Kinetic study on aminolysis of 4-pyridyl benzoate and O-4-pyridyl thionobenzoate in acetonitrile: Factors influencing reactivity and reaction mechanism

Article abstract of DOI:10.1002/bkcs.10906

A kinetic study on nucleophilic substitution reactions of 4-pyridyl benzoate (2a) andO-4-pyridyl thionobenzoate (2b) with a series of cyclic secondary amines in acetonitrile at 25.0°C is reported. Plots of pseudo-first-order rate constant (k_obsd) vs. [amine] are linear and pass through the origin for the reactions of 2a but curve upward for those of 2b. The upward curvature observed for the reactions of 2b is typical for reactions that proceed through a stepwise mechanism with a zwitterionic intermediate T^±, which decomposes to the products via uncatalyzed and catalyzed routes competitively. The reaction of 2a has been suggested to proceed through a stepwise mechanism with T^±, in which expulsion of the leaving group occurs in the rate-determining step on the basis of a linear Br?nsted-type plot with β_nuc = 0.77. The catalyzed reaction of 2b from T^± has been proposed to proceed through a concerted mechanism with a six-membered cyclic transition state rather than via a stepwise pathway with an anionic intermediate T-. Factors influencing reactivity and reaction mechanism are discussed in detail.

Full text of DOI:10.1002/bkcs.10906

Article
DOI: 10.1002/bkcs.10906
BULLETIN OF THE
I.-H. Um et al.
KOREAN CHEMICAL SOCIETY
Kinetic Study on Aminolysis of 4-Pyridyl Benzoate and O-4-Pyridyl
Thionobenzoate in Acetonitrile: Factors Influencing Reactivity and
Reaction Mechanism
Ik-Hwan Um, Min-Young Kim, and Jae-In Lee‡
†,*
†
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea.
*
E-mail: ihum@ewha.ac.kr
Department of Chemistry and Plant Resources Research Institute, Duksung Women’s University, Seoul
32-714, Korea
Received July 7, 2016, Accepted July 27, 2016, Published online September 21, 2016
‡
1
A kinetic study on nucleophilic substitution reactions of 4-pyridyl benzoate (2a) and O-4-pyridyl thio-
ꢀ
nobenzoate (2b) with a series of cyclic secondary amines in acetonitrile at 25.0 C is reported. Plots of
pseudo-first-order rate constant (kobsd) vs. [amine] are linear and pass through the origin for the reac-
tions of 2a but curve upward for those of 2b. The upward curvature observed for the reactions of 2b
is typical for reactions that proceed through a stepwise mechanism with a zwitterionic intermediate
ꢁ
T , which decomposes to the products via uncatalyzed and catalyzed routes competitively. The reac-
ꢁ
tion of 2a has been suggested to proceed through a stepwise mechanism with T , in which expulsion
of the leaving group occurs in the rate-determining step on the basis of a linear Brønsted-type plot
ꢁ
with βnuc = 0.77. The catalyzed reaction of 2b from T has been proposed to proceed through a con-
certed mechanism with a six-membered cyclic transition state rather than via a stepwise pathway with
−
an anionic intermediate T . Factors influencing reactivity and reaction mechanism are discussed in
detail.
Keywords: Aminolysis, O-4-Pyridyl thionobenzoate, Catalyzed reaction, Brønsted plot, Concerted
mechanism
Introduction
nature of the reaction medium governs the reaction mech-
anism. On the other hand, aminolysis of 2,4-dinitrophenyl
diphenylphosphinate (a P=O electrophile) has been reported
to proceed through a concerted mechanism on the basis of
Nucleophilic displacement reaction of esters with amines is
recognized as one of the most important reactions in organic
1
6
and bioorganic chemistry. Numerous studies have been
a linear Brønsted-type plot with βnuc = 0.5 ꢁ 0.1, while
performed to investigate the reaction mechanism using vari-
the corresponding reaction of 2,4-dinitrophenyl benzenesul-
ous linear free energy relationships (e.g., Brønsted-type,
fonate (an SO electrophile) has been proposed to proceed
through a stepwise mechanism, in which the RDS changes
from breakdown of the addition intermediate to its forma-
2
2–8
Hammett, Yukawa-Tsuno equations, etc.).
Reactions of
carboxylic esters with amines have been reported to proceed
either through a concerted mechanism or via a stepwise
pathway with one or two intermediates (e.g., a zwitterionic
tion as the incoming amine becomes more basic than the
7
nucleofuge by 4 to 5 pK units.
a
ꢁ
−
tetrahedral intermediate T and its deprotonated form T )
depending on reaction conditions such as nature of the reac-
tion medium (e.g., protic or aprotic solvents), type of the
We have recently reported that reactions of 2-pyridyl
benzoate (1a) with a series of cyclic secondary amines in
MeCN proceed through a forced concerted mechanism on
8a
electrophilic center (e.g., C=O, C=S, P=O, SO , etc.), basic-
the basis of a linear Brønsted-type plot with βnuc = 0.61.
2
2–8
ity of the incoming amine and leaving group, etc.
The reaction was originally expected to proceed through a
stepwise mechanism with a six-membered cyclic intermedi-
Reactions of 2,4-dinitrophenyl benzoate with a series of
cyclic secondary amines in MeCN have been reported to
proceed through a concerted mechanism on the basis of a
ate (i.e., TS ), which could be stabilized through the H-
I
bonding interaction. Nevertheless, we have proposed that
the H-bonding interaction forces the reaction to proceed
5
a
linear Brønsted-type plot with βnuc = 0.40. In contrast,
the corresponding reactions carried out in aqueous medium
have been suggested to proceed through a stepwise mech-
anism with a change in the rate-determining step (RDS) on
8a
through a concerted mechanism. In contrast, the corre-
sponding reaction of O-2-pyridyl thionobenzoate (1b, a
thiono analog of 1a) has been reported to proceed through
5
b
ꢁ
the basis of a biphasic Brønsted-type plot, indicating that
a stepwise mechanism with T , which decomposes to the
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products through catalyzed and uncatalyzed routes, on the
(a)
(b)
4
basis of upward curvature observed in the plots of k
vs. [amine].
obsd
0
0
.25
.20
8c
3
2
1
O
C
X
X
O
H
Ph
Ph C O
Ph C O
N
ꢀ
N
N
ꢀ
0.15
0.10
0.05
N
X = O(1a), S(1b) X = O(2a), S(2b)
TSI
Our study has now been extended to aminolysis of 4-
pyridyl benzoate (2a) and O-4-pyridyl thionobenzoate (2b)
in MeCN to investigate effect of changing the electrophilic
center from C=O to C=S on reactivity and reaction mech-
anism. The kinetic data obtained in this study have been
compared with those reported for the corresponding reac-
slope = 55.7 ± 0.5
intercept= 0.128 ± 0.02
2
R = 0.9997
0
.00
0
0
0.00
.00
0.02
[
0.04
0.06
0.02
0.04
0.06
amine ] / M
[ amine ] / M
Figure 1. Plot of kobsd vs. [amine] (a) and kobsd/[amine]
vs. [amine] (b) for the reaction of O-4-pyridyl thionobenzoate
8
c
tions of 1a and 1b to explore effects of modification of
the leaving group from 2-pyridyloxide to 4-pyridyloxide on
reactivity and reaction mechanism.
ꢀ
(2b) with piperidine in MeCN at 25.0 ꢁ 0.1 C.
A similar result is demonstrated for the reaction of 1a.
However, 1a is more reactive than 2a although the former
possesses a more basic leaving group than the latter (e.g.,
Results and Discussion
pK = 11.62 and 11.06 for 2-hydroxypyridine and 4-hydro-
xypyridine, respectively).
a
The kinetic study was carried out spectrophotometrically
under pseudo-first-order conditions, e.g., [amine] >> [sub-
strate]. Pseudo-first-order rate constants (kobsd) were calcu-
9
The effect of amine basicity on reactivity is illustrated in
Figure 2. The Brønsted-type plots for the reactions of 1a
and 2a are linear with βnuc = 0.61 and 0.77, respectively. A
βnuc value of 0.77 is a lower limit for reactions reported
previously to proceed through a stepwise mechanism, in
lated from the slope of linear plots of ln (A∞ – A ) vs. t.
t
The plots of kobsd vs. [amine] were linear and passed
through the origin for the reactions of 2a (Figure not
shown). Thus, the second-order rate constants (k ) were
N
ꢁ
which expulsion of the leaving group from T occurs in
calculated from the slope of the linear plots and are sum-
marized in Table 1 together with those reported previously
for the corresponding reactions of 1a for comparison. In
contrast, the plot of kobsd vs. [amine] curves upward for the
reaction of 2b with piperidine (Figure 1(a)). Similarly
curved plots were observed for the reactions of 2b with all
the other amines used in this study.
2
–5
the RDS.
Thus, we propose that the aminolysis of 2a
ꢁ
proceeds through a stepwise mechanism with T as an
intermediate. In contrast, the aminolysis of 1a has been
reported to proceed through a forced concerted mechanism
with a six-membered cyclic TS (i.e., TS ), where the intra-
I
molecular H-bonding transforms the leaving group from
strongly basic 2-pyridyloxide (pK = 11.62 in H O) to
a
2
weakly basic 2-pyridinium oxide (pK = 0.75 in H O) or
its tautomer 2-pyridone. Such cyclic TS is structurally not
possible for the reaction of 2a. Accordingly, the nucleofuge
Reactions of 1a and 2a (C O Electrophiles). Table 1
a
2
9
shows that the k value for the reaction of 2a decreases as
N
the amine basicity decreases, e.g., it decreases from 0.0351
−
1 −1
for the reaction of 2a is highly basic 4-pyridyloxide (pK =
to 0.000701 M
s
as the pK of the conjugate acid of the
a
a
1
1.06 in H O). This idea accounts for the contrasting reac-
incoming amine decreases from 18.8 to 16.6, respectively.
2
tion mechanism and for the kinetic result that 1a is more
reactive than 2a.
Table 1. Summary of kinetic data for reactions of 2-pyridyl
benzoate (1a) and 4-pyridyl benzoate (2a) with cyclic secondary
Reactions of 1b and 2b (C S electrophiles). As shown
in Figure 1(a), the plot of kobsd vs. [amine] curves upward
for the reaction of 2b with piperidine. Such upward curva-
ture is typical for reactions that proceed through a stepwise
ꢀ
a
amines in MeCN at 25.0 ꢁ 0.1 C
−
1 −1
N
k /M s
Amines
pKa
1a
2a
ꢁ
mechanism with T , which decomposes to the products via
1
2
3
4
Piperidine
3-Methylpiperidine
Piperazine
1-(2-Hydroxyethyl)-
piperazine
18.8 0.0558
18.6 0.0438
18.5 0.0433
0.0351
—
2–5
uncatalyzed and catalyzed routes competitively.
Thus,
the second-order and third-order rate constants (Kk and
2
0.0208
Kk , respectively) were calculated from Eq. (2), which is
3
17.6 0.00859 0.00326
simplified from Eq. (1) under the assumption k−1 >> k +
2
k3[amine]. As shown in Figure 1(b), the plot of kobsd
amine] vs. [amine] exhibits an excellent linear correlation
with a positive intercept, indicating that the above
/
5
Morpholine
16.6 0.00266 0.000701
[
a
The data for the reactions of 1a were taken from Ref. 8c.
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Aminolysis of 4-Pyridyl Benzoate and O-4-Pyridyl Thionobenzoate
KOREAN CHEMICAL SOCIETY
to 16.6, respectively (i.e., ca. 20 times decrease in the Kk2
value). The Kk3 values for the reactions of 1b and 2b
ꢁ
= 0.61 ± 0.03
nuc
2
-1
-2
-3
-4
exhibit a similar trend. In contrast, the Kk value for the
2
R = 0.996
1
−1 −1
2
reaction of 1b decreases only from 10.6 to 6.12 M s
although the amine basicity decreases over 2 pK units.
a
3
ꢀ
ꢀ
Py
O
S
ꢀ
S
4
O
Ph
C
N
Ph C
H
N
ꢀ
ꢀ
N
ꢀ
N
H
H
5
TSII
TSIII
The reaction of 1b has previously been reported to pro-
ꢁ
ceed through a stepwise mechanism with T , which
ꢁ
= 0.77 ± 0.04
nuc
2
decomposes to the products via uncatalyzed and catalyzed
8c
R = 0.997
routes. Both uncatalyzed and catalyzed reactions of 1b
have been suggested to proceed through a six-membered
cyclic TS (i.e., TS and TS for the uncatalyzed and cata-
II
III
16
17
18
19
20
lyzed reaction, respectively: Note that the OPy in TSIII
represents 2-pyridyloxide), which would gain stability to a
certain degree through the intramolecular H-bonding inter-
action. However, such cyclic TS is structurally not possi-
ble for the uncatalyzed reaction of 2b. This idea accounts
pK + log (p/q)
a
Figure 2. Brønsted-type plots for the reactions of 2-pyridyl benzo-
ate (1a, ●) and 4-pyridyl benzoate (2a, ○) with cyclic secondary
8c
ꢀ
amines in MeCN at 25.0 ꢁ 0.1 C.
for the kinetic result that the Kk value for the reaction of
2
1
b is much larger than that for the corresponding reac-
assumption is valid. Similarly linear plots were observed
for the reactions of 2b with the other amines studied
tion of 2b.
The effect of amine basicity on the rate constants is illus-
trated in Figure 3 for the reactions of 1b and 2b. The
Brønsted-type plots exhibit excellent linear correlations
when the pK and rate constants were statistically corrected
using p and q (p = 2, while q = 1 except q = 2 and 4 for
(
Figure not shown). Thus, the Kk and Kk values for the
2 3
reactions of 2b were calculated from the intercept and slope
of the linear plots, and are summarized in Table 2 together
with those reported previously for the corresponding reac-
tions of 1b for comparison.
a
the uncatalyzed and catalyzed reactions with piperazine,
10
ꢀ
ꢁ
respectively). It is noted that a βnuc value of 0.12 for the
uncatalyzed reaction of 1b is unusually small. However,
this also supports our previous proposal that the uncata-
2
kobsd = k k ½amineꢂ + k k ½amineꢂ =ðk−1 + k + k ½amineꢂÞ
1
2
1
3
2
3
ð1Þ
lyzed reaction of 1b proceeds through TS , in which a pro-
II
ton transfer from the aminium moiety of TS to the N atom
kobsd=½amineꢂ = Kk2 + Kk3½amineꢂ, where K = k1=k−1 ð2Þ
II
of the leaving group occurs simultaneously with expulsion
8c
of the leaving group.
We propose that the reaction of 2b proceeds through a
stepwise mechanism with T , which decomposes to the
As shown in Table 2, the Kk value for the reaction of
2
ꢁ
2
b decreases as the amine basicity decreases, e.g., it
−1 −1
products via uncatalyzed and catalyzed routes as shown in
decreases from 0.128 to 0.00623 M s as the pK of the
a
ꢁ
Scheme 1. The catalyzed reaction of 2b from T proceeds
conjugate acid of the incoming amine decreases from 18.8
Table 2. Summary of kinetic data for reactions of O-2-pyridyl thionobenzoate (1b) and O-4-pyridyl thionobenzoate (2b) with cyclic
ꢀ
a
secondary amines in MeCN at 25.0 ꢁ 0.1 C
−
1 −1
−2 −1
Kk
2
/M s
3
Kk /M s
Amines
pKa
1b
2b
1b
2b
1
2
3
4
5
a
Piperidine
3-Methylpiperidine
Piperazine
1-(2-Hydroxyethyl)piperazine
Morpholine
18.8
18.6
18.5
17.6
16.6
10.6
10.9
21.8
8.37
6.12
0.128
0.104
0.106
0.0216
0.00623
396
284
796
84.0
23.5
55.7
42.9
86.1
8.47
2.64
The kinetic data for the reactions of 1b were taken from Ref. 8c.
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Aminolysis of 4-Pyridyl Benzoate and O-4-Pyridyl Thionobenzoate
KOREAN CHEMICAL SOCIETY
(
a)
(b) 3
significantly, since the negative charge in TSIV is deloca-
lized. Moreover, if the catalyzed reaction of 2b proceeds
1
2
3
2
1
through TS , the leaving group becomes 4-hydroxypyri-
4
3
IV
1
0
1
2
5
dine, which is a much better nucleofuge than the strongly
basic 4-pyridyloxide anion.
4
2
1
0
β
= 0.12 ± 0.02
5
nuc
2
Effect of Changing Electrophilic Center from C O to
C S on Reactivity and Reaction Mechanism. From
Tables 1 and 2, one can notice that 2a is less reactive than its
thiono analog 2b. Furthermore, the reaction of 2a has been
proposed to proceed through a stepwise mechanism with a
R
= 0.960
β
= 0.61 ± 0.02
nuc
2
R
= 0.998
-
-
β
= 0.55 ± 0.02
nuc
2
R
β
= 0.998
= 0.61 ± 0.03
ꢁ
−
zwitterionic intermediate as modeled by T (C–O ), which
decomposes to the products in the RDS, on the basis of a lin-
ear Brønsted-type plot with βnuc = 0.77. In contrast, the
upward curvature shown in Figure 1(a) indicates that the reac-
tion of 2b proceeds through a stepwise mechanism with a
nuc
2
R
= 0.996
-
1
16
17
18
19
20
16
17
18
19
20
pK + log(p/q)
pK + log(p/q)
a
a
Figure 3. Brønsted-type plots for the reactions of O-2-pyridyl
thionobenzoate (1b, ●) and O-4-pyridyl thionobenzoate (2b, ○)
ꢁ
−
zwitterionic intermediate as modeled by T (C–S ), which
decomposes to the products via uncatalyzed and catalyzed
routes competitively. These results demonstrate clearly that
modification of the electrophilic center from C=O to C=S
affects not only the reactivity but also the reaction mechanism.
ꢀ
with cyclic secondary amines in MeCN at 25.0 ꢁ 0.1 C: Kk
2
(a) and Kk (b).
3
through a concerted mechanism with a six-membered cyclic
TS as modeled by TS . It is noted that TS is similar to
TSIII reported previously for the catalyzed reaction of 1b
except OPy, which represents 2-pyridyloxide in TSIII and
4
IV
IV
O
C
S
C
Ph
OPy
Ph
OPy
HN
HN
-pyridyloxide in TSIV.
It has traditionally been understood that a second amine
T
(C O )
T
(C S )
molecule in the catalyzed reaction deprotonates the ami-
ꢁ
− 2–5
nium moiety of T to form an anionic intermediate T .
It is well-known that anions are highly destabilized in
MeCN due to electronic repulsion between the anion and
Thus, one might suggest that the catalytic reaction of 2b
ꢁ
−
also proceeds with two intermediates (T and T ). How-
11
the negative dipole end of MeCN. The negative charge in
ꢁ
ever, we propose that the catalyzed reaction of 2b from T
−
ꢁ
−
the C–O moiety of T (C–O ) would be localized on the
proceeds through a concerted mechanism with a six-
membered cyclic TS as modeled by TSIV (Scheme 1) rather
than via a stepwise pathway with a discrete anionic inter-
−
small O atom. In contrast, the negative charge in the C–S
ꢁ
−
moiety of T (C–S ) would be dispersed on the large and
ꢁ
−
polarizable S atom. Accordingly, T (C–S ) would be less
−
−
mediate T . This is because the anionic T would be highly
destabilized in MeCN due to electronic repulsion between
the anionic solute and the negative dipole end of the reac-
ꢁ
−
unstable than T (C–O ) in MeCN. This idea accounts for
the kinetic results that Kk for the uncatalyzed reaction of
2
the thiono compound (2b) is larger than the second-order
rate constant (k ) for the corresponding reaction of its oxy-
1
1
tion medium, MeCN. Furthermore, if the catalyzed reac-
−
N
tion proceeds through a stepwise mechanism with T as an
gen analog (2a).
intermediate, the leaving group is highly basic 4-
pyridyloxide anion, which would be strongly destabilized
in MeCN. In contrast, if the catalyzed reaction proceeds
through a concerted mechanism with a cyclic TS as mod-
Since π-bonding energy of a C=S bond is weaker than
that of a C=O bond, one might expect that the tendency of
−
ꢁ
−
the C–S moiety of T (C–S ) to form a C=S bond (and to
−
expel the leaving group) is weaker than that of the C–O
eled by T , the electronic repulsion could be reduced
IV
ꢁ
−
moiety of T (C–O ). In fact, Castro et al. have suggested
ꢁ
that the lifetime of T is longer for reactions of substrates
ꢀ
possessing a C=S electrophilic center than for those of sub-
strates bearing a C=O electrophilic center. Thus, one can
suggest that the longer lifetime of T (C–S ) induces the
reactions of substrates possessing a C=S electrophilic center
S
S
S
S
ꢀ
k1
k2
uncat.
H
N
12
Ph C OPy
Ph C OPy
HN
Ph C OPy
HN
Ph C
k-1
ꢁ
−
HN
OPy
N
ꢀ
T
Py
S
S
(
e.g., 1b and 2b) to proceed through both uncatalyzed and
cat.
O
Ph C
Ph
C
N
H
N
OPy =
O
N
HN
catalyzed routes.
k3 HN
ꢀ
H
HOPy
Conclusions
TSIV
Scheme 1. The reaction mechanism proposed for the aminolysis
of 2b in MeCN.
The current study has shown that reactivity and reaction
mechanism are strongly influenced not only by the nature
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Aminolysis of 4-Pyridyl Benzoate and O-4-Pyridyl Thionobenzoate
KOREAN CHEMICAL SOCIETY
of the electrophilic center (C=O vs. C=S) but also by the
leaving group (2-pyridyloxide vs. 4-pyridyloxide): (1) Ami-
nolysis of 2a proceeds through a stepwise mechanism with
Acknowledgments. This research was supported by Basic
Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Edu-
cation (2015-R1D1A1A-01059624).
ꢁ
−
an addition intermediate T (C–O ), in which expulsion of
the leaving group occurs in the RDS. This is in contrast to
the reaction mechanism reported previously for the corre-
sponding reaction of 1a. (2) The reactions of 1b and 2b
proceed through a stepwise mechanism with an addition
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ꢁ
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−
2
3
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(
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ꢁ
−
oxygen analogs 1a and 2a, respectively, because T (C–S )
ꢁ
−
would be less unstable than T (C–O ) in MeCN. (5) The
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−
−
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. (a) I. H. Um, S. E. Jeon, J. A. Seok, Chem. Eur. J. 2006, 12,
1
237; (b) I. H. Um, K. H. Kim, H. R. Park, M. Fugio,
Y. Tsuno, J. Org. Chem. 2004, 69, 3937.
6. I. H. Um, J. Y. Han, Y. H. Shin, J. Org. Chem. 2009,
4, 3073.
7. I. H. Um, J. Y. Hong, J. A. Seok, J. Org. Chem. 2005,
0, 1438.
8. (a) I. H. Um, A. R. Bae, T. I. Um, J. Org. Chem. 2014,
9, 1206; (b) I. H. Um, M. Y. Kim, A. R. Bae,
J. M. Dust, E. Buncel, J. Org. Chem. 2015, 80, 217;
c) M. Y. Kim, I. H. Um, Bull. Korean Chem. Soc. 2016,
7, 1401.
Materials. 4-Pyridyl benzoate (2a) and O-4-pyridyl thio-
nobenzoate (2b) were prepared from the reaction of 4-
hydroxypyridine with benzoyl chloride and thionobenzoyl
7
8
chloride, respectively, as reported previously. The purity
7
was checked by the melting point and spectral data such as
1
13
8
H and C NMR spectra as described previously. Other
7
chemicals, including MeCN and all the amines used, were
of the highest quality available.
(
3
Kinetics. The kinetic study was carried out using a UV–
Vis spectrophotometer for slow reactions (e.g., t1/2 ≥ 10 s)
or a stopped-flow spectrophotometer for fast reactions (e.g.,
t1/2 < 10 s) equipped with a constant temperature circulat-
9
. (a) I. H. Um, S. J. Hwang, S. R. Yoon, S. E. Jeon, S. K. Bae,
J. Org. Chem. 2008, 73, 7671; (b) I. H. Um, J. A. Seok,
H. T. Kim, S. K. Bae, J. Org. Chem. 2003, 68, 7742.
1
1
0. R. P. Bell, The Proton in Chemistry, Methuen, London,
959, p. 159.
1. E. Buncel, R. A. Stairs, Solvent Effects in Chemistry, 2nd ed.,
Wiley, New York, 2016, p. 1.
ing bath to maintain the reaction temperature at
1
ꢀ
2
5.0 ꢁ 0.1 C. The reactions were carried out under
pseudo-first-order conditions, in which the concentration of
the amine was kept in excess over that of the substrate as
described previously.
12. E. A. Castro, M. Angel, D. Arellano, J. G. Santos, J. Org.
Chem. 2001, 66, 6571.
8
Bull. Korean Chem. Soc. 2016, Vol. 37, 1577–1581
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de 1581