Kinetic study on SNAr reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes with Azide ion: Effect of changing nucleophile from hydroxide to Azide ion on reaction mechanism and reactivity

Article abstract of DOI:10.1002/bkcs.10333

Second-order rate constants (k_N3-) for S_NAr reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes (2a-2h) with (Formula presented.) in 80 mol % H₂O/20 mol % DMSO at 25.0 ± 0.1 °C have been measured spectrophotometrically. The Bronsted-type plot is linear with β_1g=-0.38. The Hammett plots correlated with (Formula presented.) constants exhibit highly scattered points. In contrast, the Yukawa-Tsuno plot results in an excellent linear correlation with _ρY = 1.02 and r = 0.51, indicating that a negative charge develops partially on the O atom of the leaving Y-substituted-phenoxy moiety in the transition state. Accordingly, the reactions have been concluded to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs in the rate-determining step. Comparison of k_N3- with the k_OH- values reported previously for the corresponding reactions with OH has revealed that (Formula presented.) is only 6- to 26-fold than OH toward substrates 2a-2h, although the former is over 11 pK_a units less basic than the latter. Solvation and polarizability effects have been suggested to be responsible for the unusual reactivity shown by (Formula presented.) and OH. Effects of changing nucleophile from OH to (Formula presented) on reaction mechanism and reactivity are discussed in detail.

Full text of DOI:10.1002/bkcs.10333

Article
DOI: 10.1002/bkcs.10333
BULLETIN OF THE
H.-O. Seo et al.
KOREAN CHEMICAL SOCIETY
Kinetic Study on SNAr Reactions of 1-(Y-Substituted-phenoxy)-
2,4-dinitrobenzenes with Azide Ion: Effect of Changing Nucleophile from
Hydroxide to Azide Ion on Reaction Mechanism and Reactivity
*
Hyeon-Ok Seo, Min-Young Kim, So-Yeop Han, and Ik-Hwan Um
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea.
*E-mail: ihum@ewha.ac.kr
Received February 26, 2015, Accepted March 4, 2015, Published online June 24, 2015
Second-order rate constants (kN3−) for SNAr reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes
(2a–2h) with N3− in 80 mol % H2O/20 mol % DMSO at 25.0 0.1 ꢀC have been measured spectrophotomet-
rically. The Brønsted-type plot is linear with βlg = −0.38. The Hammett plots correlated with σYo and σY− con-
stants exhibit highly scattered points. In contrast, the Yukawa–Tsuno plot results in an excellent linear
correlation with ρY = 1.02 and r = 0.51, indicating that a negative charge develops partially on the O atom
of the leaving Y-substituted-phenoxy moiety in the transition state. Accordingly, the reactions have been con-
cluded to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs in the rate-
determining step. Comparison of kN3− with the kOH− values reported previously for the corresponding reactions
with OH− has revealed that N3− is only 6- to 26-fold less reactive than OH− toward substrates 2a–2h, although
theformerisover 11pKa unitslessbasicthanthelatter. Solvationandpolarizabilityeffects havebeensuggested
tobe responsiblefor theunusualreactivity shown byN3−andOH−. Effects of changing nucleophile fromOH− to
N3− on reaction mechanism and reactivity are discussed in detail.
Keywords: SNAr reaction, 1-Phenoxy-2,4-dinitrobenzene, Rate-determining step, Hammett plot,
Yukawa–Tsuno plot
Introduction
complex and restoration of aromaticity on expulsion of the
leaving group.
Numerous studies on nucleophilic substitution reactions of
activated aromatic compounds with various nucleophiles have
been carried out due to their importance not only in biological
process but also in synthetic applications.1–10 In fact, nucleo-
philic aromatic substitution (SNAr) reactions of aromatic com-
pounds activated by one or more powerful electron-
withdrawing groups (e.g., NO2, SO2CF3, F3CSO NSO2CF3,
etc.) have been reported to be useful in organic synthesis
including improved methods of stereoselective reaction,7 in
preparation of electrophilic derivatives of water soluble poly-
mers as well as poly(aryl ethers),8 and in some possible envi-
ronmental remediation protocols.9
The rate-determining step (RDS) in SNAr reactions has
been reported to be dependent on reaction conditions (e.g.,
reaction medium, type of nucleophile and leaving group,
etc.).2,3,10 We have shown that expulsion of the leaving group
from the Meisenheimer complex occurs in the RDS for SNAr
reactions of 1-fluoro-2,4-dinitrobenzene (1a) with secondary
amines in MeCN.10a In contrast, expulsion of the leaving
group has been reported to occur after the RDS for the corre-
sponding reactions carried out in H2O and for those with
primary amines inMeCN or for those of other 1-X-2,4-dinitro-
benzenes, e.g., X = Cl (1b), Br (1c), and I (1d), with primary
and secondary amines not only in MeCN but also in H2O.10b
SNAr reactions have generally been understood to proceed
through a stepwise mechanism, i.e., nucleophilic attack at
the position bearing a leaving group to form a σ-bonded
adduct, commonly referred to as a Meisenheimer complex,
followed by expulsion of the leaving group.1–6 There is a
similarity between nucleophilic substitution reactions of car-
boxylic esters and SNAr reactions of activated aromatic com-
pounds possessing a leaving group, e.g., nucleophilic
addition step rehybridizes the electrophilic center from sp2
to sp3 to yield a tetrahedral intermediate and expulsion of
the leaving group in the subsequent step regenerates the
sp2 center. However, a fundamental difference in SNAr reac-
tions is loss of aromaticity on formation of a Meisenheimer
SNAr reactions with anionic nucleophiles have also been
carried out.10d We have recently reported that SNAr reactions
of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes (2a–2h)
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Article SNAr Reactions of 1-(Y-Substituted-phenoxy)-2,4-dinitrobenzenes with Azide Ion KOREAN CHEMICAL SOCIETY
with alkali-metal ethoxides (EtO−M+; M+ = Li+, Na+, K+) in
anhydrous ethanol proceed through a stepwise mechanism
with a Meisenheimer complex, in which expulsion of the leav-
ing group occurs after the RDS, on the basis of the kinetic
results that σo constants resulted in a much better Hammett
correlation than σ− constants.10d A similar result has been
reported for the reactions of 2a–2h with OH− in 80 mol %
H2O/20 mol % DMSO.10e
Our study has now been extended to SNAr reactions of 2a–
2h with N3− to obtain further information on the reaction mech-
anism (Scheme 1). Azide ion has been reported to be highly
regioselective in epoxide-ring opening to give α-azidoalco-
hols.11 Azide ion is also known to be useful in synthesis of
1,2,3-triazoles from epoxides and in cycloaddition of [3+2]
azide-alkyne and azide-nitrile which leads to triazoles and tet-
razoles, respectively.11,12 Although N3− is a weak base, it is
known to be an excellent nucleophile especially toward polar-
izable substrates.13,14 We have compared the kinetic results
obtained from the current SNAr reactions with those reported
previously for the corresponding reactions with OH−10e to
investigate effects of changing nucleophile from strongly
basic OH− to weakly basic N3− on reaction mechanism and
reactivity.
ion is up to 26-fold more reactive than azide ion. The differ-
ence in reactivity of N3− and OH− will be discussed in detail
in the following section.
To deduce reaction mechanism, Brønsted-type plots have
been constructed. As shown in Figure 1, the Brønsted-type
plot for the reactions with N3− is linear with βlg = −0.38. The
plot for the corresponding reactions with OH− is also linear
with βlg = −0.16. It is noted that a βlg value of −0.3 0.1 is typ-
ical for reactions reported previously to proceed through a
stepwise mechanism with formation of an intermediate being
the RDS.10,15,16 In fact, the SNAr reactions of 2a–2h with OH−
weresuggestedtoproceedthroughastepwisemechanismwith
a Meisenheimer complex, in which expulsion of the leaving
group occurs after the RDS, on the basis of a linear
Brønsted-type plot with βlg = −0.16.10e Thus, one might pro-
pose that the SNAr reactions of 2a–2h with N3− proceed also
through a stepwise mechanism with formation of a Meisenhei-
mer complex being the RDS.
To examine validity of the above proposal, Hammett plots
forthereactionsof2a–2hwithN3−havebeenconstructedusing
σo and σ− constants in Figure 2. It is well-known that useful
Table 1. Summary of second-order rate constants for the reactions of
1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes (2a–2h) with N3−
and OH− in 80 mol % H2O/20 mol % DMSO at 25.0 0.1 ꢀC.a
Results and Discussion
Y
pKa
103kN3−/M−1 −1
s
103kOH−/M−1 −1
s
Y-PhOH
The kinetic study was carried out under pseudo-first-order con-
ditions with the nucleophile concentration in excess (≥20-fold)
over the substrate concentration. All the reactions in this study
obeyed first-order kinetics with quantitative liberation of Y-
substituted-phenoxide ionand/or itsconjugateacid(i.e., Y-sub-
stituted-phenol). Pseudo-first-order rate constants (kobsd) were
calculatedfrom theequationln(A∞ – At) = −kobsdt + C.Thesec-
ond-orderrateconstants(KN3−) were calculated from the slope
of linear plots of kobsd vs. [N3−]. The KN3− values calculated
in this way and relevant pKa values are summarized in Table 1
for the reactions of 2a–2h with N3− together with the kOH
−values reported previously for the corresponding reactions
with OH− for comparison. It is estimated from replicate runs
that the uncertainty in the rate constants is less than 3%.
Deduction of Reaction Mechanism. As shown in Table 1,
the second-order rate constant for the reactions of 2a–2h with
N3− increases as the basicity of the leaving group decreases,
e.g., KN3− increases from 0.137 × 10−3/M−1s−1 to 0.924 ×
10−3 and 1.99 × 10−3/M−1s−1 as the pKa of the conjugate acid
of the leaving Y-substituted-phenoxide ion decreases from
10.19 to 8.05 and 7.14, in turn. A similar result is shown for
the corresponding reactions with OH−, although hydroxide
2a
2b
2c
2d
4-Me
H
4-Cl
3-Cl
10.19
0.137
0.175
0.357
0.495
0.924
1.53
3.62
4.05
5.77
6.50
8.56
11.4
7.70
12.1
9.95
9.38
9.02
8.05
7.95
7.66
7.14
2e 4-COMe
2f 4-CN
2g 4-CHO
2h 4-NO2
1.19
1.99
a The kinetic data for the reactions with OH− were taken from Ref. 10e.
(a)
(b)
-2
-3
-4
-5
-1
-2
-3
2h
2f
2g
2e
2h
2g
2f
2d
2c
2e
2d
2c
2b
2b
2a
2a
β
lg = −0.38
β
lg = −0.16
R2 = 0.944
R2 = 0.984
OAr
N3
ArO N3
NO2
NO2
NO2
k2
7
8
9
10 11
k1
7
8
9
10 11
Y−PhOH
pKa
Y−PhOH
N3
ArO
pKa
k-1
Figure 1. Brønsted-type plots for the SNAr reactions of 1-(Y-substi-
tuted-phenoxy)-2,4-dinitrobenzenes (2a–2h) with (a) N3− and (b)
OH− in 80 mol % H2O/20 mol % DMSO at 25.0 0.1 ꢀC. The iden-
tity of points is given in Table 1.
NO2
NO2
NO2
2a-2h
Scheme 1. SNAr reactions of 2a–2h with N3−.
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BULLETIN OF THE
Article SNAr Reactions of 1-(Y-Substituted-phenoxy)-2,4-dinitrobenzenes with Azide Ion KOREAN CHEMICAL SOCIETY
information on reaction mechanism including the RDS can be
obtained from Hammett plots correlated with σo and σ− con-
stants. If the reactions of 2a–2h with N3− proceed through a
stepwise mechanism in which expulsion of the leaving group
occurs after the RDS (i.e., k−1 << k2), no negative charge
would develop on the O atom of the leaving Y-substituted-
phenoxy moiety in the transition state (TS). In this case, σo
constants should result in a better Hammett correlation than
σ− constants. In contrast, if the reactions proceed through a
concerted mechanism or via a stepwise pathway with expul-
sion of the leaving group being the RDS (i.e., k−1 >> k2), a neg-
ative charge would develop partially on the O atom of the
leaving group in the TS. In this case, σ− constants should result
in a better Hammett correlation than σo constants. This is
because the negative charge developing on the O atom of
the leaving Y-substituted-phenoxy moiety can be delocalized
to the substituent Y through resonance interactions.
reactions of esters with various nucleophiles (e.g., neutral
amines as well as anionic nucleophiles such as OH−, N3−,
and CN−).15,19
Â
À
ÁÃ
log kY=kH = ρY σYo + r σY− – σYo
ð1Þ
As shown in Figure 3, the Yukawa–Tsuno plot results in an
excellent linear correlation with ρY = 1.02 and r = 0.51. It is
noted that the r value in Eq. (1) represents resonance demand
of the reaction center or extent of the resonance contribution,
–
o
while the term (σY – σY ) is the resonance substituent con-
stant that measures the capacity for π-delocalization of the
π-electron acceptor substituent.17,18 Thus, the r value of
0.51 found for the reactions of 2a–2h with N3− is a clear indi-
cation that a negative charge develops partially on the O atom
of the leaving group which can be delocalized to the substitu-
ent Y through resonance interactions. This is possible only for
reactions in which expulsion of the leaving group occurs in the
RDS either in a concerted mechanism or in a stepwise pathway
with the second step (i.e., expulsion of the leaving group)
being the RDS.
o
As shown in Figure 2, the Hammett plot correlated with σY
constants for the reactions with N3− results in a poorer correla-
tion coefficient (R2 = 0.968). A slightly better Hammett corre-
lation is demonstrated when σY constants are used (R2 =
−
However, one can exclude a possibility that the SNAr reac-
tions of 2a–2h with N3− proceed through a concerted mechan-
ism. This is because SNAr reactions are generally understood
to proceed through a stepwise mechanism with a Meisenhei-
mer complex. Thus, the current SNAr reactions are concluded
to proceed through a stepwise mechanism, in which expulsion
of the leaving group from a Meisenheimer complex occurs in
the RDS. This mechanism is clearly in contrast to the mechan-
ism proposed basedonthe linear Brønsted-type plotwith βlg =
−0.38 (i.e., a stepwise mechanism in which expulsion of the
leaving group occurs after the RDS).
Effect of Changing Nucleophile from OH− to N3− on Reac-
tion Mechanism and Reactivity. It is generally understood
that expulsion of the leaving group from a Meisenheimer com-
plex occurs rapidly after the RDS to regain the lost
aromaticity.2–6 In fact, we haverecently reported that theSNAr
0.984). However, both Hammett plots exhibit highly scattered
points. This is in contrast to our previous report that σYo con-
stants result in amuch better Hammett correlation (R2 = 0.991)
than σY constants (R2 = 0.941) for the reactions of 2a–2h
–
with OH−.10e Thus, one cannot obtain conclusive information
on the reaction mechanism from these poorly correlated Ham-
mett plots.
To obtain more decisive information on the reaction mech-
anism for the reactions of 2a–2h with N3−, Yukawa–Tsuno plot
has been constructed. The Yukawa–Tsuno equation (Eq. (1))
was originally derived to account for the kinetic results
obtained from solvolysis of benzylic systems in which a pos-
itive charge developspartially atthereaction center.17,18 How-
ever, we have shown that Eq. (1) is highly effective to clarify
ambiguities in reaction mechanisms for SNAr reactions of
2a–2h with amines as well as for nucleophilic substitution
(a)
(b)
Figure 2. Hammett plots correlated with (a) σYo and (b) σY- constants
for the SNAr reactions of 1-(Y-substituted-phenoxy)-2,4-
dinitrobenzenes (2a–2h) with N3− in 80 mol % H2O/20 mol % DMSO
at 25.0 0.1 ꢀC.
Figure 3. Yukawa–Tsuno plot for the SNAr reactions of 1-(Y-substi-
tuted-phenoxy)-2,4-dinitrobenzenes (2a–2h) with N3− in 80 mol %
H2O/20 mol % DMSO at 25.0 0.1 ꢀC.
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BULLETIN OF THE
Article SNAr Reactions of 1-(Y-Substituted-phenoxy)-2,4-dinitrobenzenes with Azide Ion KOREAN CHEMICAL SOCIETY
reactions of 2a–2h with OH− and C2H5O− proceed through a
stepwise mechanism, in which expulsion of the leaving group
occurs after the RDS, on the basis of excellent Hammett cor-
relations with σo constants.10d,e However, expulsion of the
leaving group has been concluded to occur in the RDS for
the current SNAr reactions of 2a–2h with N3− on the basis of
the linear Yukawa–Tsuno plot with ρY = 1.02 and r = 0.51.
This demonstrates clearly that modification of nucleophile
fromOH− toN3−changestheRDSfromformation ofananionic
Meisenheimer complex to its breakdown.
centered electrophile) to O-4-nitrophenyl thionobenzoate
(3b, a C S centered electrophile).14 In contrast, kOH
−
decreases from 4.20 to 0.470/M−1s−1 on changing electrophile
from3ato3b.14 Itisnoted thatOH− is 1.8 × 103-foldmorereac-
tive than N3− toward 3a. On the contrary, N3− is ca. 23-fold
more reactive than more basic OH− toward the polarizable
substrate 3b.14
To account for such a change in the RDS upon modification
of nucleophile from strongly basic OH− to weakly basic N3−,
It is noted that the electrophilic center of substrates 2a–2h is
highly polarizable due to the polarizable π-electrons in the
benzene ring. Thus, one can suggest that the high reactivity
exerted by N3− toward polarizable substrates 2a–2h is due to
strong interactions between polarizable N3− and substrates
2a–2h, while the unusually decreased reactivity of OH− is
caused by weak interactions between nonpolarizable OH−
and polarizable substrates. This is in accord with the hard–soft
acids and bases (HSAB) principle.20
Meisenheimer complexes have been depicted as MCOH
−
and MCN3− for the reactions with OH− and N3−, respectively.
It is noted that OH− is much more basic and a poorer nucleo-
fuge than all the Y-substituted-phenoxides employed in this
study, while N3− is less basic and a better nucleofuge than
the aryloxides. This explains why expulsion of the leaving
group occurs after the RDS for the reactions with OH− (i.e.,
k
−1 << k2) but in the RDS for the corresponding reactions with
N3− (i.e., k−1 >> k2).
Conclusions
Thecurrentstudyhasallowedustoconcludethefollowing:(1)
The Brønsted-type plot for the reactions of 2a–2h with N3− is
linear with βlg = −0.38. (2) The Hammett plot correlated with
σY− constants results in a slightly better correlation than that
correlated with σYo constants. In contrast, the Yukawa–Tsuno
plot exhibits an excellent linear correlation with ρY = 1.02 and
r = 0.51, indicating that a negative charge develops partially
on the O atom of the leaving Y-substituted-phenoxy moiety
intheTS. (3)TheSNArreactions withN3−areconcludedtopro-
ceed through a stepwise mechanism, in which expulsion of the
leaving group occurs in the RDS. This is in contrast to the reac-
tion mechanism reported previously for the corresponding
reactions with OH−. The difference in basicity of the nucleo-
phile is responsible for the difference in the reaction mechan-
ism. (4) Azide ion is only 6- to 26-fold less reactive than
hydroxide ion toward substrates 2a–2h, although the former
is over 11 pKa units less basic than the latter. (5) Solvation
and polarizability effects are responsible for the unusual reac-
tivity shown by N3− and OH−.
It is noted that N3− is only 6–26 times less reactive than OH−,
although the former is over 11 pKa units less basic than the lat-
ter (Table 1). Thus, one can suggest that N3− is significantly
more reactive (or OH− exhibits unusually lower reactivity)
than would be expected from its basicity. This argument is
consistent with the report that N3− deviates positively from
the linear Brønsted-type plot for the reactions of 4-nitrophenyl
acetate with a series of aryloxides, while OH− exhibits nega-
tive deviation significantly from the linear Brønsted-type
plot.13 Jencks et al. have attributed the decreased reactivity
shown by strongly basic OH− to solvation effect because
OH− is known to be strongly solvated in H2O through H-
bonding interactions.13 In contrast, one might expect that
weakly basic N3− would be weakly solvated in the current reac-
tion medium. Thus, one can suggest that solvation effect is one
plausible reasonwhy N3−is only up to26-fold less reactive than
OH− although the former is over 11 pKa units less basic than
the latter.
Another possibility that might account for the high reactiv-
ity of N3− (or unusually low reactivity of OH−) compared with
its basicity is polarizability effect. Azide ion is known to be
highly polarizable and to exhibit significantly enhanced reac-
tivity toward polarizable electrophiles.13,14 In contrast,
hydroxide ion is a nonpolarizable base and often exhibits unu-
sually low reactivity toward polarizable electrophiles.14 In
fact, we have previously reported that kN3− increases from
0.00234 to 11.0/M−1s−1 (i.e., 4.7 × 103-fold increase) on chan-
ging the substrate from 4-nitrophenyl benzoate (3a, a C O
Experimental Section
Materials. Substrates 2a–2h were prepared from the reaction
of 1-fluoro-2,4-dinitrobenzene with the respective Y-substi-
tuted-phenol in anhydrous ether in the presence of triethyla-
mine as reported previously.10c The crude product was
purified through column chromatography. The purity of 2a–
2h was checked by means of the melting point and 1H
NMR characteristics. Other chemicals used in this study were
of the highest quality available. Doubly glass distilled water
was further boiled and cooled under nitrogen just before
use. Due to low solubility of 2a–2h in pure H2O, 80 mol %
H2O/20 mol % DMSO was used as the reaction medium.
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BULLETIN OF THE
Article SNAr Reactions of 1-(Y-Substituted-phenoxy)-2,4-dinitrobenzenes with Azide Ion KOREAN CHEMICAL SOCIETY
A. Zare, A. Parhami, M. N. S. Rad, G. R. Nejabat, Can. J. Chem.
2006, 84, 979; (c) S. E. Snyder, J. R. Carey, A. B. Shvets,
W. H. Pirkle, J. Org. Chem. 2005, 70, 4073.
Kinetics. The kinetic study was performed using a UV–Vis
spectrophotometer equipped with a constant temperature cir-
culating bath to maintain the reaction temperature at 25.0
0.1 ꢀC. The reaction was followed by monitoring the appear-
ance of the leaving Y-substituted-phenoxide ion. All the reac-
tions were carried out under pseudo-first-order conditions in
which nucleophile concentrations were at least 20 times
greater than the substrate concentration. All solutions were
prepared freshly just before use under nitrogen and transferred
by gas-tight syringes. The reaction was initiated by adding 5 μ
L of a 0.01 M solution of the substrates 2a–2h in CH3CN by a
10-μL syringe to a 10-mm quartz UV cell containing 2.50 mL
of the thermostatted reaction mixture made up of solvent and
aliquot of the NaN3 stock solution.
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D. J. Brunelle, D. A. Singelton, Chemosphere 1985, 14, 173;
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Product Analysis. Y-Substituted-phenoxide ion (and/or its
conjugate acid) was liberated quantitatively and identified
as one of the products by comparison of the UV–Vis spectrum
at the end of reaction with the authentic sample under the
experimental condition.
11. (a) F. Alonso, Y. Moglie, G. Radivoy, M. Yus, J. Org. Chem.
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Acknowledgments. Publication cost of this paper was sup-
ported by the Korean Chemical Society.
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