Reducing reagents for organic chemistry NaBH4, LiAlH4 etc..

organic chemistry reagents for reduction

1. REDUCTIONS

2. DiisobutylaluminumHydride (DIBAL-H): i-Bu
2AlH
•Reduces esters to the corresponding aldehydes, and lactones to
lactols(low temp reactions)

3. (DIBAL-H): Examples

4. (DIBAL-H): Examples

5. (DIBAL-H): Examples

6. DIBAL-H
Reduces nitriles to the corresponding aldehydes/amines

7. DIBAL-H
Nitriles are reduced to imines, which hydrolyze upon work-up to furnish aldehydes.

8. DIBAL-H
Reduction of Vinyl Esters to Allylic Alcohols

10. Hydride derivatives of B

11. Borohydride reducing agents
•Sources of nucleophilic hydrogen
•Reactivity and selectivity depends upon cations and ligands
•Reduction of aldehydes and ketones
•Reduction of carboxylic and ester groups require special conditions
LiBH
4NaBH
4
Zn(BH
4)
2 NaBH
3CN
Li-and K-trialkylborohydride
M(BR
nH
x)
y
Controls reactivity
Controls selectivity

12. NaBH
4
•Mild and selective reducing agent
•Reduces aldehydes and ketones to the corresponding alcohols at or below 25
°C.
•Under these conditions, esters, epoxides, lactones, carboxylic acids, nitro
groups, and nitriles are not reduced
•The reactions are performed in polar solvents (THF, DME, diglyme, hydroxyl
solvent and water)
•The reagent slowly reacts with solvent:
MeOH(30 min) > EtOH(slow) > iPrOH(stable) > tBuOH(stable).

13. NaBH
4
Examples

14. NaBH
4
Examples

15. NaBH
4

16. LiBH
4
•Selective reduction of esters and lactones to the corresponding alcohols in
the presence of carboxylic acids, tertiary amides, and nitriles.
•Groups that gets reduced: Aldehydes, ketones, epoxides, and several other
functional groups
•The reactions are performed in polar solvents (THF, DME, diglyme, hydroxyl
solvent and water)

17. LiBH
4

18. Zn(BH
4)
2
•Less basic than NaBH
4, and suitable for the reduction of base-sensitive
compounds
•Zinc cation has a better coordinating ability than either Na
+
or Li
+
•The reagent of choice for chelation-controlled, stereoselectivereductions of
acyclic ketones

19. Zn(BH
4)
2
Reduces aliphatic esters and carboxylic acids
Hydroboratingagent for alkenes, dienes, and alkynes

20. Zn(BH
4)
2
Reagent of choice in instances of potential competing 1,4-reduction.

21. Zn(BH
4)
2
Stereoselectivereduction

22. Sodium Cyanoborohydride(NaBH
3CN)
•Less reactive than NaBH
4.
•Mild reducing agent used primarily for the reduction of aldehydes and
ketones.
•Stable in aqueous solutions even at pH > 3 (permits activation of C=O by
protonation).
•Reactivity depends upon the pH of the medium

23. Sodium Cyanoborohydride(NaBH
3CN)
Reduces aldehydes and ketones at ~pH 3-4
Chemoselectivity

24. NaBH
3CN: Reductive amination (Borchreduction)
Reductive amination at around pH 6-7: method for preparation of sec-and tert-amines

25. NaBH
3CN: Deoxygenation of Tosylhydrazones
•Reduction of tosylhydrazonesto hydrocarbons: selective method for carbonyl
deoxygenation.
•Hindered carbonyl groups can be readily reduced to the corresponding hydrocarbon.
•Esters, amides, nitriles, nitro groups, and alkyl halides are not reduced under the
reaction conditions.
•Electron-poor aryl carbonyl groups resistant reduction.

26. NaBH
3CN: Deoxygenation of Tosylhydrazones

27. NaBH
3CN: Deoxygenation of Tosylhydrazones

28. Effect of counter ions in NaBH
4reductions
•Changing the counter-cation can have a profound effect on the reactivity of NaBH
4
•Addition of CeCl
3(LucheReduction) gives very selective 1,2-reduction of conjugated aldehydes and
ketones
•Co-ordination of the carbonyl and cerium result in increased hardness and more d+ character thus
more reactive to "H–"
•Tethering effect
•Reaction is under kinetic control (irreversible)
Use of Ni2+ or Co2+ results in a reversible complexation and a thermodynamically controlled reaction which results in
predominantly 1,4-reduction

29. LucheReduction (NaBH
4 + CeCl
3)
•Regioselectivereduction of α, -unsaturated carbonyl compounds to the
corresponding allylic alcohols.

31. Lithium or Potassium Trialkylborohydride
•Addition of electron-donating groups (inductive effect) increases
reducing power
•One of the best reducing reagents
•Especially good at hydrogenolysis(superior to LiAlH4)
Super Hydride LiBHEt
3

33. Lithium or Potassium Trialkylborohydride
•Aldehydes, ketones, and esters are rapidly and quantitatively reduced to the corresponding alcohols
•Reduces primary alkyl bromides and tosylatesto the corresponding hydrocarbons
Super Hydride LiBHEt
3

34. Lithium or Potassium Trialkylborohydride
•Large reagents, very good at diastereoselectivereactions (substrate
control)
•Very reactive and give preferential 1,4-reduction
Selectride

35. Lithium or Potassium Trialkylborohydride

36. Lithium or Potassium Trialkylborohydride
1,4 addition: RegioselectiveenolateformationSelectride

37. Regio-and chemoselectivereductions
1,2-additions
i-Bu
2AlH, Zn(BH
4)
2, 9-BBN, Luchereagent
1,2 and 1,4-additions
NaBH
4, LiAlH
4, and Li[AlH(OR)
3]

38. Regioselectivityof enonereductions
Nature of reagent Substituents on substrates Reaction conditions

39. Regioselectivityof enonereductions
Substituents on substrates