Activation of Benzylic Carbons by Coordination

The negative charge on the benzylic position of the side chain is stabilized by the electron-withdrawing effect of coordinated Cr(CO)3. In complex 234, attack of a carbanion occurs at the /f-carbon of the double bond to generate the stabilized anion 

235 at the benzylic carbon, which is trapped by an electrophile such as Mel. Thus two substituents are introduced from the opposite side of Cr(CO)3 to give the cis product 

Another effect of the coordination is that the benzylic cation is also stabilized [63]. This stabilization is explained by delocalization of the positive charge due to the interaction of the d-orbital of Cr with the 71-orbital of the benzylic carbon, caused by the coordination of Cr(CO)3. Facile stereospecific Friedel- Crafts-type cyclization of the complex of the optically active benzyl alcohol 248 gave the tetrahydrobenzazepine 249 with retention of the stereochemistry, and the free amine 250 with 98% ee was 

As an example of the activation of the side-chain a-carbon by the coordination, the alkylation of 251 with 1,3-dibromopropane produced 252. No such an alkylation is possible using NaH without complex formation [65], 

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