Kinetics and Mechanism of the Pyridinolysis of O-Ethyl S-Aryl Thiocarbonates in Aqueous Solution

Article abstract of DOI:10.1021/jo960781f

The reactions of a series of 3- and 4-substituted pyridines with S-(4-nitrophenyl), 3-(2,4-dinitrophenyl), and S-(2,4,6-trinitrophenyl) O-ethyl thiocarbonates (NPTC, DNPTC, and TNPTC, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 deg C, ionic strength 0.2 M (KCl). Pseudo-first-order rate coefficients (k_obsd) are found under amine excess. Plots of k_obsd vs free amine concentration are linear and pH independent. The Broensted-type plot for NPTC is linear whereas those for DNPTC and TNPTC are biphasic with slopes β₁ = 0.2 (high pK_a) for both, and β₂ = 0.9 and 0.8 (low pK_a) for DNPTC and TNPTC, respectively. The curvature center on the pK_a axis is pK_a⁰ = 8.6 (DNPTC) and 7.3 (TNPTC). The Broensted plots are consistent with the presence of a zwitterionic tetrahedral intermediate on the reaction path where its breakdown is rate determining for NPTC and there is a change in the rate-limiting step for the reactions of the other two substrates. Comparison of the stepwise reactions of the present work with the concerted ones of DNPTC and TNPTC with secondary alicyclic amines shows that the latter amines greatly destabilize T^+/- relative to isobasic pyridines. Comparison of the present reactions with other pyridinolysis indicates that substitution of the oxygen phenoxy atom by sulfur increases pK_a⁰ and the change of methyl to ethoxy as the remaining group also enlarges the pK_a⁰ and besides destabilizes T^+/-.

Full text of DOI:10.1021/jo960781f

5982
J . Org. Chem. 1996, 61, 5982-5985
Kin etics a n d Mech a n ism of th e P yr id in olysis of O-Eth yl S-Ar yl
Th ioca r bon a tes in Aqu eou s Solu tion
Enrique A. Castro,* Mar´ıa I. Pizarro, and J ose´ G. Santos*
Facultad de Quimica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
Received April 29, 1996X
The reactions of a series of 3- and 4-substituted pyridines with S-(4-nitrophenyl), S-(2,4-
dinitrophenyl), and S-(2,4,6-trinitrophenyl) O-ethyl thiocarbonates (NPTC, DNPTC, and TNPTC,
respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 °C, ionic strength
0.2 M (KCl). Pseudo-first-order rate coefficients (kobsd) are found under amine excess. Plots of
k
obsd vs free amine concentration are linear and pH independent. The Bro¨nsted-type plot for NPTC
is linear whereas those for DNPTC and TNPTC are biphasic with slopes â1 ) 0.2 (high pKa) for
both, and â2 ) 0.9 and 0.8 (low pKa) for DNPTC and TNPTC, respectively. The curvature center
o
on the pKa axis is pKa ) 8.6 (DNPTC) and 7.3 (TNPTC). The Bro¨nsted plots are consistent with
the presence of a zwitterionic tetrahedral intermediate on the reaction path where its breakdown
is rate determining for NPTC and there is a change in the rate-limiting step for the reactions of
the other two substrates. Comparison of the stepwise reactions of the present work with the
concerted ones of DNPTC and TNPTC with secondary alicyclic amines shows that the latter amines
greatly destabilize T( relative to isobasic pyridines. Comparison of the present reactions with other
pyridinolysis indicates that substitution of the oxygen phenoxy atom by sulfur increases pKao and
the change of methyl to ethoxy as the remaining group also enlarges the pKao and besides destabilizes
T(.
In tr od u ction
influence of the nucleophile by comparison of the present
reactions with those of secondary alicyclic amines with
the same substrates,8 (iii) assess the influence of the
leaving group by comparison with the pyridinolysis of
methyl aryl carbonates, and (iv) study the influence of
the substrate remaining group by comparison with the
pyridinolysis of aryl thiolacetates.7
Although the mechanisms of the aminolyses of acetate1
and benzoate2 esters and diaryl3 and alkyl aryl carbon-
ates4 are well understood, much less is known about
those concerning the thio derivatives of the above com-
pounds.
There have been lately some reports on the aminolysis
mechanisms of aryl thiobenzoates5 and thiolacetates6,7
and aryl O-ethyl thiolcarbonates.8 Nevertheless, more
kinetic studies are needed in order to clarify the mech-
anism of the latter reactions and to investigate the
influence of the nucleophile, the nucleofuge, and the
substrate remaining (“nonleaving”) group on the mech-
anism and specifically on the stability of the tetrahedral
intermediates formed in some of these reactions.3,4,7,8
In the present work we undertake a mechanistic study
on the pyridinolysis of O-ethyl substituted-phenyl thiol-
carbonates with the aim to (i) shed more light on the
reaction mechanisms of thiolcarbonates, (ii) examine the
Exp er im en ta l Section
Ma ter ia ls. 4-Nitrophenyl,9 2,4-dinitrophenyl,8a and 2,4,6-
trinitrophenyl8b O-ethyl thiolcarbonates were synthesized as
described. The series of pyridines were purified as previously
reported.10
Kin etic Mea su r em en ts. These were carried out in aque-
ous solution at 25.0 ( 0.1 °C, ionic strength 0.2 M (KCl), by
following spectrophotometrically the release of the correspond-
ing substituted benzenethiolate anion at 412 nm (the 4-nitro
derivative) or 400 nm (the other substrates), by the instru-
ments and methods described.7,8b The initial substrate con-
centration was (5-6) × 10-5 M, and the amine was at least in
10-fold excess over the substrate.
The reactions were followed for 4-5 half-lives, except the
slow reactions of 3-carbamoylpyridine (nicotinamide) with
O-ethyl 4-nitrophenyl thiolcarbonate, where the initial rate
method was used (reactions followed up to 2-5%). Pseudo-
first-order rate coefficients (kobsd) were found throughout. The
experimental reaction conditions and the kobsd values are
shown in Table 1.
P r od u ct Stu d ies. Some reactions were studied at 315 nm,
where an initial increase followed by a decrease of absorbance
was observed. Presumably this intermediate is 1-(methoxy-
carbonyl) substituted-pyridinium ion, in analogy with the
intermediates observed in the pyridinolysis of methyl chloro-
formate10 and methyl 2,4-dinitrophenyl carbonate.4b
4-Nitro, 2,4-dinitro, and 2,4,6-trinitro benzenethiolate an-
ions were identified as one of the final products of the
reactions. This was possible by comparison of the UV-vis
X Abstract published in Advance ACS Abstracts, J uly 15, 1996.
(1) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
7018.
(2) Koh, H. J .; Lee, H. C.; Lee, H. W.; Lee, I. Bull. Korean Chem.
Soc. 1995, 16, 839. Castro, E. A.; Valdivia, J . L. J . Org. Chem. 1986,
51, 1668.
(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.
(5) Lee, I.; Shim, C. S.; Lee, H. W. J . Chem. Res. (S) 1992, 90. Oh,
H. K.; Shin, C. H.; Lee, I. Bull. Korean Chem. Soc. 1995, 16, 657. Um,
I.-H.; Kwon, H.-J .; Kwon, D.-S.; Park, J .-Y. J . Chem. Res. (S) 1995,
301. Oh, H. K.; Shin, C. H.; Lee, I. J . Chem. Soc., Perkin Trans. 2 1995,
1169.
(6) Um, I.-H.; Choi, K.-E.; Kwon, D.-S. Bull. Korean Chem. Soc.
1990, 11, 362.
(7) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991, 63.
(8) (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.
(9) Al-Awadi, N.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1986,
1581.
(10) Bond, P. M.; Castro, E. A.; Moodie, R. B. J . Chem. Soc., Perkin
Trans. 2 1976, 68.
S0022-3263(96)00781-5 CCC: $12.00 © 1996 American Chemical Society

Pyridinolysis of O-Ethyl S-Aryl Thiocarbonates
J . Org. Chem., Vol. 61, No. 17, 1996 5983
Ta ble 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e P yr id in olysis of O-Eth yl S-Ar yl Th ioca r bon a tesa
Ta ble 2. Va lu es of p Ka for P yr id in es a n d kN for th e
P yr id in olysis of O-Eth yl S-Ar yl Th ioca r bon a tesa
pyridine
substituent
no. of
runs
kN/s-1 M-1
pyridine
substituent
pH
102[N]totb/M
103kobsd/s-1
pKa
NPTCb
DNPTb
31
14
0.33
0.12
TNPTCb
O-Ethyl S-(4-Nitrophenyl) Thiocarbonate (NPTC)
4-(dimethylamino)
4-(dimethylamino)
4-amino
3,4-dimethyl
4-methyl
9.87
9.37
6.77
6.25
5.86
5.37
3.43
3.4
1.6
0.014
0.0066
38
21
2.8
1.3
0.64
0.21
0.010
9.57
9.87
10.17
9.12
9.42
9.72
7.37
7.72
8.6
8.8
9.0
7.0
7.5
0.9-8.0
0.8-8.0
0.5-5.0
0.5-5.0
0.5-5.0
0.5-5.0
5.0-15
4.0-13
3.0-30
3.0-30
8.0-30
3.0-30
3.0-30
3.0-30
9.45-92.3
6
7
7
7
7
5
7
5
7
7
5
7
7
7
5
6
16.7-138
10.6-108
4-amino
2.98-27.9
4.24-41.6
5.26-53.5
0.68-1.95
0.65-1.77
3.74-22.0
2.80-20.0
7.10-21.7
0.036-0.32
0.042-0.33
0.038-0.31
0.005-0.009
0.006-0.012
3-methyl
-
-
none
0.0011
0.018
3-carbamoyl
2 × 10-5
4 × 10-4
3,4-dimethyl
4-methyl
a
Both the pKa and kN values were obtained in aqueous solution
at 25.0 °C, ionic strength 0.2 M (KCl); pKa values from ref 7.
b
Abbreviations defined in Table 1.
nonec
8.0
3-carbamoyl
7.0c 10-30
9.0d
3.0-30
O-Ethyl S-(2,4-Dinitrophenyl) Thiocarbonate (DNPTC)
4-(dimethylamino)
9.57
9.87
10.17
9.12
9.42
9.72
6.47
6.77
7.07
5.95
6.25
6.55
5.67
5.97
6.32
7.00
8.00
0.08-1.3
0.06-0.60
0.06-0.60
0.1-1.5
0.1-0.9
0.07-0.8
3.0-30
1.5-15
2.5-10
4.0-10
4.0-30
3.0-30
3.0-28
3.0-25
2.4-22
3.0-30
3.0-30
3.5-132
8.2-87
4.9-120
7.6-68
7.3-64
6.5-68
4.0-34
2.6-25
5.5-22
1.7-4.2
2.6-20
2.7-24
4.3-37
4.9-36
4.8-35
0.02-0.14
0.03-0.15
12
10
10
6
6
7
7
7
4
4
6
7
6
6
7
7
7
4-amino
3,4-dimethyl
4-methyl
none
3-carbamoylc
F igu r e 1. Bro¨nsted-type plots obtained in the reactions of
O-ethyl 4-nitrophenyl thiolcarbonate with pyridines (0, this
work) and secondary alicyclic amines (b, ref 8c), in aqueous
solution at 25.0 °C, ionic strength 0.2 M. The latter plot is
statistically corrected.8
O-Ethyl S-(2,4,6-Trinitrophenyl) Thiocarbonate (TNPTC)
4-dimethylamino
9.57
9.87
10.17
9.12
9.42
9.72
6.47
6.77
7.07
5.96
6.26
6.56
5.56
5.86
6.16
5.67
5.97
6.32
4.90
5.10
5.30
0.13-1.3
0.07-0.7
0.06-0.6
0.08-1.0
0.07-0.9
0.06-0.6
2.0-20
0.9-9.5
1.5-10
4.0-10
3.0-30
2.0-20
3.0-30
1.0-25
3.0-20
3.0-25
3.0-25
2.4-22
3.0-30
3.0-30
8.0-30
1.67-16.7
0.97-12.2
0.69-14.7
4.90-73.8
4.80-95.0
7.10-87.0
25.1-84.3
15.0-137
30.8-196
18.8-39.8
20.5-195
18.6-165
7.6-68.9
3.0-78.0
14.0-85.0
5.6-39.3
6.7-42.7
6.5-43.9
0.83-3.53
0.92-3.30
1.42-3.44
7
7
7
and kN ar the rate coefficients for hydrolysis and pyridi-
nolysis, respectively, of the thiolcarbonates.
4-amino
9
12
7
3,4-dimethyl
4-methyl
6
7
5
4
7
7
7
7
5
5
6
8
5
7
6
kobsd ) k0 + kN [N]
(1)
The k0 values were negligible compared to the ami-
nolysis term in eq 1, except in the reactions of the
substrates with nicotinamide, where the aminolysis rate
was relatively slow. At pH 5-7 the k0 values (found as
intercepts of plots of eq 1) were 4 × 10-6 s-1 (the
mononitro derivative), 2 × 10-5 s-1 (dinitro), and 7 × 10-4
s-1 (trinitro).
The second-order rate coefficients for aminolysis (kN)
were obtained as the slopes of plots of eq 1 and were pH
independent. These values, together with those of the
pKa of the conjugate acids of the pyridines, are shown in
Table 2.
With the pKa and kN values of Table 2 the Bro¨nsted-
type plots in Figures 1-3 were obtained. Also shown in
the figures, for comparison, are the plots found in the
reactions of the same substrates with secondary alicyclic
amines.8 The latter Bro¨nsted-type plots have been
statistically corrected.8,11 The linear plot for the pyridi-
nolysis of O-ethyl 4-nitrophenyl thiolcarbonate (NPTC)
has a slope of 0.8 (Figure 1). The curved lines for the
dinitro (DNPTC) and trinitro (TNPTC) derivatives were
calculated by means of a semiempirical equation (eq 2)
3-methyl
none
3-carbamoyle
a
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
Concentration of total amine (free base plus protonated forms).
b
c
d
In the presence of 0.005 M phosphate buffer. In the presence
e
of 0.005 M borate buffer. In the presence of 0.005 M acetate
buffer.
spectra at the end of the reactions with those of authentic
samples of the benzenethiols, under the same experimental
conditions.
Resu lts a n d Discu ssion
Plots of kobsd against free amine concentration at
constant pH were linear, in accordance with eq 1, where
N represents the substituted pyridine free base, and k0
(11) Bell, R. P. The Proton in Chemistry; Methuen: London, 1959;
p 159.

5984 J . Org. Chem., Vol. 61, No. 17, 1996
Castro et al.
a change in the rate-determining step, from breakdown
of T( to products (k2 step) to T( formation (k1 step) as
the amine basicity increases.1-4,7,8,10,12 The linear Bro¨n-
sted-type plot for the pyridinolysis of NPTC (Figure 1) is
consistent with the mechanism depicted by eq 3, where
the k2 step is rate limiting.
O
C
O–
C
O
C
k1
k2
EtO
SAr
EtO
SAr
EtO
N+
(3)
k–1
N+
T±
N
ArS–
Nonlinear Bro¨nsted-type plots have also been inter-
preted through structural variation of the transition state
in a single step (concerted) mechanism, according to the
Hammond postulate13 and Marcus and Murdoch equa-
tions.14 Nevertheless, we think the subject reactions of
this work are stepwise for the following reasons, although
a concerted mechanism for the pyridinolysis of the two
more reactive thiolcarbonates cannot be rigorously ex-
cluded.
F igu r e 2. Bro¨nsted-type plots obtained in the reactions of
O-ethyl 2,4-dinitrophenyl thiolcarbonate with pyridines (0,
this work) and secondary alicyclic amines (b, ref 8a). The latter
plot is statistically corrected.8 Experimental conditions as in
Figure 1.
(1) Given the rather small pKa range of the pyridines
studied here (ca. 7 pKa units) a slight and continous
variation of the Bro¨nsted slope is expected.14,15 Although
this could change for very low intrinsic barriers, the
relatively large free-energy of activation of the reactions
of this work (∆G* ca. 15 kcal/mol for the fastest reaction)
makes it difficult to reconcile the rather sharp Bro¨nsted
break for the pyridinolysis of the dinitro (DNPTC) and
trinitro (TNPTC) thiolcarbonates (Figures 2 and 3) with
the one-step process. We have tried to fit an equation
based on transition state variation in a single step15 to
our experimental Bro¨nsted plots, without success, as in
previous attempts.15,16
(2) The magnitude of the Bro¨nsted slopes at low pKa
for the pyridinolysis of DNPTC and TNPTC (â2 ) 0.9 and
0.8, respectively) and that for NPTC in the whole pKa
range (â2 ) 0.8) are in line with the values obtained in
the aminolysis of similar substrates where a stepwise
mechanism has been found with the k2 step of eq 3 as
rate determining.1-4,6,7,8c,10 Furthermore, the linearity of
the Bro¨nsted plots at low pKa for DNPTC and TNPTC is
not in accord with a structural variation of the transition
state in a single step, since a continuous Bro¨nsted
curvature is predicted in this case.14,15
F igu r e 3. Bro¨nsted-type plots obtained in the reactions of
O-ethyl 2,4,6-trinitrophenyl thiolcarbonate with pyridines (0,
this work) and secondary alicyclic amines (b, ref 8b). The latter
plot is statistically corrected.8 Experimental conditions as in
Figure 1.
based on the existence of a tetrahedral intermediate on
the reaction path.4,7,8,10 Similar equations have been
reported, which satisfactorily accounts for stepwise mech-
anisms.3,12
(3) In concerted mechanisms where curved Bro¨nsted-
type plots have been found the nature of the “nonleaving”
group of the substrate does not affect the position of the
o
log(kN/koN) ) â2(pKa - pKa ) - log[(1 + a)/2]
curvature center on the pKa axis (pKa ).17 In the pyridi-
o
nolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl thi-
olacetates the pKa values obtained are 6.6 and 4.9,
(2)
o
o
log a ) (â2 - â1)(pKa - pKa )
respectively,7 which are very different to those found in
the present reactions (pKao ) 8.6 and 7.3 for DNPTC and
TNPTC, respectively). Substantial pKao shifts have also
been found in other stepwise aminolysis reactions by
substitution of the “nonleaving” group of the sub-
strate.3,4,16
In eq 2, â1 and â2 are the Bro¨nsted slopes at high and
low pKa, respectively, and ko and pKa are the corre-
o
N
sponding values at the center of the curvature.
The Bro¨nsted curves for pyridinolysis in Figures 2 and
3 were calculated with the following parameters:
DNPTC: log ko ) 0.87, pKa ) 8.6, â1 ) 0.20 and â2 )
o
N
(13) Hammond, G. S. J . Am. Chem. Soc. 1955, 77, 334.
(14) Marcus, R. A. J . Phys. Chem. 1968, 72, 891. Murdoch, J . R. J .
Am. Chem. Soc. 1972, 94, 4410. Albery, W. J .; Kreevoy, M. M. Adv.
Phys. Org. Chem. 1978, 16, 87. More O’Ferrall, R. A. J . Chem. Soc.,
Perkin Trans. 2 1981, 1084. Murdoch, J . R. J . Phys. Chem. 1983, 87,
1571.
0.90. TNPTC: log ko ) 0.70, pKao ) 7.3, â1 ) 0.20, and
N
â2 ) 0.80. The errors of the slopes are (0.1, and those
of pKa and log ko are (0.2 and 0.1, respectively.
o
N
The Bro¨nsted-type curves can be explained by the
(15) Castro, E. A.; Moodie, R. B. J . Chem. Soc., Chem. Commun.
1973, 828.
(16) (a) Castro, E. A.; Iban˜ez, F.; Saitua, A. M.; Santos, J . G. J .
Chem. Res. (S) 1993, 56. (b) Castro, E. A.; Iban˜ez, F.; Salas, M.; Santos,
J . G.; Sepulveda, P. J . Org. Chem. 1993, 58, 459.
mechanism described in eq 3, where N represents the
substituted pyridine. The Bro¨nsted break results from
(12) Cullum, N. R.; Renfrew, A. H. M.; Rettura, D.; Taylor, J . A.;
Whitmore, J . M. J .; Williams, A. J . Am. Chem. Soc. 1995, 117, 9200.
(17) Song, B. D.; J encks, W. P. J . Am. Chem. Soc. 1989, 111, 8479.

Pyridinolysis of O-Ethyl S-Aryl Thiocarbonates
J . Org. Chem., Vol. 61, No. 17, 1996 5985
In the reactions of DNPTC and TNPTC with secondary
alicyclic amines in water linear Bro¨nsted-type plots with
slopes â ) 0.56 and 0.48, respectively, have been obtained
(see Figures 2 and 3).8a,b The magnitude of the slopes
together with the fact that the predicted hypothetical
break for stepwise reactions was not observed led to the
conclusion that the reactions are concerted.8a,b The fact
that in the pyridinolysis of the above substrates a
tetrahedral intermediate is formed in the reaction path
(T( in eq 3) indicates that substitution of a pyridine by
a secondary alicyclic amine implies a substantial desta-
bilization of T(; this is of such magnitude as to change
the mechanism from stepwise to enforced concerted.18
It has been found that pyridines are expelled more
slowly than isobasic quinuclidines (tertiary alicyclic
amines) from the zwitterionic tetrahedral intermediate
formed in the aminolysis of phenyl 4-nitrophenyl carbon-
ate.3 This means that tertiary alicyclic amines destabi-
lize kinetically the intermediate relative to pyridines,
which is in line with our findings.
Similar destabilizations of tetrahedral intermediates
have been reported in the aminolysis of 2,4-dinitrophenyl
methyl carbonate,16a 2,4-dinitrophenyl acetate,19 2,4-
dinitrophenyl and 2,4,6-trinitrophenyl thiolacetates,7 and
2,4-dinitrophenyl and 2,4,6-trinitrophenyl O-ethyl dithio-
carbonates.16b,20 In all these works the above destabiliza-
tion has been attributed to a faster nucleofugality from
the tetrahedral intermediate of a secondary alicyclic
amine compared to an isobasic pyridine.
In the pyridinolysis of 2,4-dinitrophenyl and 2,4,6-
trinitrophenyl thiolacetates biphasic Bro¨nsted-type plots
with break centers at pKa 6.6 and 4.9, respectively, were
obtained.7 The corresponding values found in this work
for DNPTC and TNPTC are larger, i.e. substitution of
methyl by ethoxy as the remaining (“nonleaving”) group
o
of the substrate increases the value of pKa . According
to the hypothesis of the tetrahedral intermediate pKao is
linearly dependent on log(k-1/k2).22 On the other hand
it is known that the above change increases both the k-1
and k2 values from tetrahedral intermediates similar to
those formed in the reactions of the present work.23 This
means that substitution of methyl by ethoxy as the
substrate “nonleaving” group enlarges k-1 more than k2,
implying a kinetic destabilization of the addition inter-
mediate.
Other examples of tetrahedral intermediates instabil-
ity caused by similar changes are the following: (i) The
stepwise acetyl transfer between substituted pyridines
and primary and secondary amines,24 which shifts to a
concerted reaction when methoxycarbonyl is being trans-
ferred.25 The concerted mechanism is enforced by the
high instability caused to the tetrahedral intermediate
by the change of a methyl group by methoxy. (ii) The
pyridinolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl
acetates4c,d compared to the corresponding methyl
carbonates,4b,d where a pKao increase of 0.5 and 1.5 units
was found for the dinitro and trinitrophenyl carbonates,
respectively, in relation to the acetates. (iii) The reac-
tions of secondary alicyclic amines with 2,4-dinitrophenyl
acetate19 and 2,4-dinitrophenyl methyl carbonate,16a where
a pKao increase of 0.4 unit was observed for the carbonate.
(iv) The reactions of the latter amines with 4-nitrophenyl
thiolacetate22 compared to O-ethyl 4-nitrophenyl thiol-
It is not clear to us why pyridines should stabilize a
tetrahedral intermediate more than isobasic alicyclic
amines. Gresser and J encks have argued that this
should be due to “a significant contribution of resonance
stabilization by electron donation from the pyridine to
the oxygen leaving group in the transition state for the
breakdown of the intermediate”.3
carbonate;8c the carbonate showing a larger pKa by 0.2
o
units.
Another type of destabilization of a tetrahedral inter-
mediate is observed by comparison of the aminolysis of
acetyl chloride26 and benzoyl fluorides17 in water. The
former reactions are stepwise, whereas the latter are
concerted, i.e. the change of methyl by phenyl as the acyl
group produces a great destabilization of the addition
intermediate. This was attributed to resonance with the
benzene ring which accelerates the departure of both the
amine and F- from the intermediate (enhancement of k-1
and k2), so that the addition “intermediate” does not have
a significant lifetime and the concerted mechanism is
enforced.17
We are at present investigating the kinetics of the
pyridinolysis of O-ethyl substituted phenyl dithiocarbon-
ates in water in order to compare it with the kinetics of
the reactions of this work, and hence assess the influence
of the thiocarbonyl group on the kinetics and mechanisms
of these reactions.
The pyridinolysis of 2,4-dinitrophenyl and 2,4,6-trini-
trophenyl methyl carbonates exhibit biphasic Bro¨nsted-
type plots with the curvature center at pKa ) pKao ) 7.8
and 6.5, respectively.4b,d These values are smaller than
o
those obtained in this work for DNPTC and TNPTC (pKa
) 8.6 and 7.3, respectively). In other words, the change
of the oxygen atom of the leaving group attached to the
carbonyl carbon by a sulfur atom results in an increase
of pKao (assuming that substitution of methoxy by ethoxy
does not affect the pKao value). This is not in agreement
with the results obtained in the pyridinolysis of the
corresponding acetates and thiolacetates, where substitu-
tion of oxygen by sulfur in the leaving group produces a
o
decrease of pKa of 0.7 and 0.1 units for the dinitro and
trinitro derivatives, respectively.4c,d,7 The discrepancy
observed for the carbonates and thiolcarbonates could
arise from a difference between the methoxy and ethoxy
groups, although also other reasons should be invoked,
in view of the similar basicities and Hammett σ values
of the groups concerned.21
Ack n ow led gm en t. The financial support given to
this work by “Fondo Nacional de Desarrollo Cientifico
y Tecnologico” (FONDECYT) of Chile is gratefully
acknowledged.
(18) 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) Castro, E. A.; Ureta, C. J . Org. Chem. 1990, 55, 1676.
(20) Castro, E. A.; Araneda, C. A.; Santos, J . G., unpublished results
on the pyridinolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl
O-ethyl dithiocarbonates.
(21) Albert, A.; Serjeant, E. P. The Determination of Ionization
Constants; Chapman and Hall: London, 1971. McDaniel, D. H.; Brown,
H. C. J . Org. Chem. 1958, 23, 420. Hansch, C.; Leo, A.; Taft, R. W.
Chem. Rev. 1991, 91, 165.
J O960781F
(22) Castro, E. A.; Ureta, C. J . Org. Chem. 1989, 54, 2153.
(23) Castro, E. A.; Cubillos, M.; Iban˜ez, F.; Moraga, I.; Santos, J . G.
J . Org. Chem. 1993, 58, 5400.
(24) Fersht, A. R.; J encks, W. P. J . Am. Chem. Soc. 1970, 92, 5442.
(25) Chrystiuk, E.; Williams, A. J . Am. Chem. Soc. 1987, 109, 3040.
(26) Palling, D. J .; J encks, W. P. J . Am. Chem. Soc. 1984, 106, 4869.