Stereo-preference in the degradation of the erythro and threo isomers of b-O-4-type lignin model compounds in oxidation processes Ⅲ: in the reaction with chlorine- and manganese-based oxidants

1. Stereo-preference in the degradation of the erythro and
threo isomers of b-O-4-type lignin model compounds in
oxidation processes Ⅲ: in the reaction with chlorine- and
manganese-based oxidants
P. Posoknistakul et al, J. Wood Sci. (2018).
https://doi.org/10.1007/s10086-018-1714-z
Seminar 2018/05/25 Isamu Katsuyama
1

2. Main points of this paper
1. Stereochemistry of b-O-4-type compounds
The erythro and threo isomers
2. Chemical properties of some oxidants
NaClO, NaClO2, MnO2, KMnO4
3. Degradation of each isomers in the presence of
the oxidants
Question: Is the reactivity same or different?
2

3. 3
1. Stereochemistry; erythro and threo isomers
A
B
Erythro isomer

4. 4
A
B
Threo isomer

5. 2. Chemical properties of some oxidants
NaClO
 A strong oxidant typically for oxidation of alcohols
 It is soluble in water.
5
NaClO H2O
ClOHClO H
HClO NaOH++
+ (pka = 7.58)
ClO
ClO
ClO
pH = pka,
[ClO
-
] / [HClO] = 1
pH > pka,
[ClO
-
] / [HClO] > 1
pH < pka,
[ClO-
] / [HClO] < 1
pH in solution
13.3
6.3
1.0
HClO < <
HClO >
HClO >>

6. NaClO2
 A oxidant typically for oxidation of aldehydes, alcohols, etc.
 It is soluble in water.
6
NaClO2
ClO2
H2O HClO2 NaOH
HClO2 H
++
+ (pka = 1.94)
ClO2
ClO2
ClO2
HClO2 < <
HClO2 <
HClO2 >
pH in solution
13.3
6.3
1.0
pH = pka,
[ClO2
-
] / [HClO2] = 1
pH > pka,
[ClO2
-
] / [HClO2] > 1
pH < pka,
[ClO2
-
] / [HClO2] < 1

7. 7
MnO2
• A mild oxidant typically for oxidation of allylic and benzylic
alcohols, etc.
→ It is very useful for selective oxidation.
• It is not soluble in organic solvent and water.

8. 8
KMnO4
• A powerful oxidant typically for oxidation of alcohols,
aldehydes, alkenes, etc.
• It is soluble in MeOH and water.
• It is not soluble in most organic solvent.

9. 3. Degradation of isomers in the presence of the oxidants
 Example of previous work
9
CAN or Lignin peroxidase
Fenton’s reagent No stereo-preference
Stereo-preference in degradation
of compound 1T
C. Bohlin et al, J. Mol Catal B Enzym, 45, 21 (2007).
C. Bohlin et al, ibid., 35, 100 (2005).

10. 10
This work
Oxidants = NaClO, NaClO2, MnO2, KMnO4
Is stereo-preference in degradation of 1E and 1T observed?

11. ➀ Oxidation in hypochlorite systems (NaClO)
 (a) alkaline pH in solution (ClO-)
Low reactivity
No clear difference
 (b) neutral pH in solution (HClO)
No clear difference
← Main reactive site may be aromatic
ring; side chain was not affected.
11
pH 13.3
at 70 ℃
pH 6.3
at 70 ℃

12. ② Oxidation in chlorite systems (NaClO2)
 (a) alkaline pH in solution (ClO2
-)
Low reactivity
No clear difference
 (b) neutral pH in solution
(ClO2
. generated from ClO2
-)
No clear difference
←Main reactive site may be aromatic
ring; side chain was not affected.
 (c) acidic pH in solution (HClO2)
Degradation of compound 1T was
slightly greater.
12
pH 13.3
at 70 ℃
pH 6.3
at 70 ℃
pH 1.0
at 70 ℃

13. 13
Reaction of alkenes with HClO, ClO2
.

14. ③ Oxidation in a manganese dioxide system
(MnO2)
 acidic pH in solution
Degradation of compound 1T
was clearly more rapid than that
of compound 1E.
→ It is presumed that MnO2
aggregates can approach 1T
more easily than 1E.
14
pH 1.0
at room temp.

15. ④ Oxidation in permanganate systems (KMnO4)
 (a) acidic pH in solution
Degradation of compound 1T
was clearly grater than that of
compound 1E.
→Large size of MnO4
- may be
related to the T preference.
 (b) neutral pH in solution
Degradation of 1T was also grater
than that of 1E. But, reactivity
was not great.
15
pH 2.0
at room
temp.
pH 6.3
at room
temp.

16. Conclusions
Oxidation in hypochlorite systems (NaClO) and chlorite
systems (NaClO2)
No clear stereo-preference was observed.
Oxidation in a manganese dioxide system (MnO2) and
permanganate systems (KMnO4)
A clear 1T stereo-preference was observed.
16