Regiodirecting effects of substituents

The classic rf cyclohexadienyl complexes of Fe(CO)3 first investigated by Fischer and Fischer [56], and subsequently by many research groups (including my own) provide a wealth of examples of studies of regiodirecting effects of substituents. With simple alkyl groups, steric effects send the nucleophile to the co position... 

The total synthesis of the carbazomycins emphasizes the utility of the iron-mediated synthesis for the construction of highly substituted carbazole derivatives. The reaction of the complex salts 6a and 6b with the arylamine 20 leads to the iron complexes 21, which prior to oxidative cyclization have to be protected by chemoselective 0-acetylation to 22 (Scheme 13). Oxidation with very active manganese dioxide followed by ester cleavage provides carbazomycin B 23a [93] and carbazomycin C 23b [94]. The regioselectivity of the cyclization of complex 22b to a 6-methoxycarbazole is rationalized by previous results from deuterium labeling studies [87] and the regiodirecting effect of the 2-methoxy substituent of the intermediate tricarbonyliron-coordinated cyclo-hexadienylium ion [79c, 79d]. Starting from the appropriate arylamine, the same sequence of reactions has been applied to the total synthesis of carbazomycin E (carbazomycinal) [95]. 

For discussion of synthesis design based on the manipulation of competing regiodirecting effects, see [4] this same concept has been discussed in terms of matched and mismatched pairs of substituents (see [5]), by an extension of the nomenclature used for double stereodifferentiation, but since double stereodifferentiation effects can arise... 

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