Kinetics and Mechanism of the Pyridinolysis of 2,4-Dinitrophenyl and 2,4,6-Trinitrophenyl O-Ethyl Dithiocarbonates

Article abstract of DOI:10.1021/jo961275t

The title reactions are subjected to a kinetic study in water, 25.0°C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order rate coefficients are found, which are linearly related to the free amine concentration. No dependence of the slopes of these plots (k_N) on the pH values was observed. The Bro?nsted-type plots (log k_N vs pK_a of the pyridines) are biphasic with slopes β₁ = 0.2 (high pK_a) for both reaction series and β₂ = 1.0 and 0.9 (low pK_a) for the dinitro and the trinitro derivatives, respectively, and with the curvature center at pK_a = pK_a⁰ = 6.9 and 5.6 for the dinitro and the trinitro compounds, respectively. These results can be explained by the formation of a zwitterionic tetrahedral intermediate (T±) in a stepwise reaction. Comparison of these Bro?nstedtype plots with those in the reactions of the same substrates with secondary alicyclic amines shows that the latter amines are better nucleofuges from T± than isobasic pyridines. Comparison of the Bro?nsted-type plots for the dinitro and trinitro derivatives obtained in this work with those for the pyridinolysis of S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) O-ethyl thiocarbonates indicates that substitution of S^- by O^- in T^±; increases the amine/ArS^- nucleofugality ratio from T^±.

Full text of DOI:10.1021/jo961275t

1
26
J . Org. Chem. 1997, 62, 126-129
Kin etics a n d Mech a n ism of th e P yr id in olysis of 2,4-Din itr op h en yl
a n d 2,4,6-Tr in itr op h en yl O-Eth yl Dith ioca r bon a tes
Enrique A. Castro,* Carlos A. Araneda, and J os e´ G. Santos*
Facultad de Qu ı´ mica, Pontificia Universidad Cat o´ lica de Chile, Casilla 306, Santiago 22, Chile
Received J uly 8, 1996X
The title reactions are subjected to a kinetic study in water, 25.0 °C, ionic strength 0.2 M (KCl).
Under amine excess, pseudo-first-order rate coefficients are found, which are linearly related to
the free amine concentration. No dependence of the slopes of these plots (k
was observed. The Br o¨ nsted-type plots (log k vs pK of the pyridines) are biphasic with slopes â
0.2 (high pK ) for both reaction series and â ) 1.0 and 0.9 (low pK
trinitro derivatives, respectively, and with the curvature center at pK
N
) on the pH values
N
a
1
)
a
2
a
) for the dinitro and the
0
a
) pK
a
) 6.9 and 5.6 for the
dinitro and the trinitro compounds, respectively. These results can be explained by the formation
(
of a zwitterionic tetrahedral intermediate (T ) in a stepwise reaction. Comparison of these Br o¨ nsted-
type plots with those in the reactions of the same substrates with secondary alicyclic amines shows
(
that the latter amines are better nucleofuges from T than isobasic pyridines. Comparison of the
Br o¨ nsted-type plots for the dinitro and trinitro derivatives obtained in this work with those for the
pyridinolysis of S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) O-ethyl thiocarbonates indicates
-
-
(
-
(
that substitution of S by O in T increases the amine/ArS nucleofugality ratio from T .
(
In tr od u ction
mediate (T ) was deduced from the biphasic Br o¨ nsted-
type plots obtained.9
-11
Although the kinetics of the aminolysis of aryl esters
and carbonates have been extensively studied and their
The object of this work is to shed more light into the
mechanism of the aminolysis of dithiocarbonates and to
investigate the influence of the nature of the amine on
1
-4
mechanisms are well established,
those of the thio-
analogues have been given less attention. Among the
latter there are in the literature some mechanistic studies
(
the mechanism and stability of T , by comparing the
present reactions with those of the same substrates with
5
on the aminolysis of 4-nitrophenyl thionobenzoate, S-(4-
10
secondary alicyclic amines.
Also of interest is to
nitrophenyl) thioacetate,6 and S-phenyl thiobenzoates
examine the influence of S in T( by comparison of the
-
and phenyl dithiobenzoates.7
reactions of this work with the pyridinolysis of the
corresponding dinitrophenyl and trinitrophenyl thiolcar-
bonates.11
We have recently investigated the mechanism of the
reactions of secondary alicyclic amines with S-aryl dithio-
acetates8 and O-ethyl S-aryl dithiocarbonates.
9,10
We
have also kinetically studied the pyridinolysis of S-(2,4-
dinitrophenyl) and S-(2,4,6-trinitrophenyl) O-ethyl thio-
carbonates (or thiolcarbonates).11 In some of these
reactions the existence of a tetrahedral addition inter-
Exp er im en ta l Section
Ma ter ia ls. The substituted pyridines were purified as
described,12 and the substrates, O-ethyl 2,4-dinitrophenyl
dithiocarbonate (DNPDTC) and O-ethyl 2,4,6-trinitrophenyl
X
Abstract published in Advance ACS Abstracts, December 1, 1996.
1) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
018.
1
0
(
dithiocarbonate (TNPDTC), were prepared as reported.
7
6
Kin etic Mea su r em en ts. These were performed spectro-
photometrically by following the substituted benzenethiolate
anion release at 400 nm in a Perkin-Elmer Lambda 3 instru-
ment. The reactions were started by addition of a stock
solution (7-10 µL) of the substrate in acetonitrile into to the
kinetic solutions (2.5 mL) contained in 1-cm cells placed in
the thermostated compartment (25.0 ( 0.1 °C) of the spectro-
photometer. The initial substrate concentration was (4-6) ×
(2) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
970.
(3) Fife, T. H.; Hutchins, J . E. C. J . Am. Chem. Soc. 1981, 103, 4194.
Brunelle, D. J . Tetrahedron Lett. 1982, 23, 1739. Castro, E. A.; Gil, F.
J . J . Am. Chem. Soc. 1977, 99, 7611. Castro, E. A.; Iba n˜ ez, F.; Lagos,
S.; Schick, M.; Santos, J . G. J . Org. Chem. 1992, 57, 2691.
(4) Kovach, I. M.; Belz, M.; Larson, M.; Rousy, S.; Schowen, R. L. J .
Am. Chem. Soc. 1985, 107, 7360. Neuvonen, H. J . Chem. Soc., Perkin
Trans. 2 1987, 159. Yoh, S.-D.; Kang, J .-K.; Kim, S.-H. Tetrahedron
-
5
1
0
M.
1
3
988, 44, 2167. Knowlton, R. C.; Byers, L. D. J . Org. Chem. 1988, 53,
862. Castro, E. A.; Ureta, C. J . Org. Chem. 1990, 55, 1676. Koh, H.
All reactions were carried out under amine excess over the
substrate. Pseudo-first-order rate constants (kobsd) were found
throughout from ln(A - A) vs time plots, where A and A are
J .; Lee, H. C.; Lee, H. W.; Lee, I. Bull. Korean Chem. Soc. 1995, 16,
39.
5) Campbell, P.; Lapinskas, B. A. J . Am. Chem. Soc. 1977, 99, 5378.
Um, I.-H.; Kwon, H.-J .; Kwon, D.-S.; Park, J .-Y. J . Chem. Res., Synop.
8
∞
∞
(
the absorbances at “infinity” and at variable times, respec-
tively. The reactions of each amine were measured at three
pH values, maintained constant during the reactions by partial
acidification of the amine (in most cases) or by addition of an
external buffer. The experimental conditions of the reactions
and the values of kobsd are shown in Tables 1 and 2.
1
995, 301.
6) Um, I. H.; Choi, K. E.; Kwon, D. S. Bull. Korean Chem. Soc. 1990,
1, 362.
7) Lee, I.; Shim, C. S.; Lee, H. W. J . Chem. Res., Synop. 1992, 90.
(
1
(
Oh, H. K.; Shin, C. H.; Lee, I. Bull. Korean Chem. Soc. 1995, 16, 657.
Oh, H. K.; Shin, C. H.; Lee, I. J . Chem. Soc., Perkin Trans. 2 1995,
1
169.
8) (a) Castro, E. A.; Ib a´ n˜ ez, F.; Santos, J . G.; Ureta, C. J . Chem.
P r od u ct St u d ies. The presence of 2,4-dinitro- or 2,4,6-
trinitrobenzenethiolate anions as one of the products of the
(
Soc., Perkin Trans. 2 1991, 1919. (b) Castro, E. A.; Ib a´ n˜ ez, F.; Santos,
J . G.; Ureta, C. J . Org. Chem. 1992, 57, 7024.
(
9) Cabrera, M.; Castro, E. A.; Salas, M.; Santos, J . G.; Sep u´ lveda,
P. J . Org. Chem. 1991, 56, 5324.
10) Castro, E. A.; Ib a´ n˜ ez, F.; Salas, M.; Santos, J . G.; Sep u´ lveda,
P. J . Org. Chem. 1993, 58, 459.
(11) Castro, E. A.; Pizarro, M. I.; Santos, J . G. J . Org. Chem. 1996,
61, 5982.
(12) Bond, P. M.; Castro, E. A.; Moodie, R. B. J . Chem. Soc., Perkin
Trans. 2 1976, 68.
(
S0022-3263(96)01275-3 CCC: $14.00 © 1997 American Chemical Society

Pyridinolysis of Dithiocarbonates
J . Org. Chem., Vol. 62, No. 1, 1997 127
Ta ble 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
Ta ble 3. Va lu es of p Ka for Su bstitu ted P yr id in iu m Ion s
a n d kN for th e P yr id in olysis of DNP DTC a n d TNP DTC
a
a
th e P yr id in olysis of DNP DTC
1
pyridine
substituent
no. of
runs
kN/s- M-1 c
pyridine
substituent
FNb 10 [N]tot/M 10 kobsd/s
2
c
3
-1
pH
pKab
DNPDTC
TNPDTC
4
4
3
4
3
-dimethylamino 9.58 0.33
.88 0.50
0.06-0.90
0.05-0.50
0.10-0.50
0.12-0.85
0.70-1.3
0.55-1.5
2.0-18
2.9-40
3.6-33
6.8-42
2.2-15
17-38
22-45
32-173
38-183
23-171
4.8-62
15-95
27-78
13-33
16-53
24-92
53-85
49-80
36-66
1.2-1.5
7
5
5
5
5
5
8
7
7
6
7
6
4
4
5
6
6
6
6
6
6
4
4
3
4
3
-N(CH3)2
-NH2
,4-(CH3)2
-CH3
9.87
9.37
6.77
6.25
5.86
5.37
3.43
2.97
13
4.8
1.0
0.55
13
5.5
2.9
1.1
0.79
0.32
0.014
0.006
9
1
0.18 0.67
9.07 0.33
-amino
9
9
.37 0.50
.67 0.67
-CH3
0.30
none
3
3
0.062
4.7 × 10
,4-dimethyld
-methyl
8.20 0.96
8
8
-4
-CONH2
-Cl
.50 0.98
.80 0.99
3.0-17
2.0-16
a
5.95 0.33
3.0-30
Both the pKa and kN values were obtained in aqueous solution
b
6
6
.25 0.50
.55 0.67
3.0-30
at 25.0 °C, ionic strength 0.2 M (KCl). Values taken from ref 13.
These values are subject to a standard deviation of ca. 10%.
c
7.0-21
-methyl
5.56 0.33
4.0-20
5
6
.86 0.50
.16 0.67
5.0-30
4.0-40
nonee
7.00 0.98 25-75
7
8
.50 0.99 25-75
.00 1.00 20-70
3
-carbamoyle
7.00 1.00 20-70
7
8
.50 1.00 20-80
.00 1.00 20-80
0.56-0.82
0.37-0.57
a
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
b
c
Fraction of amine free base. Concentration of total amine (free
d
base plus protonated forms). In the presence of borate buffer 5
-
3
e
×
10 M. In the presence of phosphate buffer 5 × 10-3 M.
Ta ble 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
a
th e P yr id in olysis of TNP DTC
pyridine
substituent
no. of
runs
2
3
-1
pH
FN
10 [N]tot/M 10 kobsd/s
F igu r e 1. Br o¨ nsted-type plots obtained in the pyridinolysis
of DNPDTC (4) and TNPDTC (9) in water, 25.0 °C, ionic
strength 0.2 M (KCl).
4
4
3
4
3
-dimethylamino 9.58 0.33 0.10-0.35
7.9-19
8.5-46
7.6-39
2.9-13
2.4-17
4.7-24
1.4-14
3.0-23
13-22
7.2-72
18-162
46-151
8.7-79
21-156
25-217
6.9-31
8.4-67
9.1-83
1.1-4.2
1.0-4.5
1.1-4.3
6
6
7
6
6
4
6
6
6
6
7
7
7
6
7
5
7
7
6
7
5
5
6
6
9
.88 0.50 0.10-0.60
1
0.18 0.67 0.06-0.42
9.07 0.33 0.15-0.65
-amino
9
9
.37 0.50 0.10-0.60
.67 0.67 0.15-0.65
Resu lts a n d Discu ssion
,4-dimethyl
-methyl
-methyl
6.47 0.33 0.10-1.3
The kinetic law obtained in the present reactions is
-
6
7
.77 0.50 0.20-1.5
.07 0.67 0.68-1.1
given by eqs 1 and 2, where ArS is the leaving group
(
2,4-dinitrobenzenethiolate or 2,4,6-trinitrobenzenethi-
olate anions), kobsd is the pseudo-first-order rate constant,
and k are the rate constants for hydrolysis and
5.95 0.33 2.0-20
6
6
5.56 0.33 3.0-30
5
6
5.67 0.67 2.5-15
5
6
4.90 0.97 2.5-25
5
5
.25 0.50 3.0-30
.55 0.67 7.0-21
k
0
N
aminolysis of the substrate, respectively, and N and S
represent the free amine and the substrate, respectively.
.86 0.50 5.0-40
.16 0.67 4.0-40
none
d[ArS-]
.97 0.80 2.5-25
.32 0.90 2.5-25
)
kobsd[S]
(1)
(2)
dt
kobsd ) k + k [N]
3
3
-carbamoylb
.10 0.98 2.6-26
.30 0.99 2.5-25
0
N
-chloroc
7.00 1.00 3.0-8.0
7
8
0.47-0.73
0.36-0.73
0.33-0.64
0 N
The value of k was negligible compared to that of k -
.50 1.00 3.0-8.0
.00 1.00 3.0-8.0
[N], except for the reactions of TNPDTC with 3-chloro-
pyridine and those of DNPDTC with pyridine and
nicotinamide.
a
b
Experimental conditions and symbols as in Table 1. In the
-
3
c
presence of acetate buffer 5 × 10
M. In the presence of
-
3
phosphate buffer 5 × 10 M.
The values of k
of eq 2, i.e., kobsd vs [N] at constant pH. The k
were independent of pH for all the reactions. These
values are shown in Table 3, together with the pK values
of the conjugate acids of the amines under the same
N
were obtained as the slopes of plots
N
values
reactions of DNPDTC and TNPDTC, respectively, was deduced
by comparison of the UV-vis spectra at the end of some
reactions with those of authentic samples of the substituted
benzenethiols under the same experimental conditions.
The accumulation and later decomposition of an intermedi-
ate was observed in the reactions of DNPDTC with pyridine
and nicotinamide. This was achieved by HPLC at 254 nm,
using as eluant a solution of acetate buffer in methanol at pH
a
1
3
experimental conditions of the kinetic measurements.
With the k and pK values of Table 3, the Br o¨ nsted-
type equation for the reactions under study was plotted
Figure 1). The lines in the plots were calculated by
N
a
(
4
.9. This intermediate could not be identified, but it is
means of a semiempirical equation (eq 3, see derivation
in the Appendix) based on the existence of a zwitterionic
presumably 1-(ethoxythiocarbonyl)piridinium ion (reaction
with pyridine) and 1-(ethoxythiocarbonyl)-3-carbamoylpyri-
dinium ion (reaction with nicotinamide). A similar intermedi-
ate, 1-(methoxycarbonyl)pyridinium ion, was detected by UV
spectrophotometry in the reaction of methyl chloroformate
(
tetrahedral addition intermediate (T ) on the reaction
(13) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991,
63.
1
2
with pyridine.

1
28 J . Org. Chem., Vol. 62, No. 1, 1997
Castro et al.
Sch em e 1
This steric hindrance must be compensated by a larger
electron withdrawal effect from the trinitrophenylthio
group of TNPDTC relative to the dinitro analogue in
DNPDTC. A similar situation occurs in the reactions of
the same substrates with secondary alicyclic amines,
where the steric hindrance of the reactions of the trinitro
derivative is such that the k
N
values are slightly smaller
for TNPDTC compared to the dinitro derivative, when
amine attack is rate determining.10 Also, the rate
constant for attack (k
1
) of substituted quinuclidines on
2
,4-dinitrophenyl phenyl carbonate is smaller than that
of the same amines on 3,4-dinitrophenyl phenyl carbon-
2
ate, and this has been attributed to a steric effect by
the o-nitro in the former substrate.2
Figure 1 shows that the reactivity of TNPDTC is higher
than that of DNPDTC when the k
rate determining. In this case k ) K
higher reactivity of TNPDTC should be due to a much
larger k value and a similar K ()k /k-1) value for both
reactions; this can be explained as follows. A much larger
2
step of Scheme 1 is
N
1 2
k
(see above). This
path and a change in the rate-determining step from its
decomposition to its formation as the basicity of the
2
1
1
1
,2,9-13
nucleophile (amine) increases.
In eq 3, â
1
and â
2
-
-
k
2
(
is expected for TNPS leaving relative to DNPS from
0
0
1 + a
2
T of Scheme 1, due to the much lower basicity of the
log (k /k ) ) â (pK - pK ) - log
N
N
2
a
a
former anion (pK
.4, respectively).13 A similar K
reactions since the k values are similar (see above), and
a
of TNPSH and DNPSH are 1.4 and
3
1
is reasonable for both
0
log a ) (â - â )(pK - pK )
(3)
1
2
1
a
a
the values of k-1 for both reactions should be alike in view
-
are the slopes at high and low pK
a
values, respectively,
are the corresponding values for an
amine at the center of the curvature. The Br o¨ nsted lines
were calculated with the following parameters: â ) 0.2
for the reactions of both substrates, â ) 1.0 and 0.9 for
DNPDTC and TNPDTC, respectively, log k
that the higher basicity of DNPS (stronger electronic
0
0
-
and pK
a
and k
N
push) compared to TNPS should be compensated by a
stronger steric push by TNPS, resulting in a similar push
from both groups in T to expel the amine.2
(
1
0
The fact that the pK
a
value is lower for the TNPDTC
2
0
N
) 0.078 and
reactions compared to DNPDTC (Figure 1) is in agree-
0
0
pK
0.17 and pK
The values of â
a
) 6.9 for the reactions of DNPDTC, and log k
N
)
ment with the hypothesis of the tetrahedral intermediate.
0
0
-
a
) 5.6 for the pyridinolysis of TNPDTC.
and â are in accord to the correspond-
The stepwise reactions exhibit a decrease of pK
a
as
the leaving group of the substrate decreases its
1
2
basicity.1
,2,9,10,13-15
ing values found in other similar aminolyses: the reac-
2
This is due to the larger k for the less
1
tions of primary amines with aryl acetates, quinuclidines
basic nucleofuge which needs a less basic amine (lower
2
2
with diaryl carbonates, secondary alicyclic amines with
a 2
pK ) to satisfy the relation k ) k-1.
4
-nitrophenyl, 2,4-dinitrophenyl and 2,4,6-trinitrophenyl
Nonlinear Br o¨ nsted-type plots were also found in the
reactions of DNPDTC and TNPDTC with secondary
alicyclic amines in water. These were explained through
the presence of a zwitterionic tetrahedral intermediate
on the reaction pathway and a change in the rate-
9
,10
O-ethyl dithiocarbonates,
pyridines with methyl chlo-
roformate,12 and pyridines and secondary alicyclic amines
with 2,4-dinitrophenyl and 2,4,6-trinitrophenyl thiolac-
etates.13
10
determining step.10 The pK
0
values are 9.2 and 8.4 for
According to the rate law and the Br o¨ nsted-type plots
obtained in the present work, and the intermediate and
product studies, we propose Scheme 1 as the most
probable mechanism for the present reactions, where Ar
is 2,4-dinitrophenyl or 2,4,6-trinitrophenyl, and -Nd
represents the substituted pyridine.
a
1
0
the aminolysis of DNPDTC and TNPDTC, respectively.
According to the hypothesis of the tetrahedral inter-
mediate it can be shown that the value of pK is linearly
a
0
1
4,16
related to the log of the k-1/k
2
ratio, through eq 4.
0
In Scheme 1, the k
at high pK values, i.e., for amines such that k-1 , k
this case k ) k and the Br o¨ nsted slope is â . On the
other hand, the k step should be rate limiting at low
pK values where k-1 . k , and k ) K (K ) k
In this case the Br o¨ nsted slope is â . Near the center of
the Br o¨ nsted curvature there is no clear rate-determining
step and k ) k /(k-1 + k ). A nonlinear Br o¨ nsted-type
relationship should occur in this pK region due to the
sum of terms in the denominator of the k equation. At
the very center of the Br o¨ nsted curvature, k-1 ) k , i.e.,
should leave the tetrahedral
1
step should be rate determining
log(k-1/k ) ) (â - â ) (pK - pKa)
(4)
2
2
1
a
a
2
. In
N
1
1
Since for the reactions of DNPDTC and TNPDTC with
secondary alicyclic amines, the values of â and â are
.8 and 0.2, respectively, very similar to those for the
2
2
1
a
2
N
1
k
2
1
1
/k-1).
10
0
2
0
pyridinolysis of these substrates, the value of pK
4
k
a
in eq
is decisive in assessing the relative magnitude of the
2
-1/k ratios when comparing the reactions of a given
N
1
k
2
2
a
substrate with pyridines and alicyclic amines. Since the
N
0
a
pK values for the aminolyses of DNPDTC and TNPDTC
2
are greater than those for the corresponding pyridinoly-
ses, it follows from eq 4 that for a given secondary
0
a pyridine of pK
intermediate at a rate equal to that of ArS .
It can be seen in Figure 1 that the k values for the
a
) pK
a
-
2,9-13
N
(
14) Castro, E. A.; Ureta, C. J . Org. Chem. 1989, 54, 2153.
(15) J encks, W. P.; Gilchrist, M. J . Am. Chem. Soc. 1968, 90, 2622.
16) According to ref 14, d[log(k ) d(pK ). Integrating
this expression between k /k-1 ) 1 at the corresponding pK
and k /k-1 at the corresponding pK , one gets log(k /k-1) ) (â
(pK - pK ). Multiplying this by -1, one obtains eq 4.
reactions of both substrates with the two most basic
pyridines are very similar. This fact can be explained
by steric hindrance of the second o-nitro group of TNP-
DTC to the rate-determining attack of these pyridines.
(
2
/k-1)] ) (â
2
- â
1
a
0
2
a
) pK
- â
a
,
2
a
2
2
1
)
0
a
a

Pyridinolysis of Dithiocarbonates
J . Org. Chem., Vol. 62, No. 1, 1997 129
1
4
alicyclic amine the k-1/k
of an isobasic pyridine.
It is known that the value of the rate constant for
expulsion of the nucleofuge from a zwitterionic tetrahe-
2
ratio is greater compared to that
found in the aminolysis of S-phenyl thioacetate, with
0
13
0
an estimated pK
a
) 12.2, shows a larger pK
a
than that
calculated for the same aminolysis of phenyl dithioac-
0
8a
etate, pK
a
ca. 10.
Also, the biphasic Br o¨ nsted-type
dral intermediate (k
2
) is not significantly dependent on
plots obtained in the reactions of secondary alicyclic
the basicity or nature of the amino moiety.1 This is due
,2
9
amines with O-ethyl 4-nitrophenyl dithiocarbonate and
O-ethyl S-(4-nitrophenyl) thiocarbonate20 show the same
to the lack of an electron pair on the nitrogen atom of
0
the cationic amino group in the zwitterionic intermediate
feature, the latter exhibiting a larger pK
a
()10.7) than
(
0
(
T ), which prevents a “push” from this moiety to expel
the former (pK
a
) 9.6). A last example is the linear
Br o¨ nsted-type plot found in the aminolysis of 4-nitro-
> 11) compared to the biphasic
plot obtained in the same reactions of 4-nitrophenyl
1
,2
the leaving group.
0
phenyl benzoate (pK
a
Since the k-1/k
amine relative to an isobasic pyridine and since k
2
ratio is larger for a secondary alicyclic
is
2
0
ca. 9.2.5
thionobenzoate, with pK
a
independent of the amine nature it follows that k-1 is
larger for the former amine, i.e., the secondary alicyclic
amines are better nucleofuges from T than isobasic
pyridines.
(
Ack n ow led gm en t. We thank “Fondo Nacional de
Desarrollo Cient ´ı fico y Tecnol o´ gico” of Chile for financial
assistance.
The above is in agreement with the results of Gresser
and J encks on the aminolysis of diaryl carbonates:
(
Ap p en d ix
quinuclidines are better nucleofuges from T than iso-
2
basic pyridines. Also in accord is the finding that the
Equation 3 was deduced as follows: Applying the
0
pK
a
values for the reactions of 2,4-dinitrophenyl and
(
steady state condition to T in Scheme 1, eq 1a results.
2
,4,6-trinitrophenyl thiolacetates with secondary alicyclic
amines are larger than those for the corresponding
reactions of pyridines with the same substrates;13 i.e., the
former amines are expelled faster than isobasic pyridines
k ) k k /(k + k2)
(1a)
N
1
2
-1
At the center of the Br o¨ nsted curvature k-1 ) k
2
;
0
0
therefore, k
N
1
) k /2 is obtained, where the 0 super-
from the tetrahedral intermediate formed in these reac-
tions.13
scripts denote the k values which correspond to a pyridine
0
of pK
a
) pK
a
. At the center of the Br o¨ nsted plot the
intercept, therefore
0, which together with the latter equation
In the pyridinolysis of S-(2,4-dinitrophenyl) and S-(2,4,6-
trinitrophenyl) O-ethyl thiocarbonates (DNPTC and
straight lines at low and high pK
a
0
0
0
k
1
k
2
/ k-1 ) k
1
TNPTC, respectively), biphasic Br o¨ nsted-type plots were
0
yields eq 2a.
found with the center of curvature at pK
a
) pK
a
) 8.6
and 7.3, respectively.11 Comparison of these values with
0
0
0
0
0
k k /k
-1
) k1 ) 2kN
(2a)
those obtained in this work for DNPDTC and TNPDTC,
1
2
respectively, indicates that, according to eq 4, the k-1/k
2
Rearranging and applying logarithms to eq 1a, eq 3a
results.
ratio for a given pyridine is larger for a thiolcarbonate
relative to the corresponding dithiocarbonate. In other
-
-
(
words, substitution of S by O in T increases the k-1
ratio for a given pyridine. Since it has been found in
similar reactions that the above substitution enlarges
/
log k ) log (k k / k ) - log ((k / k-1) + 1) (3a)
N
1
2
-1
2
k
2
On the other hand,
d[log(k k /k )] ) â d(pK )
8
,17
both k-1 and k
for thiolcarbonates relative to that for dithiocarbonates
is due to a larger k-1 and less large k for the former
2
,
2
it means that the larger k-1/k ratio
(4a)
1
2
-1
2
a
2
Integrating this expression between k 0 0/k-1 and k
k k /
0
-
(
1 2 1 2
substrate. That is, the greater ability of O in T ,
0
-
a a
k-1, and between the corresponding pK and pK , and
compared to S , to form a double bond and expel a leaving
group18 favors amine expulsion from T( relative to
taking into account eq 2a, the following equation is
obtained:
-
-
DNPS or TNPS leaving.
In the aminolysis (secondary alicyclic amines) of 4-ni-
0
0
log(k k /k-1) ) log k + â (pK - pK ) + log 2 (5a)
0
1
2
N
2
a
a
trophenyl dithioacetate a pK
a
value of ca. 9 can be
value
value is 10.5 for the
biphasic Br o¨ nsted-type plot obtained in the same ami-
calculated on the basis of the k-1 values and the k
2
From eq 4a, and knowing that d(log k
gets the expression d[log(k
Integrating this from 1 to k /k-1 and from pK
a is obtained.
1
)/d(pK
2
a
) ) â
1
, one
).
eq
found.8
b,19
The experimental pK
a
0
2
/k-1)] ) (â
- â
1
) d(pK
a
0
2
a
to pK
a
1
4
0
nolysis of S-(4-nitrophenyl) thioacetate. The larger pK
a
6
for the reactions of the carbonyl derivative is in line with
our results. Similarly, the linear Br o¨ nsted-type plot
(
â2-â1)(pKa-pKa0)
k2/k-1 ) 10
(6a)
(
17) Castro, E. A.; Ib a´ n˜ ez, F.; Santos, J . G.; Ureta, C. J . Org. Chem.
993, 58, 4908.
18) Hill, S. V.; Thea, S.; Williams, A. J . Chem. Soc., Perkin Trans.
1983, 437. Cottrell, T. L. The Strengths of Chemical Bonds, 2nd ed.;
By replacing eqs 5a and 6a in eq 3a, the final equation
1
(eq 3 of the text) results.
(
2
J O961275T
Butterworth: London, 1959; pp 275-276. Kwon, D. S.; Park, H. S.;
Um, I. H. Bull. Korean Chem. Soc. 1991, 12, 93.
(
19) From Table II of ref 8b, the value of k-1 which equals k
2
()3 ×
(20) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1994,
59, 3572.
1
07
s ) is for an amine (hypothetical) of pK ca. 9.2.
-1
a