in organic reaction

1. Role of Proton
Feroze Hussain (8828)
Presented by
DEPARTMENT OF CHEMISTRY
CENTRAL UNIVERSITY OF HARYANA
1
M.Sc. 3rd
semester

2. Contents• Introduction
• Role of proton
• Reactions
• Mechanism for the role of H2SO4
• Mechanism for isotopic labelling
• Advantages and disadvantages
• Conclusions
• Acknowledgement
• References
2

3. Per acids used are: inert solvent
Perbenzoic acid (C6H5COOOH) CH2Cl2
m -Chloroperbenzoic acid (m-CPBA) CHCl3
peroxytriflouroacetic acid (CF3COOOH etc CCl4
Epoxidation of olefins
Olefins can be converted into epoxides on treating with per acids/per oxides in
inert solvents, is known as epoxidation of olefins as shown in scheme 1.
3

4. Epoxidation of olefins
+
CCl4
+ C6H5COOH
Cl COOOH
Cyclohexene 3-Chlorobenzoperoxoic
acid
O
Benzoic acid7-Oxabicyclo
[4.1.0]hexane
CH3H3C
H3C H
+
CH3
H3C
H3C
H
O C6H5COOH+
CCl4
Benzoic acid2,2,3-Trimethyloxirane
C6H5COOOH
Benzoperoxoic
acid
2-Methylbut-
2-ene
4

5. • In presence of Bronsted acid (Eg; H2SO4 ) - highly efficient catalyst
• In absence of Bronsted acid (Eg; H2SO4 ) - doesn’t work
Scheme 2
Proton-promoted and anion-enhanced epoxidation of olefins
5
MnII
(Dbp-MCP)(OTf)2
Condition Non heme Mn catalyst

6. • yields and enantioselectivities are highly dependent on Mn catalyst and the Bronsted acid
and are highly increased in the presence of H2SO4 .
• yields and enantioselectivities are high and similar irrespective of the oxidants in the presence
of H2SO4, suggesting that a common epoxidizing intermediateis generated in the reactions of
[MnII
(Dbp-MCP)]2+
and other oxidants such as peracids, alkyl hydroperoxides or
iodosylbenzene etc.
Epoxidation of olefins by H2O2 in the presence of Mn catalyst
6
Observations

7. H2SO4 plays an important role to facilitate:
• the formation of a high-valent Mn-oxo species and to increase the
oxidizing power
• enantioselectivity of the Mn-oxo oxidant in olefin epoxidation
reaction.
• Density functional theory (DFT) calculations support experimental
results such as the formation of a Mn(V)-oxo species as an epoxidizing
intermediate.
Role of H2SO4
7

8. Further it has been observed that protons play important roles not only in the formation
of metal-oxo species via the O−O bond cleavage of metal-hydroperoxo species, but also in
the increase of the reactivity of the metal-oxo species in the olefin epoxidation reactions
by non-heme metal catalysts and H2O2 as shown in Scheme(3).
Scheme (3). Effects of Proton on :-
(A) the Hydroperoxide O−O Bond Cleavage
(B) the Reactivity of Metal-Oxo Species
Role of Proton
8

9. Table : 1 Catalytic Epoxidation of Olefins by MnII
(Dbp-MCP)(OTf)2
and H2O2 in the Presence of H2SO4 or under Other Reaction
Reactions with different Substrates and reaction conditions
9

10. Asymmetric epoxidation
First performed by Prof . Herbert
Optical activity (ee) was less
In 1980,Prof . Karl Barry Sharpless performed the
same with high optical activity (ee)
Very commonly used in industries and research
Ring forming reaction
Substrate for the asymmetric epoxidation must be
allylic alcohol
Enantioselective synthesis

11. Asymmetric epoxidation
Sharpless asymmetric epoxidations : An enantioselective chemical reaction to prepare
2,3-epoxyalcohols from primary and secondary allylic alcohols as shown in scheme (5)
R2
R1
CH2
R3
HO
O
HO
HO
O
2) Cat. Ti(tOPr)
1)+DET
1)-DET
4) TBHP
3)
HO
O
At -20o
C
11
Scheme (5) : Showing Sharpless
asymmetric epoxidations

12. Asymmetric Epoxidation by MnII
(Dbp-MCP)(OTf)2
As MnII
(Dbp-MCP)(OTf)2 is chiral catalyst , it has been examined in the asymmetric
epoxidation of olefins by H2O2 in the presence of H2SO4 , the epoxidation of chalcone
derivatives by MnII
(Dbp-MCP)(OTf)2 and H2O2 in the presence of H2SO4 afforded high yields of
epoxide products with excellent enantioselectivities (>97% enantiomeric excess (ee) values as
shown in scheme (6)) .
Scheme (6) 12

13. Table(2)
Asymmetric Epoxidation by MnII
(Dbp-MCP)(OTf)2
13
Derivative yield ee
Chalcone high high
Chromene Low to moderate high
Cinnamate Low to moderate high
Styrene moderate moderate
Observation

14. Asymmetric Epoxidation of Chalcone by Various Oxidants Catalyzed by MnII
(Dbp-MCP)(OTf)2
in the absence and presence of H2SO4 has been studied and observed yields as well as
is increased effectively in the presence of H2SO4 as shown in table (3)
entry oxidant H2SO4 yield (%) ee (%)
1 m-CPBA no 38 37
yes 87 95
2 t-BuOOH no 18 45
yes 56 97
3 Cumyl-OOH no 8 30
yes 33 96
4 PhIO no 25 68
yes 78 97
Effect of H2SO4 on yields and ee with different oxidants
Table (3)
14

15. Mechanism for the role of H2SO4
(A) Mechanism for the Formation of Metal-Oxo Intermediates in the Reaction of
a non-heme Metal(II) Complex and H2O2
(B) Heterolytic and Homolytic O−O Bond cleavage mechanisms in the Presence and Absence
of H2SO4, Respectively, in the reaction of MnII
(Dbp-MCP)(OTf)2 and ROOH has been
proposed as shown in scheme (7)
scheme (7)
15

16. Mechanism for isotopic labelling to study the formation of
High-Valent Mn-Oxo Species as intermediate
(A)Proposed Mechanism for the Use of 18
O-
Labeled Water Experiment to probe the
Intermediacy of a High-Valent Mn-Oxo
Species in the Catalytic Epoxidation of
Olefins by MnII
(Dbp-MCP)(OTf)2 and
H2O2.
(B) Reaction Scheme Showing the Source of
Oxygen Found in Epoxide Product as
shown in Scheme (8).
Scheme 8
16

17.  As current interest in chemistry is to have efficient and sustainable work , it is
highly efficient method for the epoxidation of Olefins and also involves the
use of environment friendly oxidant H2O2 .
 Catalytic epoxidation :-
• gives high yields and ee.
• have mild reaction conditions.
• is catalysed by earth abundant metals.
Advantages
17

18. Disadvantages
• It has not been explained why catalytic epoxidation with HCl does not takes place
as it is also a Bronsted acid .
• It has been observed that the yield as well as ee is high due to presence of SO4
2-
anion,
but why it is so, has not been explained .
18

19. Conclusions
• It is very nice reaction with significant yields and ee.
• Protons plays an important role for cleavage of peroxide bond as well as for
activating Metal-Oxo complex formed as intermediate .
• This reaction is not possible in the absence of Bronsted acid .
• This reaction also need MnII
(Dbp-MCP)(OTf)2 , which is compulsory .
• Anions of Bronsted acids also plays important role depending upon anions .
19
• For chalcone and its derivatives it observed as it is dependent on oxidant
too.

20. • For simple olefins yields and ee are independent on the nature of oxidant
in the presence of H2SO4 and is found almost similar indicating the
formation of high valent metal oxo complex as intermediate .
• An unexpected novel effect of sulfuric acid in the catalytic (asymmetric)
epoxidation of olefins by aqueous catalyzed by a non-heme manganese
complex is seen.
• It is really an environmental sound reaction as it gives high yields and ee
and is catalysed by using earth-abundant transition metals (e.g., Fe and
Mn) and environmentally sound oxidants (e.g., H2O2)
Conclusions
20

21. 21
• Prof. A.J. Varma ( Dean, School of Chemical Science, CUH)
• Dr. Azaj Ansari (Astt. Prof.)
• Ravan Kumar (M.Sc. 2nd
year)
• Shashikant Tiwari
ACKNOWLEDGEMENT

22. References
Miao, C.; Wang, B.; Wang, Y.; Xia, C.; Lee, Y.-M.; Nam, W. J.
Am. Chem. Soc. 2016, 138, 936-943
22