, retard complexation. Protection of these functional groups as acetals or esters followed by chromium complexation and hydrolysis procedures give the corresponding tricarbonylchromium complexes of benzaldehyde or benzoic acid in good yields. Electron-donating substituents accelerate the rate of complexation. The chemical consequences of chromium complexation include (1) the activation of the aromatic ring to nucleophilic addition; (2) enhancement of acidity in the side chain; (3) enhancement of the rate of solvolysis in the side chain; (4) enhancement of acidity of aromatic hydrogens: (5) control of reactions by steric effects of the tricarbonylchromium fragment. After the reaction, the tricarbonylchromium fragment can be easily removed by mild oxidation. When the arene ring is disubstituted with different substituents at the ortho and meta positon, the complexes are planar chiral and the substrates are useful for asymmetric synthesis.