Question

The reagent(s) required for the following conversion are :

(Reactant: cyclohexane ring with \(\mathrm{CO_2Et}\), \(\mathrm{CO_2H}\) and \(\mathrm{CN}\) substituents at positions 1, 3 and 5; product: cyclohexane ring with \(\mathrm{CH_2OH}\), \(\mathrm{CO_2H}\) and \(\mathrm{CHO}\) at those positions.)

• A. (i) \(\mathrm{B_2H_6}\); (ii) \(\mathrm{SnCl_2/HCl}\); (iii) \(\mathrm{H_3O^+}\)
• B. (i) \(\mathrm{LiAlH_4}\); (ii) \(\mathrm{H_3O^+}\)
• C. (i) \(\mathrm{B_2H_6}\); (ii) DIBALH; (iii) \(\mathrm{H_3O^+}\)
• D. (i) \(\mathrm{NaBH_4}\); (ii) Raney Ni/\(\mathrm{H_2}\); (iii) \(\mathrm{H_3O^+}\)

Hint:

Choose reagents that reduce only \(\mathrm{COOH}\) (to \(\mathrm{CH_2OH}\)) and \(\mathrm{CN}\) (to \(\mathrm{CHO}\)) while the ester ends up as \(\mathrm{COOH}\) by hydrolysis.

Answer 1

The Correct Option is A

Required changes: \(\mathrm{-CO_2H\to -CH_2OH}\), \(\mathrm{-CN\to -CHO}\), \(\mathrm{-CO_2Et\to -CO_2H}\).
Step (i) \(\mathrm{B_2H_6}\) selectively reduces the carboxylic acid group to a primary alcohol; the ester and nitrile are left untouched under these conditions.
Step (ii) \(\mathrm{SnCl_2/HCl}\) (Stephen reduction) converts the nitrile to an aldimine salt.
Step (iii) \(\mathrm{H_3O^+}\) hydrolyses the aldimine to \(\mathrm{-CHO}\) and also hydrolyses the ester \(\mathrm{-CO_2Et}\) to \(\mathrm{-CO_2H}\).
\(\mathrm{LiAlH_4}\) (option 2) would reduce all three groups, DIBALH (option 3) would attack the ester and nitrile, and \(\mathrm{NaBH_4}\) (option 4) cannot reduce the acid, nitrile or ester. Hence option (1).
Answer: (1) (i) \(\mathrm{B_2H_6}\); (ii) \(\mathrm{SnCl_2/HCl}\); (iii) \(\mathrm{H_3O^+}\)