Substitution with arenechromium tricarbonyl

Substituent effects in addition of earbanions to 7r-arenechromium tricarbonyl complexes have been examined. Methyl and chloro substituents give mixtures of ortho- and mefa-substitution, with the amount of mem-isomer increasing with bulk of the anions. Methoxyl and dimethylamino substituents are strongly meta -directing. The more hindered 3-position of 1,2-dimethoxybenzene is substituted even by a tertiary carbanion. Naphthalene shows 99% a -substitution. A trimethylsilyi group is strongly para-directing. 

Substituted arenechromium tricarbonyl aldehydes have been reacted with enoxysilanes derived from cyclanones [545] or thiol propionates [546], After de-complexation, the expected aldols are detained with excellent selectivities (Figure 6.99). 

Other arenechromium tricarbonyls. Semmelhack has extended his study of the reactions of carbanions with benzenechromium tricarbonyl to anisole-and toluenechromium tricarbonyl. The distribution of products depends on the particular anion used, but almost complete absence of para-substitution is consistently observed. A preference for meta-substitution over orfAo-substitution is general. 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

Nucleophilic aromatic substitution can also be promoted by jr-complex formation but chromium complexes are particularly useful for realizing this type of activation as palladium or nickel do not lead to stable JT-complexes with arenes. For example, Jt-arenechromium tricarbonyl complexes react easily with nucleophiles, even when electrodonating substituents are present on the aromatic ring. This reaction has been used in two steps for the synthesis of carenone B (a sesquiterpene), one of these steps being an mrm-molecular nucleophilic attack with formation of a spirobicyclic system [22]. 

---