Kinetic study of the phenolysis of O-methyl and O-phenyl O-2,4-dinitrophenyl thiocarbonates and O-ethyl 2,4-dinitrophenyl dithiocarbonate

Article abstract of DOI:10.1021/jo034538+

The reactions of a series of phenols with O-methyl O-2,4-dinitrophenyl thiocarbonate (MDNPTOC), O-phenyl O-2,4-dinitrophenyl thiocarbonate (PDNPTOC), and O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC) are studied kinetically in water, at 25.0 °C and an ionic strength of 0.2 M (KCl). All reactions show pseudo-first-order kinetics under an excess of phenol over the substrate, and are first order in phenoxide anion. The reactions of EDNPDTC show a linear Bronsted-type plot of slope β = 0.67, suggesting a concerted mechanism. On the other hand, the phenolyses of MDNPTOC and PDNPTOC exhibit linear Bronsted-type plots of slopes β = 0.27 and 0.28, respectively, consistent with stepwise mechanisms where the formation of an anionic tetrahedral intermediate (T^-) is rate determining. By comparison of the kinetics and mechanisms of the reactions under investigation with similar reactions, the following conclusions arise: (i) Substitution of S^- by O^- in the intermediate T^- destabilizes this species. (ii) The change of DNPO in T^- to DNPS also destabilizes this intermediate. (iii) Substitution of MeO by PhO as the nonleaving group of the substrate does not affect the kinetics, probably by a compensation of electronic and steric effects. (iv) The change of an amino group in a tetrahedral intermediate to a phenoxy group destabilizes the intermediate.

Full text of DOI:10.1021/jo034538+

Kin etic Stu d y of th e P h en olysis of O-Meth yl a n d O-P h en yl
O-2,4-Din itr op h en yl Th ioca r bon a tes a n d O-Eth yl 2,4-Din itr op h en yl
Dith ioca r bon a te
Enrique A. Castro,* David Arellano, 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 April 28, 2003
The reactions of a series of phenols with O-methyl O-2,4-dinitrophenyl thiocarbonate (MDNPTOC),
O-phenyl O-2,4-dinitrophenyl thiocarbonate (PDNPTOC), and O-ethyl 2,4-dinitrophenyl dithiocar-
bonate (EDNPDTC) are studied kinetically in water, at 25.0 °C and an ionic strength of 0.2 M
(KCl). All reactions show pseudo-first-order kinetics under an excess of phenol over the substrate,
and are first order in phenoxide anion. The reactions of EDNPDTC show a linear Brønsted-type
plot of slope â ) 0.67, suggesting a concerted mechanism. On the other hand, the phenolyses of
MDNPTOC and PDNPTOC exhibit linear Brønsted-type plots of slopes â ) 0.27 and 0.28,
respectively, consistent with stepwise mechanisms where the formation of an anionic tetrahedral
intermediate (T-) is rate determining. By comparison of the kinetics and mechanisms of the reactions
under investigation with similar reactions, the following conclusions arise: (i) Substitution of S-
by O- in the intermediate T- destabilizes this species. (ii) The change of DNPO in T- to DNPS also
destabilizes this intermediate. (iii) Substitution of MeO by PhO as the nonleaving group of the
substrate does not affect the kinetics, probably by a compensation of electronic and steric effects.
(iv) The change of an amino group in a tetrahedral intermediate to a phenoxy group destabilizes
the intermediate.
In tr od u ction
tetrahedral intermediate (T-) and a change in the rate-
determining step, from T- breakdown to T- formation
with the increase of the phenoxide basicity.
To extend our kinetic investigations on the phenolysis
of thiocarbonyl compounds, in this work we examine the
mechanisms of the phenolyses of O-methyl and O-phenyl
O-2,4-dinitrophenyl thiocarbonates (MDNPTOC and PD-
NPTOC, respectively) and of O-ethyl 2,4-dinitrophenyl
dithiocarbonate (EDNPDTC).
The kinetics and mechanisms of the phenolysis of
esters have been subjected to numerous investigations,1
but there have been only a few studies on the kinetics of
the phenolysis of carbonates,2 thiolesters,3 thiolcarbon-
ates,4 and thionocarbonates.5 The phenolysis of carbon-
ates and thiolcarbonates is ruled by concerted mecha-
nisms,2,4 but the phenolyses of O-methyl O-4-nitrophenyl
and O-bis(4-nitrophenyl) thiocarbonates were shown to
be stepwise.5 For the latter reactions biphasic Brønsted-
0
type plots were obtained, with breaks (pKa ) at pKa 7.1,
which were explained by the formation of an anionic
* Corresponding author.
(1) (a) Buncel, E.; Um, I. H.; Hoz, S. J . Am. Chem. Soc. 1989, 111,
971. (b) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1989,
111, 2647. (c) Kwon, D.-S.; Lee, G.-J .; Um, I.-H. Bull. Korean Chem.
Soc. 1990, 11, 262. (d) Ba-Saif, S. A.; Colthurst, M. J .; Waring, M. A.;
Williams, A. J . Chem. Soc., Perkin Trans. 2 1991, 1901. (e) Stefanidis,
D.; Cho, S.; Dhe-Paganon, S.; J encks, W. P. J . Am. Chem. Soc. 1993,
115, 1650. (f) Colthurst, M. J .; Nanni, M.; Williams, A. J . Chem. Soc.,
Perkin Trans. 2 1996, 2285. (g) Maude, A. B.; Williams, A. J . Chem.
Soc., Perkin Trans. 2 1997, 179. (h) Um, I.-H.; Yoon, H.-W.; Lee, J .-S.;
Moon, H.-J .; Kwon, D.-S. J . Org. Chem. 1997, 62, 5939. (i) Um, I. H.;
Buncel, E. J . Org. Chem. 2000, 65, 577. (j) Um, I. H.; Lee, E. J .; Buncel,
E. J . Org. Chem. 2001, 66, 4859.
Our goal is to evaluate the effect of the electrophilic
(CO vs CS), leaving, and nonleaving groups on the
kinetics and mechanism, by comparing the title reactions
between them and with the phenolysis of similar carbon-
ates and thiocarbonates.
(2) (a) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 2001,
66, 3129. (b) Castro, E. A.; Angel, M.; Pavez, P.; Santos J . G. J . Chem.
Soc., Perkin Trans. 2 2001, 2351. (c) Castro, E. A.; Pavez, P.; Santos
J . G. J . Org. Chem. 2002, 67, 4494.
(3) (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.
(4) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 1999, 64,
2310.
Exp er im en ta l Section
Ma ter ia ls. The series of phenols employed were purified
by distillation or recrystallization. EDNPDTC was synthesized
as described.6
(5) Castro, E. A.; Angel, M.; Arellano, D.; Santos J . G. J . Org. Chem.
2001, 66, 6571.
(6) Castro, E. A.; Iba´n˜ez, F.; Salas, M.; Santos, J . G.; Sepu´lveda, P.
J . Org. Chem. 1993, 58, 459.
10.1021/jo034538+ CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/09/2003
6192
J . Org. Chem. 2003, 68, 6192-6196

Phenolysis of Thiocarbonates
TABLE 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e P h en olysis of O-Meth yl O-2,4-Din itr op h en yl
Th ioca r bon a te (MDNP TOC)a
PDNPTOC was synthesized by a modification of literature
procedures,7-9 as follows: To a solution of 2,4-dinitrophenol
(0.3 g, 1.6 mmol) dissolved in dichloromethane (20 mL) in a
Schlenk round-bottomed flask was added pyridine in 40%
excess, slowly under nitrogen atmosphere. The solution was
rapidly transferred to a compensation funnel, under nitrogen.
In another Schlenk round-bottomed flask, O-phenyl chlo-
rothioformate (0.22 mL) was dissolved in anhydrous dichlo-
romethane (20 mL) under nitrogen. The compensation funnel
was attached to the flask and the 2,4-dinitrophenoxide solution
added dropwise with stirring. The mixture was left overnight
with stirring under nitrogen at ambient temperature. The
reaction mixture was washed with a saturated NH4HSO4
solution and then with water. The organic layer was dried with
MgSO4 and filtered and the solvent was evaporated off. The
product was purified by column chromatography (silica gel,
chloroform/hexane 70/30). The pure product, PDNPTOC,
melted at 106-108 °C and was identified as follows:
1H NMR (200 MHz, CDCl3) δ 7.2-7.5 (m, 5H), 7.64 (d, 1H,
J ) 8.9 Hz), 8.61 (dd, 1H, J ) 2.7, 8.9 Hz), 9.07 (d, 1H, J )
8.9 Hz); 13C NMR (50 MHz, CDCl3) δ 191.9, 153.5, 150.2, 145.7,
141.3, 130.0, 129.6, 127.4, 127.3, 122.2, 121.4; IR (KBr) 1347,
1537 (C-NO2), 1294 (CdS) cm-1. Anal. Calcd for C13H8N2O6S:
C, 48.75; H, 2.52; N, 8.75; S, 10.01. Found: C, 48.89; H, 2.46;
N, 8.53; S, 9.77.
phenoxide
substituent pH
no. of
runs
b
FN
103[ArOH]tot/Mc 103kobsd/s-1
4-OCH3
H
10.0 0.33
10.3 0.50
10.6 0.67
9.6 0.33
9.9 0.50
10.2 0.67
8.3 0.33
8.6 0.50
8.9 0.67
7.5 0.33
7.8 0.50
8.1 0.67
6.8 0.33
7.1 0.50
7.4 0.67
5.0 0.33
5.3 0.50
5.6 0.67
6.0-80.0
6.0-60.0
6.0-80.0
6.0-80.0
6.0-80.0
6.0-80.0
2.0-25.0
2.0-25.0
2.0-25.0
0.80-10.0
0.80-10.0
0.80-10.0
6.0-80.0
8.0-80.0
6.0-60.0
6.0-60.0
6.0-60.0
6.0-80.0
2.22-32.6
3.89-35.3
6.75-70.0
1.77-21.0
2.20-27.0
3.44-39.2
0.25-3.51
0.43-5.42
0.62-6.94
0.092-1.10
0.134-1.60
0.162-2.10
0.25-4.86
0.62-7.71
0.66-7.20
0.124-0.830
0.161-1.28
0.222-2.10
7
6
7
7
7
7
7
7
7
7
7
7
7
6
6
6
6
7
3-CN
4-CN
2,6-F2
2,3,4,5,6-F5
a
In aqueous solution, at 25.0 °C, ionic strength 0.2 M (KCl).
Fraction of free phenoxide. c Concentration of total phenol (free
b
MDNPTOC was prepared as described above for PDNPTOC,
but using O-methyl chlorothioformate.9 It melted at 75.6-76.0
°C, and was identified as follows:
phenoxide plus conjugate acid).
TABLE 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e P h en olysis of O-P h en yl O-2,4-Din itr op h en yl
Th ioca r bon a te (P DNP TOC)a
1H NMR (200 MHz, CDCl3) δ 4.22 (s, 3H), 7.53 (d, 1H, J )
8.9 Hz), 8.56 (dd, 1H, J ) 2.7, 8.9 Hz), 8.99 (d, 1H, J ) 8.9
Hz); 13C NMR (50 MHz, CDCl3) δ 193.0, 150.1, 150.2, 145.5,
141.4, 129.4, 127.4, 122.0, 61.7; IR (KBr) 1346, 1538 (C-NO2),
1308 (CdS) cm-1. Anal. Calcd for C8H6N2O6S: C, 37.21; H,
2.32; N, 10.85; S, 12.40. Found: C, 36.83; H, 2.10; N, 10.58; S,
12.35.
phenoxide
substituent pH
no. of
b
FN
103[ArOH]tot/Mc 103kobsd/s-1 runs
4-OCH3
H
10.0 0.33
10.3 0.50
10.6 0.67
9.6 0.33
9.9 0.50
10.2 0.67
8.3 0.33
8.6 0.50
8.9 0.67
7.5 0.33
7.8 0.50
8.1 0.67
6.8 0.33
7.1 0.50
7.4 0.67
5.0 0.33
5.3 0.50
5.6 0.67
8.0-60.0
6.0-75.0
8.0-75.0
2.0-25.0
2.0-25.0
2.5-25.0
6.5-80.0
6.5-80.0
6.5-60.0
6.5-80.0
6.5-60.0
6.5-60.0
25-80.0
6.5-80.0
6.5-80.0
8.0-80.0
8.0-80.0
8.0-80.0
6.58-31.4
6.35-37.8
8.26-63.0
0.420-5.30
0.874-8.15
1.10-13.0
0.813-9.89
0.980-16.7
1.81-17.9
0.156-5.12
0.671-7.20
0.500-8.74
0.968-3.80
0.580-5.88
0.500-6.38
0.128-1.25
0.170-2.00
0.200-2.77
5
7
6
7
7
6
7
7
6
7
6
5
4
6
7
6
6
6
O-Bis(phenyl) thiocarbonate8 (one of the products in the
reaction of phenol with PDNPTOC), O-methyl O-phenyl thio-
carbonate7 (one of the products in the reaction of phenol with
MDNPTOC), and O-ethyl O-phenyl thiocarbonate10 (one of the
products in the reaction of phenol with EDNPDTC) were
prepared as reported.
Kin etic Mea su r em en ts. These reactions were followed
spectrophotometrically (300-500 nm) by monitoring the cor-
responding 2,4-nitrophenoxide or 2,4-dinitrobenzenethiolate
anions. The rate constants were determined at 400 (reactions
of MDNPTOC and EDNPDTC) and 360 nm (PDNPTOC). The
reactions were studied in aqueous solutions, at 25.0 ( 0.1 °C,
and an ionic strength of 0.2 M (maintained with KCl). At least
a 10-fold excess of total phenol (phenoxide anion plus phenol)
over the substrate was employed in all reactions.
3-CN
4-CN
2,6-F2
2,3,4,5,6-F5
Pseudo-first-order rate coefficients (kobsd) were found in all
cases. These were obtained by means of the kinetic software
of the spectrophotometer, after at least 4 half-lives, except for
the slowest reaction (EDNPDTC with pentafluorophenol)
where the initial rate method was used.11
a
In aqueous solution, at 25.0 °C, ionic strength 0.2 M (KCl).
Fraction of free phenoxide. c Concentration of total phenol (free
b
phenoxide plus conjugate acid).
The experimental conditions of the reactions and the kobsd
values are shown in Tables 1-3.
dinitrophenol and 2,4-dinitrobenzenethiol, under the same
reaction conditions.
P r od u ct Stu d ies. 2,4-Dinitrophenoxide anion was identi-
fied as one of the products in the phenolysis of MDNPTOC
and PDNPTOC, and 2,4-dinitrobenzenethiolate anion as one
of the products in the reactions of EDNPDTC. This was
achieved by comparison of the UV-vis spectra after completion
of these reactions with those of authentic samples of 2,4-
In the reactions of MDNPTOC, PDNPTOC, and EDNPDTC
with phenoxide anion, O-methyl O-phenyl thiocarbonate,
O-bis(phenyl) thiocarbonate, and O-ethyl O-phenyl thiocar-
bonate, respectively, were found as the other product of these
reactions. All these products were identified by HPLC, using
authentic samples as standards. HPLC conditions: column,
Eurospher C-18 (10 cm, 7µm); eluent, acetonitrile/water 70/
30; isocratic mode, 0.7 mL/min.
(7) Castro, E. A.; Cubillos, M.; Santos, J . G.; Tellez, J . J . Org. Chem.
1997, 62, 2512.
(8) Castro, E. A.; Santos, J . G.; Tellez, J .; Uman˜a, M. I. J . Org. Chem.
1997, 62, 6568.
(9) Al-Kazimi, H. R.; Tarbell, D. S.; Plant, D. J . Am. Chem. Soc.
Resu lts a n d Discu ssion
The rate law obtained for all the reactions studied is
given by eqs 1 and 2, where P, S, and ArO- represent
the product (either 2,4-dinitrophenoxide or 2,4-dini-
trobenzenethiolate anions), a substrate, and a substituted
1955, 77, 2479.
(10) Al-Awadi, N.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1988,
177.
(11) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1987,
109, 6362.
J . Org. Chem, Vol. 68, No. 16, 2003 6193

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 P h en olysis of O-Eth yl 2,4-Din itr op h en yl
Dith ioca r bon a te (EDNP DTC)a
phenoxide
substituent pH FN 103[ArOH]tot/Mc
no. of
runs
103kobsd/s-1
b
4-OCH3
10.0 0.33
10.3 0.50
10.6 0.67
9.4 0.26
9.7 0.41
10.0 0.58
8.7 0.33
9.0 0.50
9.3 0.67
7.5 0.33
7.8 0.50
8.1 0.67
6.0-80.0
10.0-80.0
10.0-80.0
6.0-60.0
6.0-60.0
6.0-60.0
6.0-60.0
6.0-60.0
6.0-60.0
10.0-90.0
10.0-90.0
10.0-90.0
10.0-85.0
10.0-85.0
10.0-85.0
12.9-118
6
5
5
5
5
5
5
5
5
5
5
5
5
5
5
28.9-183
32.6-253
H
1.80-14.3
3.01-25.9
4.03-35.3
1.34-9.42
1.74-13.6
2.11-17.9
0.25-1.92
0.39-2.80
0.53-3.45
0.0010-0.111
0.0019-0.111
0.0021-0.0994
3-Cl
4-CN
2,3,4,5,6-F5 6.5d 0.94
6.8d 0.97
F IGURE 1. Brønsted-type plots for the phenolysis of MDNP-
TOC (O), PDNPTOC (9), and EDNPDTC (b) in water, at 25.0
°C, ionic strength 0.2 M (KCl). The slopes (â) are 0.27, 0.28,
and 0.67, respectively.
7.0d 0.98
a
In aqueous solution, at 25.0 °C, ionic strength 0.2 M (KCl).
Fraction of free phenoxide. c Concentration of total phenol (free
phenoxide plus conjugate acid). In the presence of phosphate
buffer 0.005 M.
b
d
The Brønsted-type plots for the phenolysis of the two
thionocarbonates, MDNPTOC and PDNPTOC, are linear
(see Figure 1), with slopes (â) 0.27 and 0.28, respectively.
These values are in accordance with those found in the
stepwise phenolyses of O-methyl O-4-nitrophenyl and
O-bis(4-nitrophenyl) thiocarbonates, when formation of
the anionic tetrahedral intermediate (T-) is the rate-
determining step.5 The â values also agree with those
found in the reactions of pyridines with alkyl aryl
carbonates,12 secondary alicyclic amines (SAA) with
methyl 4-nitrophenyl carbonate,13 quinuclidines and SAA
with diaryl carbonates,14,15 pyridines and SAA with aryl
thiolacetates,16 SAA with diaryl thionocarbonates,17and
SAA and pyridines with dithiocarbonates,6,18 when for-
mation of the zwitterionic tetrahedral intermediate (T()
is rate limiting (â ) 0.1-0.3).
TABLE 4. Va lu es of p Ka of P h en ols a n d kN for th e
P h en olysis of O-Meth yl O-2,4-Din itr op h en yl
Th ioca r bon a te (MDNP TOC), O-P h en yl
O-2,4-Din itr op h en yl Th ioca r bon a te (P DNP TOC), a n d
O-Eth yl 2,4-Din itr op h en yl Dith ioca r bon a te (EDNP DTC)a
102kN/s-1 M-1
phenoxide
substituent
pKa
MDNPTOC
PDNPTOC
EDNPDTC
4-OCH3
H
3-Cl
3-CN
4-CN
10.3
9.9
9.0
8.6
7.8
7.1
5.3
124 ( 4
71 ( 3
130 ( 3
71 ( 1
451 ( 2
97 ( 4
43 ( 1
46 ( 1
23 ( 1
13.0 ( 0.5
5.0 ( 0.1
41.0 ( 0.5
31.0 ( 0.3
19.0 ( 0.5
4.0 ( 0.1
5.9 ( 0.2
2,6-F2
2,3,4,5,6-F5
0.13 ( 0.01
a
Both the pKa and kN values were determined in aqueous
Therefore, the â values obtained in the phenolysis of
the two thionocarbonates studied in this work suggest
that these reactions are ruled by a stepwise mechanism,
where the formation of an anionic tetrahedral intermedi-
ate (T-) is rate limiting (Scheme 1, R ) methyl or phenyl,
DNP ) 2,4-dinitrophenyl).
solution, at 25.0 °C, ionic strength 0.2 M (KCl).
phenoxide nucleophile, respectively, and k0 and kN are
the rate coefficients for hydrolysis and phenolysis of the
substrates, respectively.
d[P]
dt
If the mechanism for the phenolysis of the above
thionocarbonates is stepwise the pKa value for the center
) kobsd[S]
(1)
0
of the Brønsted curvature (pKa ) should be 4.1 (the pKa
kobsd ) k0 + kN[ArO-]
(2)
of 2,4-dinitrophenol). This value lies outside the pKa
range of the series of phenols employed, preventing,
therefore, the observation of the Brønsted break. Nev-
The value of k0 was much lower than those of kN[ArO-]
in eq 2, except in the slow reactions of pentafluorophenol
with the three substrates, where the phenolysis term in
eq 2 was also small. The rate coefficient k0 measures the
attack of water and/or that of OH- to the substrate,
depending on the pH values. In the particular case of the
reactions of EDNPDTC with pentafluorophenoxide, where
phosphate buffer was used, the k0 values also include the
catalysis by the buffer (presumably base-catalyzed hy-
drolysis). The values of kN for all reactions were obtained
as the slope of linear plots of kobsd vs [ArO-]. The kN
values were found to be pH independent. These kN values
are shown in Table 4, together with the pKa values of
the series of phenols employed.
0
ertheless, the predicted pKa value of 4.1 for a stepwise
mechanism confirms that for the reactions of the thiono-
(12) (a) Castro, E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99, 7611.
(b) Castro, E. A.; Freudenberg, M. J . Org. Chem. 1980, 45, 906. (c)
Castro, E. A.; Iban˜ez, F.; Lagos, S.; Schick, M.; Santos, J . G. J . Org.
Chem. 1992, 57, 2691.
(13) Castro, E. A.; Aliaga, M.; Campodo´nico, P.; Santos, J . G. J . Org.
Chem. 2002, 67, 8911.
(14) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963.
(15) Castro, E. A.; Andu´jar, M.; Campodo´nico, P.; Santos, J . G. Int.
J . Chem. Kinet. 2002, 34, 309. Castro, E. A.; Andu´jar, M.; Toro, A.;
Santos, J . G. J . Org. Chem. 2003, 68, 3608.
(16) (a) Castro, E. A.; Ureta, C. J . Org. Chem. 1989, 54, 2153. (b)
Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991, 63.
(17) Castro, E. A.; Saavedra, C.; Santos, J . G.; Uman˜a, M. I. J . Org.
Chem. 1999, 64, 5401. Castro, E. A.; Galvez, A.; Leandro, L.; Santos,
J . G. J . Org. Chem. 2002, 67, 4309.
Figure 1 shows the Brønsted-type plots obtained with
the values of kN and pKa in Table 4 (measured under the
same experimental conditions).
(18) Castro, E. A.; Araneda, C. A.; Santos, J . G. J . Org. Chem. 1997,
62, 126.
6194 J . Org. Chem., Vol. 68, No. 16, 2003

Phenolysis of Thiocarbonates
SCHEME 1
methyl O-4-nitrophenyl thiocarbonate, which show step-
wise mechanisms.5 The destabilization of a tetrahedral
intermediate by substitution of S- by O- has been
explained by the greater ability of O- in the intermediate
to form a double bond and expel a nucleofuge, compared
to S-, due to a stronger π-bonding energy of the carbonyl
group relative to thiocarbonyl.21
The Brønsted-type plot for the phenolysis of EDNPDTC
is linear, with slope â ) 0.67 (Figure 1), which is in
accordance with a concerted mechanism. Analogous â val-
ues have been obtained for the concerted phenolyses of
2,4-dinitrophenyl and 2,4,6-trinitrophenyl O-ethyl thiol-
carbonates,4 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-
trinitrophenyl methyl carbonates,2a diaryl carbonates,2c,5
aryl acetates,1b,d,e and acetic anhydride.1d
carbonates with the phenols employed in this work the
rate-determining step is the formation of the intermedi-
ate T- in Scheme 1.
The phenolysis of O-methyl O-4-nitrophenyl thiocar-
0
bonate shows a biphasic Brønsted plot with pKa ) 7.1,
which is the pKa of 4-nitrophenol, a clear indication that
this mechanism is stepwise.5 The fact that the formation
of the intermediate T- is rate limiting for the reactions
of the same phenols with MDNPTOC is in accordance
with the previous finding, since 2,4-dinitrophenoxide is
a better nucleofuge from T- than 4-nitrophenoxide from
the corresponding intermediate, and this should decrease
the pKa0 value for the phenolysis of MDNPTOC compared
to that for the reactions of the same phenols with the
mononitro derivative.
The value of â ) 0.67 for the phenolysis of EDNPDTC
is near the lower limit of the â values found for stepwise
reactions when breakdown of the tetrahedral intermedi-
ate to products is the rate-determining step (â ) 0.8-
1.0).6,7,12-14,16,18,22 Nevertheless, the stepwise mechanism
can be ruled out on the following grounds: For the
pyridinolyses of O-ethyl O-2,4-dinitrophenyl thiocarbon-
ate and EDNPDTC stepwise mechanisms were found, as
evidenced by biphasic Brønsted plots with the pKa values
0
at the curvature center (pKa ) of 6.8 and 6.9, respec-
tively.7,18 This suggests that the leaving abilities of 2,4-
dinitrophenoxide (DNPO-) and 2,4-dinitrobezenethiolate
(DNPS-) from the corresponding zwitterionic tetrahedral
intermediates (2 and 3, respectively) are similar.18 This
is due to the fact that the lower basicity of DNPS- than
DNPO- (pKa of the corresponding phenols are 3.4 and
4.1, respectively)16b is compensated by an intrinsic lower
nucleofugality of an ArS- group compared to an isobasic
Ar′O- group.23 On the other hand, for the stepwise
phenolysis of MDNPTOC the predicted Brønsted break
The similar kN values exhibited by the phenolysis of
MDNPTOC and PDNPTOC (Table 4 and Figure 1) is
noteworthy. Larger kN values for the latter reactions
could be expected if only electronic effects were consid-
ered, since the phenoxy group is more electron withdraw-
ing than methoxy inductively (σI ) 0.37 and 0.29,
respectively) and less electron donating by resonance (σR
) -0.40 and -0.56, respectively).19 Nevertheless, the
phenoxy group is bulkier than methoxy and should offer
a greater steric hindrance to nucleophilic attack than
methoxy; therefore, the electronic advantage of the
phenoxy group should be compensated by its steric
disadvantage.
It has been found that the phenolysis of methyl 2,4-
dinitrophenyl carbonate is driven by a concerted mechan-
ism;2a in contrast, the phenolysis of MDNPTOC shows a
stepwise path (this work). This suggests that substitution
of S- in intermediate 1 (R ) Me) by O- destabilizes this
intermediate to the point of mechanistic change.
0
(pKa ) is 4.1 (see above). If the phenolysis of EDNPDTC
were stepwise, and assuming that DNPS- has a similar
leaving ability as DNPO- from the corresponding anionic
tetrahedral intermediates (4 and 5, respectively), the
0
value of pKa for the hypothetical stepwise mechanism
of EDNPDTC would be ca. 4.1. This analysis assumes
that the change of EtO to MeO in an anionic tetrahedral
intermediate does not alter the rate microcoefficients k-1
and k2 (shown in Scheme 1) involved in the stepwise
process. This is reasonable since these rate microcoeffi-
cients are not influenced by the change of the above
substituents in a zwitterionic tetrahedral intermediate.7
Therefore, if the phenolysis of EDNPDTC were stepwise
the Brønsted â value would be similar to those found for
the phenolysis of the thionocarbonates (â ) 0.27, 0.28),
i.e., formation of the tetrahedral intermediate would be
rate limiting. This is true even if DNPS- leaves 4 faster
than DNPO- from 5 (see below). Since for the phenolysis
of EDNPDTC the Brønsted â value is 0.67, the stepwise
mechanism for this reaction can be excluded.
The above result is in agreement with the fact that the
reactions of SAA with 2,4-dinitrophenyl and 2,4,6-trini-
trophenyl O-ethyl thiolcarbonates are concerted,20 whereas
the reactions of the same amines with 2,4-dinitrophenyl
and 2,4,6-trinitrophenyl O-ethyl dithiocarbonates are
stepwise.6 Moreover, the phenolyses of bis(4-nitrophenyl)
carbonate5 and methyl 4-nitrophenyl carbonate2a are
ruled by concerted mechanisms, in opposition to the same
reactions of O-bis(4-nitrophenyl) thiocarbonate and O-
(19) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(20) Castro, E. A.; Iba´n˜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.
The above mechanistic change, from stepwise for the
phenolysis of MDNPTOC to concerted for that of EDN-
J . Org. Chem, Vol. 68, No. 16, 2003 6195

Castro et al.
PDTC, is surprising on the basis that DNPO- and DNPS-
leave the zwitterionic tetrahedral intermediates 2 and
3, respectively, at similar rates. Nevertheless, it is
possible that DNPS- leaves faster than DNPO- from the
corresponding anionic tetrahedral intermediates 4 and
5, respectively. In fact, DNPS- is a better nucleofuge than
DNPO- from the tetrahedral intermediates (6 and 7)
The aminolysis (SAA) of EDNPDTC follows a stepwise
mechanism, as evidenced by the biphasic Brønsted-type
plot obtained.6 The fact that the phenolysis of the same
substrate is concerted (this work) is in agreement with
the destabilization of zwitterionic tetrahedral intermedi-
ates by substitution of an amino group by ArO. Examples
of this type of destabilization are the stepwise reactions
of SAA with (i) 4-nitrophenyl and 2,4-dinitrophenyl
acetates,22,24 compared with the concerted phenolyses of
these compounds,1d,e (ii) O-ethyl 4-nitrophenyl thiolcar-
bonate,25 and (iii) O-phenyl and O-4-nitrophenyl chlo-
rothioformates,26 in contrast to the concerted phenolyses
of the same substrates.4,27
formed in the pyridinolysis of the corresponding thiolac-
etate and acetate, as judged by the pKa0 values of 6.6 and
7.3 found in the reactions of 2,4-dinitrophenyl thiolace-
tate16b and 2,4-dinitrophenyl acetate,12b respectively.
Another possibility is that the anionic intermediate (4)
formed in the phenolysis of EDNPDTC is more unstable
thermodynamically than that (5) formed in the same
reactions of MDNPTOC, the former reaction bypassing
therefore intermediate 4 and choosing the less energetic
concerted path.
Ack n ow led gm en t. We thank FONDECYT of Chile
(project 2000051) for financial assistance to this work.
P.P. thanks CONICYT for a Doctoral scholarship.
Su p p or tin g In for m a tion Ava ila ble: Experimental condi-
tions, values of the concentrations of total substituted phenols,
and kobsd for the reactions of SAA with MDNPTOC (Table S1),
PDNPTOC (Table S2), and EDNPDTC (Table S3). This mate-
rial is available free of charge via the Internet at http://pubs.
acs.org.
J O034538+
(21) Hill, S. V.; Thea, S.; Williams, A. J . Chem. Soc., Perkin Trans.
2 1983, 437.
(22) (a) J encks, W. P.; Gilchrist, M. J . Am. Chem. Soc. 1968, 90,
2622. (b) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974,
96, 7018.
(24) Castro, E. A.; Ureta, C. J . Org. Chem. 1990, 55, 1676.
(25) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1994,
59, 3572.
(26) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1997,
62, 4395.
(23) Douglas, K. T.; Alborz, M. J . Chem. Soc., Chem. Commun. 1981,
551. Douglas, K. T. Acc. Chem. Res. 1986, 19, 186.
(27) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1998,
63, 6820.
6196 J . Org. Chem., Vol. 68, No. 16, 2003