Benzylic Activation and Stereochemical Control in Reactions of Tricarbonyl(arene)chromium Complexes

Abstract

This chapter covers reactions of ( 6 -arene) chromium complexes that occur in the side chain of 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( 6 -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 CO 2 H, 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.

Catalytic Asymmetric C–H Arylation of (η6-Arene)Chromium Complexes: Facile Access to Planar-Chiral Phosphines

“…Unsymmetrically substituted metallocenes and (η 6 -arene)chromium complexes are two notable families of planar-chiral transition-metal organometallic complexes, and they have extensively been applied as stoichiometric auxiliaries and/or starting materials for asymmetric synthesis of biologically interesting substances. , Derivatives of ferrocene have been extensively investigated as optically active ligands in catalysis . In contrast, the use of (arene)chromium complexes as chiral ligands has been significantly less explored with only a small selection of such ligands reported to date (Figure ).…”

Palladium-Catalyzed Allylic Substitution with (η6-Arene–CH2Z)Cr(CO)3-Based Nucleophiles

“…17 However, the chromium-free complexes are often the desired products. Arene complexes of the type (η 6 -arene)Cr(CO) 3 can be decomplexed simply by exposure to light and air to afford the corresponding arenes.…”

“…As shown in Scheme 2, however, the double allylation proceeded smoothly at ambient temperature and the cis -diallylation product 9 was isolated in 76% yield. 17,18 Presumably this approach could be coupled with ring closing metathesis to afford bicyclic scaffolds.…”

“…The observed double inversion is consistent with the ability of the Cr(CO) 3 moiety to stabilize benzyl anions by delocalization of the negative charge onto the chromium to “soften” the nucleophilicity of the organolithium. 16,17 …”

Efficient Enantioselective Syntheses of Sertraline, 2‐Epicatalponol and Catalponol from Tetralin‐1,4‐dione

“…It is well established in reactions of arene tricarbonylchromium complexes that nucleophilic and electrophilic reagents approach the complexed arene on the face opposite to the metal [16,17] The structural assignment of 19 rests on the precedence of diastereoselective methylation of the enolate of complex 17, [18] on NOESY measurements…”