The diastereoselective functionalisation of arene tricarbonylchromium complexes containing a benzylic heteroatom substituent
Treatment of (S)-tricarbonyl(α-methylbenzyldimethylamine)chromium (4) with n-butyl-lithium or t-butyl-lithium gives rise to a highly regioselective ortho deprotonation via co-ordination of the base to the benzylic nitrogen atom. In the latter case a highly diastereoselective substitution is observed consistent with reaction via the least hindered transition state. Determination of the stereochemistry of the product (S)-tricarbonyl(2,α-dimethylbenzyldimethylamine)chromium (5) by single crystal X-ray analysis is described. The regioselectivity of metallation of tricarbonyl(α-methylbenzyl methyl ether)chromium (14) and tricarbonyl (ethyl α-methylbenzyl sulphide)chromium (20) is also described. Treatment of compound (14) with t-butyl-lithium gives regioselective α-deprotonation with concomitant suppression of the Wittig rearrangement.