Question

Question: Devise a stepwise mechanism for the following reaction. The reaction does not take place by direct electrophilic aromatic substitution at C2 . (Hint: The mechanism begins with addition of an electrophile at C3.)

Short answer

Answer

Step-by-step solution

Electrophilic aromatic substitution

Electrophilic aromatic substitution occurs when the electron-rich compound attacks the electrophile and substitutes on its most electron-dense place.

The substitution depends upon the formation of the most stable intermediate.

Indole

The Lewis acid (boron trifluoride) is an electron-deficient compound. Therefore, the pyrrole group in the indole transfers its electron (third position) to the ${\mathrm{BF}}_3$group, which accelerates the attack of the alcohol group at the second position.

Explanation

The mechanism of the indole cyclization in the presence of Lewis acid is shown as:

Mechanism