Kinetics and mechanism of the benzenethiolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates

Article abstract of DOI:10.1021/jo034082w

The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 °C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Bronsted-type plots for the nucleophilic rate constants are linear with slopes β = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Bronsted plot was found for the reactions of DNPC and DNPTC at pK_a ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's hard and soft acids and bases principle.

Full text of DOI:10.1021/jo034082w

Kin etics a n d Mech a n ism of th e Ben zen eth iolysis of
2,4-Din itr op h en yl a n d 2,4,6-Tr in itr op h en yl Meth yl Ca r bon a tes a n d
S-(2,4-Din itr op h en yl) a n d S-(2,4,6-Tr in itr op h en yl)
Eth yl Th iolca r bon a tes
Enrique A. Castro,* Paulina Pavez, and J ose´ G. Santos*
Facultad de Quı´mica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
ecastro@puc.cl
Received J anuary 23, 2003
The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC,
respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC
and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic
investigation in water, at 25.0 °C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-
first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant.
However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate
anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants
are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluoroben-
zenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which
show acid dependence. The Brønsted-type plots for the nucleophilic rate constants are linear with
slopes â ) 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively.
No break in the Brønsted plot was found for the reactions of DNPC and DNPTC at pKa ca. 4.1 and
3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzene-
thiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups
involved. Comparison of the kinetic results obtained in this work with those for the concerted
phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward
carbonates than isobasic phenoxide anions. This is explained by Pearson’s “hard and soft acids
and bases” principle.
In tr od u ction
knowledge, on the thiolysis of aryl carbonates or S-aryl
thiolcarbonates.
There have been many reports in the literature on the
kinetics and mechanisms of the phenolysis of esters and
thioesters,1-3 but only a few on the mechanisms of the
same reactions of carbonates and thiocarbonates.4 Con-
cerning the thiolysis of the above compounds, there have
been a few studies on the kinetics and mechanism of the
reactions of esters and thioesters,1,5 but none, to our
We have found that the phenolyses of S-(4-nitrophen-
yl), S-(2,4-dinitrophenyl), and S-(2,4,6-trinitrophenyl)
ethyl thiocarbonates (NPTC, DNPTC, and TNPTC, re-
spectively) are driven by concerted mechanisms.4a The
phenolyses of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-
trinitrophenyl methyl carbonates (NPC, DNPC, and
TNPC, respectively) were also found to be concerted on
the basis of the linear Brønsted-type plots with slopes â
ca. 0.5-0.7 obtained.4b
To extend our investigations on the mechanisms of the
reactions of aryl carbonates, we perform in the present
work a kinetic study of the benzenethiolysis of DNPC,
TNPC, DNPTC, and TNPTC. The aim of this work is as
follows: (i) to assess the influence of the leaving group
of the substrate on the kinetics and mechanism, by
comparing the title reactions; (ii) to evaluate the effect
of the nucleophile, by comparison of the mechanism of
the reactions under study with that for the phenolysis
of the same substrates;4a,b and (iii) to compare the kinetics
* To whom correspondence should be addressed. Fax (56 2) 6864744.
(1) (a) Hupe, D. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 451.
(b) Pohl, E. R.; Wu, D.; Hupe, D. J . J . Am. Chem. Soc. 1980, 102, 2759.
(c) Pohl, E. R.; Hupe, D. J . J . Am. Chem. Soc. 1980, 102, 2763.
(2) Buncel, E.; Um, I. H.; Hoz, S. J . Am. Chem. Soc. 1989, 111, 971.
Kwon, D.-S.; Lee, G.-J .; Um, I.-H. B. Korean. Chem. Soc. 1990, 11,
262. Park, Y. S.; Kim, C. K.; Lee, B. S.; Lee, I.; Lim, W. M.; Kim, W.
K. J . Phys. Org. Chem. 1995, 8, 32. Um, I.-H.; Yoon, H.-W.; Lee, J .-S.;
Moon, H.-J .; Kwon, D.-S. J . Org. Chem. 1997, 62, 5939. Um, I.-H.; Lee,
E.-J .; Buncel, E. J . Org. Chem. 2001, 66, 4859.
(3) (a) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc.
1987, 109, 6362. (b) Ba-Saif, S. A.; Colthurst, M. J .; Waring, M. A.;
Williams, A. J . Chem. Soc., Perkin Trans. 2 1991, 1901. (c) Hengge,
A. J . Am. Chem. Soc. 1992, 114, 6575. (d) Maude, A. B.; Williams, A.
J . Chem. Soc., Perkin Trans. 2 1997, 179. (e) Stefanidis, D.; Cho, S.;
Dhe-Paganon, S.; J encks, W. P. J . Am. Chem. Soc. 1993, 115, 1650.
(4) (a) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 1999,
64, 2310. (b) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 2001,
66, 3129. (c) Castro, E. A.; Angel, M.; Pavez, P.; Santos, J . G. J . Chem.
Soc., Perkin Trans. 2 2001, 2351. (d) Castro, E. A.; Angel, M.; Arellano,
D.; Santos, J . G. J . Org. Chem. 2001, 66, 6571. (e) Castro, E. A.; Pavez,
P.; Santos, J . G. J . Org. Chem. 2002, 67, 4494.
(5) Guanti, G.; Dell Erba, C.; Cevasco, G.; Narisano, E. Chem.
Commun. 1978, 613. Shames, S. I.; Byers, L. D. J . Am. Chem. Soc.
1981, 103, 6170. Douglas, K. T. Acc. Chem. Res. 1986, 19, 186.
10.1021/jo034082w CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/09/2003
3640
J . Org. Chem. 2003, 68, 3640-3645

Benzenethiolysis of Aryl Methyl Carbonates and Thiolcarbonates
TABLE 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 Ben zen eth iola te An ion s w ith
2,4-Din itr op h en yl Meth yl Ca r bon a te (DNP C)a
TABLE 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 Ben zen eth iola te An ion s w ith
2,4,6-Tr in itr op h en yl Meth yl Ca r bon a te (TNP C)a
benzenethiolate
substituent
no. of
benzenethiolate
substituent
no. of
pH 103 [ArSH]totb/M 102 kobsd/s-1 runs
pH 103 [ArSH]totb/M 103 kobsd/s-1 runs
4-methoxy
4-chloro
3-chloro
4-nitroe
2.5c
3.0c
3.5c
3.5d
4.0d
4.5d
4.0d
4.5d
5.0d
4.5d
0.25-1.0
0.10-0.70
0.10-0.70
0.25-0.80
0.25-0.80
0.10-0.80
0.10-0.80
0.25-1.0
0.10-0.60
0.01
0.80-12
2.4-18
6.5-38
7.6-13
14-25
17-66
9.2-15
16-41
20-56
6
5
5
4
4
5
4
5
4
4-methoxy
6.0c
6.5c
7.0c
6.0c
6.5c
7.0c
6.4c
6.6c
7.0c
8.0d
8.5d
9.0d
0.10-1.0
0.10-0.70
0.050-0.70
0.50-10
0.50-10
0.50-5.0
0.50-10
0.50-10
0.50-10
0.25-3.5
0.25-5.0
0.25-5.0
0.50-10
0.50-10
1.0-10
2.0-24
5
4
5
6
6
4
5
6
6
5
6
6
6
6
5
6.2-44
5.0 - 52
1.4-48
5.5-96
6.4-72
0.50-120
2.2-153
7.6-181
1.9-33
2.3-48
2.8-53
none
4-chloro
3-chloro
10
5
4
2,3,4,5,6-pentafluoro 2.4
1.0-10
0.50-5.0
0.40-3.0
1.7-20
0.80-12
1.6-7.8
2.7
3.0
5
2,3,4,5,6-pentafluoro 8.0d
0.10-0.31
0.18-0.43
0.67-0.94
a
b
8.5d
9.0d
In water, at 25.0 °C, ionic strength 0.2 M (KCl). Total
concentration of substituted benzenethiol (acid plus conjugate
base). c In the presence of citrate buffer 0.01 M. In the presence
d
a
b
of acetate buffer 0.01 M. e For this reaction (carried out with an
equimolar concentration of reactants), second-order kinetics were
obtained.
In water, at 25.0 °C, ionic strength 0.2 M (KCl). Total
concentration of substituted benzenethiol (acid plus conjugate
base). c In the presence of phosphate buffer 0.01 M. In the
d
presence of borate buffer 0.01 M.
TABLE 3. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Rea ction s of Ben zen eth iola te An ion s w ith
of the back reactions of the benzenethiolyses of the above
carbonates with the phenolysis of S-aryl ethyl thiolcarbon-
ates.4a
S-(2,4-Din itr op h en yl) Eth yl Th ioca r bon a te (DNP TC)a
benzenethiolate
substituent
no. of
runs
pH 103 [ArSH]totb/M 103 kobsd/s-1
none
8.0c
8.5c
9.0c
8.0c
8.5c
9.0c
7.5c
8.5c
9.0c
0.50-10.0
0.50-10.0
0.50-10.0
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
1.00-10.0
1.00-10.0
1.00-10.0
6.22-63.3
6.50-65.2
7.71-138
4
4
5
4
4
4
4
4
4
4
4
4
4-chloro
3-chloro
1.00-9.49
1.14-9.46
0.58-9.47
0.25-3.80
0.45-4.00
0.55-4.60
0.0074-0.054
0.0073-0.052
0.0065-0.055
2,3,4,5,6-pentafluoro 4.5d
5.0d
5.5d
Exp er im en ta l Section
Ma ter ia ls. The benzenethiols were used as purchased. The
substrates DNPC,6a,b TNPC,6c DNPTC,7a and TNPTC7b were
synthesized as previously reported.
a
b
In water, at 25.0 °C, ionic strength 0.2 M (KCl). Total
concentration of substituted benzenethiol (acid plus conjugate
base). c In the presence of borate buffer 0.01 M. In the presence
d
Kin etic Mea su r em en ts. Kinetic measurements were car-
ried out spectrophotometrically in the range 300-500 nm
(appearance of the substituted phenoxide or benzenethiolate
anions) by means of a diode array spectrophotometer. The
reactions were investigated in aqueous solutions, 25.0 ( 0.1
°C, ionic strength 0.2 M (maintained with KCl), and at least
a 10-fold excess of total benzenethiol over the substrate was
employed, except in the reactions of 4-nitrobenzenethiolate
with TNPC and TNPTC. For the latter reactions (followed at
400 nm), an equimolar concentration (1 × 10-5 M) of the
reactants was employed and second-order kinetics were ob-
tained.
With the above exceptions, the initial concentration of the
substrates was 3 × 10-5 M and pseudo-first-order rate coef-
ficients (kobsd) were found in all cases. The experimental
conditions of the reactions and the kobsd values obtained are
shown in Tables 1-4.
of acetate buffer 0.01 M.
of the UV-vis spectra after completion of the reactions with
those of authentic samples under the same conditions.
Resu lts a n d Discu ssion
The rate law obtained for most of the reactions under
investigation (the exceptions being the reactions of 4-ni-
trobenzenethiolate with TNPC and TNPTC) is given by
eq 1, where P is 2,4-dinitrophenoxide, 2,4,6-trinitrophen-
oxide, 2,4-dinitrobenzenethiolate, or 2,4,6-trinitroben-
zenethiolate anions, S represents the substrate (DNPC,
TNPC, DNPTC, or TNPTC), and kobsd is the pseudo-first-
order rate coefficient.
P r od u ct Stu d ies. For the benzenethiolysis of DNPC,
TNPC, DNPTC, and TNPTC, one of the products was identi-
fied as 2,4-dinitrophenoxide, 2,4,6-trinitrophenoxide 2,4-dini-
trobenzenethiolate, and 2,4,6-trinitrobenzenethiolate anions,
respectively. The identification was achieved by comparison
d[P]
dt
) kobsd[S]
(1)
For the above reactions, the plots of kobsd against
benzenethiolate (ArS-) concentration are linear, accord-
ing to eq 2, where k0 and kN are the rate coefficients for
hydrolysis and thiolysis of the substrates, respectively.
(6) (a) Pianka, M. J . Sci. Food Agric. 1960, 17, 47. (b) Castro, E. A.;
Gil, F. J . J . Am. Chem. Soc. 1977, 99, 7611. (c) Castro, E. A.; Iba´n˜ez,
F.; Lagos, S.; Schick, M.; Santos, J . G. J . Org. Chem. 1992, 57, 2691.
(7) (a) Castro, E. A.; Iba´n˜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.
kobsd ) k0 + kN[ArS-]
(2)
J . Org. Chem, Vol. 68, No. 9, 2003 3641

Castro et al.
constants (kH in eq 3), and the pKa of benzenethiols8 are
shown in Table 5.
TABLE 4. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Rea ction s of Ben zen eth iola te An ion s w ith
S-(2,4,6-Tr in itr op h en yl) Eth yl Th ioca r bon a te (TNP TC)a
Table 5 includes the kN values obtained for the reac-
tions of 4-nitrobenzenethiolate with TNPC and TNPTC,
under second-order conditions. These reactions could not
be studied under pseudo-first-order conditions, as in the
case of the other benzenethiolates, due to the high
absorbance of this nucleophile, which prevented its use
in great excess over the substrate. The hydrolysis reac-
tion was checked in the absence of 4-nitrobenzenethi-
olate: No significant reaction was observed during the
time the reaction with benzenethiolate is complete. The
back reaction was not significant as indicated by the good
second-order kinetics obtained. This is reasonable due
to the poor nucleophilicity (and also low concentration)
of 2,4,6-trinitrophenoxide or 2,4,6-trinitrobenzenethiolate
anions toward the alkyl S-(4-nitrophenyl) thiolcarbonates
compared to that of 4-nitrobenzenethiolate anion toward
TNPC and TNPTC. Therefore, the latter reactions can
be completely described by eq 4.
benzenethiolate
substituent
no. of
pH 103 [ArSH]totb/M 102 kobsd/s-1 runs
none
2.5c
3.0c
3.5c
2.5c
3.0c
3.5c
2.5c
3.0c
3.5c
4.5e
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.10-1.00
0.01
0.82-4.60
1.80-8.00
2.50-15.4
4.80-10.5
4.90-11.2
5.00-9.80
2.10-8.20
2.30-9.00
3.00-9.70
6
5
5
6
6
4
6
6
6
4-chloro
3-chloro
4-nitrod
10
4
4
2,3,4,5,6-pentafluoro 4.5e
1.00-10.0
1.00-10.0
1.00-10.0
2.80-14.8
2.90-15.2
3.00-13.8
5.0e
5.5e
4
a
b
In water, at 25.0 °C, ionic strength 0.2 M (KCl). Total
concentration of substituted benzenethiol (acid plus conjugate
base). c In the presence of citrate buffer 0.01 M. For this reaction
d
(carried out with an equimolar concentration of reactants), second-
order kinetics were obtained. e In the presence of acetate buffer
0.01 M.
RO-CO-XTNP + NPS- f RO-CO-SNP +
TNPX- (4)
(R ) Me, X ) O; R ) Et, X ) S; TNP )
2,4,6-trinitrophenyl; NP ) 4-nitrophenyl)
For the reactions of DNPC and DNPTC with 4-ni-
trobenzenethiolate, the determination of kN was not
possible due to the fact that the UV-vis spectra of the
leaving groups, 2,4-dinitrophenoxide and 2,4-dinitroben-
zenethiolate anions, are very similar to that of 4-ni-
trobenzenethiolate. Even under second-order experimen-
tal conditions the absorbance differences observed during
the reactions are very small.
The acid catalysis found for the reactions of some
benzenethiolate anions with TNPC and TNPTC was
presumably observed because these were the only reac-
tions carried out at low pH (2.4-3.5) values. It is
reasonable to assume that the other reactions would
show the same catalysis at low pH values. A similar
catalysis was found in the addition of thiol anions to
acetaldehyde9a and in the reactions of 4-cyanopyridine
with 2,4-dinitrophenyl acetate and DNPC.9b The much
larger kH value obtained for the reaction of TNPC with
4-methoxybenzenethiolate compared to that for the reac-
tion of the same substrate with the pentafluoro derivative
(Table 5) can be attributed to a general-acid catalysis (by
the acid form of the citrate buffer) for the former reaction
since this was carried out in the presence of external
buffer whereas the latter was studied without external
buffer. The acid-catalyzed path was not further investi-
gated.
F IGURE 1. Plots of kobsd vs concentration of free 4-methoxy-
benzenethiolate, obtained in the reaction of this anion with
2,4,6-trinitrophenyl methyl carbonate (TNPC), at pH 2.5 (b),
pH 3.0 (9), and pH 3.5 (O).
The values of both the slope (kN) and the intercept (k0)
are pH independent, except those for the reactions of
TNPC with 4-methoxy- and pentafluorobenzenthiolate
anions and TNPTC with 4-chloro-, 3-chloro-, and unsub-
stituted benzenethiolate anions. For these reactions, the
plot of kobsd vs [ArS-] is linear at each pH (see Figure 1
as an example), but the slope increases as the pH
decreases. The plot (not shown) of the slopes in Figure 1
vs [H+] is linear, with slope kH (the acid-catalyzed rate
constant) and intercept kN (the nucleophilic rate con-
stant). Namely, for these reactions, kobsd behaves kineti-
cally according to eq 3. The values of kN and kH in
aqueous solution, 25.0 °C, ionic strength 0.2 M are shown
in Table 5.
The Brønsted-type plots for the benzenethiolysis of
DNPTC and TNPTC, obtained with the kN values and
the pKa of the benzenethiols (taken from Table 5), are
(8) (a) Castro, E. A.; Cubillos, M.; Iban˜ez, F.; Moraga, I.; Santos, J .
G. J . Org. Chem. 1993, 58, 5400. (b) De Maria, P.; Fini, A.; Hall, F. M.
J . Chem. Soc., Perkin Trans. 2 1973, 1969. (c) J encks, W. P.; Salvesen,
K. J . Am. Chem. Soc. 1971, 93, 4433.
kobsd ) k0 + kN[ArS-] + kH[ArS-][H+]
(3)
(9) (a) Gilbert, H. F.; J encks, W. P. J . Am. Chem. Soc. 1977, 99,
7931. (b) Castro, E. A.; Borquez, M. T.; Parada, P. M. J . Org. Chem.
1986, 51, 5072.
The values obtained for the nucleophilic rate con-
stants (kN in eqs 2 and 3), the acid-catalyzed rate
3642 J . Org. Chem., Vol. 68, No. 9, 2003

Benzenethiolysis of Aryl Methyl Carbonates and Thiolcarbonates
TABLE 5. Va lu es of p Ka of Ben zen eth iols a n d kN for th e Rea ction s of Ben zen eth iola tes w ith Ar yl Meth yl Ca r bon a tes
a n d S-Ar yl Eth yl Th iolca r bon a tesa
kN/s-1 M-1
10-3 kN/s-1 M-1
kN/s-1 M-1
10-3 kN/s-1 M-1
benzenethiolate substituent
pKa of benzenethiol
DNPC
TNPC
DNPTC
TNPTC
4-methoxy
none
4-chloro
3-chloro
4-nitro
6.5b
6.4b
6.0b
5.5d
4.6b
2.7f
108 ( 7
500 ( 80c
-
26 ( 1
13.9 ( 0.4
108 ( 5c
19.7 ( 0.4
10.1 ( 0.3
23.3 ( 0.7
4.3 ( 0.2
9.3 ( 0.2
4.2 ( 0.4
6.8 ( 4.4c
2.2 ( 1.1c
5.3 ( 0.2e
0.029 ( 0.006c
-1.41 + 1.00pKa
1.4 ( 0.1e
pentafluoro
0.025 ( 0.002
-3.94 + 0.87pKa
0.005 ( 0.001
-4.79 + 0.94pKa
0.013 ( 0.006
-1.32 + 0.91pKa
Brønsted-type equationg
a
b
The kN values were measured in aqueous solution, at 25.0 °C and an ionic strength of 0.2 M (KCl). Values of pKa taken from ref 8a,
measured in aqueous solution, at 25.0 °C and an ionic strength of 0.2 M. c Values obtained from eq 3. The values of kH obtained from this
equation for the reactions of TNPC are (2 ( 1) × 108 s-1 M-2 (4-methoxybenzenethiolate) and (8 ( 2) × 103 s-1 M-2 (pentafluoroben-
zenethiolate), respectively, and for the reactions of TNPTC are (1.1 ( 0.5) × 108 s-1 M-2 (benzenethiolate), (6 ( 3) × 107 s-1 M-2 (4-
chlorobenzenethiolate), and (2 ( 1) × 107 s-1 M-2 (3-chlorobenzenethiolate). Value of pKa determined by interpolation of the linear
d
relationship between the pKa values measured in aqueous solution at 25.0 °C and an ionic strength of 0.2 M (ref 8a) and the thermodynamic
pKa values in water at 25.0 °C (ref 8b). e Values of kN measured under second-order conditions (see text). f Value of pKa measured in
g
aqueous solution at 25.0 °C and an ionic strength of 1.0 M (ref 8c). log kN as a function of pKa.
F IGURE 2. Brønsted-type plots obtained in the benzenethi-
olysis of S-(2,4-dinitrophenyl) ethyl thiolcarbonate (DNPTC,
b) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonate (TNPTC,
O), in aqueous solution, 25.0 °C, ionic strength 0.2 M. The
slopes are â ) 0.9.
F IGURE 3. Brønsted-type plots obtained in the benzenethi-
olysis of 2,4-dinitrophenyl methyl carbonate (DNPC, b) and
2,4,6-trinitrophenyl methyl carbonate (TNPC, O), in aqueous
solution, 25.0 °C, ionic strength 0.2 M. The slopes are â ) 0.9
and 1.0, respectively.
shown in Figure 2. The plots are linear and have the
same slope (â ) 0.9) within the error limits.
absence of this curvature within the pKa range of the
nucleophiles employed is a conclusive proof that the
mechanism of these reactions is concerted.14 The hypo-
thetical intermediate T- in the reaction of DNPTC could
be so unstable that either it does not exist (enforced
concerted mechanism) or its high energy skews the
energy surface of the reaction to favor the concerted
mechanism.15 If the hypothetical intermediate T- in the
reaction of DNPTC were so unstable, T- for the TNPTC
benzenethiolysis would be even more unstable in view
of the better nucleofuge involved (2,4,6-trinitrobenzeneth-
iolate) compared with 2,4-dinitrobenzenethiolate ion.
Therefore, if the benzenethiolysis of DNPTC were con-
certed it is more likely that the same reaction of TNPTC
would also be concerted.
The magnitudes of these Brønsted slopes are consistent
with those found in the stepwise pyridinolysis of the same
thiolcarbonates when decomposition to products of a
zwitterionic tetrahedral intermediate (T() is rate limit-
ing.10 The slope values are also in accord with those
obtained in the aminolysis of aryl acetates,11a diaryl
carbonates,11b and acetyl chloride,12 when breakdown of
the intermediate T( to products is the rate-determining
step. Therefore, it would seem that the benzenethiolysis
of DNPTC and TNPTC would proceed through a tetra-
hedral intermediate (anionic in these cases), with its path
to products being rate limiting. Nevertheless, if the
mechanism were stepwise, the Brønsted-type plot for
DNPTC would be biphasic, with a value of pKa at the
The Brønsted-type plots for the benzenethiolysis of
DNPC and TNPC, obtained with the kN values and the
pKa of the benzenethiols (Table 5), are shown in Figure
3. The plots are linear with slopes of 0.9 for DNPC and
1.0 for TNPC.
0
curvature center (pKa ) of 3.4, corresponding to pKa value
of 2,4-dinitrobenzenethiol.13 This is the pKa at which the
hypothetical anionic tetrahedral intermediate (T-) would
break to reactants and products with equal rates.14 The
(10) Castro, E. A.; Pizarro, M. I.; Santos, J . G. J . Org. Chem. 1996,
61, 5982.
(11) (a) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974,
96, 7018. (b) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977,
99, 6963, 6970.
(13) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991,
63.
(14) (a) Williams, A. Acc. Chem. Res. 1989, 22, 387. (b) Williams, A.
Chem. Soc. Rev. 1994, 23, 93.
(15) J encks, W. P. Acc. Chem. Res. 1980, 13, 161. J encks, W. P.
Chem. Soc. Rev. 1981, 10, 345.
(12) Palling, D. J .; J encks, W. P.J . Am. Chem. Soc. 1984, 106, 4869.
J . Org. Chem, Vol. 68, No. 9, 2003 3643

Castro et al.
The magnitudes of these Brønsted slopes are consistent
with those found in the stepwise pyridinolysis of the same
carbonates when the decomposition to products of a
tetrahedral intermediate (1) is rate limiting.6b,c The
values of â are also in accord with those obtained in the
stepwise aminolysis of similar acetates, carbonates, and
their thiol derivatives when breakdown of the zwitteri-
onic tetrahedral intermediate to products is rate determin-
ing.9-13 Nevertheless, the similar values of â seem
fortuitous, and we believe the reactions of benzenethi-
olates with DNPC and TNPC are not stepwise (through
the tetrahedral intermediate 2) but concerted, as will now
be shown. The existence of a very similar intermediate
(3) was ruled out in the phenolysis of S-aryl ethyl
thiolcarbonates.4a This reaction is the microscopic reverse
of the reaction under the present investigation and
therefore must have the same mechanism. It is known
that the change of methoxy to ethoxy in a similar
intermediate does not alter significantly the rate con-
stants for its decomposition nor its thermodynamic
stability.16 Therefore, it is reasonable that intermediate
2 is either too unstable to exist or very unstable. In either
case the mechanism of the reactions of benzenethiolates
with DNPC and TNPC would be concerted.
DNPC reactions, covering a pKa range 2.7-6.5, indicates
that this mechanism is not stepwise, confirming therefore
the concerted process. The thiolysis of TNPC should also
be concerted in view of the even more unstable “inter-
mediate” that would be formed. These results augment
those on the phenolysis of thiolcarbonates due to the
microscopic reversibility.
The instability of the putative anionic tetrahedral
intermediate 2 should be extremely high taking into
account the groups attached to its central carbon. For
the phenolysis of aryl acetates, Williams and co-workers
have argued that two O-aryl groups are sufficient to
destabilize an anionic tetrahedral intermediate to the
point of nonexistence.3a
The fact that the tetrahedral intermediate 4 (Ar ) NP
or DNP), formed in the thiolysis of 4-nitrophenyl and 2,4-
dinitrophenyl acetates,1a is more stable than the putative
one (2 Ar ) NP or DNP) in the phenolysis of S-(4-
nitrophenyl) ethyl thiolcarbonate4a and the thiolysis of
2,4-dinitrophenyl methyl carbonate (this work) indicates
that the change of Me to MeO or EtO destabilizes
intermediate 4. A similar destabilization has been found
in the aminolysis of thiolacetates and thiolcarbonates.
The aminolyses (secondary alicyclic amines) of 2,4-
dinitrophenyl and 2,4,6-trinitrophenyl thiolacetates are
stepwise,13 through intermediate 5 (Ar ) DNP or TNP),
whereas the same aminolyses of S-(2,4-dinitrophenyl)
and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates are
concerted.7 The latter result indicates that intermediate
6 (Ar ) DNP or TNP) is either too labile to qualify as an
intermediate (enforced concerted mechanism) or it exists
but it is so unstable that the concerted process is
favored.15
Another reason to believe that the benzenethiolysis of
DNPC is concerted is that there is no Brønsted break
for this reaction (Figure 3) near the pKa of 2,4-dinitro-
phenol (pKa 4.1 under the experimental conditions). If
these reactions were stepwise there would be a break in
0
the Brønsted plot at a pKa value (pKa ) for which the rate
constant for Ar′S- expulsion (k-1) from the intermediate
2 (OAr ) 2,4-dinitrophenoxy, DNPO) equals that for
DNPO- expulsion (k2). Although it is known that ArO-
is a better nucleofuge compared to an isobasic Ar′S-, as
judged by elimination rate constants from the conjugate
bases of the corresponding esters (E1cB mechanism),17
presumably the rate constant k2 (DNPO- expulsion from
2) is not greater than k-1 (isobasic Ar′S- expulsion from
2). In fact, for the stepwise thiolysis of 4-nitrophenyl and
2,4-dinitrophenyl acetates, the Brønsted breaks are
The greater reactivity of TNPC than DNPC and of
TNPTC than DNPTC toward benzenethiolates (Table 5
and Figures 2 and 3) can be explained by the presence
of a third nitro group in the trinitro substrates, which
would destabilize the ground state of these compounds
compared to those of the dinitro derivatives.18 This would
make the former compounds more prone to nucleophilic
attack.18 Furthermore, the trinitro nucleofuges should be
expelled faster from these substrates than the more basic
dinitro leaving groups from DNPC and DNPTC, respec-
tively.
0
centered at pKa ) 7.8 and 5.0, respectively, which are
close to the pKa of 4-nitrophenol (NPOH) and DNPOH.1a
Similarly, for the stepwise thiolysis of 4-nitrophenyl
formate and pivalate the Brønsted pKa is ca. 7.1b The
0
Figure 4 shows a comparison of the Brønsted plots for
the nucleophilic rate constants for the benzenethiolysis
(this work) and phenolysis4b of DNPC and TNPC, and
Figure 5 shows a comparison of the same reactions4a of
DNPTC and TNPTC. It can be seen that in all cases
benzenethiolysis is faster than phenolysis toward a
carbonyl carbon within the pKa range studied. A similar
result was found in the reactions of 4-nitrophenyl acetate1a
and 4-nitrophenyl formate:1b benzenethiolates are more
similar values of k2 and k-1 for sulfur and oxygen isobasic
groups has been attributed to the fact that both groups
are attached to the same compound (e.g., intermediate
2) and the larger leaving ability of ArO than an isobasic
Ar′S is compensated by a smaller push by Ar′S compared
to that by the isobasic ArO.1a Therefore, if the ben-
0
zenethiolysis of DNPC were stepwise the pKa value
would be ca. 4. The lack of a Brønsted break for the
(16) Castro, E. A.; Cubillos, M.; Santos, J . G.; Tellez, J . J . Org. Chem.
1997, 62, 2512.
(17) Douglas, K. T. Acc. Chem. Res. 1986, 19, 186.
(18) Neuvonen, H.; Neuvonen, K.; Koch, A.; Kleinpeter, E.; Pasanen,
P. J . Org. Chem. 2002, 67, 6995.
3644 J . Org. Chem., Vol. 68, No. 9, 2003

Benzenethiolysis of Aryl Methyl Carbonates and Thiolcarbonates
F IGURE 5. Brønsted-type plots obtained in the benzenethi-
olysis (b, this work) and phenolysis (O, ref 4a) of (a) 2,4-
dinitrophenyl ethyl thiolcarbonate (DNPTC) and (b) 2,4,6-
trinitrophenyl ethyl thiolcarbonate (TNPTC), in aqueous
solution, 25.0 °C, ionic strength 0.2 M.
F IGURE 4. Brønsted-type plots obtained in the benzenethi-
olysis (b, this work) and phenolysis (O, ref 4b) of (a) 2,4-
dinitrophenyl methyl carbonate (DNPC) and (b) 2,4,6-trini-
trophenyl methy carbonate (TNPC), in aqueous solution, 25.0
°C, ionic strength 0.2 M.
concerted reactions which do not follow the reactivity-
selectivity principle. For instance, in the concerted (SN2)
reactions of anilines with benzyl bromides and benzoyl
chlorides in acetonitrile, the magnitude of the Hammett
Fnuc constant increases with the increasing reactivity of
the substrates.21 Also in the concerted phenolyses of aryl
acetates the transition state is tighter for the more basic
nucleophiles.22 Shaik has also found that SN2 (concerted)
reactions usually break the reactivity-selectivity and
Bell-Evans-Polanyi principles.23 Similar findings have
been reported by Lee24 and Williams.25
reactive toward these substrates than isobasic phenoxide
anions. This was attributed to the softer nature of the
sulfur nucleophile, relative to the oxygen nucleophile,19
that would prefer to bind to the relatively soft carbonyl
carbon. This can be confirmed by the fact that ben-
zenenethiolates show an additional rate enhancement,
relative to isobasic phenoxide ions, toward 4-nitrophenyl
thiolacetate, which possesses a softer electrophilic group
than that of the corresponding carbonate.1a
The Brønsted slopes found for the concerted phenolyses
of DNPC, TNPC, DNPTC, and TNPTC are 0.48, 0.52,
0.77, and 0.61, respectively.4a,b These slopes are smaller
than those obtained in the concerted benzenethiolysis of
these compounds (this work): â ) 0.9 for DNPC, DNPTC,
and TNPTC and â )1.0 for TNPC. This means that the
bond formation between the sulfur nucleophiles and the
carbonyl carbon is more advanced in the transition state,
Ack n ow led gm en t. We thank the financial assis-
tance given to this work by FONDECYT (Chile), Project
No. 2000051. P.P. thanks CONICYT for a Doctoral
scholarship.
relative to reactants, than that between the oxygen
nucleophiles and the same carbon atom. The larger
Brønsted slopes for the faster reactions of benzenethi-
olates is against the reactivity-selectivity principle and
the Hammond postulate.20 Nevertheless, there are many
J O034082W
(21) Lee, I. Chem. Soc. Rev. 1995, 24, 223.
(22) Reference 14a, see especially Figure 5.
(23) Shaik, S. S. Acta Chem. Scand. 1990, 44, 205 (see especially p
214).
(24) Lee, I.; Lee, H. W. Collect. Czech. Chem. Commun. 1999, 64,
1529 (see especially p 1546).
(25) Williams, A. Concerted Organic and Bio-organic Mechanisms;
CRC Press: Boca Raton, 2000; p 42.
(19) Pearson, R. G. J . Chem. Educ. 1968, 45, 581. Pearson, R. G. J .
Org. Chem. 1989, 54, 1423.
(20) Hammond, G. S. J . Am. Chem. Soc. 1955, 77, 334.
J . Org. Chem, Vol. 68, No. 9, 2003 3645