Concerted mechanisms of the reactions of methyl aryl carbonates with substituted phenoxide ions

Article abstract of DOI:10.1021/jo010022j

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC, DNPC, and TNPC, respectively) with substituted phenoxide ions are subjected to a kinetic study in water at 25.0 °C, ionic strength 0.2 M (KCl). Production of the leaving groups (the nitro derivatives) is followed spectrophotometrically. Under excess of the phenoxide ions pseudo-first-order rate coefficients (k_obsd) are found throughout. Plots of k_obsd vs substituted phenoxide concentration at constant pH are linear, with the slope (k_N) independent of pH. The Broensted-type plots (log k_N vs pK_a of the phenols) are linear with slopes β = 0.67, 0.48, and 0.52 for the phenolysis of NPC, DNPC, and TNPC, respectively. The magnitudes of these Broensted slopes are consistent with a concerted mechanism. In the particular case of the phenolysis of NPC the expected hypothetical curvature center of the Broensted plot for a stepwise mechanism should be pK_a⁰ = 7.1 (the pK_a of 4-nitrophenol). This curvature does not appear within the pK_a range of the substituted phenols studied (5.3-10.3), indicating that these reactions are concerted. The phenolysis of DNPC and TNPC should also be concerted in view of the even more unstable tetrahedral intermediates that would be formed if the reactions were stepwise. The reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate perhaps to the point of nonexistence. The k_N values for the title reactions are larger than those for the concerted phenolysis of the corresponding ethyl S-aryl thiolcarbonates. The k_N values found in the present reactions are subjected to a dual regression analysis as a function of the pK_a, of both the nucleophile and leaving group, the coefficients being βN = 0.5 and β_ig = -0.3, respectively. These coefficients are consistent with a concerted mechanism.

Full text of DOI:10.1021/jo010022j

J . Org. Chem. 2001, 66, 3129-3132
3129
Con cer ted Mech a n ism s of th e Rea ction s of Meth yl Ar yl
Ca r bon a tes w ith Su bstitu ted P h en oxid e Ion s
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 8, 2001
The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC,
DNPC, and TNPC, respectively) with substituted phenoxide ions are subjected to a kinetic study
in water at 25.0 °C, ionic strength 0.2 M (KCl). Production of the leaving groups (the nitro
derivatives) is followed spectrophotometrically. Under excess of the phenoxide ions pseudo-first-
order rate coefficients (kobsd) are found throughout. Plots of kobsd vs substituted phenoxide
concentration at constant pH are linear, with the slope (kN) independent of pH. The Bro¨nsted-type
plots (log kN vs pKa of the phenols) are linear with slopes â ) 0.67, 0.48, and 0.52 for the phenolysis
of NPC, DNPC, and TNPC, respectively. The magnitudes of these Bro¨nsted slopes are consistent
with a concerted mechanism. In the particular case of the phenolysis of NPC the expected
0
hypothetical curvature center of the Bro¨nsted plot for a stepwise mechanism should be pKa ) 7.1
(the pKa of 4-nitrophenol). This curvature does not appear within the pKa range of the substituted
phenols studied (5.3-10.3), indicating that these reactions are concerted. The phenolysis of DNPC
and TNPC should also be concerted in view of the even more unstable tetrahedral intermediates
that would be formed if the reactions were stepwise. The reactions of the same substrates with
pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral
intermediate by a phenoxy group destabilizes the intermediate perhaps to the point of nonexistence.
The kN values for the title reactions are larger than those for the concerted phenolysis of the
corresponding ethyl S-aryl thiolcarbonates. The kN values found in the present reactions are
subjected to a dual regression analysis as a function of the pKa, of both the nucleophile and leaving
group, the coefficients being âN ) 0.5 and âlg ) -0.3, respectively. These coefficients are consistent
with a concerted mechanism.
In tr od u ction
methyl 2,4,6-trinitrophenyl carbonates with the purpose
of clarifying the mechanisms of the phenolysis of aryl
carbonates. In this paper we also compare these mech-
anisms with those for the pyridinolysis of the same
substrates3 and the phenolysis of other carboxylic deriva-
tives5 and thiolcarbonates.6
The kinetics and mechanisms of the aminolysis and
pyridinolysis of carboxylic acid derivatives, such as esters
and carbonates, have been extensively investigated.1-4
Nevertheless, the mechanisms of the reactions of phe-
noxide anions with these substrates have been much less
studied.5
In the present work we undertake a kinetic and
mechanistic study of the reactions of phenoxide anions
with methyl 4-nitrophenyl, methyl 2,4-dinitrophenyl, and
Exp er im en ta l Section
Ma ter ia ls. The substituted phenols (Aldrich) were purified
either by distillation or recrystallization. The substrates were
synthesized as previously reported.3 Methyl phenyl carbonate
(one of the products from the reactions of the substrates with
phenoxide anion) was prepared as described previously.4a
Deter m in a tion of p Ka . The pKa values of 3-chloro and 2,6-
difluoro phenols were determined spectrophotometrically, at
239 and 232 nm, respectively, in water at 25.0 ( 0.1 °C, ionic
strength 0.2 M (KCl), by the method described.7 The pKa
determination of the other phenols has been reported previ-
ously.8
(1) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
7018.
(2) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
6970.
(3) (a) Bond, P. M.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2
1976, 679. (b) Castro, E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99,
7611. (c) Castro, E. A.; Iban˜ez, F.; Lagos, S.; Schick, M.; Santos, J . G.
J . Org. Chem. 1992, 57, 2691.
(4) (a) Castro, E. A.; Freudenberg, M. J . Org. Chem. 1980, 45, 906.
(b) Neuvonen, H. J . Chem. Soc., Perkin Trans. 2 1987, 159. (c) Yoh,
S.-D.; Kang, J -K.; Kim, S.-H. Tetrahedron 1988, 44, 2167. (d) Knowlton,
R. C.; Byers, L. D. J . Org. Chem. 1988, 53, 3862. (e) Lee J -P.; Yoh, S.
D. Bull. Korean Chem. Soc. 1996, 17, 211. (f) Hess, R. A.; Hengge, A.
C.; Cleland, W. W. J . Am. Chem. Soc. 1997, 119, 6980. (g) Koh, H. J .;
Shin, C. H.; Lee, H. W.; Lee, I. J . Chem. Soc., Perkin Trans. 2 1998,
1329. (h) Colthurst, M. J .; Kanagasoorian, A. J . S. S.; Wong, M. S. O.;
Contini, C.; Williams, A. Can. J . Chem. 1998, 76, 678. (i) Koh, H. J .;
Lee, H. W.; Lee, I. Can. J . Chem. 1998, 76, 710. (j) Um, I.-H.; Park,
Y.-M.; Shin, E.-H. Bull. Korean Chem. Soc. 1999, 20, 392. (k) Cho, B.
R.; Kim, N. S.; Kim, Y. K.; Son, K. H. J . Chem. Soc., Perkin Trans. 2
2000, 1419. (l) Um, I.-H.; Min, J .-S.; Ahn, J .-A.; Hahn, H.-J . J . Org.
Chem. 2000, 65, 5659. (m) Koh, H. J .; Han, K. L.; Lee, H. W.; Lee, I.
J . Org. Chem. 2000, 65, 4706.
Kin etic Mea su r em en ts. The reactions were followed spec-
trophotometrically in the range 300-500 nm (appearance of
(5) (a) Hupe, D. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 451.
(b) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1987,
109, 6362. (c) Buncel, E.; Um, I. H.; Hoz, S. J . Am. Chem. Soc. 1989,
111, 971. (d) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem.
Soc. 1989, 111, 2647. (e) Stefanidis, D.; Cho, S.; Dhe-Paganon, S.;
J encks, W. P. J . Am. Chem. Soc. 1993, 115, 1650. (f) Colthurst, M. J .;
Williams, A. J . Chem. Soc., Perkin Trans. 2 1997, 1493.
(6) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 1999, 64,
2310.
(7) Albert, A.; Serjeant, E. P. The Determination of Ionization
Constants; Chapman and Hall: London, 1971.
10.1021/jo010022j CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/05/2001

3130 J . Org. Chem., Vol. 66, No. 9, 2001
Castro et al.
Ta ble 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 P h en oxid e An ion s w ith Meth yl
4-Nitr op h en yl Ca r bon a te (NP C)a
Ta ble 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 P h en oxid e An ion s w ith Meth yl
2,4,6-Tr in itr op h en yl Ca r bon a te (TNP C)a
c
c
phenoxide
substituent
103 [phenol]tot
(M)
104 kobsd
(s-1
no. of
runs
phenoxide
substituent
103 [phenol]tot
(M)
104 kobsd
(s-1
no. of
runs
b
b
pH
FN
)
pH
FN
)
4-MeO
H
10.0
10.3
10.6
9.6
0.33
0.50
0.66
0.33
0.50
0.66
0.33
0.50
0.66
0.33
0.50
0.67
0.33
0.50
0.66
1.00
1.0-40
1.0-40
1.0-40
1.0-40
2.5-40
2.5-40
4.0-40
1.0-40
1.0-40
6.0-80
4.0-80
4.0-80
8.0-40
8.0-40
8.0-40
8.0-80
12.0-359
34.3-502
53.0-542
4.79-100
8.66-168
14.4-204
5.74-53.7
2.66-68.7
4.00-89.6
0.391-7.40
0.462-11.6
0.848-15.7
0.42-1.72
0.68-2.29
0.768-3.15
0.308-1.04
7
8
8
8
7
7
6
8
8
7
8
8
4
4
4
6
H
9.9
10.2
8.7
0.50
0.66
0.33
0.50
0.66
0.33
0.50
0.67
0.33
0.50
0.67
1
0.8-8.0
0.8-8.0
1.0-40
4.0-40
4.0-40
10-400
10-400
10-400
80-400
10-400
10-400
40-800
40-800
40-800
2.0-9.2
5
6
6
6
6
8
8
8
4
8
7
8
8
8
0.86-3.9
0.67-6.1
0.80-8.4
1.30-10.1
0.15-5.24
0.20-7.0
0.29-10.7
0.108-3.5
0.195-4.79
0.40-4.35
0.233-3.6
0.211-3.65
0.23-3.70
3-Cl
9.0
9.9
9.3
10.2
8.7
4-CN
7.5
3-Cl
7.8
9.0
8.1
9.3
2,6 -F2
2,3,4,5,6-F5
6.8
4-CN
2,6-F2
7.5
7.1
7.8
7.4
8.1
8.0d
8.5d
9.0d
6.8
1
7.1
1
7.4
a
d
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
2,3,4,5,6-F5
8.0
b
Free phenoxide fraction of total phenol. c Total concentration of
a
d
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
substituted phenol (acid plus conjugate base). In the presence
of borate buffer 0.01 M.
Free phenoxide fraction of total phenol. c Total concentration of
b
d
substituted phenol (acid plus conjugate base). In the presence
of borate buffer 0.01 M.
Ta ble 4. Va lu es of p Ka of P h en ols a n d kN for th e
P h en olysis of Meth yl Ar yl Ca r bon a tesa
Ta ble 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 P h en oxid e An ion s w ith Meth yl
2,4-Din itr op h en yl Ca r bon a te (DNP C)a
kN (s-1 M-1
)
phenoxide
substituent
pKa
NPC
DNPC
TNPC
c
103 [phenol]tot
(M)
104 kobsd
(s-1
no. of
runs
4-MeO
H
3-Cl
4-CN
2,6-F2
2,3,4,5,6-F5
10.3b 2.23 ( 0.05
11.1 ( 0.5
3.9 ( 0.1
2.2 ( 0.1
0.41 ( 0.02
0.35 ( 0.02
0.033 ( 0.002
phenoxide
substituent
9.9b 0.77 ( 0.01
87 ( 5
b
pH
FN
)
9.0
7.8b 0.029 ( 0.001
7.1 0.011 ( 0.0005
5.3b 0.001 ( 0.0002
0.33 ( 0.01
39 ( 1
4-MeO
H
10.0
10.3
10.6
9.6
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
1
1.0-40
1.0-40
2.5-40
1.0-40
1.0-40
1.0-40
1.0-40
1.0-40
1.0-40
4.0-40
2.5-40
2.5-40
8.0-40
1.0-40
1.0-40
4.0-80
4.0-80
4.0-80
7.93-168
13.4-226
30.1-268
2.0-49.8
7
8
7
8
8
8
8
8
8
6
7
7
4
8
8
8
8
8
3.8 ( 0.1
1.8 ( 0.1
0.45 ( 0.02
a
9.9
3.66-76.0
6.14-112
0.67-33.2
1.14-44.9
1.90-57.2
0.60-5.80
0.55-6.72
0.741-11.5
0.882-4.85
0.207-7.02
0.25-9.33
0.201-2.39
0.225-2.86
0.29-3.0
Both the pKa and kN values were determined in aqueous
solution at 25.0 °C, ionic strength 0.2 M (KCl). b Values taken from
ref 6.
10.2
8.7
3-Cl
9.0
9.3
achieved by comparison of the UV-vis spectra after completion
of the reactions with those of authentic samples under the
same conditions. The reactions of unsubstituted phenoxide ion
with the three substrates yield methyl phenyl carbonate as
the other product of the reactions, as indicated by comparison
of the HPLC spectrum of an authentic sample with those
obtained after completion of the reactions. HPLC conditions:
column, Eurospher C-18 (10 cm, 7 µm); eluant, acetonitrile/
water ) 70/30; isocratic mode, 0.7 mL/min.
4-CN
7.5
7.8
8.1
2,6-F2
2,3,4,5,6-F5
6.8
7.1
7.4
8.0d
d
8.5
9.0
1
1
d
a
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
Free phenoxide fraction of total phenol. c Total concentration of
b
Resu lts a n d Discu ssion
d
substituted phenol (acid plus conjugate base). In the presence
of borate buffer 0.01 M.
The phenolyses of the title substrates are governed
kinetically by eq 1, where kobsd is a pseudo-first-order rate
the leaving nitrophenoxide anions) by means of a Hewlett-
Packard 8453 diode array spectrophotometer. All reactions
were studied in aqueous solution at 25.0 ( 0.1 °C, ionic
strength 0.2 M (KCl). In most cases, the substituted phenol
and its conjugate base acted as buffer, and in a few cases, the
buffer was borate 0.01 M.
kobsd ) ko + kN [ArO-]
(1)
coefficient, ko and kN are the rate coefficients for hydroly-
sis and phenolysis, respectively, and ArO- represents the
nucleophile, a substituted phenoxide ion. The values of
ko and kN were obtained as the intercept and slope,
respectively, of linear plots of kobsd vs [ArO-] at constant
pH.
For almost all the reactions the values of ko and kN
are independent of pH. Only for the reaction of methyl
2,4-dinitrophenyl carbonate (DNPC) with pentafluoro-
phenoxide anion was there observed an increase of the
ko value with the increase of pH.
Table 4 shows the kN values obtained in the present
reactions and the pKa values of the phenols found under
the same experimental conditions as those of the kinetic
measurements. The Bro¨nsted-type plots obtained with
the kN values for the phenolysis of the three substrates
are shown in Figure 1. These plots are linear with slopes
In all cases the initial substrate concentration was 2 × 10-5
M, and the substituted phenol was at least in a 10-fold excess
over the substrate.
Under these conditions, pseudo-first-order rate coefficients
(kobsd) were found by fitting of the first-order rate equation to
the experimental points; nevertheless, for the slowest reactions
(the mononitro substrate with 2,6-difluoro and pentafluoro
phenols) the initial rate method was used.5b The experimental
conditions of the reactions and the kobsd values are shown in
Tables 1-3.
P r od u ct Stu d ies. For the phenolysis of the 4-nitro, 2,4-
dinitro, and 2,4,6-trinitro substrates one of the products was
identified as 4-nitrophenoxide, 2,4-dinitrophenoxide, and 2,4,6-
trinitrophenoxide ions, respectively. The identification was
(8) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1998,
63, 6820.

Reactions of Methyl Aryl Carbonates
J . Org. Chem., Vol. 66, No. 9, 2001 3131
Clearly, the magnitude of the Bro¨nsted slope alone is
not sufficient to decide whether the mechanism is con-
certed or stepwise. A proof for a stepwise reaction
(through a tetrahedral intermediate) would be to observe
a Bro¨nsted break (due to change in the limiting step)
centered at the pKa of the leaving group, provided the
attacking and leaving groups are of the same nature.14,15
If the phenolysis of NPC were stepwise the hypotheti-
cal break of the Bro¨nsted-type plot would be at pKa )
7.1, which is the pKa of 4-nitrophenol in water. The pKa
range of the conjugate acids of the nucleophiles, i.e.,
substituted phenols, covered in this work is 5.3-10.3.
Since no break is observed in the Bro¨nsted plot for NPC
in Figure 1, it can be concluded that the mechanism for
the phenolysis of NPC is concerted.
For the phenolyses of DNPC and TNPC the hypotheti-
cal Bro¨nsted breaks for stepwise mechanisms are outside
the pKa range of the phenols, in view of the low pKa of
2,4-dinitro- and 2,4,6-trinitro phenols (pKa 4.0 and 0.33,
respectively).7 If these reactions were stepwise, formation
of the tetrahedral intermediate would be rate-determin-
ing, since expulsion of the leaving groups from the
intermediate would be much faster than that for the
attacking phenoxide ions. The Bro¨nsted slopes obtained
for these reactions (Figure 1) are larger than the usual
â values found for similar reactions when formation of
the tetrahedral intermediate is the rate-limiting step (â
) 0.1-0.3).1,2,5a,15,16 This analysis indicates that the
reactions of DNPC and TNPC are concerted.
Another reason to believe that the phenolyses of DNPC
and TNPC are governed by concerted mechanisms is the
fact that the same reaction of NPC is concerted. If the
hypothetical intermediate that would be formed in the
reactions of NPC (if the mechanism were stepwise) is
either too unstable to exist (enforced concerted process)
or very unstable but existent,14,17 the change of 4-nitro-
phenoxy to 2,4-dinitro- or 2,4,6-trinitrophenoxy would
render the intermediate even more unstable. Therefore,
it is reasonable that the phenolyses of DNPC and TNPC
are also concerted.
The reactions of the title substrates with pyridines in
aqueous solution are stepwise, with the formation of a
zwitterionic tetrahedral intermediate (T().3 This conclu-
sion arises from the large slope value of the linear
Bro¨nsted plot for the pyridinolysis of NPC,3a and the
biphasic behavior of the Bro¨nsted plots in the reactions
of DNPC3b and TNPC.3c The former result was explained
by the breakdown of T( to products as being rate-
limiting.3a The biphasic Bro¨nsted plots for the pyridi-
nolyses of DNPC and TNPC were interpreted through a
change in rate-determining step, from breakdown to
formation of T(, as the nucleophile becomes stronger.3b,c
The fact that the pyridinolyses of the title substrates
are stepwise, whereas their phenolyses are concerted,
means that the tetrahedral intermediates in the former
reactions are much destabilized by the change of a
pyridine to a phenoxy group.
F igu r e 1. Bro¨nsted-type plots obtained in the phenolysis of
4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl meth-
yl carbonates (NPC, DNPC, and TNPC, respectively) in
aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
â ) 0.67 ( 0.03, 0.48 ( 0.03, and 0.52 ( 0.06 for the
reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-
trinitrophenyl methyl carbonates, respectively (NPC,
DNPC, and TNPC, respectively).
The fact that these Bro¨nsted plots are linear is in
contrast with the sharp breaks found in the Bro¨nsted
plots for the phenolysis of bis(4-nitrophenyl) phenylphos-
phonate in water and in water/dimethyl sulfoxide mix-
tures.9 These Bro¨nsted breaks, centered at pKa values
much larger than that of 4-nitrophenol, cannot be ex-
plained by a stepwise mechanism (through a pentacoor-
dinate intermediate) and a change in rate-determining
step.9 The breaks were attributed to the occurrence of
solvational imbalance phenomena in the transition state
of the reactions.9 Nevertheless, these Bro¨nsted breaks are
not observed in the reactions of the present work (Figure
1) nor in the phenolyses of ethyl S-aryl thiolcarbonates,6
aryl chlorothionoformates,8 aryl acetates,5b,d 4-nitro-
phenyl diphenylphosphinate, and 4-nitrophenyl diphenyl
phosphate.10
The values of the Bro¨nsted slopes obtained in the title
reactions are in agreement with those found in the
concerted reactions in water of phenoxide ions with aryl
esters and analogous carboxylic derivatives, such as
1-acetoxy-8-hydroxynaphthalene (â ) 0.48),11 4-chloro
2-nitrophenyl acetate (â ) 0.64 ( 0.05),5d acetic anhy-
dride (â ) 0.58 ( 0.05),12 2,4-dinitrophenyl acetate (â )
0.57 ( 0.03),12 3-nitrophenyl, 4-nitrophenyl, and 3,4-di-
nitrophenyl formates (â
) 0.64, 0.51, and 0.43,
respectively),5e and the corresponding acetates (â ) 0.66,
0.59, and 0.53, respectively).5e
On the other hand, concerted phenolyses of carboxylic
compounds in water with larger Bro¨nsted slopes (â )
0.8-1) have also been reported.5d Even larger â values
have been found in the reactions of phenoxide anions with
acetic anhydride in solvents less polar than water, e.g.,
â ) 1.3 and 1.6 in acetonitrile and chlorobenzene,
respectively.13
This is consistent with the fact that the pyridinolyses
of aryl acetates,1,3c,4a,18 aryl benzoates,19 ethyl S-aryl
(13) Maude A. B.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1997,
179.
(9) Terrier, F.; Moutiers, G.; Xiao, L.; Le Guevel, E.; Guir, F. J . Org.
Chem. 1995, 60, 1748.
(10) Bourne, N.; Chrystiuk, E.; Davis, A. M.; Williams, A. J . Am.
Chem. Soc. 1988, 110, 1890. Ba-Saif, S.; Waring, M. A.; Williams, A.
J . Am. Chem. Soc. 1990, 112, 8115.
(11) Hibbert, F.; Malana, M. A. J . Chem. Soc., Perkin Trans. 2 1990,
711.
(14) Williams, A. Acc. Chem. Res. 1989, 22, 387.
(15) Castro, E. A. Chem. Rev. 1999, 99, 3505.
(16) Castro, E. A.; Garcia, P.; Leandro, L.; Quesieh, N.; Rebolledo,
A.; Santos, J . G. J . Org. Chem. 2000, 65, 9047.
(17) (a) J encks, W. P. Chem. Soc. Rev. 1981, 10, 345. (b) Williams,
A. Chem. Soc. Rev. 1994, 23, 93.
(12) Ba-Saif, S. A.; Colthurst, M.; Waring, M. A.; Williams, A. J .
Chem. Soc., Perkin Trans. 2 1991, 1901.
(18) Bond, P. M.; Castro, E. A.; Moodie, R. B. J . Chem. Soc., Perkin
Trans. 2 1976, 68.

3132 J . Org. Chem., Vol. 66, No. 9, 2001
Castro et al.
thiolcarbonates,20 and acetic anhydride21 are stepwise,
whereas the phenolyses of the same substrates are
concerted.5b,d,f,6,12
Since the reactions of phenoxide ions with 4-nitro-
phenyl acetate are concerted, the tetrahedral “intermedi-
ate” 1 (PN ) 4-nitrophenyl) either does not exist or is
very unstable.5b On the other hand, it is known that the
change of methyl by a methoxy group destabilizes the
tetrahedral intermediate;22 therefore the “intermediate”
2 should be even more unstable than 1. This indicates
that the phenolysis of NPC should be concerted, as found
in the present work.
The phenolysis of ethyl S-(4-nitrophenyl) thiolcarbon-
ate was found to be driven by a concerted mechanism.6
This means that structure 3 should also be very unstable.
On the other hand, the change of a methoxy group by
ethoxy should not significantly affect the stabilization of
a tetrahedral intermediate.23 The change of 4-nitro-
phenoxy to 4-nitrobenzenethio should stabilize slightly
structure 3,6 despite the relatively large difference of the
basicities involved (pKa values of their conjugate acids
7.1 and 4.6, respectively), which would suggest that SPN
should leave faster than OPN. Nonetheless, it should be
taken into account that phenoxides are better nucleofuges
than isobasic benzenethiolates from tetrahedral struc-
tures.24
The nucleophilic rate constants (kN) found in this work
are larger than those obtained in the concerted phenoly-
sis of the corresponding ethyl S-aryl thiolcarbonates.6
This result is in accord with the larger kN values found
in the concerted reactions of secondary alicyclic amines
with TNPC25 compared with ethyl S-(2,4,6-trinitrophenyl)
thiolcarbonate.26b Also in line are the results in stepwise
aminolyses: (i) the pyridinolyses of NPC, DNPC, and
TNPC are faster3 than those of the corresponding ethyl
S-aryl thiolcarbonates,20 (ii) anilines are more reactive
toward DNPC than ethyl S-(2,4-dinitrophenyl) thiol-
carbonate,26d and (iii) quinuclidines react faster with
phenyl 4-nitrophenyl carbonate2 than ethyl S-(4-nitro-
phenyl) thiolcarbonate.26c This difference in reactivity
presumably arises from the bulkier S atom, relative to
the O atom, which inhibits the attack to the thiolcarbon-
ates by the nucleophile.
F igu r e 2. Logarithmic plot of experimental kN vs calculated
kN (through eq 2) for the phenolysis of methyl aryl carbonates
in aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
regression analysis (n ) 17, R2 ) 0.97). In this expression
N and lg refer to nucleophile and leaving group, respec-
tively; the pKa coefficients (âN ) 0.5 and âlg ) -0.3) are
subjected to an error of ( 0.1.
A logarithmic plot of the experimental kN vs the
calculated one through eq 2 is shown in Figure 2; the
slope is unity.
The sensitivity of kN to the nucleophile (âN ) 0.5) is in
agreement with those exhibited in several phenolysis
when the reaction mechanism is concerted, as was
mentioned earlier.5a,e,11,12 For the concerted aminolysis
(secondary alicyclic amines, anilines and quinuclidines)
of carbonates and similar compounds in water the value
of âN is 0.5-0.6.15,17b,22,26
On the other hand, the sensitivity of kN to the leaving
group basicity (âlg ) -0.3) is in agreement with those
exhibited in the concerted aminolysis of ethyl S-aryl
thiolcarbonates in water (âlg ) -0.19)26b and the con-
certed phenolysis of aryl formates (âlg from -0.31 to
-0.48),5e although smaller than those obtained in the
concerted phenolysis of aryl acetates (âlg from -0.50 to
-0.63).5e
The values of the rate constants for the phenolyses of
4-nitrophenyl and 2,4-dinitrophenyl acetates5d,e are larger
than those found for the reactions of the same nucleo-
philes with NPC and DNPC, respectively (this work). In
other words, substitution of Me by MeO lowers the kN
value, as found in the stepwise aminolyses of these
substrates.3a,b,4a This can be explained by the larger
electron-releasing effect exerted by MeO compared to that
of Me, which inhibits amine attack to the carbonyl
center3a decreasing, therefore, the rate constants for both
the stepwise and concerted mechanisms.
With the kN values found in the present reactions
together with the pKa values for both the nucleophiles
and leaving groups (Table 4), eq 2 can be deduced by dual
log kN ) - 3.2 + 0.5 pKa(N) - 0.3 pKa(lg) (2)
The kN values for the phenolysis of 4-nitrophenyl
formate are larger than those for the same reactions of
NPC. This can be accounted for by the lower steric
hindrance and the absence of an electron-releasing effect
by the formate esters.5e
(19) Castro, E. A.; Steinfort, G. B. J . Chem. Soc., Perkin Trans. 2
1983, 453. Castro, E. A.; Santander, C. L. J . Org. Chem. 1985, 50,
3595. Castro, E. A.; Valdivia, J . L. J . Org. Chem. 1986, 51, 1668.
(20) Castro, E. A.; Pizarro, M. I.; Santos, J . G. J . Org. Chem. 1996,
61, 5982.
(21) Castro, C.; Castro, E. A. J . Org. Chem. 1981, 46, 2939.
(22) Chrystiuk, E.; Williams, A. J . Am. Chem. Soc. 1987, 109, 3040.
(23) Castro, E. A.; Cubillos, M.; Santos, J . G.; Tellez, J . J . Org. Chem.
1997, 62, 2512.
(24) Douglas, K. T. Acc. Chem. Res. 1986, 19, 186.
(25) Unpublished results from this laboratory.
(26) (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. (c) Castro, E. A.; Mun˜oz, P.; Santos, J . G. J .
Org. Chem. 1999, 64, 8298. (d) Castro, E. A.; Leandro, L.; Milla´n, P.;
Santos, J . G. J . Org. Chem. 1999, 64, 1953.
Ack n ow led gm en t. We thank “Fondo Nacional de
Investigaciones Cient´ıficas y Tecnolo´gicas” (FOND-
ECYT) of Chile, project 2000051, for financial support
to this work. P.P. thanks CONICYT of Chile for a
scholarship.
J O010022J