Electrophiles for the Electrophilic Aromatic Substitution

The electrophilic aromatic substitution reaction can be used to add bromo (-Br), chloro (-C1), nitro (-NO2), and sulfonic acid (-SO3H) groups to 

Introduction to Strategies for Organic Synthesis, xst Edition. Laurie S. Starkey. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

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Tricarbonyliron-coordinated cyclohexadienylium ions 569 were shown to be useful electrophiles for the electrophilic aromatic substitution of functionally diverse electron-rich arylamines 570. This reaction combined with the oxidative cyclization of the arylamine-substituted tricarbonyl(ri -cyclohexadiene)iron complexes 571, leads to a convergent total synthesis of a broad range of carbazole alkaloids. The overall transformation involves consecutive iron-mediated C-C and C-N bond formation followed by aromatization (8,10) (Schemes 5.24 and 5.25). 

Now we have an excellent electrophile for the electrophilic aromatic substitution reaction. The hydroxyl lone pair is a much better electron-releasing group than the methyl, so the electrophile adds ortho to the best electron-releasing group, hydroxyl (the para position is already occupied by methyl). The Dg step removes the proton from the carbon that the electrophile attacked to rearomatize the ring. 

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