Reducing agents are chemo-selective as well as stereoselective.

1. Presented by:-
Kavita Singhal
I.D. 52618
Ph.D 3rd yr.
Department of Chemistry

2. The selectivity
This term arises from the presence of more than one
functional group.
There are three main types of selectivity –
• Chemoselectivity- which functional group will react.
• Regioselectivity- where it will react.
• Sterioselectivity- how it will react(steriochemistry of
products)

3. Chemoselectivity

4. What is chemoselectivity?
• Preferential selectivity of one functional group over
other.
• Two different functional group and unequal reactivity.
• Reaction of one of two identical functional groups.
• Reaction of a group once, when it may react again.

5. What are Chemoselective
reducing agent?
If an organic compound contains more than one
different functional groups or more than one like
functional groups that are not equivalent and, if a
reagent reduce exclusively or predominately with one
of them, the reaction is said to be chemoselective and
those reagent are termed as chemoselective reducing
agent.

6. Chemoselective reducing agent-
• K[BH(OAc)3 ]
• Zn(BH4)2
• NaBH4.CeCl3 –EtOH
• NaBH4 EtOH-H2O
• BH3 .THF or BH3 .SMe2
• DIBAL-H , n-BuLi

7. • K[BH(OAc)3 ]
This reagent involves the reduction of aldehydes in the presence of
ketones.
CHO
O
K[BH(OAc)3]
C6H6
O
OH

8. • Zn(BH4 )2
Zinc borohydride is a mild reducing agent permitting the
reduction of aldehydes in presence of ketones. Moreover it
selectively reduces a nonconjugated keto group in the presence
of a conjugated keto group.
O
CHO
Zn(BH4)2
THF
O
OH
O
O
Zn(BH4)2
DME
OH
O

9. • NaBH4 .CeCl3 –EtOH
This reagent reduces chemoselectively a keto group in presence of
an aldehyde and an ester group.
The reason for the selective reduction of keto group in presence of
an aldehyde is that the CHO group undergo preferential
acetialisation with CeCl3 and trimethylorthoformate.
O
CHO
COOCH3
NaBH4.CeCl3-EtOH
HC(OCH3)3
H3O+
CHO
COOCH3
OH
H

10. • NaBH4 EtOH-H2O
Selective reduction of keto group in presence of an ester can be
accomplished by using NaBH4 in ethanol and H20 .
Cl COOEt
O
NaBH4
EtOH, H2O
Cl COOEt
OH

11. • BH3 .THF or BH3 .SMe2
The facile reduction of the –COOH group by BH3 .THF or BH3
.SMe2 has been employed for chemoselective reductions of
carboxylic group in presence of ester or lactone .
OO
COOH
CH3
BH3.SMe2
H2O
OO
CH2OH
CH3
HOOC
COOEt
BH3.THF
H2O
COOEt
HO

12. • DIBAL-H, n-BuLi
Treatment of DIBAL-H with one equivalent of n-BuLi generate
a reductant that selectively reduces esters in the presence of
amides or nitriles.
NC COOCH3
O
H+
,H2O
DIBAL-H,n-BuLi NC
O
OH
Et3N COOCH3
O
DIBAL-H,n-BuLi Et3N
O
H+
,H2O
OH

13. Regioselectivity

14. What is regioselectivity?
 Regioselectivity is the preference for one orientation
over another in the arrangement of a reaction product.
 A regioselective reaction is one in which one direction
of bond making or breaking occurs preferentially over
all other possible directions.

15. Regioselective reducing agent
• NaBH4 .CeCl3
• LiAlH(OR)3
• K[BH(s-Bu)3]

16. Reduction of α,β-unsaturated aldehyde and
ketones
Hydride reductions of α,β-unsaturated aldehyde and ketones may
proceed via 1,2 or 1,4-additons to furnish the corresponding allylic
alcohols or saturated aldehyde and ketones respectively.
O
O-
H
1,2-addition
H-
H2O
HO H
O O-
1,4-addition
H-
H2O
O

17. • NaBH4.CeCl3 (Luche reagent)
This reagent favours 1,2 additions.
OHO
NaBH4.CeCl3
MeOH

18. • LiAlH(OMe)3
This reagent favours both 1,2 and 1,4 additions.
O
H
OH
LiAlH(OMe)3,THF
O
LiAlH(OMe)3,THF
O

19. K[BH(s-Bu)3]
• This reagent is termed as K-Selectride.
• This gives both the 1,4 or 1,2 addition.
• This reduces β-unsubstituted cyclohexenones to
cyclohexanones(1,4-addition) and β-substituted
cyclohexenones to the corresponding allylic alcohols(1,2-
additions)
O
HO
H
R
K[BH(s-Bu)3]
R=CH3
R
K[BH(s-Bu)3]
R=H
R
O

20. Sterioselectivity

21. What is Sterioselectivity?
Sterioselectivity is the property of a chemical reaction in
which a single reactant forms an unequal mixture of
sterioisomers.
Sterioselective process in which one sterioisomer is
predominates over another when two or more may be
formed-
•If the product are enantiomers, the reaction is
enantioselective;
•If they are diasteriomers , the reaction is
diasterioselective;

22. Enantioselective reductions-
Reduction which involves the formation of enantiomer of
chiral molecule are termed as enantioselective reduction.
Ph Me
O
LiAlH4
Ph Me
H OH
+
Ph Me
H
OH
(R)(S)

23. Diasterioselective reductions-
Reductions which involves the fomation of diasteriomers are
termed as diasterioselective reductions.
O
H
Me H-
1
H
HO
Me2
1R,2S
diasterioisomers
1
OH
H
Me2
1S,2S
+

24. Sterioselective reducing agent-
• BINAL-H
• Baker’s yeast
• Alpine borane

25. BINAL-H-
BINAL-H is prepared by adding binapthol to a solution of LiAlH4
in THF followed by one equivalent of either methanol or ethanol.
It reduces acetylenic ketones, aromatic ketones, & α,β unsaturated
ketones with high optical purities.
R
n-C5H11
O
(R)-BINAL-H
THF
R
n-C5H11
HO
H
R
n-C5H11
O
(S)-BINAL-H
THF
R
n-C5H11
OHH

27. baker’s yeast-
This reagent involves the enzymatic reductions of keto grouops.
Enzymes provide an alternative to methods for the enantioselective
reduction of prochiral ketones.
COOEt
O
baker's yeast
H2O, sucrose COOEt
OH
S
H2O, sucrose
O
(S)
baker's yeast HO O
O
(S)

28. Alpine borane-
Alpine borane reduces aldehydes, α-keto esters and acetylenic
ketones with excellent enantioselectivity.
Ph D
O
(R)-Alpine-Borane
OH
H Ph
D
O
(R)-Alpine-Borane
NaOH-H2O2
H
HO H

29. References:
1. George S. Zweifel, Michael Nantz, and Peter Somfai. 2007. Modern
Organic Synthesis.
2. Jonathan Clayden, Stuart Warren, Nick Greeves. 2012. Organic
Chemistry.
3. S.Jagdamba,Yadav.L.D.S.2015. Organic Synthesis.
4. http://en.wikipedia.org/wiki/Reducing agent.

30. THANK
YOU…