Reducing agents guide template

Reducing agents guide: NaBH₄, LiAlH₄, DIBAL-H, H₂/Pd-C, Lindlar catalyst, Na/NH₃ and NaBH₃CN, each with its conditions, chemoselectivity and a substrate → product example.

What the template shows

• Hydrides, catalytic hydrogenation and dissolving metals: what each reduces, and a worked example for each
• What it does
• MeOH or EtOH, 0 °C → rt
• Mild hydride: aldehydes and ketones → alcohols. Esters, acids and amides are left alone.
• Et₂O or THF; then H₃O⁺
• Strong hydride: esters and acids → 1° alcohols, amides → amines, nitriles → amines, ketones → alcohols.
• i-Bu₂AlH (1 equiv), CH₂Cl₂ or toluene, −78 °C
• Ester → aldehyde (the tetrahedral intermediate survives at −78 °C); nitrile → aldehyde after work-up.
• C=C and C≡C → alkane (syn), Ar–NO₂ → ArNH₂, removes benzyl groups; esters and arenes survive.
• H₂, Pd/CaCO₃, Pb(OAc)₂, quinoline
• Poisoned catalyst: alkyne → cis (Z) alkene, syn addition; stops before the alkane.
• Dissolving metal: internal alkyne → trans (E) alkene via a radical anion; arenes give Birch 1,4-dienes.
• amine, MeOH, pH ≈ 6
• Reductive amination: reduces the iminium ion much faster than the ketone, giving the amine in one pot.
• Hydride strength: LiAlH₄ > DIBAL-H > NaBH₄ > NaBH₃CN. Choose the mildest reagent that reduces the group you want, so the others survive.
• NaBH₄ leaves esters alone, so a keto-ester is reduced only at the ketone; LiAlH₄ would reduce both.
• DIBAL-H stops at the aldehyde only at −78 °C with one equivalent; warmer or with excess it gives the alcohol.
• Alkyne geometry: Lindlar (H₂ on a poisoned Pd surface, syn) gives Z alkenes; Na/NH₃(l) (electron, proton, electron, proton) gives E alkenes.
• NaBH₃CN is stable at pH ≈ 6, where the iminium ion forms and is reduced much faster than the ketone (reductive amination); NaBH(OAc)₃ is a less toxic alternative.
• Every part of this figure is editable · sources on the template card

Drawn with 128 atoms, 31 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 23 Textbook
• Wade, Organic Chemistry, 8th ed., Pearson 2013 Textbook
• Smith, March's Advanced Organic Chemistry, 7th ed., Wiley 2013 Textbook