Arene chromium tricarbonyl complexes decomplexation

The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Because of the instability of hydroxy-substituted arene chromium tricarbonyl complexes, yields of the latter tend to be low. High yields of arene complexes can, however, be obtained by in situ silylation of the crude product of the benzannulation reaction [336]. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. Treatment of the benzannulation products with phosphines also leads to decomplexed phenols [272]. 

Direct nucleophilic addition of potassium enolates derived from bis(trimethylsilyl)ketene acetals to aromatic chromium-complexed aromatic ethers affords meta substituted products (Scheme 124). A very high degree of asymmetric induction is obtained upon reaction of chiral arene chromium tricarbonyl complexes. For example, alkylation of complex (80) gave (81) after decomplexation (Scheme 125). ... 

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

A further chiral auxiliary-based tactic exploited tricarbonyl( 76-arene)chromium complexes of aromatic imines 71, which reacted under ultrasound (US) irradiation with a-bromoesters in a predictable stereochemical course to give comparable amounts of /S-aminoesters and / -lactams, as outlined in equation 44127. Chromium decomplexation is eventually achieved by photochemical oxidation under air. 

Nucleophilic Substitution. Some carbon nucleophiles add to tricarbonyl(arene)chromium complexes to yield anionic ii -cyclohexadienyl complexes (2) (eq 8), which give the substituted arenes via decomplexation by oxidation with iodine. Protolysis of the intermediate cyclohexadienylchromium complexes (2) generate cyclohexadienes, and reaction with electrophiles generates either the arene chromium complexes or produces the acylated species. 

Still another route for selective formylation makes use of the tricarbonyl-chromium complexes of calix[4]arenes. Reaction of the tetrapropyl ether of 4 with Cr(CO)3 produces a complex in which a Cr(CO)3 moiety is associated with a single aromatic residue of the calixarene. Treatment of the complex with BuLi and then D2O, Mel, or DMF followed by decomplexation with I2 yields mono-substituted calix[4]arenes carrying D at a p- or m-position. Me at a p-position, and CHO at a p-position, respectively." ... 

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