Substituent-induced regioselective hydroxylation of the aromatic C-H bond on naphthalene with metachloroperbenzoic acid catalyzed by F20TPPMnCl

Article abstract of DOI:10.1002/poc.3012

The regioselective hydroxylation of the aromatic C-H bond on a series of naphthalenes with different β-substituent R (R = H, Me, Et, i-Pr, OMe, COOH, Br, etc.) was studied, and the substituent effect on the regioselectivity was investigated. The electron-donating substituent afforded the aromatic C-H bond hydroxylation at the 1α position with more than 80% selectivity, while the electron-withdrawing substituent afforded the aromatic C-H bond hydroxylation at the 4α position with more than 60% selectivity of β-substituted naphthalene hydroxylated with metachloroperbenzoic acid catalyzed by tetrakis(pentafluorophenyl)porphyrin manganese(III) chloride. The research showed that the steric and electronic effects of the substituent appeared to play a significant role in determining the regioselectivity, and the electronic effect was of more importance than the steric effect of the substituent in the current situation. The studies may provide additional proofs for the stepwise mechanism of the aromatic C-H bond hydroxylation through a cationic intermediate. Copyright

Full text of DOI:10.1002/poc.3012

Research Article
Received: 18 April 2012,
Revised: 1 June 2012,
Accepted: 8 July 2012,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.3012
Substituent-induced regioselective
hydroxylation of the aromatic C–H bond on
naphthalene with metachloroperbenzoic acid
catalyzed by F TPPMnCl
2
0
a
a,b
a
a
Chang-Di Chen , Wen-Bing Sheng , Guo-Jun Shi and Can-Cheng Guo *
The regioselective hydroxylation of the aromatic C–H bond on a series of naphthalenes with different b-substituent
R (R = H, Me, Et, i-Pr, OMe, COOH, Br, etc.) was studied, and the substituent effect on the regioselectivity was
investigated. The electron-donating substituent afforded the aromatic C–H bond hydroxylation at the 1a position
with more than 80% selectivity, while the electron-withdrawing substituent afforded the aromatic C–H bond
hydroxylation at the 4a position with more than 60% selectivity of b-substituted naphthalene hydroxylated with
metachloroperbenzoic acid catalyzed by tetrakis(pentafluorophenyl)porphyrin manganese(III) chloride. The research
showed that the steric and electronic effects of the substituent appeared to play a significant role in determining
the regioselectivity, and the electronic effect was of more importance than the steric effect of the substituent in
the current situation. The studies may provide additional proofs for the stepwise mechanism of the aromatic C–H
bond hydroxylation through a cationic intermediate. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: regioselective hydroxylation; aromatic C–H bond; naphthalene; substituent
naphthalene increased to ~10:1 using manganese b-polynitro-
porphyrin as catalyst and ammonium mandelate as co-catalyst.
INTRODUCTION
The selective hydroxylation of the aromatic C–H bond of
naphthalene and its derivatives, which is of enormous impor-
tance for chemical synthesis and chemical degradation in
The origin cause of the regioselectivity in this case has not been
clarified yet, but it was believed that ammonium mandelate
might facilitate the heterolytic scission of the O–O bond of
[
1–5]
III
V
organic chemistry and drug metabolism in organisms,
has
Mn OOH species to generate Mn oxo species through “push–
[
25]
been a desirable but challenging task in the past few decades.
Although many efforts have been made to achieve the goal
pull” mechanism,
which resulted in decreasing the extent of
the radical character of the Mn-oxo moiety in metalloporphyrin
intermediate, and it might help increase the selectivity as a
[5–8]
effectively,
the technology and the intrinsic cause of the
[
12,13]
regioselectivity of the hydroxylation of the aromatic C–H bond
result. Safari et al.
showed that the aromatic C–H bonds
on naphthalene and its derivatives have not been improved very
of b-methyl-naphthalene and b-naphthaldehyde could be
hydroxylated at the 4a and 3b position with their ratio of
~3:1 and ~10:1, respectively, with metachloroperbenzoic acid
(m-CPBA) using iron(III) porphyrin as catalyst in aprotic solvent.
The preference of the 4a position rather than the 1a position
might be ascribed to the bulky interaction between Fe(III)OOAr
intermediate with the substituent at the b position.
As additional dimension, despite the high regioselectivity of
the aromatic C–H bond hydroxylation of naphthalene, it is not
regiospecific. And the regioselectivity of the aromatic C–H bond
of substituted naphthalene, which is more complicated, has not
been fully studied. What are the reasons for the generally high
[9,10]
well until now.
Recently, biologically inspired metallopor-
phyrin catalyzed hydroxylation of the aromatic C–H bond of
naphthalene and its derivatives has received considerable
[5,6]
interest
because of the advantages of mild condition and
efficiency of the catalytic oxidation.
The difficulty of the regioselective hydroxylation of the
aromatic C–H bond on naphthalene and its derivatives lies in
the fact that they have aromatic C–H bonds at a and b positions,
and there are many other factors such as substrate structures,
[
11,12]
porphyrin structures, central metals of metalloporphyrins,
and experimental conditions that can influence the regioselec-
[12,13]
tivity.
In addition, it is often impacted by the problem of
the free-radical chain reaction usually associated with metallo-
[14–19]
[20]
* Correspondence to: C.-C. Guo, College of Chemistry and Chemical Engineering,
Hunan University, Changsha, 410082, China.
E-mail: ccguo@hnu.edu.cn
porphyrin.
Chauhan et al.
demonstrated that iron(III)
porphyrin can catalyze the hydroxylation of the aromatic C–H
bond on naphthalene at the a and b position with their ratio
of ~2.5:1 using H
to control because H
which will decrease the selectivity through a Russell-type
2
O
2
as oxidant. However, the selectivity is hard
a C.-D. Chen, W.-B. Sheng, G.-J. Shi, C.-C. Guo
College of Chemistry and Chemical Engineering, Hunan University, Changsha,
2 2
O
decomposes easily to hydroxyl radical,
410082, China
[
21–23]
[24]
mechanism.
Mansuy et al.
showed that the ratio of the
b W.-B. Sheng
aromatic C–H bond hydroxylation at the a and b position of
Hunan University of Chinese Medicine, Changsha, 410082, China
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

C.-D. CHEN ET AL.
but incomplete regiospecificity? And how does it operate to
adjust the aromatic C–H bond hydroxylation?
added. In the typical procedure, a solution of 0.3 mmol m-CPBA in 1 mL
CH
of 0.3 mmol cyclohexene, 0.3 mmol b-substituted naphthalene, and
.3 mmol F20TPPMnCl in 1 mL CH CN:CH Cl (1:1) solvent at ambient
3 2 2
CN:CH Cl (1:1) solvent was added dropwise to a solution consisting
In order to gain insight into the factors that determine the
regioselectivity of the aromatic C–H bond hydroxylation on
naphthalene derivatives, we studied the hydroxylation of the
aromatic C–H bond on b-substituted naphthalenes catalyzed
by tetrakis(pentafluorophenyl)porphyrin manganese(III) chloride
0
3
2
2
temperature. After 1 h, the concentration of cyclohexene was analyzed
by GC–MS SIM program. In the blank experiment, the procedure was
the same as the procedure described previously without the adding of
b-substituted naphthalene.
3 2 2
(F20TPPMnCl) with m-CPBA in CH CN/CH Cl (1:1) solvent and
further explored the substituent effect on the regioselectivity.
The studies may provide additional proofs for the stepwise
mechanism through a cationic intermediate of the hydroxylation
of the aromatic C–H bond of naphthalene and its derivatives.
Calculation method
Full geometry optimizations for reactants, products, and acylperoxo-
manganese(III) porphyrin intermediate were performed at semi-empirical
PM3 level implemented in the software package Hyperchem 7.0
(
Hypercube Inc., USA). The ground spin states of F20TPPMnCl and its
EXPERIMENTAL
intermediate were at S = 1 state where the bonding energy and total
energy were the lowest. Frontier molecular orbital energies E(HOMO),
E(LUMO), total energy, and ΔH were calculated after the geometry optimi-
zations performed by PM3 method and identified by vibration analysis. All
the calculations were performed on a Gateway NV48 PC (2GHz, 2 GB).
Materials and instruments
The tetrakis(pentafluorophenyl)porphyrin manganese(III) chloride was
prepared by the established synthesis procedures.
[26,27]
m-CPBA was
purchased from Aladdin (Pudong New Area, Shanghai, China) and further
purified through washing with phosphate buffer (pH = 7.4). Its purity was
analyzed using iodometric method. Other reagents were purchased from
Aladdin and used without further purification except that acetonitrile
and dichloromethane were distilled from sodium before used. The gas
chromatography–mass spectrometry (GC–MS) qualitative and quantitative
analysis was performed on a Class-5000 GC–MS spectrometer (Shimadzu
RESULTS AND DISCUSSION
The regioselective hydroxylation of b-substituted
naphthalene
In the studies of the hydroxylation of b-substituted naphthalenes
with m-CPBA and without any catalyst in CH CN:CH Cl (1:1)
3 2 2
(China) Co., Ltd., West Huaihai Road, Shanghai, China). The UV–Vis spectrum
was recorded on a PE L-17 spectrometer (PerkinElmer (China), Inc., Chaoyang
District, Beijing, China). All the melting points were measured using an
Electrothermal 9100 Melting Point apparatus (Tianjin Reliant Instrument
Co., Ltd., Hexi District, Tianjin, China).
solvent, the conversion of b-substituent naphthalenes was less
than 5% even after stirring the reaction mixture for 24 h at
ambient temperature. However, with the use of F20TPPMnCl as
catalyst, the highest conversion of b-substituent naphthalenes
reached 86.20% (Table 1), which indicated that F20TPPMnCl
played an important role in the hydroxylation of b-substituent
naphthalenes.
Typical procedure of b-substituted naphthalene hydroxylation
A solution of 0.36 mmol m-CPBA in 1 mL CH
added dropwise to a solution consisting of 0.3 mmol b-substituted
naphthalene and 0.3 mmol F20TPPMnCl in 1 mL CH CN:CH Cl (1:1)
3 2 2
CN:CH Cl (1:1) solvent was
From Table 1, the oxidation system with F TPPMnCl as
catalyst, m-CPBA as oxidant, and CH CN:CH Cl (1:1) as aprotic
3 2 2
20
3
2
2
solvent at ambient temperature. After 1 h, the reaction mixture was
analyzed by GC–MS SIM program using nitrobenzene as internal standard.
In the blank experiment, the procedure was the same as the procedure
described previously without the adding of the catalyst of F20TPPMnCl.
The products were isolated by column chromatography and identified by
GC–MS spectra and confirmed with their authentic samples firstly. Then,
the position of hydroxyl groups was distinguished using their melting points
according to the adequate differences of the melting points of all kinds of
solvent could transform a variety of b-substituted naphthalenes
to b-substituted naphthols with high regioselectivity. And it
was shown that the hydroxylation of the aromatic C–H bond
on b-substituted naphthalene preferred at the substituted
aromatic ring (2 + 4) with more than 80% selectivity rather than
the non-substituted one of the naphthalene ring even when
the substituent was electron-withdrawing. Further, the hydroxyl-
ation preferred at the 1a position (2) with more than 80%
selectivity with the electron-donating substituent and at the 4a
position (4) with more than 60% selectivity with the electron-
withdrawing substituent in the substituted aromatic ring of
b-substituted naphthalene. In addition, the quinonic products
were in negligible yields detected by GC–MS spectra, so the reac-
tion could be served as a model for studying the regioselective
hydroxylation of the aromatic C–H bond on naphthalene ring.
[28]
naphthols, and the results were compared with literature reports
:
ꢀ
ꢀ
+
1
2
2
2
2
-naphthol: mp ( C): 94.5 (lit.; 94–96), GC/MS (m/z) 144(M );
+
-naphthol: mp ( C): 121 (lit.; 120–122), GC/MS (m/z) 144(M );
-methyl-1-naphthol: mp ( C): 62.5 (lit.; 64–66), GC/MS (m/z) 158(M );
-ethyl-1-naphthol: mp ( C): 68.5 (lit.; 68–70), GC/MS (m/z) 172(M );
-isopropyl-1-naphthol: mp ( C): 47.5 (lit.; 45–50), GC/MS (m/z) 186
M ); 2-methoxy-1-naphthol: mp ( C): 58 (lit.; 56–61), GC/MS (m/z) 174
M ); 2-bromo-4-naphthol: mp ( C): 61 (lit.; 60–63), GC/MS (m/z) 222(M );
-bromo-1-naphthol: mp ( C): 45 (lit.; 44–46), GC/MS (m/z) 222(M );
-hydroxy-2-naphthoic acid: mp ( C): 221 (lit.; 220–225), GC/MS (m/z)
ꢀ
+
ꢀ
+
ꢀ
+
ꢀ
(
(
+
ꢀ
+
ꢀ
+
2
4
1
7
7
7
ꢀ
+
Bulky interaction of substituent with phenyl groups on
metalloporphyrin
88(M ).
ꢀ
+
-methyl-1-naphthol: mp ( C): 83.5 (lit.; 80–84), GC/MS (m/z) 158(M );
-ethyl-1-naphthol: mp ( C): 53 (lit.; 53–57), GC/MS (m/z) 172(M );
-isopropyl-1-naphthol: mp ( C): 83.5 (lit.; 83–84), GC/MS (m/z) 186
M ); 7-methoxy-1-naphthol: mp ( C): 106 (lit.; 105–111), GC/MS (m/z) 174(M ).
ꢀ
+
To explain the phenomenon that the aromatic C–H bond
hydroxylation preferred at the substituted aromatic ring
ꢀ
ꢀ
+
+
(
(2 + 4), we first studied the metalloporphyrin intermediate using
b-methyl-naphthalene as model substrate (Fig. 1).
Competitive kinetic method
From Fig. 1, the lmax band of the reaction mixture decreased
from 470 to 410 nm when m-CPBA was added to the
solution of F20TPPMnCl and then changed back to 470 nm when
b-methyl-naphthalene was added further. Because the lmax
The relative rate of b-substituted naphthalene was studied using
cyclohexene as reference substrate and determined by the decreasing
of cyclohexene concentration when b-substituted naphthalene was
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HYDROXYLATION OF THE AROMATIC C–H BOND
a
Table 1. The hydroxylation of b-substituted naphthalenes
b
b
Substrate
Conv. (%)
Yield (%)
Selectivity (%)
2
3
4
2
3
4
c
R = H
R = Me
R = Et
R = i-Pr
R = OMe
R = Br
54.50
86.20
70.70
60.80
76.00
29.30
11.50
44.00
65.50
53.70
45.60
60.80
3.20
93.00
93.30
92.43
91.94
92.82
15.46
33.68
—
—
4.70
4.40
4.00
4.70
4.50
Trace
Trace
Trace
Trace
Trace
13.00
6.30
6.70
7.57
8.06
7.18
21.74
Trace
Trace
Trace
Trace
Trace
62.80
66.32
R = COOH
3.20
a
ꢁ3
ꢀ
Substrate:m-CPBA:F TPPMnCl = 1:1.2:10 , 27 C, 1 h. 0.3 mmol substrate in 2 mL CH CN/CH Cl (1:1) solvent.
It is based on substrate.
20
3
2
2
b
c
It contained 3.10% 2-naphthol in 44.00% of all of the naphthols obtained.
Figure 1. Real-time UV–Vis spectra of the reaction mixture during hydroxylation of b-methyl-naphthalene with m-CPBA in CH
3
CN:CH
2
Cl
2
(1:1) solvent
band of 470 nm and that of 410 nm is the characteristic absorption
and (2), and the contribution of the electronic and the steric
effect of the substituent on the hydroxylation could be calcu-
[
29]
band of F TPPMnCl and acylperoxo-manganese(III) porphyrin,
2
0
[
32]
respectively, it was suggested that the metalloporphyrin inter-
mediate was acylperoxo-manganese(III) porphyrin in the current
situation.
lated using Eqn (3)
:
ꢀ
ꢁ
ꢂ
2
lgkrel ¼ 0:43Es ꢁ 1:25s þ 0:13 R ¼ 0:96
lgkrel ¼ 2:421EðHOMOÞ ꢁ 0:002Volume
(1)
(2)
Further, we studied the quantitative structure–activity relation-
ship of the b-substituted naphthalene hydroxylation (Table 2).
To gain insight into the substituted effect on the relative rate of
b-substituted naphthalene (lgkrel), we used Taft’s electronic
ꢀ
ꢁ
2
þ 22:478 R ¼ 0:91
*
[30,31]
constant (s ) and steric constant (E
s
) of the substituent
in
our system.
The plot of lgkrel versus s and E
*
s
and that of lgkrel versus
electronic effect ð%Þ ¼ ½jrj=jrj þ jdjꢃ
Volume and E(HOMO) of b-substituted naphthalene are pre-
sented in Fig. 2, and their equations are presented in Eqns (1)
ꢂ 100; steric effect ð%Þ
¼ 1 ꢁ electronic effect
(3)
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C.-D. CHEN ET AL.
a
Table 2. Quantitative structure–activity relationship descriptors and relative rates of b-substituted naphthalenes
3
kR (ꢄ10ꢁ3
Substrate
Volume (Å )
E(HOMO) (eV)
)
lgkrel = lgk /k
R
nap.
R = H
R = Me
R = Et
R = i-Pr
R = t-Bu
R = s-Bu
R = t-Pent
R = s-Pent
R = s-Hept
R = s-Oct
R = OMe
R = Br
456.96
509.06
559.91
604.61
641.29
647.76
678.98
700.03
785.20
838.82
532.86
518.26
532.85
586.70
ꢁ8.84
ꢁ8.76
ꢁ8.75
ꢁ8.76
ꢁ8.77
ꢁ8.76
ꢁ8.76
ꢁ8.72
ꢁ8.73
ꢁ8.78
ꢁ8.69
ꢁ9.03
ꢁ9.08
ꢁ9.36
62.08 ꢅ 2.04
97.97 ꢅ 2.35
130.03 ꢅ 2.68
103.13 ꢅ 1.88
42.25 ꢅ 0.88
47.12 ꢅ 0.86
36.47 ꢅ 0.76
28.2 ꢅ 0.66
27.5 ꢅ 0.66
26.5 ꢅ 0.66
195 ꢅ 2.98
0.00
0.22
0.34
0.24
ꢁ0.14
ꢁ0.10
ꢁ0.21
ꢁ0.32
ꢁ0.33
ꢁ0.35
0.52
ꢁ0.40
ꢁ0.77
ꢁ1.69
23.5 ꢅ 0.58
10 ꢅ 0.40
R = COOH
R = SO3H
1.2 ꢅ 0.03
a
The quantum chemical descriptor Volume meant the van der Waals molecular volume. All the values were calculated with
semi-empirical PM3 level in the software package Hyperchem 7.0.
*
2
Figure 2. The plot of lgkrel versus s and E
s
of the substituent (left) and plot of lgkrel versus Volume and E(HOMO) of the substituent (right): R is
squared correlation coefficient, F is variance ratio, and SEE is standard error of estimate; the critical value of F at confidence level a = 0.05 is 3.98
From Eqn (1), the value of r, which represented the sensitivity
factor for the reaction to electronic effect, was ꢁ1.25, and the
value of d, which represented the sensitivity factor for the
reaction to steric effect, was 0.43.
From Eqn (3), the percentage contribution of the electronic
effect and the steric effect of the substituent on the hydroxyl-
ation was 74.4% and 25.6%, respectively, which indicated that
the electronic effect played more important role than the steric
effect in the hydroxylation.
3.78 nm when the hydroxylation occurred at 4a or 1a position
on the substituted aromatic ring was longer than that between
the bromine atom and any of the four phenyl groups of
2.73 nm when the hydroxylation occurred at 8a position on the
non-substituted aromatic ring.
So the bulky interaction energy of the bromine atom and the
phenyl groups was lower when the hydroxylation occurred at
the substituted aromatic ring. In the result, the aromatic C–H
bond hydroxylation preferred at the substituted aromatic ring
of b-bromine-naphthalene where the steric interaction energy
barrier was lower according to the principle that the lowest
energy barrier reaction pathway was preferred.
Further, from Eqn (2), the value of the weight factor of E
(HOMO) was 2.421, and that of the weight factor of Volume
was ꢁ0.002, which indicated that the frontier molecular orbital
energy E(HOMO) of the substrate had the more important effect
on the hydroxylation. Because the value of E(HOMO) was
[
33]
Thermodynamic regulation of s-adduct intermediate by
substituent
controlled by the electronic effect of the substituent,
it
indicated that the electronic effect played the more important
role in the hydroxylation, which confirmed the result obtained
from Eqn (3).
[
34]
Very recently, density functional theory calculations suggested
that the aromatic C–H bond hydroxylation occurred via a
[
35]
Finally, we compared the distance of the bromine atom
tetrahedral s-adduct. Nam et al. found inverse kinetic isotope
effect of the aromatic C–H bond hydroxylation, that is, k /k < 1,
[
32]
and the phenyl groups on the s-adduct intermediate
b-bromine-naphthalene as a model for b-substituted naphthalene
Fig. 3).
From Fig. 3, it was shown that the shortest distance between
using
H
D
2
3
which consisted with the sp -to-sp hybridization change when
the tetrahedral s-adduct formed. Further, from Eqn (1) studied
earlier, the value of r was ꢁ1.25, which indicated a cationic
intermediate generated from the hydroxylation of the aromatic
(
the bromine atom and any of the four phenyl groups of
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HYDROXYLATION OF THE AROMATIC C–H BOND
Figure 3. The s-adduct intermediate derived from b-bromine-naphthalene and F20TPPMnOOAr hydroxylated at 4a position (left), 1a position
(middle), and 8a position (right)
[
32]
C–H bond. Moreover, the studies in the section of “Bulky inter-
action of substituent with phenyl groups on metalloporphyrin”
showed that the hydroxylation preferred at the substituted
aromatic ring (2 + 4). Besides, the energy gap of E(HOMO) of b-
methyl-naphthalene as model for b-substituted naphthalene and
E(LUMO) of F TPPMnOOAr of 6.13 eV was smaller (Scheme 1), so
the HOMO was provided by b-methyl-naphthalene and the
LUMO was provided by F20TPPMnOOAr according to the frontier
molecular orbital theory. And it indicated an electrophilic
attack on the p-system of the naphthalene ring by acylperoxo-
manganese(III) porphyrin intermediate.
With the results earlier, we proposed that the b-substituted
naphthalene hydroxylation involved an initial electrophilic
attack on the p-system of the naphthalene ring by acylperoxo-
manganese(III) porphyrin intermediate to produce a tetrahedral
cationic s-adduct (Scheme 2).
2
0
To gain insight into the regulation of the hydroxylation
position by the substituent in Scheme 2, we chose R = Me
and R = Br as model for the electron-donating and electron-
withdrawing substituent, respectively (Table 3).
From Table 3, when R = Me, the value of ΔH of step II hydrox-
ylated at 1a position (R-cationic 1) of 102.97 kcal/mol was
lower than that hydroxylated at 4a position (R-cationic 2) of
141.69 kcal/mol, so step II occurred via attacking the 1a position
according to the principle that the lowest energy barrier
pathway was preferred. Similarly, when R = Br, the value of ΔH
of step II hydroxylated at 4a position (R-cationic 2) of
142.13 kcal/mol was lower than that hydroxylated at 1a position
(R-cationic 1) of 145.18 kcal/mol, so step II occurred via attacking
the 4a position. Finally, the total value of ΔH of the hydroxylation
was ꢁ53.1 and ꢁ57.01 kcal/mol for R = Me and R = Br, respec-
tively, which meant that the hydroxylation was exothermic and
could occur easily.
Scheme 1. The energy of frontier molecular orbital of F20TPPMnOOAr
(left) and b-methyl-naphthalene (right)
I
ArO O
m-CPBA
MnIII
F20TPPMnCl
CH CN/CH Cl
2
3
2
+OAr-
R
OMn(Porp)
III
II
MnIII
+
+
R
R
H
OH
major
ArO O
MnIII
MnIII
R-cationic 1
or
II
ArO O
OH
H
OMn(Porp)
MnIII
III
+OAr-
+
+
R
R
R
R-cationic 2
Scheme 2. The mechanism pathway of b-substituted naphthalene hydroxylation
major
J. Phys. Org. Chem. (2012)
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C.-D. CHEN ET AL.
a
Table 3. The thermodynamic data of each step
f
Step
I
Category
Reagent
Substrate
m-CPBA
ΔH (kcal/mol)
ΔH (kcal/mol)
ꢁ13.82
ꢁ48.31
ꢁ731.33
ꢁ772.98
30.94
F20TPPMnCl
Product
Reagent
Product
F TPPMnOOAr
20
II
II
b-Methyl-naphthalene
ꢁ
OAr
ꢁ99.1
R-cationic 1 (R = Me)
R-cationic 2 (R = Me)
b-Bromine-naphthalene
ꢁ539.97
ꢁ501.25
47.97
102.97
141.69
Reagent
Product
ꢁ
OAr
ꢁ99.1
R-cationic 1 (R = Br)
R-cationic 2 (R = Br)
R-cationic 1 (R = Me)
b-Methyl-naphthol
F TPPMnOAr
ꢁ480.73
ꢁ483.78
ꢁ539.97
ꢁ11.86
ꢁ769.46
ꢁ481.78
3.26
145.18
142.13
III
III
Reagent
Product
ꢁ142.25
ꢁ185.32
20
Reagent
Product
R-cationic 2 (R = Br)
b-Bromine-naphthol
F TPPMnOAr
ꢁ769.46
20
P
P
a
f
f
ΔH of each step could be calculated using the equation ΔH = ΔH (product) ꢁ ΔH (reagent).
The origin cause of the regioselectivity
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the aromatic C–H bond hydroxylation derived from the intermo-
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J. Phys. Org. Chem. (2012)
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