Kinetics and Mechanism of the Benzenethiolysis of O-Ethyl S-(2,4-Dinitrophenyl) and O-Ethyl S-(2,4,6-Trinitrophenyl) Dithiocarbonates and O-Methyl O-(2,4-Dinitrophenyl) Thiocarbonate

Article abstract of DOI:10.1021/jo0352530

Reactions of O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC), O-ethyl 2,4,6-trinitrophenyl dithiocarbonate (ETNPDTC), and O-methyl O-(2,4-dinitrophenyl) thiocarbonate (MDNPTOC) with a series of benzenethiolate anions in aqueous solution, at 25.0 °C and an ionic strength of 0.2 M (KCl), are subjected to a kinetic investigation. Under excess benzenethiolate, these reactions obey pseudo-first-order kinetics and are first order in benzenethiolate. Nonetheless, similar reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with the ethyl trinitrophenyl ester (ETNPDTC), which showed overall second-order kinetics. The nucleophilic rate constants (k_N) are pH independent, except those for the reactions of ETNPDTC with the X-benzenethiolates with X = H, 4-Cl, and 3-Cl, which increase as pH decreases. The Bronsted-type plots (log k_N vs pK _a of benzenethiols) are linear with slopes β = 0.66 for the reactions of both ethyl dinitrophenyl ester (EDNPDTC) and ethyl trinitrophenyl ester (ETNPDTC) and β = 0.58 for those of the thiocarbonate ester (MDNPTOC). For the benzenethiolysis of MDNPTOC and EDNPDTC, no breaks were found in the Bronsted-type plots at pK_a 4.1 and 3.4, respectively, consistent with concerted mechanisms. Benzenethiolysis of the ethyl trinitrophenyl ester (ETNPDTC) should also be concerted in view of the even more unstable tetrahedral intermediate that would have been formed had this reaction been stepwise. ETNPDTC is more reactive toward benzenethiolate anions than EDNPDTC due to the better leaving group involved in the former substrate. The k_N values found for the reactions of EDNPDTC with benzenethiolates are larger than those obtained for the concerted reactions of the same substrate with isobasic phenoxide anions. This is explained by Pearson's hard and soft acids and bases principle. The concerted mechanism for the benzenethiolysis of MDNPTOC, in contrast to the stepwise mechanism found for the phenolysis of this substrate, is attributed to the greater kinetic instability of the hypothetical tetrahedral intermediate formed in the former reaction, due to the greater nucleofugality of ArS^- compared with an isobasic ArO^-. Benzenethiolates are more reactive toward MDNPTOC and EDNPDTC than the corresponding carbonate and thiolcarbonate, respectively. This is also in accordance with the HSAB principle, since benzenthiolates are relatively soft bases that prefer to bind to a relatively soft thiocarbonyl center rather than a relatively hard carbonyl center.

Full text of DOI:10.1021/jo0352530

Kin etics a n d Mech a n ism of th e Ben zen eth iolysis of O-Eth yl
S-(2,4-Din itr op h en yl) a n d O-Eth yl S-(2,4,6-Tr in itr op h en yl)
Dith ioca r bon a tes a n d O-Meth yl O-(2,4-Din itr op h en yl)
Th ioca r bon a te
Enrique A. Castro,* Paulina Pavez, and J ose´ G. Santos*
Facultad de Qu´ımica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
ecastro@puc.cl
Received August 27, 2003
Reactions of O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC), O-ethyl 2,4,6-trinitrophenyl
dithiocarbonate (ETNPDTC), and O-methyl O-(2,4-dinitrophenyl) thiocarbonate (MDNPTOC) with
a series of benzenethiolate anions in aqueous solution, at 25.0 °C and an ionic strength of 0.2 M
(KCl), are subjected to a kinetic investigation. Under excess benzenethiolate, these reactions obey
pseudo-first-order kinetics and are first order in benzenethiolate. Nonetheless, similar reactant
concentrations were used in the reactions of 4-nitrobenzenethiolate anion with the ethyl trinitro-
phenyl ester (ETNPDTC), which showed overall second-order kinetics. The nucleophilic rate
constants (kN) are pH independent, except those for the reactions of ETNPDTC with the
X-benzenethiolates with X ) H, 4-Cl, and 3-Cl, which increase as pH decreases. The Brønsted-
type plots (log kN vs pKa of benzenethiols) are linear with slopes â ) 0.66 for the reactions of both
ethyl dinitrophenyl ester (EDNPDTC) and ethyl trinitrophenyl ester (ETNPDTC) and â ) 0.58 for
those of the thiocarbonate ester (MDNPTOC). For the benzenethiolysis of MDNPTOC and
EDNPDTC, no breaks were found in the Brønsted-type plots at pKa 4.1 and 3.4, respectively,
consistent with concerted mechanisms. Benzenethiolysis of the ethyl trinitrophenyl ester (ETNP-
DTC) should also be concerted in view of the even more unstable tetrahedral “intermediate” that
would have been formed had this reaction been stepwise. ETNPDTC is more reactive toward
benzenethiolate anions than EDNPDTC due to the better leaving group involved in the former
substrate. The kN values found for the reactions of EDNPDTC with benzenethiolates are larger
than those obtained for the concerted reactions of the same substrate with isobasic phenoxide anions.
This is explained by Pearson’s “hard and soft acids and bases” principle. The concerted mechanism
for the benzenethiolysis of MDNPTOC, in contrast to the stepwise mechanism found for the
phenolysis of this substrate, is attributed to the greater kinetic instability of the hypothetical
tetrahedral “intermediate” formed in the former reaction, due to the greater nucleofugality of ArS-
compared with an isobasic ArO-. Benzenethiolates are more reactive toward MDNPTOC and
EDNPDTC than the corresponding carbonate and thiolcarbonate, respectively. This is also in
accordance with the HSAB principle, since benzenthiolates are relatively soft bases that prefer to
bind to a relatively soft thiocarbonyl center rather than a relatively hard carbonyl center.
In tr od u ction
reactions of carbonates and thiocarbonates,5,6 there have
been only a few investigations on the kinetics and
mechanism of the benzenethiolysis of esters and thioes-
ters2,7 and only one on the latter reactions of carbonates
and thiolcarbonates.8 Furthermore, we are not aware of
any reports on the kinetics of the benzenethiolysis of
thiocarbonyl compounds.
Although there is abundant literature on the kinetics
and mechanisms of the phenolysis of esters and thioes-
ters,1-4 and there have been some reports on the same
* Corresponding author.
(1) Castro, E. A. Chem. Rev. 1999, 99, 3505 and references therein.
(2) (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.
(3) 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.
(4) (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.
(5) (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.
(6) Castro, E. A.; Arellano, D.; Pavez, P.; Santos, J . G. J . Org. Chem.
2003, 68, 6192.
(7) (a) Guanti, G.; Dell Erba, C.; Cevasco, G.; Narisano, E. Chem.
Commun. 1978, 613. (b) Shames, S. I.; Byers, L. D. J . Am. Chem. Soc.
1981, 103, 6170. (c) Douglas, K. T. Acc. Chem. Res. 1986, 19, 186.
(8) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 2003, 68,
3640.
10.1021/jo0352530 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/18/2003
9034
J . Org. Chem. 2003, 68, 9034-9039

Benzenethiolysis of dithiocarbonates
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 O-Eth yl
2,4-Din itr op h en yl Dith ioca r bon a tea
We have found that the phenolysis of ethyl S-(2,4-
dinitrophenyl) dithiocarbonate (EDNPDTC) is concerted,6
whereas the corresponding reaction of methyl 2,4-dini-
trophenyl thionocarbonate (MDNPTOC) is governed by
a stepwise mechanism.6 This difference of mechanism
was explained by the greater nucleofugality of 2,4-
dinitrobenzenethiolate than 2,4-dinitrophenoxide from
the corresponding hypothetical tetrahedral intermedi-
ates, which kinetically destabilizes the former “interme-
diate”.6
Recently, we have described the benzenethiolysis of
methyl 2,4-dinitrophenyl and methyl 2,4,6-trinitrophenyl
carbonates (MDNPC and MTNPC, respectively) and of
ethyl S-(2,4-dinitrophenyl) and ethyl S-(2,4,6-trinitro-
phenyl) thiolcarbonates (EDNPTC and ETNPTC, respec-
tively).8 These reactions were found to be concerted on
the basis of the linear Brønsted plots obtained and the
absence of a break at the expected pKa for a hypothetical
stepwise mechanism.8
b
benzenethiolate
substituent
103 [ArSH]tot
(M)
103 kobsd
(s-1
no. of
runs
pH
)
none
4.5c
5.0c
5.5c
4.5c
5.0c
5.5c
8.0d
8.5d
9.0d
0.10-0.80
0.10-1.0
0.10-1.0
0.10-1.0
0.10-1.0
0.10-1.0
0.10-1.0
0.10-1.0
0.10-1.0
1.0-10
0.83-13.2
3.40-26.0
4.00-64.0
0.99-6.11
1.91-29.0
6.30-69.0
40.0-181
33.0-202
28.0-183
2.30-24.0
2.90-27.0
2.95-29.0
5
4
6
5
4
5
4
5
5
4
4
4
4-chloro
3-chloro
2,3,4,5,6-pentafluoro 4.5c
5.0c
5.5c
1.0-10
1.0-10
a
In aqueous solution, at 25.0 °C and an ionic strength of 0.2 M
b
(KCl). Total concentration of substituted benzenethiol (acid plus
conjugate base). c In the presence of acetate buffer 0.01 M. In
d
the presence of borate buffer 0.01 M.
To extend our kinetic studies on the reactions of
benzenethiolates, in the present work we carry out a
kinetic investigation of the benzenethiolysis of EDNP-
DTC, ETNPDTC, and MDNPTOC. The following are the
goals of this work. (i) To examine the influence of the
leaving group of the substrate on the kinetics and
mechanism, by comparing the three reactions series. (ii)
To assess the effect of the nucleophile, by comparison of
the title reactions with the phenolysis of the same
substrates.6 (iii) To evaluate the change of carbonyl to
thiocarbonyl as an electrophilic center by comparison of
the reactions under study with the benzenethiolysis of
carbonates and thiolcarbonates.8 (iv) To compare the
kinetics of the back reactions of the benzenethiolysis of
the dinitrophenyl thiocarbonate with the phenolysis of
the dinitrophenyl dithiocarbonate.6
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 O-Eth yl
2,4,6-Tr in itr op h en yl Dith ioca r bon a tea
b
benzenethiolate
substituent
103 [ArSH]tot
(M)
102 kobsd
(s-1
no. of
runs
pH
)
none
2.5c
3.0c
3.5c
2.5c
3.0c
3.5c
2.5c
3.0c
3.5c
4.5e
0.10-1.0
0.10-0.80
0.10-1.0
0.10-0.80
0.10-0.80
0.10-1.0
0.10-1.0
0.10-1.0
0.10-1.0
0.01
28.0-80.0
50.0-98.0
62.0-114
32.0-98.5
62.0-149
76.2-156
15.0-63.3
17.9-92.2
21.1-104
6
4
5
4
5
6
6
4
5
4-chloro
3-chloro
4-nitrod
10
5
5
2,3,4,5,6-pentafluoro 4.5e
0.25-1.0
0.25-1.0
0.40-1.0
12.7-57.0
20.0-67.0
46.0-83.0
5.0e
5.5e
4
a
In aqueous solution, at 25.0 °C and an ionic strength of 0.2 M
b
(KCl). Total concentration of substituted benzenethiol (acid plus
conjugate base). c In the presence of citrate buffer 0.01 M. For
d
this reaction (carried out with an equimolar concentration of
reactants), second-order kinetics were obtained. e In the presence
of acetate buffer 0.01 M.
The initial concentration of the substrates was (0.5-3) ×
10-5 M, and pseudo-first-order rate coefficients (kobsd) were
found, except in the particular case mentioned above. The
experimental conditions of the reactions and the kobsd values
obtained are shown in Tables 1-3.
Exp er im en ta l Section
P r od u ct Stu d ies. For the benzenethiolysis of MDNPTOC,
EDNPDTC, and ETNPDTC, one of the products was identified
as 2,4-dinitrophenoxide, 2,4-dinitrobenzenethiolate, and 2,4,6-
trinitrobenzenethiolate anions, respectively. This was carried
out by comparison of the UV-vis spectra after completion of
these reactions with those of authentic samples under the
same experimental conditions.
Ma ter ia ls. The series of benzenethiols were used as pur-
chased. The substrates: EDNPDTC,9 ETNPDTC,9 and MD-
NPTOC6 were synthesized as described previously.
Kin etic Mea su r em en ts. These were performed spectro-
photometrically (diode array) in the range 300-500 nm by
following the appearance of the corresponding 2,4-dinitrophen-
oxide or 2,4-dinitro- or 2,4,6-trinitro-benzenethiolate anions.
The reactions were carried out in aqueous solutions, at 25.0
( 0.1 °C, and an ionic strength of 0.2 M (maintained with KCl).
In the reactions of EDNPDTC and ETNPDTC with ben-
zenethiolate, the other product was identified as O-ethyl
S-phenyl dithiocarbonate (EPDTC) by comparison, through
HPLC, of a sample at the end of the reaction with an authentic
sample10 as a standard. HPLC conditions: column, Eurospher
C-18 (10 cm, 7µm); eluant, acetonitrile/water 70/30; isocratic
mode, 0.5 mL/min.
At least
a 10-fold excess of total benzenethiol over the
substrate was used, except in the reaction of 4-nitroben-
zenethiolate with ETNPDTC. For this reaction (followed at 400
nm), equimolar concentrations (1 × 10-5 M) of the reagents
were used and second-order kinetics were obtained.
(9) Grupta, S. P.; Dureja, P. J . Ind. Chem. Soc. 1973, 50, 546. Castro,
E. A.; Iban˜ez, F.; Salas, M.; Santos, J . G.; Sepu´lveda, P. J . Org. Chem.
1993, 58, 459.
(10) Cox, J . R.; Gladys, C. L.; Field, L.; Pearson, D. E. J . Org. Chem.
1960, 25, 1083.
J . Org. Chem, Vol. 68, No. 23, 2003 9035

Castro et al.
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 O-Meth yl
O-(2,4-Din itr op h en yl) Th ioca r bon a tea
with the X-benzenethiolate anions with X ) H, 3-Cl, and
4-Cl, which were conducted at low pH values. For the
latter reactions, the plots of kobsd vs [ArS-] are linear at
each pH, showing an increase of the slope with the
decrease of the pH value. For these reactions, kobsd can
be described by eq 3, where kN and kH are the nucleophilic
and the acid-catalyzed rate constants, respectively. The
same kinetic behavior was observed for the reactions of
some benzenethiolates with MTNPC and ETNPTC at low
pH values.8 The values of kN and kH found for the
reactions in this work, together with the pKa values of
the substituted benzenethiols,11 are shown in Table 4.
b
benzenethiolate
substituent
103 [ArSH]tot
(M)
103 kobsd
(s-1
no. of
runs
pH
)
4-methoxy
6.0c 0.05-1.00
6.5c 0.05-0.50
7.0c 0.10-1.00
8.5d 0.25-2.00
8.0d 0.10-2.00
8.5d 0.25-2.00
9.0d 0.25-2.00
8.5d 0.25-5.00
9.0d 0.25-5.00
0.88-46.8
14.2-90.0
16.1-147
0.90-13.0
3.22-67.5
11.6-70.1
12.0-73.5
5.15-110
7.55-117
0.528-6.98
0.564-7.07
0.60-7.30
6
5
6
6
7
6
6
6
6
6
6
6
4-methyl
4-chloro
3-chloro
2,3,4,5,6-pentafluoro 8.0d 0.80-10.0
8.5d 0.80-10.0
kobsd ) k0 + kN[ArS-] + kH[ArS-][H+]
(3)
9.0d 0.80-10.0
a
In aqueous solution, at 25.0 °C and an ionic strength of 0.2 M
Table 4 also includes the kN value for the reaction of
4-nitrobenzenethiolate with ETNPDTC, which was de-
termined under second-order conditions. This reaction
could not be studied under pseudo-first-order conditions
because of the high absorbance of this nucleophile.8 The
hydrolysis reaction was checked in the absence of 4-ni-
trobenzenethiolate: no significant reaction was observed
during the time the reaction with 4-benzenethiolate is
complete. The back reaction was not significant as judged
by the good second-order kinetics found. The same
behavior was observed for the reactions of this nucleo-
phile with methyl 2,4,6-trinitrophenyl carbonate and
ethyl 2,4,6-trinitrophenyl thiolcarbonate.8
b
(KCl). Total concentration of substituted benzenethiol (acid plus
conjugate base). c In the presence of phosphate buffer 0.01 M. In
d
the presence of borate buffer 0.01 M.
Resu lts a n d Discu ssion
The kinetic equation obtained for the title reactions,
with the exception of the reaction of 4-nitrobenzenethio-
late with ETNPDTC, is given by eq 1, where P is 2,4-
dinitrophenoxide, 2,4-dinitrobenzenethiolate, or 2,4,6-
trinitrobenzenethiolate anions, S represents the sub-
strate (MDNPTOC, EDNPDTC, or ETNPDTC), and kobsd
is the pseudo-first-order rate constant.
For the reactions of MDNPTOC and EDNPDTC with
4-nitrobenzenethiolate ion, the determination of kN was
not possible since the UV-vis spectra of the leaving
groups, 2,4-dinitrophenoxide and 2,4-dinitrobenzenethio-
late anions, are very similar to that of NPS-. Even under
second-order experimental conditions, the differences in
absorbance of the above compounds during the reactions
are very small. This was also the case for the reactions
of NPS- with the corresponding dinitro carbonate and
thiolcarbonate.8
The acid catalysis exhibited by the reactions of some
benzenethiolate anions with ETNPDTC was probably due
to the fact that these were the only reactions carried out
at low pH (2.5-3.5) values. A similar catalysis was found
in the benzenethiolysis of MTNPC and ETNPTC,8 in the
addition of thiol anions to acetaldehyde12a and in the
d[P]
dt
) kobsd[S]
(1)
For these reactions, linear plots of kobsd versus ben-
zenethiolate (ArS-) concentration were obtained, accord-
ing to eq 2, where k0 and kN are the rate coefficients for
hydrolysis and benzenethiolysis of the substrates, re-
spectively. The values of k0 and kN were determined as
the intercept and slope, respectively, of the linear plots
mentioned above.
kobsd ) k0 + kN[ArS-]
(2)
The values of both k0 and kN were found to be pH
independent, except those for the reactions of ETNPDTC
TABLE 4. Va lu es of p Ka of Ben zen eth iols a n d kN a n d Br øn sted -Typ e Equ a tion s for th e Ben zen eth iolysis of MDNP TOC,
EDNP DTC, a n d ETNP DTCa
kN (s-1 M-1
)
benzenethiolate
substituent
pKa of benzenethiol
MDNPTOC
EDNPDTC
ETNPDTC
4-methoxy
4-methyl
none
4-chloro
3-chloro
6.5b
6.4b
6.4b
6.0b
5.5d
4.6b
2.7f
199 ( 8
70 ( 2
865 ( 35
307(10
172 ( 6
(1.7 ( 0.2) × 104c
(1.1 ( 0.2) × 104c
(6.7 ( 1.2) × 103c
(1.6 ( 0.1) × 103e
61 ( 4
34 ( 1
22.7 ( 0.6
4-nitro
pentafluoro
Brønsted-type
equationg
0.71 ( 0.01
-1.76 + 0.58 pKa
2.7 ( 0.2
-1.36 + 0.66 pKa
0.069 + 0.66 pKa
a
b
Values of kN in aqueous solution, at 25.0 °C and an ionic strength of 0.2 M (KCl). Values of pKa taken from ref 11a, in aqueous
solution, at 25.0 °C and an ionic strength of 0.2 M. c Values of kN obtained from eq 3. The values of kH obtained from this equation for the
reactions of ETNPDTC are (1.8 ( 0.2) × 108 s-1 M-2 (benzenethiolate), (9 ( 1) × 107 s-1 M-2 (4-chlorobenzenethiolate), and (1.5 ( 0.2)
× 107 s-1 M-2 (3-chlorobenzenethiolate). Value of pKa determined by interpolation of the linear relationship between the pKa values
d
measured in aqueous solution, at 25.0 °C and an ionic strength 0.2 M (ref 11a), and the thermodynamic pKa values in water at 25.0 °C
(ref 11b). e Value of kN measured under second-order conditions (see text). f Value of pKa measured in aqueous solution at 25.0 °C and an
g
ionic strength of 1.0 M (ref 11c). Log kN as a function of pKa of benzenethiols.
9036 J . Org. Chem., Vol. 68, No. 23, 2003

Benzenethiolysis of dithiocarbonates
SCHEME 1
zenethiolate (TNPS-) is a better nucleofuge from 2 than
DNPS- from 1. Therefore, it is very probable that the
benzenethiolysis of ETNPTC, as well as that of EDNP-
DTC, would also be driven by a concerted mechanism
(shown in Scheme 1).
F IGURE 1. Brønsted-type plots obtained for the benzenethio-
lysis of O-ethyl S-(2,4,6-trinitrophenyl) dithiocarbonate (b),
O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (O), and O-
methyl O-(2,4-dinitrophenyl) thiocarbonate (2) in aqueous
solution, at 25.0 °C and an ionic strength of 0.2 M.
reactions of 4-cyanopyridine with 2,4-dinitrophenyl ace-
tate and methyl 2,4-dinitrophenyl carbonate,12b all of
which were conducted at low pH values.
Figure 1 shows the Brønsted-type plots obtained for
the benzenethiolysis of MDNPTOC, EDNPDTC, and
ETNPDTC, obtained with the kN and pKa values in Table
4.
The substrate ETNPDTC is more reactive toward
benzenethiolates than EDNPDTC (Figure 1); this should
be due to the presence of a third nitro group in ETNP-
DTC, which would speed both the attack of the nucleo-
phile and the nucleofugality of the leaving group.
As seen in Figure 1, benzenethiolates react faster with
EDNPDTC than with MDNPTOC. This result can be
explained by the soft nature of benzenethiolates, which
favor the binding to a softer electrophilic center such as
the thiocarbonyl group of a dithiocarbonate rather than
the harder thiocarbonyl center of a thionocarbonate. This
is in agreement with Pearson’s “hard and soft acids and
bases” (HSAB) principle.17 The difference of the nonleav-
ing groups (MeO and EtO) of these two substrates should
not be an important factor regarding the difference in
reactivity of these substrates (see below).18
With the kN values obtained for the benzenethiolysis
of both dithiocarbonates (Table 4), together with the pKa
values of the benzenethiols as both nucleophiles (Table
4) and nucleofuges (pKa 3.4 and 1.4 for 2,4-dinitro- and
2,4,6-trinitrobenzenethiols, respectively15), eq 4 can be
obtained by dual regression analysis (n ) 9, R2 ) 0.994).
In this equation, pKa (N) and pKa (lg) are those corre-
sponding to the nucleophile and leaving group, respec-
tively.
The Brønsted-type plots for EDNPDTC and ETNPDTC
are linear with slope (â) values of 0.66 for both, in
accordance with concerted mechanisms.1,4b,e,5,6,13 Never-
theless, the magnitude of the â value alone is not
sufficient to prove a concerted mechanism. Definite proof
requires evidence that there is no break in the Brønsted-
type plot at the predicted pKa for a hypothetical stepwise
mechanism.14 If the benzenethiolysis of EDNPDTC were
stepwise, the Brønsted plot would have a break centered
0
at a pKa (pKa ) value of 3.4, which is the pKa of 2,4-
dinitrobenzenethiol.15 At this pKa, a (hypothetical) sub-
stituted benzenethiolate nucleophile (ArS-) would leave
the putative anionic tetrahedral intermediate (1) as fast
as 2,4-dinitrobenzenethiolate (DNPS-). Since the ben-
zenethiolysis of EDNPDTC shows no break at pKa 3.4
(Figure 1), the stepwise mechanism can be ruled out for
this reaction series.14 This means that compound 1 is
either very unstable or does not exist, the latter situation
corresponding to an enforced concerted mechanism.16 If
the putative intermediate 1 is so unstable, the hypotheti-
cal intermediate 2 in the benzenethiolysis of ETNPDTC
would be even more unstable since 2,4,6-trinitroben-
log kN ) 1.13 + 0.66 pKa (N) - 0.73 pKa (lg) (4)
(11) (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.
(12) (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.
(13) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1989,
111, 2647. Castro, E. A.; Iban˜ez, F.; Salas, M.; Santos, J . G. J . Org.
Chem. 1991, 56, 4819. Castro, E. A.; Salas, M.; Santos, J . G. J . Org.
Chem. 1994, 59, 30.
A double logarithmic plot (not shown) of the kN values
obtained experimentally (Table 4) versus those calculated
through eq 4 is linear with unity slope and zero intercept.
The sensitivity of log kN to the nucleophile basicity (âN
) 0.66) is in line with the âN values found for other
concerted mechanisms, as discussed above. On the other
hand, the sensitivity of log kN to the leaving group
basicity (âlg ) -0.73) is in accordance with the âlg values
(14) Williams, A. Acc. Chem. Res. 1989, 22, 387. Williams, A. Chem.
Soc. Rev. 1994, 23, 93.
(15) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991,
63.
(16) J encks, W. P. Acc. Chem. Res. 1980, 13, 161. J encks, W. P.
Chem. Soc. Rev. 1981, 10, 345.
(17) Pearson, R. G. J . Chem. Educ. 1968, 45, 581. Pearson, R. G. J .
Org. Chem. 1989, 54, 1423.
(18) Castro, E. A.; Cubillos, M.; Santos, J . G.; Tellez, J . J . Org. Chem.
1997, 62, 2512.
J . Org. Chem, Vol. 68, No. 23, 2003 9037

Castro et al.
F IGURE 2. Brønsted-type plots obtained for the benzenethio-
lysis (O, this work) and phenolysis (b, ref 6) of O-ethyl 2,4-
dinitrophenyl dithiocarbonate, in aqueous solution, at 25.0 °C
and an ionic strength of 0.2 M.
F IGURE 3. Bro¨nsted-type plots obtained for the benzenethio-
lyses of EDNPDTC (O, this work) and EDNPTC (b, ref 8), in
aqueous solution, at 25.0 °C and an ionic strength of 0.2 M.
benzenethiolysis of MDNPTOC and therefore must have
the same mechanism. In conclusion, it is reasonable that
intermediate 3 is either very unstable or nonexistent;
therefore, the benzenethiolysis of MDNPTOC should be
concerted.14
obtained for the concerted phenolyses of aryl acetates and
aryl formates.4e
The Brønsted-type plot (Figure 1) for the benzenethio-
lysis of MDNPTOC, obtained with the kN values and the
pKa of the benzenethiols (Table 4), is linear with a slope
of 0.58, also in agreement with a concerted mechanism.
Furthermore, there is no break in this Brønsted-type plot
near the pKa of 2,4-dinitrophenol (pKa 4.1 under the
experimental conditions).8 If an anionic tetrahedral
intermediate (3) were formed in these reactions, it would
be expected that 2,4-dinitrophenoxide (DNPO-) would
leave this intermediate as fast as an isobasic benzenethio-
late (ArS-). This is because the greater intrinsic nucleo-
fugality of a given aryloxide, compared to an isobasic
benzenethiolate,7c is counterbalanced by a greater push
exerted by the former to expel an isobasic benzenethiolate
from the tetrahedral intermediate.2a In fact, for the
stepwise thiolysis of 4-nitrophenyl and 2,4-dinitrophenyl
acetates, the Brønsted breaks are centered at pKa0 ) 7.8
and 5.0, respectively, which are close to the pKa values
of 4-nitrophenol and 2,4-dinitrophenol, respectively.2a
Likewise, for the stepwise thiolysis of 4-nitrophenyl
formate and pivalate, the Brønsted breaks are centered
at pKa ca. 7.2b The lack of a Brønsted break at pKa ca. 4
for the benzenethiolysis of MDNPTOC (Figure 1), con-
firms that this mechanism is concerted.8
The phenolysis of MDNPTOC has been found to be
stepwise, with rate-determining formation of the anionic
tetrahedral intermediate 5.6 This is in contrast to the
concerted benzenethiolysis of this compound (this work),
which bypasses the highly unstable intermediate 3.
These results indicate that the change of ArO to ArS
destabilizes intermediate 5. This can be attributed to the
greater nucleofugality of ArS- from 3, compared with that
of ArO- from 5, due to the lower basicity of ArS- relative
to ArO-.
A comparison of the Brønsted-type plots for the con-
certed benzenethiolysis of EDNPDTC (this work) and the
concerted phenolysis of the same substrate6 is shown in
Figure 2. As observed, benzenethiolates react faster than
isobasic phenoxides toward the thiocarbonyl carbon of
this substrate. This can be attributed to the softer
character of the sulfur nucleophile, relative to the oxygen
nucleophile, that would prefer to bind to the relatively
soft thiocarbonyl carbon, in accordance to Pearson’s
HSAB principle.17 Benzenethiolates are also better nu-
cleophiles than isobasic aryloxides toward the carbonyl
group of acetates,2a,b carbonates,8 and thiolcarbonates,8
despite that these are considered harder centers than
thiocarbonyl.17
The fact that the Bronsted slope for the faster ben-
ezenethiolysis of EDNPDTC (â ) 0.66) is similar to the
slower phenolysis of the same compound (â ) 0.68)6 is
against the reactivity-selectivity principle and the Ham-
mond postulate.19 Nonetheless, there are many examples
of concerted reactions that show anti Hammond behav-
ior.8,14,20 We have also found that the fast concerted
benzenethiolyses of carbonates and thiolcarbonates8 ex-
On the other hand, the existence of an anionic “inter-
mediate” (4) very similar to compound 3 was ruled out
for the phenolysis of EDNPDTC.6 The change of MeO to
EtO as the nonleaving group should not significantly
affect the stability of a tetrahedral intermediate since the
above change does not alter the microcoefficients involved
in the decomposition of zwitterionic tetrahedral inter-
mediates, for instance, those formed in the pyridinolysis
of alkyl aryl thionocarbonates.18 The concerted phenolysis
of EDNPDTC is the microscopic back reaction of the
(19) Hammond, G. S. J . Am. Chem. Soc. 1955, 77, 334.
(20) Lee, I. Chem. Soc. Rev. 1995, 24, 223. Shaik, S. S. Acta Chem.
Scand. 1990, 44, 205. Lee, I.; Lee, H. W. Collect. Czech. Chem.
Commun. 1999, 64, 1529. Williams, A. Concerted Organic and Bio-
organic Mechanisms; CRC Press: Boca Raton, FL, 2000; p 42.
9038 J . Org. Chem., Vol. 68, No. 23, 2003

Benzenethiolysis of dithiocarbonates
With the kN values obtained in this work for the
benzenethiolysis of MDNPTOC and EDNPDTC, together
with those found for the same reactions of the corre-
sponding carbonate (MDNPC) and thiolcarbonate (ED-
NPTC),8 a comparison between the four Bronsted-type
plots can be made. Figure 3 shows the Bronsted plots
for the reactions of EDNPDTC and EDNPTC, and Figure
4 exhibits those for MDNPTOC and MDNPC.
From Figure 3, it can be observed that benzenethio-
lates are more reactive toward EDNPDTC than ED-
NPTC, whereas Figure 4 shows that these nucleophiles
react faster with MDNPTOC than with MDNPC. This
clearly indicates that these soft sulfur nucleophiles have
a preference toward a soft center such as a thiocarbonyl
group over a harder carbonyl group.17
F IGURE 4. Bro¨nsted-type plots obtained for the benzenethio-
lyses of MDNPTOC (O, this work) and MDNPC (b, ref 8), in
aqueous solution, at 25.0 °C and an ionic strength of 0.2 M.
Ack n ow led gm en t. The financial assistance given
to this work by FONDECYT (Chile), project 2000051,
is gratefully acknowledged. P.P. thanks CONICYT for
a Doctoral scholarship.
hibit larger â values than the slower concerted phenolysis
of the same compounds.5a,b
J O0352530
J . Org. Chem, Vol. 68, No. 23, 2003 9039