Reducing Reagents

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).