Reduction

Reducing Agents, Applications on Functional Groups

 Lithium Aluminium Hydride LiAlH₄

Reduces acids, esters, amides, nitriles, aldehydes, ketones, epoxides, halides → alcohols/amines/alkane. Non-selective, powerful hydride donor.

 Lithium Borohydride LiBH₄

Selectively reduces esters, lactones, → alcohols; also reduces aldehydes and ketones. Less reactive than LAH.

Sodium Borohydride NaBH₄

Reduces aldehydes, ketones, → alcohols. Doesn't reduce esters, acids, or nitriles under normal conditions.

 Diisobutylaluminum Hydride (DIBAL-H) reduces esters → aldehydes at low temperatures; nitriles → imines (→ aldehydes after hydrolysis).

Lithium Triethoxyaluminohydride Li(EtO)₃AlH

Selective reduction of nitriles, tertiary amides → aldehydes.

Borane Complexes BH₃•THF

Reduces carboxylic acids, aldehydes, ketones, and alkenes. Competes with hydroboration in alkenes.

 Alane AlH₃

Similar to LAH, reduces esters, acids, amides, nitriles → alcohols/amines.

 Sodium Cyanoborohydride NaBH₃CN

Mild reductant, used for reductive amination and selective reductions, stable in mildly acidic conditions.

Samarium Iodide SmI₂

Reduces aldehydes, ketones, and alkyl halides in the presence of esters/acids; highly chemoselective.

 Ionic Hydrogenation Et₃SiH + TFA

Reduces carbocations formed from ketones, alkenes, and lactols. Doesn't reduce esters, amides, or nitriles.

Tributyltin Hydride Bu₃SnH

Radical reductions: dehalogenation, deoxygenation, decarboxylation.

Trialkylborane + H₂O B(R)₃

Tin-free radical reductions (Barton-type).

Chemoselectivity Highlights

• NaBH₄: Selective for aldehydes and ketones; does not reduce esters, acids, or nitriles.
  
  

• DIBAL-H: Reduces esters to aldehydes at low temperatures without over-reduction.
  
  

• Li(EtO)₃AlH: Selective reduction of nitriles and tertiary amides to aldehydes.
  
  

• Samarium Iodide: Reduces carbonyls without affecting esters/acids—useful for selective reductions.

Stereoselectivity & Stereospecificity

Luche Reduction (NaBH₄ + CeCl₃)

Stereoselective reduction of α,β-unsaturated carbonyls to allylic alcohols. Controls regio- and stereochemistry.

Corey–Winter Olefination: Stereospecific conversion of diols to olefins. Retains stereochemistry due to a concerted mechanism.

Eastwood Deoxygenation: Stereospecific elimination in diol systems via cyclic orthoformates.

Reduction of Tosylhydrazones: Stereoselective deoxygenation to alkanes; often retains geometry, but electron-poor aryl systems resist reduction.

Diazene-Mediated Deoxygenation: Stereospecific, where possible, via concerted elimination; otherwise, proceeds via radical pathways.

 Radical Reductions (Bu₃SnH): Often proceed with retention or controlled formation of stereochemistry, depending on radical stability and cyclisation pathways.

Examples of Selectivity

• LiAlH₄: Reduces most functional groups—non-selective.
  
  

• NaBH₄: Stops at aldehyde/ketone reduction—high chemoselectivity.
  
  

• DIBAL-H: Temperature-controlled reduction prevents over-reduction.
  
  

• Samarium Iodide: Allows selective carbonyl reductions in multifunctional molecules.
  
  

• Ionic Hydrogenation: Selective for carbocation intermediates—leaves esters, acids untouched.