Sodium cyanoborohydride and iminium reductions

Question

I understand that the LAH will reduce the nitrile into a primary amine. The addition of acid will likely destroy the acetal mask and leave us with ethylene glycol and a molecule with both a ketone and an amine functional group.

However, what does the sodium cyanoborohydride do here with the acid and heat? I'm again thinking this is a red herring; $\ce{NaBH3CN}$ only reduces iminium ions as far as we are concerned... and we have an amine. Amines don't exist in equilibrium with imines.

Answer 1

I believe the reaction effected in the third step is a reductive amination. In this case, the ketone and amine moieties are both present within the same molecule, so the reaction proceeds intramolecularly. The $\ce{TsOH}$ is an acid, presumably serving to protonate the carbonyl oxygen and activate the carbon toward nucleophilic attack by the amine. As the iminium group forms (with concomitant loss of water), it is rapidly reduced to an amine by the $\ce{NaBH3CN}$. After workup, the final product is:

Answer 2

Cyanoborohydride has a number of functions as a selective reducing agent, which are very much pH dependant:

• Reduction of aldehydes and ketones. At low pH (3-4), it effectively reduces aldehydes and ketones to the parent hydrocarbon.
• Reductive amination of aldehydes and ketones. At pH 6-8, aldehydes and ketones undergo reductive amination
• Reduction of oximes, at low pH

Have a close look at the pH of your system.