Kinetic study of the phenolysis of bis(4-nitrophenyl) carbonate, bis(4-nitrophenyl) thionocarbonate, and methyl 4-nitrophenyl thionocarbonate

Article abstract of DOI:10.1021/jo0101252

The reactions of a homogeneous series of phenols with bis(4-nitrophenyl) carbonate (BNPC), bis-(4-nitrophenyl) thionocarbonate (BNPTOC), and methyl 4-nitrophenyl thionocarbonate (MNPTOC) are subjected to a kinetic investigation in water, at 25.0 °C and ionic strength of 0.2 M (KCl). Under excess of phenol over the substrate, all the reactions obey pseudo-first-order kinetics and are first order in phenoxide anion. The reactions of BNPC show a linear Broensted-type plot with slope β=0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOC and MNPTOC show biphasic Broensted-type plots with slopes β=0.30 and 0.44, respectively, at high pK_a, and β=1.25 and 1.60, respectively, at low pK_a, consistent with stepwise mechanisms. For the reactions of both thionocarbonates, the pK_a value at the center of the BrSnsted plot (pK_a⁰) is 7.1, which corresponds to the pK_a of 4-nitrophenol. This confirms that the phenolyses of the thionocarbonates are stepwise processes, with the formation of an anionic tetrahedral intermediate. By the comparison of the kinetics and mechanisms of the title reactions with similar reactions, the following conclusions can be drawn: (i) Substitution of S- by O- in an anionic tetrahedral intermediate (T^-) destabilizes it. (ii) The change of MeO by 4-nitrophenoxy in T^- results in an increase of both the rate constant and equilibrium constant, for the formation ofT^-, and also in an enlargement of the rate coefficient for the expulsion of 4-nitrophenoxide from T^-. (iii) Substitution of an amino group in a tetrahedral intermediate by ArO destabilizes it. (iv) Secondary alicyclic amines and other amines show greater reactivity toward MNPTOC than isobasic phenoxide anions.

Full text of DOI:10.1021/jo0101252

J . Org. Chem. 2001, 66, 6571-6575
6571
Kin etic Stu d y of th e P h en olysis of Bis(4-n itr op h en yl) Ca r bon a te,
Bis(4-n itr op h en yl) Th ion oca r bon a te, a n d Meth yl 4-Nitr op h en yl
Th ion oca r bon a te
Enrique A. Castro,* Mauricio Angel, David Arellano, and J ose´ G. Santos*
Facultad de Quı´mica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
ecastro@puc.cl.
Received February 1, 2001
The reactions of a homogeneous series of phenols with bis(4-nitrophenyl) carbonate (BNPC), bis-
(4-nitrophenyl) thionocarbonate (BNPTOC), and methyl 4-nitrophenyl thionocarbonate (MNPTOC)
are subjected to a kinetic investigation in water, at 25.0 °C and ionic strength of 0.2 M (KCl).
Under excess of phenol over the substrate, all the reactions obey pseudo-first-order kinetics and
are first order in phenoxide anion. The reactions of BNPC show a linear Bro¨nsted-type plot with
slope â ) 0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOC
and MNPTOC show biphasic Bro¨nsted-type plots with slopes â ) 0.30 and 0.44, respectively, at
high pKa, and â ) 1.25 and 1.60, respectively, at low pKa, consistent with stepwise mechanisms.
0
For the reactions of both thionocarbonates, the pKa value at the center of the Bro¨nsted plot (pKa )
is 7.1, which corresponds to the pKa of 4-nitrophenol. This confirms that the phenolyses of the
thionocarbonates are stepwise processes, with the formation of an anionic tetrahedral intermediate.
By the comparison of the kinetics and mechanisms of the title reactions with similar reactions, the
following conclusions can be drawn: (i) Substitution of S- by O- in an anionic tetrahedral
intermediate (T-) destabilizes it. (ii) The change of MeO by 4-nitrophenoxy in T- results in an
increase of both the rate constant and equilibrium constant, for the formation of T-, and also in an
enlargement of the rate coefficient for the expulsion of 4-nitrophenoxide from T-. (iii) Substitution
of an amino group in a tetrahedral intermediate by ArO destabilizes it. (iv) Secondary alicyclic
amines and other amines show greater reactivity toward MNPTOC than isobasic phenoxide anions.
In tr od u ction
To extend our kinetic studies on the phenolyses of
carbonates and thiocarbonates, in this work, we inves-
tigate the kinetics and mechanisms of the phenolyses of
bis(4-nitrophenyl) carbonate (BNPC) and two thionocar-
bonates: bis(4-nitrophenyl) thionocarbonate (BNPTOC)
and methyl 4-nitrophenyl thionocarbonate (MNPTOC).
Another aim is to assess the influence of the electro-
philic (CO vs CS) and nonleaving groups and the nature
of the nucleophile on the kinetics and mechanism of these
reactions. This objective will be achieved by the compari-
son of the reactions of the present work with the
phenolysis of 4-nitrophenyl methyl carbonate3b and the
aminolysis of BNPTOC4 and MNPTOC.5
Although the kinetics and mechanisms of the phenoly-
ses of esters are well documented,1,2 only few reports have
dealt with the kinetics of the same reactions of carbonates
and thiocarbonates.3 Among the latter reactions are the
phenolyses of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-
trinitrophenyl O-ethyl thiolcarbonates3a and 4-nitrophen-
yl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl
carbonates.3b All these reactions have been found to be
governed by concerted mechanisms, on the basis of the
linear Bro¨nsted-type plots obtained and the magnitude
of their slopes.3 Moreover, for the phenolysis of 4-nitro-
phenyl methyl carbonate the concerted mechanism was
confirmed by the fact that the predicted Bro¨nsted break
for a stepwise mechanism (at pKa ) 7.1) was not observed
within the pKa of the phenols used.3b
* To whom correspondence should be addressed.
Exp er im en ta l Section
(1) (a) Guanti, G.; Cevasco, G.; Thea, S.; Dell’Erba, C.; Petrillo, G.
J . Chem. Soc., Perkin Trans. 2 1981, 327-330. (b) Buncel, E.; Um, I.
H.; Hoz, S. J . Am. Chem. Soc. 1989, 111, 971-975. (c) Ba-Saif, S.;
Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1989, 111, 2647-2652.
(d) Ba-Saif, S. A.; Colthurst, M. J .; Waring, M. A.; Williams, A. J . Chem.
Soc., Perkin Trans. 2 1991, 1901-1908. (e) Colthurst, M. J .; Nanni,
M.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1996, 2285-2291. (f)
Maude, A. B.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1997, 179-
183. (g) Kwon, D.-S.; Lee, G.-J .; Um, I.-H. Bull. Korean Chem. Soc.
1990, 11, 262-265. (h) Um, I.-H.; Yoon, H.-W.; Lee, J .-S.; Moon, H.-
J .; Kwon, D.-S. J . Org. Chem. 1997, 62, 5939-5944.
(2) Stefanidis, D.; Cho, S.; Dhe-Paganon, S.; J encks, W. P. J . Am.
Chem. Soc. 1993, 115, 1650-1656.
(3) (a) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 1999,
64, 2310-2313. (b) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem.
2001, 66, 3129-3132.
Ma ter ia ls. The phenols were purified by distillation or
recrystallization. BNPC was purchased from Aldrich. BNP-
TOC4,6 and MNPTOC7 were synthesized as reported. Bis-
(phenyl) carbonate (one of the products in the reaction of
(4) Castro, E. A.; Santos, J . G.; Tellez, J .; Uman˜a, M. I. J . Org. Chem.
1997, 62, 6568-6574.
(5) Castro, E. A.; Saavedra, C.; Santos, J . G.; Uman˜a, M. I. J . Org.
Chem. 1999, 64, 5401-5407.
(6) Al-Kazimi, H. R.; Tarbell, D. S.; Plant, D. J . Am. Chem. Soc.
1955, 77, 2479-2482.
(7) Castro, E. A.; Cubillos, M.; Santos, J . G.; Tellez, J . J . Org. Chem.
1997, 62, 2512-2517.
10.1021/jo0101252 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/01/2001

6572 J . Org. Chem., Vol. 66, No. 20, 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
Ta ble 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 Bis(4-n itr op h en yl) Th ion oca r bon a te
(BNP TOC)a
th e P h en olysis of Bis(4-n itr op h en yl) Ca r bon a te (BNP C)a
phenoxide
substituent pH
103[ArOH]tot
(M)c
102kobsd
(s-1
no. of
runs
b
FN
)
phenoxide
substituent
103[ArOH]tot
(M)c
102kobsd
(s-1
no. of
runs
b
pH
FN
)
4-OCH3
H
8.7d 0.025
9.0d 0.048
9.3d 0.090
0.50-3.0
0.50-3.0
0.50-3.0
0.5-3.0
0.520-1.65
0.680-2.53
1.41-4.27
0.19-1.70
0.44-3.06
1.22-3.76
0.58-2.39
0.95-3.30
1.27-4.07
1.87-8.26
2.60-15.6
2.62-18.7
0.58-2.06
0.76-2.18
0.80-2.19
0.203-0.347
0.188-0.382
0.206-0.387
5
5
5
6
6
6
6
6
6
6
6
6
5
5
5
5
5
5
4-OCH3
H
10.0
10.3
10.6
9.6
0.333
0.500
0.667
0.333
0.500
0.667
0.333
0.500
0.667
0.333
0.500
0.667
0.333
0.500
0.667
3.0-10.0
3.0-10.0
3.0-10.0
3.0-10.0
3.0-10.0
3.0-10.0
3.0-10.0
3.0-10.0
3.0-10.0
0.50-3.00
0.50-3.00
0.50-3.00
40.0-80.0
40.0-80.0
40.0-80.0
4.0-12.0
4.0-12.0
4.0-12.0
100-500
3.58-9.45
5.49-14.0
7.12-18.2
2.25-7.26
3.97-10.8
5.08-14.1
2.31-5.11
3.00-7.65
3.53-10.1
0.57-1.36
0.66-2.16
0.81-2.62
5.15-10.6
6.54-15.8
8.63-19.7
1.23-2.44
1.29-2.53
1.52-2.50
0.396-0.998
5
5
5
5
5
5
5
5
5
6
6
4
5
5
5
5
5
5
5
9.0
9.3
9.6
8.7
9.0
9.3
7.5
7.8
8.1
0.112
0.200
0.285
0.333
0.500
0.667
0.333
0.500
0.667
0.5-3.0
0.5-3.0
9.9
3-Cl
0.5-3.0
10.2
9.1
0.5-3.0
4-Cl
0.5-3.0
9.4
4-CN
2,6-F2
10.0-60.0
10.0-60.0
10.0-60.0
2.0-10.0
2.0-10.0
2.0-10.0
30.0-70.0
30.0-70.0
30.0-70.0
9.7
3-Cl
8.7
9.0
8.7d 0.974
9.0d 0.987
9.3d 0.993
9.3
4-CN
2,6-F2
7.5
7.8
2,3,4,5,6-F5 6.7e 0.962
7.0e 0.980
8.1
8.7d 0.974
9.0d 0.987
9.3d 0.993
8.0e 0.998
7.3e 0.990
a
In aqueous solution, at 25.0 °C, and ionic strength of 0.2 M
2,3,4,5,6-F5
(KCl). Fraction of free phenoxide. c Concentration of total phenol
b
a
d
In aqueous solution, at 25.0 °C, and ionic strength of 0.2 M
(free phenoxide plus conjugate acid). In the presence of borate
(KCl). Fraction of free phenoxide. c Concentration of total phenol
b
buffer (0.005 M). e In the presence of phosphate buffer (0.005 M).
d
(free phenoxide plus conjugate acid). In the presence of borate
buffer (0.005 M). e In the presence of phosphate buffer (0.005 M).
phenol with BNPC) was obtained from Aldrich. Phenyl 4-ni-
trophenyl carbonate (another product of the latter reaction)
was prepared by a modification8 of a procedure reported.9
Phenyl 4-nitrophenyl thionocarbonate (one of the products of
the reaction of phenol with BNPTOC) was obtained in poor
yield by the method described;6 a better yield was found by
another procedure.5 Bis(phenyl) thionocarbonate (another
product of the latter reaction) was synthesized as reported.4
Methyl phenyl thionocarbonate (one of the products of the
reaction of phenol with MNPTOC) was prepared by the same
method used for the synthesis of MNPTOC.7
Kin etic Mea su r em en ts. These were performed spectro-
photometrically by following the production of 4-nitrophenox-
ide ion at 400 nm, by means of a Hewlett-Packard 8453 diode
array spectrophotometer. The reactions were studied in aque-
ous solutions, at 25.0 ( 0.1 °C, ionic strength of 0.2 M
(maintained with KCl), and at least a 10-fold excess of total
phenol (phenoxide plus phenol) over the substrate.
Pseudo-first-order rate coefficients (kobsd) were found through-
out, mostly by means of the “infinity” method; for the slowest
reactions (BNPTOC and MNPTOC with pentafluorophenol),
the initial rate method was used.10
In the reactions of the symmetrical substrates (BNPC and
BNPTOC) with phenoxides, a slow reaction of these nucleo-
philes with the initial product (phenyl 4-nitrophenyl carbonate
and the analogous thionocarbonate, respectively) was noticed.
This reaction was much slower than the major one and,
therefore, did not interfere with the main reaction, as indicated
by the good correlation coefficients obtained.
The experimental conditions of the reactions and the kobsd
values are shown in Tables 1-3.
P r od u ct Stu d ies. In all the reactions the 4-nitrophenoxide
ion was identified as one of the products by the comparison of
the UV-vis spectra, after completion of these reactions, with
those of authentic samples of 4-nitrophenol under the same
reaction conditions.
In the reactions of BNPC and BNPTOC with unsubstituted
phenoxide anions, phenyl 4-nitrophenyl carbonate and phenyl
4-nitrophenyl thionocarbonate, respectively, were found as the
other products of these reactions. After the kinetics were
measured, a slow reaction of phenoxide with these products
was observed, yielding bis(phenyl) carbonate and bis(phenyl)
thionocarbonate, respectively. All these products were identi-
Ta ble 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 Meth yl 4-Nitr op h en yl Th ion oca r bon a te
(MNP TOC)a
phenoxide
substituent pH
103[ArOH]tot
(M)c
102kobsd
(s-1
no. of
runs
b
FN
)
4-OCH3
H
10.0 0.333
10.3 0.500
10.6 0.667
9.4 0.258
9.7 0.409
10.0 0.580
8.7 0.333
9.0 0.500
9.3 0.667
7.5 0.333
7.8 0.500
8.1 0.667
7.1 0.483
7.4 0.651
7.8 0.824
7.2d 0.988
0.6-8.0
1.0-8.0
1.0-8.0
0.6-6.0
0.6-6.0
0.6-6.0
0.6-6.0
0.6-6.0
0.6-6.0
1.0-9.0
1.0-9.0
1.0-9.0
1.0-9.0
1.0-9.0
1.0-8.0
10.0-50.0
4.0-42.5
8.7-60.4
6
5
5
5
5
5
5
5
5
5
5
5
5
5
5
4
11.2-72.3
0.89-7.23
1.31-12.9
1.87-15.8
0.79-5.90
1.04-9.06
1.32-11.1
0.26-1.77
0.43-2.86
0.54-3.91
0.102-0.818
0.127-1.07
0.194-1.28
0.0134-0.0296
3-Cl
4-CN
2,6-F2
2,3,4,5,6-F5
a
In aqueous solution, at 25.0 °C, and ionic strength of 0.2 M
(KCl). Fraction of free phenoxide. c Concentration of total phenol
(free phenoxide plus conjugate acid). In the presence of phosphate
b
d
buffer (0.005 M).
fied by HPLC, using as standards the authentic samples.
HPLC conditions were as follows: column, Eurospher C-18 (10
cm, 7 µm); eluant, acetonitrile/water ) 70:30; isocratic mode,
0.7 mL/min.
In the reactions of MNPTOC with unsubstituted phenoxide,
the presence of methyl phenyl thionocarbonate in the reaction
media was determined by HPLC, using the latter product as
reference. The HPLC conditions were as described above.
Resu lts a n d Discu ssion
The rate law obtained in all the reactions under
investigation is given by eqs 1 and 2, where NP is
4-nitrophenyl, S and ArO- represent a substrate and a
substituted phenoxide nucleophile, and k0 and kN are the
hydrolysis and nucleophilic rate constants, respectively.
(8) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963-
6970.
(9) Smith, G. G.; Ko¨sters, B. Chem. Ber. 1960, 93, 2400-2404.
(10) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1987,
109, 6362-6368.

Phenolysis of Carbonates and Thionocarbonates
J . Org. Chem., Vol. 66, No. 20, 2001 6573
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 Bis(4-n itr op h en yl) Ca r bon a te (BNP C), Bis(4-n itr op h en yl)
Th ion oca r bon a te (BNP TOC), a n d Meth yl 4-Nitr op h en yl Th ion oca r bon a te (MNP TOC)a
kN (s-1 M-1
)
phenoxide
substituent
pKa
BNPC
BNPTOC
MNPTOC
4-OCH3
H
4-Cl
3-Cl
4-CN
10.3
9.9
9.4
9.0
7.8
7.1
5.3
144 ( 6
45 ( 3
2.6 ( 0.1
2.1 ( 0.1
1.35 ( 0.04
0.47 ( 0.02
1.4 ( 0.1
19 ( 1
4.7 ( 0.2
1.10 ( 0.05
0.35 ( 0.02
0.136 ( 0.007
0.28 ( 0.01
0.062 ( 0.004
2,6-F2
2,3,4,5,6-F5
1.9 ( 0.1
0.019 ( 0.001
0.043 ( 0.002
0.0015 ( 0.0001
0.00004 ( 0.00001
a
Both the pKa and kN values were determined in aqueous solution, at 25.0 °C, and ionic strength of 0.2 M (KCl).
pKa of 4-nitrophenol) in the Bro¨nsted-type plot of Figure
1.11 If this reaction were stepwise, with the formation of
an anionic tetrahedral intermediate (T-), the rate con-
stants for the decomposition of T- to reactants and
products would be equal at pKa ) 7.1.8,11 Therefore, for
nucleophiles (phenoxides) that are less basic than 4-ni-
trophenoxide, the breakdown of T- to products would be
rate determining, and for those nucleophiles that are
more basic than 4-nitrophenoxide, formation of T- would
be the rate-limiting step. Therefore, for a stepwise
mechanism a biphasic Bro¨nsted plot is expected, with
linear relationships over the rate-determining step re-
gions and a curvature (break) between that is centered
at the pKa of the leaving group.8 The lack of such
curvature at pKa ) 7.1 rules out the stepwise mechanism
for the phenolysis of BNPC.
A referee has pointed out that the phenolysis of BNPC
could be stepwise in view of the fact that in the Bro¨nsted
plot the pentafluorophenol point falls off the straight line
through the other points. Nevertheless, we think this
falloff is too small to qualify for a stepwise mechanism.
A similar slight falloff for pentafluorophenol has been
found in the concerted phenolysis of 4-nitrophenyl ac-
etate.2 However, the stepwise mechanism for the phe-
nolysis of BNPC cannot be rigorously excluded.
F igu r e 1. Bro¨nsted-type plots for the phenolyses of bis(4-
nitrophenyl) carbonate (BNPC), bis(4-nitrophenyl) thionocar-
bonate (BNPTOC), and methyl 4-nitrophenyl thionocarbonate
(MNPTOC) in aqueous solution, at 25.0 °C with ionic strength
of 0.2 M (KCl).
The Bro¨nsted-type plots for the phenolysis of the two
thionocarbonates, BNPTOC and MNPTOC, are biphasic
(see Figure 1), with slopes â1 ) 0.30 and 0.44, respec-
tively, at high pKa, and â2 ) 1.25 and 1.60, respectively,
at low pKa. Both plots have breaks at pKa ) 7.1 (pKa of
4-nitrophenol), which is consistent with the occurrence
of stepwise mechanisms (eq 4, X ) methyl or 4-nitrophen-
yl), as discussed above.
The value of k0 was much lower than the second term
(phenolysis) of eq 2, except in the slow reactions of
pentafluorophenol with the three substrates, where the
phenolysis term was also small. The kN values were
obtained as the slopes of linear plots of kobsd vs [ArO-].
These values were found to be independent of pH. The
kN values determined for the phenolyses of the three
substrates, together with the pKa values of the different
phenols employed, are shown in Table 4.
With the use of the kN and pKa values of Table 4
(measured under the same experimental conditions) the
three Bro¨nsted-type plots of Figure 1 were obtained.
The Bro¨nsted-type plot for the phenolysis of BNPC is
linear with slope â ) 0.66 (Figure 1), which is consistent
with a concerted mechanism (eq 3, NP ) 4-nitrophenyl)
with no intermediate. Similar â values have been found
in the concerted phenolyses of 2,4-dinitrophenyl and
2,4,6-trinitrophenyl O-ethyl thiolcarbonates (â ) 0.77 and
0.61, respectively),3a 4-nitrophenyl, 2,4-dinitrophenyl, and
2,4,6-trinitrophenyl methyl carbonates (â ) 0.67, 0.48,
and 0.52, respectively),3b aryl acetates (â ) 0.53-
0.66),1c,d,2 and acetic anhydride (â ) 0.58).1d
According to our results, the tetrahedral intermediate
1, although unstable, does exist. In contrast, the tetra-
hedral species 2 is either more unstable than 1 or
nonexistent. In the former case, the reaction chooses the
path of lower free energy, which is the concerted mech-
anism. If 2 does not exist, the concerted process is
enforced, i.e., this mechanism takes place because the
stepwise route is impossible.12
In accordance to our findings, the phenolysis of methyl
4-nitrophenyl carbonate is concerted,3b whereas the reac-
(11) Williams, A. Acc. Chem. Res. 1989, 22, 387-392.
(12) J encks, W. P. Chem. Soc. Rev. 1981, 10, 345-375. Williams,
A. Chem. Soc. Rev. 1994, 23, 93-100.
The best proof that the phenolysis of BNPC is con-
certed is the fact that there is no break at pKa ) 7.1 (the

6574 J . Org. Chem., Vol. 66, No. 20, 2001
Castro et al.
The concerted phenolysis of BNPC (this work) is much
faster than the concerted reactions of the same nucleo-
philes with methyl 4-nitrophenyl carbonate3b (e.g., un-
substituted phenoxide reacts 60 times faster with the
former substrate). This means that the change of NP to
Me in BNPC decreases the value of kN. This can be
ascribed to the stronger electron-withdrawing effect of
NPO compared to MeO, as discussed above.
The reactions of secondary alicyclic (SA) amines with
BNPTOC exhibit a nonlinear Bro¨nsted-type plot with
slopes â ) 0.1 and 0.5 at high and low pKa, respectively.
The slight Bro¨nsted curvature was attributed to a
concerted mechanism, although the stepwise process was
not rigorously ruled out.4 In light of our result that the
phenolysis of this substrate is stepwise, it seems more
likely that the above aminolysis is stepwise, for the
following reasons.
The reactions of SA amines with 4-nitrophenyl and 2,4-
dinitrophenyl acetates17,18 are stepwise (eq 5) whereas
the phenolyses of these compounds are concerted.1d,2
Other examples are the stepwise aminolyses (SA amines)
of 4-nitrophenyl O-ethyl thiolcarbonate (5)19 and phenyl
and 4-nitrophenyl chlorothionoformates (6),20 compared
to the concerted phenolyses of the same substrates.3a,21
All these examples indicate that the change of an amino
group, in a zwitterionic tetrahedral intermediate, to ArO
destabilizes the intermediate.
tion of the same nucleophiles with MNPTOC is stepwise
(this work). This means that the substitution of S- in
intermediate 1 (X ) Me) by O- destabilizes the interme-
diate in such a way as to change the mechanism.
The destabilization of a tetrahedral intermediate by
the substitution of S- by O- has precedents in the
aminolysis of compounds similar to the title substrates.
The reactions of secondary alicyclic amines with 2,4-
dinitrophenyl and 2,4,6-trinitrophenyl O-ethyl thiolcar-
bonates have been found to be concerted.13 In contrast,
the reactions of the same amines with 2,4-dinitrophenyl
and 2,4,6-trinitrophenyl O-ethyl dithiocarbonates are
governed by stepwise mechanisms.14 This means that the
zwitterionic tetrahedral intermediate 3 is less unstable
than 4; i.e., species 3 is destabilized by the change of S-
to O-.
This has been attributed to the greater ability of O-
in a tetrahedral intermediate to form a double bond and
expel a nucleofuge, when compared to S-, due to a
stronger π-bonding energy of the carbonyl group relative
to thiocarbonyl.15 In agreement with this, it has been
found that expulsions of PhS- and an amine from the
zwitterionic tetrahedral intermediate formed in the ami-
nolysis of phenyl thiolacetate (similar to 4, Me in place
of EtO) are much faster than those from the tetrahedral
species formed in the same aminolysis of phenyl dithio-
acetate (similar to 3).16
According to Figure 1, BNPTOC has a larger value of
kN than MNPTOC, irrespective of which step is rate
determining. The larger reactivity of BNPTOC than
MNPTOC toward phenoxides, when the k1 step of eq 4
is rate limiting, can be ascribed to the strong electron
withdrawal from intermediate 1 by NPO relative to the
electron-releasing effect of MeO. When the k2 step of eq
4 is rate determining, kN ) K1k2, where K1 is the
equilibrium constant for the first step. In this case, the
larger kN values for the reactions of BNPTOC than those
for MNPTOC can be attributed to larger values of both
K1 and k2 for the former reactions. The larger K1 value
can be explained by the same argument given above for
k1. The larger value of k2 for BNPTOC can be ascribed
to two factors: (i) The tetrahedral intermediate formed
in the BNPTOC reactions (1, X ) NP) possesses two NPO
leaving groups, which gives it a statistical advantage
(factor of 2) over the NPO leaving from the other
intermediate (1, X ) Me). (ii) The intermediate 1 (X )
NP) has its central carbon atom more positively charged
(due to its two NPO groups) than that of 1 (X ) Me),
thereby facilitating the formation of the CS double bond
and the expulsion of the leaving group from the former
intermediate.
For the pyridinolysis of MNPTOC, a linear Bro¨nsted-
type plot of slope 1.0 was found.7 This was explained by
a stepwise mechanism where the breakdown to products
of the zwitterionic tetrahedral intermediate (T() is the
rate-determining step.7 The lack of a Bro¨nsted break for
the pyridinolysis of this substrate (compared to the break
at pKa ) 7.1 for its phenolysis) can be explained by the
superior nucleofugality from T( of a pyridine relative to
an isobasic aryl oxide from intermediate 1 (X ) Me). This
is evident from the fact that the Bro¨nsted breaks found
in the pyridinolyses of methyl 2,4-nitrophenyl carbonate,22a
2,4-nitrophenyl acetate,22b methyl 2,4,6-trinitrophenyl
carbonate,23 and 2,4,6-trinitrophenyl acetate23 are at pKa
values that are much larger than those of the leaving
groups.
In the reactions of SA amines with MNPTOC, nonlin-
ear upward plots of kobsd vs [amine] were found, which
(17) (a) J encks, W. P.; Gilchrist, M. J . Am. Chem. Soc. 1968, 90,
2622-2637. (b) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc.
1974, 96, 7018-7031.
(18) Castro, E. A.; Ureta, C. J . Org. Chem. 1990, 55, 1676-1679.
(19) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1994,
59, 3572-3574.
(13) Castro, E. A.; Iba´n˜ez, F.; Salas, M.; Santos, J . G. J . Org. Chem.
1991, 56, 4819-4821. Castro, E. A.; Salas, M.; Santos, J . G. J . Org.
Chem. 1994, 59, 30-32.
(20) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1997,
62, 4395-4397.
(21) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1998,
63, 6820-6823.
(14) Castro, E. A.; Iba´n˜ez, F.; Salas, M.; Santos, J . G.; Sepulveda,
P. J . Org. Chem. 1993, 58, 459-463.
(22) (a) Castro, E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99, 7611-
7612. (b) Castro, E. A.; Freudenberg, M. J . Org. Chem. 1980, 45, 906-
910.
(15) Hill, S. V.; Thea, S.; Williams, A. J . Chem. Soc., Perkin Trans.
2 1983, 437-446.
(16) Castro, E. A.; Iba´n˜ez, F.; Santos, J . G.; Ureta, C. J . Chem. Soc.,
Perkin Trans. 2 1991, 1919-1924.
(23) Castro, E. A.; Iba´n˜ez, F.; Lagos, S.; Schick, M.; Santos, J . G. J .
Org. Chem. 1992, 57, 2691-2694.

Phenolysis of Carbonates and Thionocarbonates
J . Org. Chem., Vol. 66, No. 20, 2001 6575
Sch em e 1
[amine]. From these plots, the values of the microscopic
rate constants for the formation of T( (k1) were obtained
by fitting.5
The k1 values for the aminolysis (SA amines) of
MNPTOC are greater than those obtained in this work
for the reactions of the same compound with isobasic
phenoxides, when the first step of eq 4 is rate determin-
ing. Amines are also more reactive than isobasic phe-
noxides toward thiocarbonyl compounds when the break-
down of the tetrahedral intermediate to products is the
rate-limiting step. Examples are the greater reactivities
of pyridine (pKa ) 5.37)4,7 than pentafluorophenoxide (pKa
) 5.3, this work) toward MNPTOC and BNPTOC, which
amount to 10-fold7 and 35-fold,4 respectively. The higher
reactivity of carbonyl derivatives toward amines com-
pared to isobasic aryloxides has been documented
previously.17a,24
were interpreted by the formation of two tetrahedral
intermediates of which one was zwitterionic (T(, Scheme
1) and another was anionic (T-).5 Base catalysis by the
amine for the conversion of T( to T- was claimed to be
responsible for the upward curved plots of kobsd vs
Ack n ow led gm en t. We thank FONDECYT of Chile
(Projects 1960460 and 1990561) for financial assistance
to this work.
(24) Bond, P. M.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2 1976,
679-682.
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