SolutionsHydrogen, Hydrides, Boranes
1. Choosing the reducing agent
What is obtained when methyl 4-oxopentanoate is reduced with: a) NaBH4 in methanol; b) LiAlH4 in ether and then H3O+; c) H2/Pd at room temperature?
- Methyl 4-hydroxypentanoate: borohydride reduces the ketone but not the ester. (The hydroxy ester can cyclize to the γ-lactone.)
- 1,4-Pentanediol: LAH reduces both the ketone and the ester.
- Methyl 4-hydroxypentanoate: hydrogenation reduces the ketone (rather slowly) but not the ester, which is much harder to hydrogenate.
2. Hydrogen and double bonds
Which product is obtained when 4-pentenal is reduced with: a) NaBH4; b) excess H2 with Pt?
- 4-Penten-1-ol: hydride attacks only the aldehyde.
- 1-Pentanol: catalytic hydrogenation reduces both the C=C and the aldehyde.
To reduce only the C=C of an isolated carbonyl compound, the ketone or aldehyde can first be protected as an acetal.