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Reduction can be defined as conversion of an atom
in a higher oxidation stage to a lower one (III → II,
IV → II or II → I), as in the transformation RCH=NH
A reduction reaction proceeds with a decrease in
oxidation state and a gain of electrons .
Reduction potential can be used to assess the
ability of a reagent to reduce molecules.
CLASSIFICATION OF REDUCTION REACTIONS
1. Catalytic hyrdogenation (H2 with metals)
2. Hydride transfer reactions, using hydride sources
such as LiAlH4, NaBH4,etc.
3. Dissolving metal reductions (Na, Li in ammonia
solution) (Birch reduction)
4. Replacement of oxygen by hydrogen
5. Removing oxygen from the substrate
6. Reduction with cleavage
7. Reductive coupling
METAL HYDRIDE REDUCTION:
Metal hydride reductions are probably the most
widely used, followed by catalytic hydrogenation.
Boron Based Reagents
• Sodium Borohydride - NaBH4
• Sodium Cyanoborohydride - NaCNBH3
• Sodium Triacetoxyborohydride - Na(OAc)3BH
• Lithium Borohydride - LiBH4
• Potassium Borohydride - KBH4
• Tetramethylammonium Borohydride
SODIUM BOROHYDRIDE - NABH4
Sodium borohydride was first prepared by reaction of sodium hydride
(NaH) with trimethylborate, B(OMe)3.
NaBH4 is less reactive than LiAlH4.
It is only powerful enough to reduce aldehydes, ketones and acid
chlorides to alcohols.
Esters, amides, acids and nitriles are largely untouched.
An aldehyde is reduced to 1O & ketone to a 2O alcohol respectively.
Selective (chemoselectivity) reagent
SODIUM BOROHYDRIDE - NABH4
Aqueous and alcohol solvents are preferred due to the excellent
solubility of NaBH4.
NaBH4 reacts with water to form hydroxyborohydride intermediates, but
these are also mild reducing agents.
It is relatively insoluble in ether solvents , so these are rarely used for
In most cases aqueous ammonium chloride, aqueous acetic acid, or
dilute mineral acids are used for hydrolysis .
An important use of NaBH4 is reduction of enamines, imines or iminium
salts, which is particularly useful in alkaloid and amino acid syntheses.
In a synthesis of (+)-majvinine,134 Cook and co-workers converted the
amine unit in 99 to the imine (100) by reaction with benzaldehyde.
Subsequent reduction with NaBH4 gave a 92% overall yield of the N-
SODIUM CYANOBOROHYDRIDE - NACNBH3
Reaction of NaBH4 with HCN gives sodium cyanoborohydride (NaBH3CN).
A remarkably stable reagent that it is very selective, and does not
decompose in acid solution (the pH should be less acidic than pH 3).
It is soluble in THF, MeOH, H2O, HMPA, DMF and sulfolane and they do not
Ketones and aldehydes are reduced in acidic media but not at neutral pH.
At pH 3-4, It reduces aldehydes & ketones
At pH 6-8, It reduces iminium ions
Less reactive than NaBH4. Used predominantly in reductive aminations
Electron withdrawing cyano group decreases the ease of hydrogen transfer.
NaBH3CN reduced iminium ions much more quickly than ketone or
Sometimes strong acids can release HCN from NaCNBH3
SODIUM TRIACETOXYBOROHYDRIDE - NA(OAC)3BH
The preparation of triacetoxyborohydride was first performed by Wartik
and Pearson through the reaction of NaBH4 and CO2 .
It can be also generated in situ from NaBH4 and acetic acid.
It is a milder and more selective reducing agent than NaBH4. The mild
nature may be attributed both to the bulky nature of the reagent and to
the inductive electron-withdrawing ability of the three acetoxy groups
which stabilize the B-H bond.
It is effective in reductive amination of aldehydes and ketones, it can
reduce N-heterocycles (indoles, quinolines, and isoquinolines), imines,
enamines, oximes, amides, aryl ketones, acetals, and other
Lithium aluminum hydride (LiAlH4)
Lithium tri-tert-butoxyaluminumhydride [Li(OtBu)3AlH]
Diisobutylaluminum hydride ([(CH3)2CHCH2]2AlH, Dibal
Sodium bis(2-methoxyethoxy)aluminium hydride (Red-Al)
L and K selectrides
LITHIUM ALUMINUM HYDRIDE (LIALH4)
Very powerful reducing reagent
Available as either powder or pallet
Used as a suspension in ether or THF
Reacts violently with water, alcohol
Reduces carbonyl, carboxylic acid & ester
Reduces nitrile, amide & aryl nitro group to amine
Reduces acetylene to olefin
Reduces C-X bond, opens epoxide
LiAlH4 is a stronger reducing agent than NaBH4 due to weaker Al-H bond
LiAlH4 is used to reduce compounds that are nonreactive toward NaBH4
([(CH3)2CHCH2]2ALH, DIBAL OR DIBAL-H)
Prepared by refluxing triisobutylaluminum
in the solvent heptane.
Dibal-H is a strong reducing reagent,
reducing most functional .
Selective reagent (alkyne to alkene, ester
or ketone to aldehyde).
Specialist reductant of nitrile to aldehyde
Under carefully controlled conditions, will partially reduce an
ester to an aldehyde.
Direct conversion of acids to aldehydes:
Acid chloride to aldehyde using Weinreb’s amide:
•Very reactive hydride donors due to inductive effect
•Bulk makes them very good at diastereoselective reactions
REDUCTION WITH DIIMIDE
Diimide (NH=NH) is a transient species generated by reaction of acids
with potassium azodicarboxylate by thermolysis of anthracene-9,10-
Diimide gives primarily cis reduction of alkenes
Reduces symmetrical π bonds faster than polarized π bonds.
1. Advanced Organic Chemistry: Part B: Reaction and Synthesis,
By Francis A. Carey, Richard J. Sundberg
2. Reaction Mechanisms in Organic Chemistry, Dr. Mukul C Ray
3. Organic Synthesis, By Michael Smith
4. Advanced Organic Chemistry, 4th Edition, Jerry March, Wiley India
5. A Guidebook to Mechanism In Organic Chemistry, 6th ed. By Peter
6. Reaction Mechanism in Organic Chemistry, By S. M. Mukherji, S.
7. Organic Reaction Mechanism, 3rd Edition, V.K. Ahluwalia and R.K.
Parashar, Narosa Publishers
8. Organic Chemistry: Vol 2, 5th Edition, I.L. Finar, Pearson Education