Reducing Reagents
| Reagent | Preferred Solvents | Functions Reduced | Reaction Work-up |
|---|---|---|---|
| Sodium Borohydride NaBH4 |
ethanol; aqueous ethanol 15% NaOH; diglyme avoid strong acids |
aldehydes to 1º-alcohols ketones to 2º-alcohols 1,2-reduction of enones is favored by CeCl3 inert to most other functions |
1) simple neutralization 2) extraction of product |
| Lithium Aluminum Hydride (LAH) LiAlH4 |
ether; THF avoid alcohols and amines avoid halogenated compounds avoid strong acids |
aldehydes to 1º-alcohols ketones to 2º-alcohols carboxylic acids to 1º-alcohols esters to alcohols epoxides to alcohols nitriles & amides to amines halides & tosylates to alkanes most functions react |
1) careful addition of water 2) dissolve aluminum salts 3) extraction of product |
| Lithium tri t-Butoxyaluminohydride LiAlH(Ot-C4H9)3 |
ether; THF avoid alcohols and amines avoid halogenated compounds avoid strong acids |
fast: acid chlorides to aldehydes (at -78 ºC) 3º-amides to aldehydes (at -78 ºC) nitriles to aldehydes (at -78 ºC) slower: aldehydes to 1º-alcohols ketones to 2º-alcohols |
1) careful addition of water 2) dissolve aluminum salts 3) extraction of product |
| Diisobutylaluminum Hydride AlH[CH2CH(CH3)2]2 |
THF; toluene avoid alcohols and amines avoid halogenated compounds avoid strong acids |
fast: acid chlorides to aldehydes (at -78 ºC) 3º-amides to aldehydes (at -78 ºC) nitriles to aldehydes (at -78 ºC) slower: aldehydes to 1º-alcohols ketones to 2º-alcohols |
1) careful addition of water 2) dissolve aluminum salts 3) extraction of product |
| Diborane B2H6 = 2 BH3 |
ether; THF sulfide complex in CH2Cl2 complexes with amines avoid alkenes & alkynes |
carboxylic acids to 1º-alcohols aldehydes to 1º-alcohols ketones to 2º-alcohols nitriles to amines esters & epoxides slowly reduced |
1) dilute acid or H2O2
2) extraction of product |
| Hydrogen & Catalyst H2 & Pt, or Pd, or Ru, or Ni Modified (poisoned) Catalyst |
alcohols, ethers, hydrocarbons or carboxylic acids |
alkenes & alkynes to alkanes (fast) nitro groups to amines (fast) imines to amines (fast) aldehydes & ketones to alcohols (slow) nitriles to amines (slow) may remove benzylic groups alkynes to alkenes acyl chlorides to aldehydes |
filter to remove catalyst |
| Reactive Metals Na, or Li, or K Mg or Al or Zn or Fe |
liq. ammonia & ether co-solvents or alcohols or amines water, alcohols, acetic acid or aqueous mineral acid |
ketones to 2º-alcohols alkynes to alkenes conjugated π-systems (e.g. aromatic rings, dienes & enones) cleaves C-X and benzylic groups cleaves activated substituents nitro groups to amines C=O (aldehyde/ketone) to CH2 |
1) quench with NH4Cl 2) extract product extract product from salts |
Many related hydride reagents, having different reductive power and selectivity, incorporate a variety of organic ligands on the hydride carrier atom. Among these are: lithium triethylborohydride (super hydride), potassium tri-sec-butylborohydride (K-Selectride), sodium bis(2-methoxyethoxy)aluminumhydride (Red-Al), sodium cyanoborohydride, sodium triacetoxyborohydride. various organosilanes and in situ generated aluminum hydride and dichlorohydride. Organoborane derivatives having modified reactivity relative to borane itself include: bis(1,2-dimethylpropyl)borane (disiamylborane), 9-borabicyclo[3.3.1]nonane (9-BBN) & catecholborane (BHcat).