Pressure Effect of Manganese(III)-Based Oxidative Free-Radical Substitution of Aromatic Compounds
Department of Materials and Life Sciences, Graduate School of Science and Technology, and Department of Chemistry, Faculty of Science
Md. Taifur Rahman and Hiroshi Nishino*
Oxidation of aromatic hydrocarbons mediated by trivalent manganese complexes has been studied extensively in the last 60’s1 and 80’s2 and the entire investigation was carried out only under the atmospheric pressure. The objective of the present study is to investigate the effect of the pressure increase on the product yield percentage of the reactions between 2-methoxynaphthalene (1a) or 1- methoxynaphthalene (1b) with trivalent manganese complexes e.g., manganese(III) acetate, [Mn(OAc)3]and tris(2,4-pentanedionato)manganese (III), [Mn(acac)3] (Scheme 1).3
OCH3
OCH3
OCH3
O Me O
Me H
OCH3
Me O
Me O H
OCH3 Me
O OAc
Me O
Mn(acac)3 AcOH , Reflux
Mn(acac)3
1a
1b
AcOH , Reflux
+
2a
3b 2b
Scheme 1
Both these reactions involve electron donor-acceptor like complexes (EDA complexes). Electron donor-acceptor like complex formation occurs (i) between the Mn(III) complex and the solvent and (ii) between the Mn(III) complex and the aromatic π-electron system, and these complex formations are very important factors for the oxidation of the aromatic compounds (Scheme 2).4-6 In this study, we
established the hypothesis of the formation of electron donor-acceptor (EDA) like complex, and
investigated the role of high pressure on the facile formation of EDA like complex between the aromatic
compounds and Mn(III) complxes. In addition, we also studied on the electrostatic interaction between the aromatic ring and Mn(III) complexes under high pressure.
H O
R'' O
O R'
C R'
O R''
R
R R
O O R'
R''
R
EDA like complex High Pressure
O R"
O R' Mn(III)
Mn(III) Mn(II)
-H+ Mn(III) Scheme 2
Here we show one example of the reaction of 2-methoxynaphthalene (1a) with Mn(acac)3 under argon pressure (1-130 atm) and the result of the pressure effect on the yield (%) (Fig. 1).
Fig.1. A change in the yield(%) of the reaction between 2-methoxynapthalene with tris (2,4-pentadionato)
manganese(III) along with the pressure change
0 10 20 30 40 50 60
1 5 10 50 80 100110130 Pressure(atm) Yield(%) of 1-Diacetalmethyl-2- methoxynaphthalene
Mol. Ratio 1:4 Mol. Ratio 1:6 Mol. Ratio 1:8 Mol. Ratio 1:10
OCH3 OCH3
Me O
Me O H
Mn(acac)3
1a
AcOH , Reflux
2a
We show another example of the reaction of 1-methoxynaphthalene (1b) with Mn(acac)3 under argon pressure (1-130 atm) and the result of the pressure effect on the total yield (%) (Fig. 2).
OCH3 OCH3
O Me O
Me
Mn(acac)3 AcOH , Reflux
1b 2b: R = H
3b: R = OAc
R
Fig.4. A Change in the Total Yield (%) of the Reaction between 1-Methoxynaphthalene (1b) with Tris (2,4-
pentanedionato)manganese(III) along with the Pressure Change
0 5 10 15 20 25 30 35 40 45 50
1 5 20 50 70 90 100120130 Pressure (atm)
Total Yield (%) of 4-Diacetylmethylated 1- Methoxynaphthalene
Mol. Ratio 1:4 Mol. Ratio 1:6 Mol. Ratio 1:8
It was found that thepressure exerted very important effects on the formation of EDA like complex by the following possible ways:
1. High pressure could compensate for the loss of entropy associated with the formation of EDA like complex .
2. The partial molar volume of EDA like complex is believed to be smaller than the added partial molar volume of the substrates. So from the general law of thermodynamics it could be assumed that the equillibrium (Step 1) would be favorably shifted towards the right direction, i.e., formation of EDA like complex, as the pressure increased (Scheme 3).
Aromatic ring + Mn(III) complex EDA Complex
Educt Radical Products Step 1
EDA Complex Step 2
Scheme 3
3. High pressure could make the approach of the Mn(III) complex to the aromatic molecule facile and allows more electrostatic interaction between the d-orbital of the Mn(III) complex and the aromatic electronic system (Scheme 4).
Mn O
O O
O H3C CH3
CH3
CH3 O
H3C O
H3C III
OCH3
Mn O
O O
O H3C CH3
CH3 O
H3C O
H3C H3C
OCH3
Donor Acceptor
(I = 7.82 eV)
Pressure
Pressure Pressure
Pressure Pressure
Pressure High pressure
One-electron transfer oxidation EDA complex
formation Scheme 4
We have concluded based on our experimental results as follows:
1. The product yield (total yield %) usually increased along with the pressure.
2. Increasing trend in the products yield (%) remained more or less consistent over the wide range of molar ratios of the substrates, i.e., aromatic compounds : Mn(III) complexes, along with the pressure increase.
3. This increasing trend in the product yield (%) resulting from the pressure gradient provided some support for the hypothesis of facile EDA like complex formation between the aromatic ring and manganese(III) complex under higher pressures.
References
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