Kinetics and mechanism of the reactions of anilines with ethyl S-aryl thiocarbonates

Article abstract of DOI:10.1021/jo982063u

The reactions of anilines with ethyl S-(2,4-dinitrophenyl) thiocarbonate (DNPTC) and ethyl S-(2,4,6-trinitrophenyl) thiocarbonate (TNPTC) are subjected to a kinetic study in aqueous solution at 25.0 °C and ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under aniline excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) VS [N] (N is the free substituted aniline) are linear and pH independent, with slope k(N). The Bronsted-type plots (log k(N) vs pK(a) of anilinium ions) are linear, with slopes β = 0.9 for DNPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T(±)) is rate determining, and β = 0.54 for TNPTC, consistent with a concerted mechanism. Consideration of the results for aminolysis from the present work and those from previous studies leads to the following conclusions. (i) The tetrahedral intermediate possessing a 2,4-dinitrobenzenethio group is more stable than that including the 2,4,6-trinitrobenzenethio group. (ii) The tetrahedral intermediate possessing the 2,4,6-trinitrobenzenethio group has no existence beyond the limit of a vibration period (concerted mechanism). (iii) Tetrahedral intermediates possessing anilino groups are less stable than those derived from pyridines but are more stable than the tetrahedral intermediates derived from secondary alicyclic amines. (iv) Anilines are more reactive toward the carbonyl group of methyl 2,4-dinitrophenyl carbonate than toward the carbonyl of DNPTC.

Full text of DOI:10.1021/jo982063u

J . Org. Chem. 1999, 64, 1953-1957
1953
Kin etics a n d Mech a n ism of th e Rea ction s of An ilin es w ith Eth yl
S-Ar yl Th ioca r bon a tes
Enrique A. Castro,* Leonardo Leandro, Pablo Milla´n, and J ose´ G. Santos*
Facultad de Quı´mica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
Received October 13, 1998
The reactions of anilines with ethyl S-(2,4-dinitrophenyl) thiocarbonate (DNPTC) and ethyl S-(2,4,6-
trinitrophenyl) thiocarbonate (TNPTC) are subjected to a kinetic study in aqueous solution at 25.0
°C and ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (400
nm) the release of the corresponding substituted benzenethiolate anion. Under aniline excess,
pseudo-first-order rate coefficients (kobsd) are found. Plots of kobsd vs [N] (N is the free substituted
aniline) are linear and pH independent, with slope kN. The Bro¨nsted-type plots (log kN vs pKa of
anilinium ions) are linear, with slopes â ) 0.9 for DNPTC, in agreement with a stepwise mechanism
where the breakdown of a tetrahedral addition intermediate (T() is rate determining, and â ) 0.54
for TNPTC, consistent with a concerted mechanism. Consideration of the results for aminolysis
from the present work and those from previous studies leads to the following conclusions. (i) The
tetrahedral intermediate possessing a 2,4-dinitrobenzenethio group is more stable than that
including the 2,4,6-trinitrobenzenethio group. (ii) The tetrahedral intermediate possessing the 2,4,6-
trinitrobenzenethio group has no existence beyond the limit of a vibration period (concerted
mechanism). (iii) Tetrahedral intermediates possessing anilino groups are less stable than those
derived from pyridines but are more stable than the tetrahedral intermediates derived from
secondary alicyclic amines. (iv) Anilines are more reactive toward the carbonyl group of methyl
2,4-dinitrophenyl carbonate than toward the carbonyl of DNPTC.
In tr od u ction
and aryl ethyl thionocarbonates,8 aryl O-ethyl dithiocar-
bonates, and aryl dithioacetates.9 Although much atten-
tion has been focused on the effects of the leaving group
on the kinetics and mechanism, much less is known about
the effects of the amine nature on the mechanisms of the
above reactions.
There is abundant literature on the kinetics and
mechanism of the aminolysis of carbonyl and thiocarbo-
nyl compounds such as acetate1 and benzoate2 esters,
diaryl3 and alkyl aryl carbonates,4 aryl thiobenzoates,5
aryl thiolacetates,6 aryl O-ethyl thiolcarbonates,7 diaryl
In some of these works, it has been concluded that
secondary alicyclic amines are better nucleofuges from
a zwitterionic tetrahedral intermediate (T() than isobasic
pyridines; this has been deduced from the pKa values at
(1) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
7018. Cho, B. R.; Kim, Y. K.; Yoon, C. M. J . Am. Chem. Soc. 1997,
119, 691. Maude, A. B.; Williams, A. J . Chem. Soc., Perkin Trans. 2
1997, 179.
(2) (a) Kirsch, J . F.; Kline, A. J . Am. Chem. Soc. 1969, 91, 1841. (b)
Menger, F. M.; Smith, J . H. J . Am. Chem. Soc. 1972, 94, 3824. (c)
O′Leary, M. H.; Marlier, J . F. J . Am. Chem. Soc. 1979, 101, 3300. (d)
Khan, M. N. J . Org. Chem. 1983, 48, 2046. (e) Castro, E. A.; Valdivia,
J . L. J . Org. Chem. 1986, 51, 1668. (f) Bell, K. H. Aust. J . Chem. 1987,
40, 1723. (g) Koh, H. J .; Lee, H. C.; Lee, H. W.; Lee, I. Bull. Korean
Chem. Soc. 1995, 16, 839.
o
the center of the break (pKa ) of nonlinear Bro¨nsted
plots.6b It has also been found that pyridines are expelled
more slowly than isobasic quinuclidines from the zwit-
terionic tetrahedral intermediate formed in the aminoly-
sis of phenyl 4-nitrophenyl carbonate.3
In the reactions of ethyl S-(2,4-dinitrophenyl) thiocar-
bonate (DNPTC) and ethyl S-(2,4,6-trinitrophenyl) thio-
carbonate (TNPTC) with secondary alicyclic amines, it
has been found that the mechanism is concerted,7a,b
whereas in the pyridinolysis of the above substrates, the
mechanism is stepwise, i.e., a tetrahedral intermediate
is formed on the reaction path.7d This fact indicates that
substitution of a pyridine by a secondary alicyclic amine
implies a substantial destabilization of T(; this is of such
(3) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
6970.
(4) (a) Bond, P. M.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2
1976, 679. (b) Castro, E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99,
7611. (c) Castro, E. A.; Freudenberg, M. J . Org. Chem. 1980, 45, 906.
(d) Castro, E. A.; Iban˜ez, F.; Lagos, S.; Schick, M.; Santos, J . G. J .
Org. Chem. 1992, 57, 2691. (e) Castro, E. A.; Iban˜ez, F.; Saitua, A. M.;
Santos, J . G. J . Chem. Res., Synop. 1993, 56.
(5) (a) Campbell, P.; Lapinskas, B. A. J . Am. Chem. Soc. 1977, 99,
5378. (b) Lee, I.; Shim, C. S.; Lee, H. W. J . Chem. Res., Synop. 1992,
90. (c) Oh, H. K.; Shin, C. H.; Lee, I. Bull. Korean Chem. Soc. 1995,
16, 657. (d) Um, I.-H.; Kwon, H.-J .; Kwon, D.-S.; Park, J .-Y. J . Chem.
Res. 1995, (S) 301, (M) 1801. (e) Oh, H. K.; Shin, C. H.; Lee, I. J . Chem.
Soc., Perkin Trans. 2 1995, 1169.
(6) (a) Um, I.-H.; Choi, K.-E.; Kwon, D.-S. Bull. Korean Chem. Soc.
1990, 11, 362. (b) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans.
2 1991, 63.
(7) (a) Castro, E. A.; Iban˜ez, F.; Salas, M.; Santos, J . G. J . Org. Chem.
1991, 56, 4819. (b) Castro, E. A.; Salas, M.; Santos, J . G. J . Org. Chem.
1994, 59, 30. (c) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem.
1994, 59, 3572. (d) Castro, E. A.; Pizarro, M. I.; Santos, J . G. J . Org.
Chem. 1996, 61, 5982.
(8) (a) Castro, E. A.; Santos, J . G.; Tellez, J .; Uman˜a, M. I. J . Org.
Chem. 1997, 62, 6568. (b) Castro, E. A.; Cubillos, M.; Santos, J . G.;
Tellez, J . J . Org. Chem. 1996, 61, 3501. (c) Castro, E. A.; Cubillos, M.;
Santos, J . G.; Te´llez, J . J . Org. Chem. 1997, 62, 2512.
(9) (a) Castro, E. A.; Cubillos, M.; Iban˜ez, F.; Moraga, I.; Santos, J .
G. J . Org. Chem. 1993, 58, 5400. (b) Castro, E. A.; Araneda, C. A.;
Santos, J . G. J . Org. Chem. 1997, 62, 126. (c) Oh, H. K.; Woo, S. Y.;
Shin, C. H.; Park, Y. S.; Lee, I. J . Org. Chem. 1997, 62, 5780. (d) Oh,
H. K.; Lee, J . Y.; Yun, J . H.; Park, Y. S.; Lee, I. Int. J . Chem. Kinet.
1998, 30, 419.
10.1021/jo982063u CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/25/1999

1954 J . Org. Chem., Vol. 64, No. 6, 1999
Castro et al.
Ta ble 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Rea ction s of An ilin es w ith O-Eth yl
magnitude as to prevent the existence of the intermediate
with a secondary amino moiety.
S-(2,4-Din itr op h en yl) Th ioca r bon a te (DNP TC)a
The comparison of the reactions of 2,4-dinitrophenyl
methyl carbonate with pyridines, secondary alicyclic
amines, and anilines suggests that anilines render T(
more stable than do secondary alicyclic amines but less
stable than do pyridines.4e
In the present work, we undertake a mechanistic study
on the reactions of DNPTC and TNPTC with anilines
with the aim to (i) shed more light on the reaction
mechanism of thiolcarbonates, (ii) examine the influence
of the amine nature by comparison of the title reactions
with those of the same substrates with pyridines7d and
secondary alicyclic amines,7a,b and (iii) examine the
influence of the leaving group by comparison of the title
reactions among them and by comparison of these reac-
tions of DNPTC with those of methyl 2,4-dinitrophenyl
carbonate with anilines.4e
b
aniline substituent pH 103[aniline]tot (M) 105kobsd (s-1) runs
4-methoxy
4-methyl
none
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
2.0-15
2.0-15
2.0-15
6.0-30
6.0-30
6.0-30
5.0-80
5.0-80
5.0-80
9.0-90
9.0-90
9.0-90
2.0-16
2.0-16
2.0-16
8.0-80
8.0-80
8.0-80
2.7-26
6
6
6
6
6
6
6
6
6
6
6
6
5
5
5
6
6
6
3.0-30
3.3-26
5.5-20
5.2-20
5.5-20
1.1-9.1
1.1-9.3
1.5-10
3-methoxy
3-acetyl
3-cyano
0.88-5.6
0.83-5.9
1.0-5.8
0.22-0.80
0.20-0.82
0.23-0.77
0.14-0.47
0.26-0.57
0.33-0.68
Exp er im en ta l Section
a
In water at 25.0 °C, ionic strength 0.2 M (KCl), in the presence
b
of phosphate buffer 0.005 M. Total concentration of substituted
aniline (acid plus conjugate base).
Ma ter ia ls. The anilines (Aldrich) were purified either by
distillation or recrystallization. DNPTC and TNPTC were
obtained as previously reported.7a,b O-Ethyl phenyl carbamate
(EtOCONHPh) was synthesized as follows. To a solution of
ethyl chloroformate (1.5 mL, 15 mmol) dissolved in acetonitrile
(10 mL) was added slowly a solution of aniline (2.8 mL, 30
mmol) in acetonitrile (10 mL). The mixture was left 30 min at
ambient temperature. Chloroform (15 mL) was added to this
mixture, and the solution washed with 10% HCl and water.
The organic layer was dried with MgSO4 and filtered under
vacuum, and the solvent was evaporated off. The crystallized
product melted at 52-53 °C (lit10 mp 52 °C) and was identified
as follows: 1H NMR (200 MHz, CDCl3) δ 7.20-7.40 (m, 4H),
7.10 (d, 1H), 6.7 (s, 1H), 4.2 (c, 2H, J ) 7.1 Hz) 1.3 (t, 3H, J )
7.1 Hz); IR (KBr) 1703 (CdO), 1599 (CdC), 1234 (C-N
Ta ble 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Rea ction s of An ilin es w ith O-Eth yl
S-(2,4,6-Tr in itr op h en yl) Th ioca r bon a te (TNP TC)a
b
aniline substituent pH 103[aniline]tot (M) 104kobsd (s-1) runs
4-methoxy
4-methyl
none
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
7.00
7.20
7.50
1.0-20
1.0-20
1.0-20
6.0-30
6.0-30
6.0-30
5.0-100
5.0-100
5.0-100
9.0-90
9.0-90
9.0-90
5.0-40
5.0-40
5.0-40
8.0-80
8.0-80
8.0-80
5.4-54
5.4-54
5.4-54
4.4-38
4.2-39
4.5-37
7.6-25
8.3-26
8.1-26
2.6-29
2.4-30
2.6-29
1.8-16
2.0-16
2.0-17
0.6-4.9
0.7-4.8
0.8-5.0
0.19-3.1
0.21-3.5
0.21-3.8
0.19-1.6
0.20-1.6
0.24-1.6
8
8
8
6
6
6
6
6
6
6
6
6
7
7
7
6
6
6
6
6
6
carbamate), 746 and 694 (CH, arom) cm-1
.
3-methoxy
3-acetyl
3-cyano
4-acetyl
Deter m in a tion of p Ka . The pKa of the 4-methylaniline was
determined spectrophotometrically at 232 nm, 25.0 ( 0.1 °C,
and ionic strength 0.2 M (KCl).
Kin etic Mea su r em en ts. These were carried out by means
of a Hewlett-Packard 8453 diode array spectrophotometer in
water, at 25.0 ( 0.1 °C, ionic strength 0.2 M (KCl), and
phosphate buffer 0.005 M. The reactions were followed at 400
nm (formation of the substituted benzenethiolate anions). All
reactions were studied under excess of the aniline over the
substrate (20-fold at least). The initial substrate concentration
was 5 × 10-5 M.
Pseudo-first-order rate coefficients (kobsd) were found for all
reactions; for most of them, kobsd was determined by means of
the “infinity” method, except for the slowest reactions (DNPTC
with 3-aminoacetophenone and with 3-aminobenzonitrile),
where the initial rate method was used. The experimental
conditions of the reactions and the kobsd values are shown in
Tables 1 and 2.
a
In water at 25.0 °C, ionic strength 0.2 M (KCl), in the presence
b
of phosphate buffer 0.005 M. Total concentration of substituted
aniline (acid plus conjugate base).
d[ArS-]
) kobsd [EtOCOSAr]
(1)
(2)
dt
P r od u ct Stu d ies. For the present reactions, one of the
products was identified as the corresponding benzenethiolate;
this was achieved by comparison of the UV-vis spectra after
completion of the reactions with authentic samples under the
same conditions. For the reactions of both substrates with
aniline, the other product was identified as O-ethyl phenyl
carbamate by comparison with an authentic sample by HPLC,
column Eurospher C-18 (10 cm, 7µm), eluant acetonitrile/water
70/30, isocratic mode 0.5 mL/min.
kobsd ) ko + kN [N]
trinitro-phenyl, ko and kN are the rate constants for
hydrolysis and aminolysis of the substrates, respectively,
and N represents the substituted aniline free base.
The second-order rate coefficients for aminolysis (kN)
were obtained as the slopes of plots of eq 2 at constant
pH and were pH independent. These values, together
with those of the pKa of the conjugated acids of the
anilines, statistically corrected with q ) 1 and p ) 3,4e,11
are shown in Table 3.
Resu lts a n d Discu ssion
The general rate law obtained in the present reactions
is given by eqs 1 and 2, where Ar is 2,4-dinitro- or 2,4,6-
With the data of Table 3, the Bro¨nsted-type plots of
Figure 1 were obtained. The slopes of the lines are â )
(10) Herrera, M.; Ruiz, V. M.; Valderrama, J . A.; Vega, J . C. An.
Quim. 1980, C76, 183.
(11) Bell, R. P. The Proton in Chemistry; Methuen: London, 1959;
p 159.

Reactions of Anilines with Ethyl S-Aryl Thiocarbonates
J . Org. Chem., Vol. 64, No. 6, 1999 1955
Ta ble 3. Va lu es of p Ka of An ilin iu m Ion s a n d kN for th e
Sch em e 1
Rea ction s of An ilin es w ith DNP TC a n d TNP TCa
103kN (s-1 M-1
)
pKa +
aniline
log(p/q)b
DNPTC
TNPTC
4-methoxyaniline
4-methylaniline
aniline
3-methoxyaniline
3-aminoacetophenone
3-aminobenzonitrile
4-aminoacetophenone
6.13
5.56
5.21
4.84
4.12
3.38
2.90
20 ( 1
180 ( 7
69 ( 2
6.1 ( 0.1
1.10 ( 0.03
0.59 ( 0.02
0.38 ( 0.02
0.046 ( 0.002
27 ( 1
17 ( 1
12.0 ( 0.3
3.6 ( 0.2
2.8 ( 0.1
a
Both pKa and kN values were determined in water at 25.0 °C,
b
ionic strength 0.2 M (KCl). Statistically corrected with p ) 3 and
q ) 1 (see text).
0.05);15 2,4-dinitrophenyl acetate (â ) 0.57 ( 0.03);15
3-nitrophenyl, 4-nitrophenyl, and 3,4-dinitrophenyl for-
mates (â ) 0.64, 0.51, and 0.43, respectively);16 and the
corresponding acetates (â ) 0.66, 0.59, and 0.53, respec-
tively).16
Nevertheless, concerted phenolysis mechanisms with
larger â values are also known, such as those of 4-nitro-
phenyl, 4-formylphenyl, and 3-nitrophenyl acetates (â )
0.75, 0.79, and 1.04, respectively),14 although these â
values are based on pKa values at an ionic strength (0.1
M) different than that of the kinetic measurements (1.0
M).16
The reactions of secondary alicyclic amines with DNPTC
and TNPTC are concerted, as indicated by the linear
Bro¨nsted plots found, with slopes â ) 0.56 and 0.48,
respectively,7a,b and the fact that the predicted Bro¨nsted
breaks, had these reactions been stepwise, were not
observed.7a,b The â value alone is not sufficient to prove
that a reaction is concerted; one must be sure that the
o
hypothetical break (pKa ) of the Bro¨nsted-type plot (due
F igu r e 1. Bro¨nsted-type plots obtained in the reactions of
anilines with O-ethyl S-(2,4-dinitrophenyl) thiocarbonate
(DNPTC) and O-ethyl S-(2,4,6-trinitrophenyl) thiocarbonate
(TNPTC) in aqueous solution, at 25.0 °C, ionic strength 0.2 M
(KCl).
to the change in the rate-determining step of a stepwise
process) is located at a pKa value within the pKa range
of the nucleophiles used in the plot.12 Unfortunately,
there are no studies on stepwise reactions of anilines with
o
thiocarbonates and related compounds in which pKa
0.9 ( 0.1 and â ) 0.54 ( 0.05 for the reactions of DNPTC
and TNPTC, respectively.
values have been found experimentally that would allow
the estimation of a hypothetical Bro¨nsted break for the
reactions of TNPTC with anilines. Nevertheless, the
great difference found in the Bro¨nsted slopes for the
reactions of the two substrates studied in this work
(Figure 1) suggests that the mechanism of the reactions
of TNPTC with anilines is concerted.
It has been reported that the reactions of DNPTC and
TNPTC with pyridines proceed through the formation of
a tetrahedral intermediate (T().7d The fact that the
reactions of the same substrates with secondary alicyclic
amines are concerted7a,b was explained by an expulsion
rate from T( of the latter amines faster than that for
isobasic pyridines. Namely, the alicyclic amines desta-
bilize kinetically the tetrahedral intermediate to the point
that it cannot exist.
The magnitude of â for the reaction of DNPTC is
in agreement with the values of the Bro¨nsted slopes
found in the stepwise aminolysis of similar substrates
when the breakdown to products of the zwitterionic
tetrahedral intermediate (T() is the rate-determining
step.1a,2e,3,4,5a,d,e,6,7c,d,8,9a-c,4e The mechanism for this reac-
tion is shown in Scheme 1, where DNP is 2,4-dinitro-
phenyl and R is a substituted phenyl group. The step k2
should be rate determining for these reactions, and the
formation of T( should be an equilibrium step.
In contrast, the value of â ) 0.54 found for the reaction
of TNPTC (Figure 1) is in accord with a concerted process
in which the structure of the transition state remains
constant with the variation of the nucleophile basicity.12
This â value is in agreement with those found in the
concerted reactions of phenoxide anions with 1-acetoxy-
8-hydroxynaphthalene (â ) 0.48);13 4-chloro 2-nitrophenyl
acetate (â ) 0.64 ( 0.05);14 acetic anhydride (â ) 0.58 (
The fact that the reactions of DNPTC with secondary
alicyclic amines are concerted,7a whereas the reactions
(14) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1989,
111, 2647.
(15) Ba-Saif, S. A.; Colthurst, M.; Waring, M. A.; Williams, A. J .
Chem. Soc., Perkin Trans. 2 1991, 1901.
(16) Stefanidis, D.; Cho, S.; Dhe-Paganon, S.; J encks, W. P. J . Am.
Chem. Soc. 1993, 115, 1650.
(12) Williams, A. Acc. Chem. Res. 1989, 22, 387.
(13) Hibbert, F.; Malana, M. A. J . Chem. Soc., Perkin Trans 2 1990,
711.

1956 J . Org. Chem., Vol. 64, No. 6, 1999
Castro et al.
of the same substrate with anilines proceed via a
tetrahedral intermediate, indicates that substitution of
an aniline by a secondary alicyclic amine implies a
substantial destabilization of the tetrahedral intermedi-
ate. This is of such magnitude as to prevent the existence
of this intermediate, and therefore, the mechanism
changes from stepwise to enforced concerted.17 This
destabilization could be due to a faster nucleofugality
from the tetrahedral intermediate of a secondary alicyclic
amine compared to that of an isobasic aniline, as found
in the reactions of methyl 2,4-dinitrophenyl carbonate.4e
On the other hand, the fact that the reactions of
TNPTC with pyridines7d are stepwise, whereas those of
the same substrate with anilines are concerted, indicates
that anilines destabilize kinetically the tetrahedral in-
termediate (T() in comparison with pyridines as a result
of a greater nucleofugality from T( of the former amines.
In summary, we can conclude that anilines destabilize
the tetrahedral intermediate relative to pyridines but
stabilize it compared with secondary alicyclic amines.
Therefore the sequence: secondary alicyclic amines >
anilines > pyridines seems to hold for the rate constants
for amine expulsion from the tetrahedral intermediate.
This is the same sequence proposed for the reactions of
this amine series with methyl 2,4-dinitrophenyl carbon-
ate, where all reactions undergo a stepwise mechanism.4b,e
By comparison of the concerted reactions of TNPTC
with the stepwise mechanism for the reactions of DNPTC
with the same series of anilines, it can be concluded that
there is a remarkable destabilization of the tetrahedral
intermediate as a result of the introduction of a third
nitro group to the nucleofuge. This destabilization must
arise from the lower basicity of TNPS relative to DNPS
(pKa 1.4 and 3.4, respectively),6b which increases its
nucleofugality from the intermediate and renders this
species highly unstable kinetically. The “intermediate”
no longer exists, and the concerted mechanism is en-
forced.17
F igu r e 2. Bro¨nsted-type plots obtained in the reactions of
anilines with O-ethyl S-(2,4-dinitrophenyl) thiocarbonate
(DNPTC) and methyl 2,4-dinitrophenyl carbonate (DNPC) in
aqueous solution, at 25.0 °C, ionic strength 0.2 M (KCl).
phenoxide anions are better leaving groups from a
tetrahedral intermediate than isobasic benzenethiolate
anions.19 If it is assumed that the greater push due to
the change of methyl by an ethoxy or methoxy group in
T( enlarges the k2 values by the same factor,3,7a the
greater reactivity toward anilines of DNPC compared
with DNPTC, when breakdown of T( is rate limiting,
must be due to K1.
From eqs 7 and 8 of ref 18 and eqs 15 and 16 of ref 6b,
the values of k-1 (expulsion of secondary alicyclic amines
and pyridines from the corresponding tetrahedral inter-
mediates) can be determined for the reactions of 2,4-
dinitrophenyl acetate18 and 2,4-dinitrophenyl thiolacetate.6b
It is found that the k-1 value for a given amine is 2 to 4
times greater for the former reaction. This is in line with
the known superior push provided by a ArO group from
T( to expel other groups, compared to the push from
ArS.20 Therefore, the value of k-1 (expulsion of an aniline)
should be greater from the intermediate 1 than from 2
in view of the stronger push exerted by DNPO in 1.
From a comparison of the Bro¨nsted-type plots obtained
in the reactions of anilines with DNPTC and methyl 2,4-
dinitrophenyl carbonate (DNPC),4e shown in Figure 2, it
can be observed that the latter substrate is more reactive
toward anilines than the former. Although these sub-
strates possess different “nonleaving” groups (EtO and
MeO), it has been found that the change from EtO to
MeO as the nonleaving group in similar substrates does
not affect the kinetics.8c Therefore, the difference in
reactivity between DNPTC and DNPC must arise from
the different leaving groups. Because breakdown to
products of the corresponding tetrahedral intermediate
is the rate-determining step for both reactions, it means
that kN ) K1k2 is greater for DNPC (K1 ) k1/k-1 and k2
are defined in Scheme 1).
The k2 values found for the expulsion of DNPS and
DNPO from the tetrahedral intermediates formed in the
reactions of 2,4-dinitrophenyl thiolacetate and acetate
with secondary alicyclic amines are closely similar, 2.4
Therefore, the larger value of K1k2 for the reactions of
anilines with DNPC relative to that with DNPTC (Figure
2) can only be explained by a larger k1 value for DNPC
to more than compensate for its larger value of k-1. A
greater value of k1 in the reactions of anilines with
DNPC, relative to DNPTC, can be explained on the basis
× 109 and 3.0 × 109
s
-1, respectively.6b,18 This is so
because the difference in basicity of these two groups (pKa
) 3.4 and 4.0 for 2,4-dinitrobenzenethiol and 2,4-dini-
trophenol, respectively),6b is compensated by the fact that
(17) J encks, W. P. Acc. Chem. Res. 1980, 13, 161. J encks, W. P.
Chem. Soc. Rev. 1981, 10, 345. Williams, A. Chem. Soc. Rev. 1994, 23,
93.
(19) (a) J ensen, J . L.; J encks, W. P. J . Am. Chem. Soc. 1979, 101,
1476. (b) Duglas, K. T. Acc. Chem. Res. 1986, 19, 186.
(20) Hupe, D. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 451.
(18) Castro, E. A.; Ureta, C. J . Org. Chem. 1990, 55, 1676.

Reactions of Anilines with Ethyl S-Aryl Thiocarbonates
J . Org. Chem., Vol. 64, No. 6, 1999 1957
of the “hard and soft acids and bases” principle:21 anilines,
which are considered borderline bases, react faster with
the carbonyl group of carbonate, considered relatively
hard, than the relatively soft carbonyl group of thiolcar-
bonate.21 Pyridines, whose absolute hardness is similar
to that of anilines,21 react faster toward the carbonyl
group of methyl 2,4-dinitrophenyl carbonate compared
to the softer thiocarbonyl group of ethyl 2,4-dinitrophenyl
thionocarbonate.8c
Ack n ow led gm en t. We thank FONDECYT of Chile
for financial assistance to this work.
(21) Pearson, R. G. J . Chem. Educ. 1968, 45, 581. Pearson, R. G. J .
Org. Chem. 1989, 54, 1423.
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