3.6.2.2 Ring Lithiation
The ring deprotonation of Cr(CO)3 -complexed arene leads to the regioselective preparation of alkylated complexes. Using chiral bases enantioselective deportonation of prochiral substrates can be accomplished (Scheme 10).
Scheme 10
3.6.2.2 Side-Chain Activation
The Cr(CO)3 group strongly stabilizes the negative charge at the a-position of alkyl side chains facilitating benzylic deprotonation. In case of substrates having two competing benzylic positions the regiochemistry can be controlled by electronic effects (Scheme 11).
Scheme 11
3.6.2.3 As Catalysts
Arene-Cr(CO)3 complexes are a good source for the ‘free' Cr(CO)3 unit that catalyzes a number of useful transformations. For example, the 1,4-hydrogenation of 1,3-dienes gives Z-configurated alkenes (Scheme 12). 1,3-Dienes that can adopt a s-cisoid conformation proceed the reaction, whereas isolated double bonds are not affected.
Scheme 12