Dearomatization via η⁶-Arene Complexes

Abstract

The chapter presents a detailed overview of the dearomatization of arenes by temporary complexation to the electrophilic complex fragments Cr(CO)₃ and Mn(CO)₃ ⁺. The activation of the η⁶-coordinated arene in the complex enables nucleophilic addition. In the case of the fragment Cr(CO)₃, the resulting anionic cyclohexadienyl complexes can be reacted with electrophiles (H⁺, RX) to yield, after decomplexation, trans-disubstituted cyclohexadienes. With H⁺, rapid isomerization results in mixtures of dienes. With anisol, this can be directed to yield synthetically useful 5-substituted cyclohexenones. With C-electrophiles (primary and secondary allyl-, benzyl-, and alkyl-iodides and -bromides), the overall reaction sequence is a trans-addition of a C-nucleophile and a C-electrophile across an arene double bond. With alkyl electrophiles, a CO insertion step yields ketone products. Highly asymmetric reactions have been developed by using either chiral directing groups on the arene, chiral nucleophiles, chiral Cr bound ligands and planar chiral arene complexes. Reactions with the Mn(CO)₃ ⁺ activating group enlarges the range of nucleophiles in the initial addition step. As this results in a neutral cyclohexadienyl complex, a reactivation by ligand exchange is often necessary at this stage in order to follow up with a second nucleophilic addition.