Probably the most common type of reagent used for reductions in organic chemistry is the hydride reagent – i.e. one which reacts as ‘H–‘ – a hydride nucleophile. It is worth noting, however, that simple hydrides e.g. NaH are not very nucleophilic and are far more likely to act as bases. Therefore the reagentsmost likely to be encountered here are of the type LiAlH4, NaBH4 etc.
Reductions on the Ketone Level: Aldehydes and ketones to alcohols
with NaBH4 This is reactive enough to reduce aldehydes and ketones but leave carboxylic acids, amides and esters alone – useful selectivity.
The Luche Reaction – ketones to alcohols, but no reduction of aldehydes This is very useful combination of reagents – NaBH4 and CeCl3 – which will selectively reduce a ketone in the presence of an aldehyde.
This is particularly useful because usually aldehydes are more reactive than ketones, so it would be very difficult to react with the ketone without reacting with the aldehyde first. In fact it is the increased reactivity of the aldehyde that this reactive uses to its advantage. Because an aldehyde is more reactive it has more of a tendency to add water as a hydrate which is then held in place by the Lewis Acidic CeCl3 – at the same time the CeCl3 activates the non-hydrated ketone to attack by NaBH4:
with LiAlH4 A much more reactive reagent than NaBH4, LiAlH4 will reduce most carbonyl groups. The mechanism goes as follows;