# Why are imines more susceptible to reduction than carbonyl compounds?

Why are imines more susceptible to reduction than carbonyl compounds? Imines react with $\ce{NaCNBH3}$ while the carbonyls don't.

Generally speaking, I think it is the reverse - carbonyls are reduced faster than imines with various sources of $\ce{H^-}$. The reason usually cited (see pages 2-4 of this reference) for this behavior is that the imine double bond is less polarized than the carbonyl double bond. In the figure below resonance structure II contributes less when X=N as compared to the case X=O, due to the decreased electronegativity of nitrogen compared to oxygen.
• @OscarLanzi Yes, take a look at the last reference (section 1.2.2.1) in my answer. They mention use of $\ce{NaBH3CN}$ and $\ce{NaBH4}$ \ $\ce{RCO2H}$ under mildly acidic conditions. – ron Apr 29 '17 at 13:57