Kinetics and mechanism of the aminolysis of 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates in aqueous ethanol

Article abstract of DOI:10.1021/jo034385q

The reactions of anilines with 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates (MPDNPC and ClPDNPC, respectively) and the latter substrate with secondary alicyclic (SA) amines are subjected to a kinetic study in 44 wt % ethanol-water solution, at 25.0 °C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of 2,4-dinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (k_obsd) are found. Plots of k_obsd vs [amine] are linear and pH-independent, with slope k_N. The Bronsted-type plots (log k_N vs pK_a of aminium ions) are linear, with slopes β = 0.68 and 0.66 for the reactions of anilines with MPDNPC and ClPDNPC, respectively, and β = 0.44 for the reactions of SA amines with ClPDNPC. The magnitude of the slope for the latter reaction indicates that its mechanism is concerted. The slope values for the reactions of anilines are in the borderline between stepwise and concerted mechanisms. The sensitivity of logk_N to the basicity of the nonleaving group (β_nlg) is ca. -0.7 for the reactions of anilines, in agreement with that found for the SA reactions (β_nlg ca. -0.6). These results suggest that the reactions of anilines are concerted, although it is also possible that both mechanisms (stepwise and concerted) operate simultaneously. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions can be drawn: (i) ClPDNPC is more reactive than MPDNPC toward the two amine series. (ii) The change of water to aqueous ethanol destabilizes a zwitterionic tetrahedral intermediate. (iii) The change of the nonleaving group from MeO to 4-methylphenoxy or 4-chlorophenoxy also destabilizes this intermediate.

Full text of DOI:10.1021/jo034385q

Kin etics a n d Mech a n ism of th e Am in olysis of 4-Meth ylp h en yl a n d
4-Ch lor op h en yl 2,4-Din itr op h en yl Ca r bon a tes in Aqu eou s Eth a n ol
Enrique A. Castro,* Paola Campodonico, Alejandra Toro, and J ose´ G. Santos*
Facultad de Quı´mica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
ecastro@puc.cl
Received March 25, 2003
The reactions of anilines with 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates
(MPDNPC and ClPDNPC, respectively) and the latter substrate with secondary alicyclic (SA) amines
are subjected to a kinetic study in 44 wt % ethanol-water solution, at 25.0 °C, and an ionic strength
of 0.2 M (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release
of 2,4-dinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (kobsd) are
found. Plots of kobsd vs [amine] are linear and pH-independent, with slope kN. The Brønsted-type
plots (log kN vs pKa of aminium ions) are linear, with slopes â ) 0.68 and 0.66 for the reactions of
anilines with MPDNPC and ClPDNPC, respectively, and â ) 0.44 for the reactions of SA amines
with ClPDNPC. The magnitude of the slope for the latter reaction indicates that its mechanism is
concerted. The slope values for the reactions of anilines are in the borderline between stepwise
and concerted mechanisms. The sensitivity of logkN to the basicity of the nonleaving group (ânlg) is
ca. -0.7 for the reactions of anilines, in agreement with that found for the SA reactions (ânlg ca.
-0.6). These results suggest that the reactions of anilines are concerted, although it is also possible
that both mechanisms (stepwise and concerted) operate simultaneously. By comparison of the
reactions under investigation between them and with similar aminolyses, the following conclusions
can be drawn: (i) ClPDNPC is more reactive than MPDNPC toward the two amine series. (ii) The
change of water to aqueous ethanol destabilizes a zwitterionic tetrahedral intermediate. (iii) The
change of the nonleaving group from MeO to 4-methylphenoxy or 4-chlorophenoxy also destabilizes
this intermediate.
In tr od u ction
ous ethanol,4a the reactions of the latter amines with
phenyl 2,4-dinitrophenyl carbonate (PDNPC) in water,4b
and the reactions of quinuclidines with MPNPC and
4-chlorophenyl 4-nitrophenyl carbonate (ClPNPC), and
the latter compound with SA amines in aqueous ethanol.4c
The quinuclidinolysis of PDNPC in water was claimed
to be stepwise, through the formation of a zwitterionic
tetrahedral intermediate (T(), on the basis of the slightly
curved Brønsted-type plot obtained.3 This plot was
explained by a change in the rate-determining step, from
breakdown of T( to its formation, with the increasing
basicity of the amine.3 Nevertheless, this mechanism has
been questioned in the light of the linear Brønsted-type
plot of slope (â) 0.4 obtained for the aminolysis (SA) of
PDNPC in water, which indicates a concerted process.4b
The reactions of SA amines with MPNPC in aqueous
ethanol exhibit a biphasic Brønsted-type plot with slopes
â1 ) 0.2 (high pKa region) and â2 ) 0.9 (low pKa region),
which was attributed to a stepwise mechanism through
a T( intermediate and a change in the rate-limiting step
with amine basicity.4a On the other hand, the reactions
of the latter amines with MPDNPC in aqueous ethanol
were claimed to be concerted (single step without T() on
the basis of the slightly curved Brønsted plot found.4a
The quinuclidinolysis of MPNPC and ClPNPC exhibit
linear Brønsted plots of slopes (â) ca. 0.9, consistent with
stepwise mechanisms with rate-determining breakdown
Although there have been many reports on the kinetics
and mechanisms of the aminolyses (several types of
amines) of alkyl aryl carbonates and their mechanisms
have been clarified,1,2 there have been only a few inves-
tigations concerning the kinetics of the aminolyses of
diaryl carbonates.3,4 The latter studies involve the reac-
tions of several nitro- and dinitrophenyl phenyl carbon-
ates with quinuclidines in water,3 the reactions of
4-methylphenyl 4-nitrophenyl and 4-methylphenyl 2,4-
dinitrophenyl carbonates (MPNPC and MPDNPC, re-
spectively) with secondary alicyclic (SA) amines in aque-
* To whom correspondence should be addressed. Fax: (56 2)
6864744.
(1) (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.; Freudenberg, M. J . Org. Chem. 1980, 45, 906.
(d) Castro, E. A.; Iban˜ez, F.; Lagos, S.; Schick, M.; Santos, J . G. J .
Org. Chem. 1992, 57, 2691. (e) Castro, E. A.; Iban˜ez, F.; Saitu´a, A. M.;
Santos, J . G. J . Chem. Res. (S) 1993, 56. (f) Castro, E. A.; Cubillos,
M.; Santos, J . G. J . Org. Chem. 2001, 66, 6000.
(2) Koh, H. J .; Lee, J . W.; Lee, H. W.; Lee, I. Can. J . Chem. 1998,
76, 710.
(3) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
6970.
(4) (a) Castro, E. A.; Andu´jar, M.; Campodo´nico, P.; Santos, J . G.
Int. J . Chem. Kinet. 2002, 34, 309. (b) Castro, E. A.; Aliaga, M.;
Campodo´nico, P.; Santos, J . G. J . Org. Chem. 2002, 67, 8911. (c) Castro,
E. A.; Andujar, M.; Toro, A.; Santos, J . G. J . Org. Chem. 2003, 68,
3608.
10.1021/jo034385q CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/24/2003
5930
J . Org. Chem. 2003, 68, 5930-5935

Aminolysis of Diaryl Carbonates
of T( to products.4c The reaction of the latter substrate
with SA amines shows a biphasic Brønsted plot with
slopes â1 ) 0.2 (high pKa region) and â2 ) 0.9 (low pKa
region), consistent with a stepwise mechanism through
an intermediate T(.4c
To extend our kinetic studies on the aminolysis of
diaryl carbonates and with the aim to shed some light
on these mechanisms, in the present work we investigate
the reactions of a series of anilines with MPDNPC and
ClPDNPC and that of the latter substrate with a series
of SA amines in aqueous ethanol. A specific objective is
to assess the influence of the nonleaving group, the amine
nature, and the solvent on the kinetics and mechanisms
of these reactions. This will be achieved by a kinetic
comparison of the title reactions between them and with
similar aminolyses in aqueous ethanol and in water.
TABLE 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
Rea ction s of Secon d a r y Alicyclic Am in es w ith
4-Ch lor op h en yl 2,4-Din itr op h en yl Ca r bon a te (ClP DNP C)a
b
103 [N]tot
(M)
103 kobsd
(s-1
no. of
runs
amine
piperidine
pH
)
6.7c 0.427-3.00 0.265-0.375
7
6
6
6
6
7
7
7.0c 0.853-3.00 0.315-0.466
7.3c 1.28-3.41
0.398-0.781
piperazine
9.41 0.824-2.40 53.2-219
9.71 0.843-2.40 83.0-324
10.01 0.411-2.88 49.0-449
8.79 0.416-2.91 4.59-35.1
1-(2-hydroxyethyl)-
piperazine
9.09 0.416-2.91 9.69-54.7
9.39 0.416-2.50 13.0-59.8
7
6
7
7
6
7
6
6
7
6
7
6
morpholine
8.18 0.20-2.40
8.48 0.20-2.40
8.78 0.20-2.00
7.33 0.20-2.40
7.63 0.20-2.40
7.93 0.20-2.40
5.07 0.42-3.00
5.37 0.40-2.40
5.67 0.45-3.16
4.30 8.76-30.7
8.50-76.0
7.10-99.6
14.0-105
3.00-20.0
7.00-27.0
8.00-36.0
0.686-3.86
0.944-4.69
0.965-6.12
0.896-2.72
1-formylpiperazine
piperazinium ion
1-(2-hydroxyethyl)-
piperazinium iond
4.60 8.92-31.2
4.90 9.11-31.5
1.09-3.13
1.36-3.76
6
6
a
In 44 wt % ethanol-water, at 25.0 °C, ionic strength 0.2 (KCl).
Exp er im en ta l Section
b
Concentration of total amine (free base plus protonated forms).
c In the presence of phosphate buffer 0.01 M. The pKa of the
conjugate acid of this reactive species is 5.6; buffer is due to partial
ionization of the dication.
d
Ma ter ia ls. The anilines5 and SA amines6 were purified
either by distillation or recrystallization. MPDNPC was pre-
pared as described.4a ClPDNPC was obtained by a modification
of a standard procedure,3 as previously reported.7 4-Meth-
ylphenyl and 4-chlorophenyl N-phenyl carbamates (ArO-CO-
NH-Ph) and 1-(4-chlorophenoxycarbonyl)morpholine (4-ClC6-
H4O-CO-NC4H8O), which are the products of the reactions of
the two substrates with aniline and ClPDNPC with morpho-
line, were prepared by a variation of a standard synthesis,8
as published for ethyl phenyl carbamate,9 from the corre-
sponding chloroformates and aniline in acetonitrile. The
crystallized (ethanol) 4-methylphenyl and 4-chlorophenyl N-
phenyl carbamates melted at 111-112 °C (lit.10 mp 108-110
°C) and 146-147 °C (lit.10 mp 148-150 °C), respectively. 1-(4-
Chlorophenoxycarbonyl)morpholine melted at 76.8-77.3 °C
(lit.11 mp 80.0 °C). These products were identified by 1H NMR
(200 MHz, CDCl3) and IR analyses.
Deter m in a tion of p Ka . The pKa values of the conjugate
acids of the anilines were determined spectrophotometrically
in 44 wt % ethanol-water solution, 25.0 °C, ionic strength 0.2
(KCl), as reported.12
Kin etic Mea su r em en ts. These were carried out by means
of a diode array spectrophotometer in 44 wt % ethanol-water
solution, at 25.0 ( 0.1 °C, ionic strength 0.2 M (KCl). The
reactions were followed at 360 nm (formation of 2,4-dinitro-
phenoxide anion).
M. In the reactions of both substrates with anilines and in
the reaction of ClPDNPC with piperidine, phosphate buffer
0.01M was used. In the other reactions the pH was maintained
by partial protonation of the SA amines.
Pseudo-first-order rate coefficients (kobsd) were found for all
reactions; these were determined by means of the spectropho-
tometer kinetic software for first-order reactions. The experi-
mental conditions of the reactions and the kobsd values are
shown in Tables 1-3.
P r od u ct Stu d ies. For the present reactions one of the
products was identified as 2,4-dinitrophenoxide anion; this was
achieved by comparison of the UV-vis spectra after completion
of the reactions with an authentic sample of 2,4-dinitrophenol
5 × 10-5 M, under the same conditions. For the reactions of
MPDNPC and ClPDNPC with aniline and the latter substrate
with morpholine, the other product was identified as 4-meth-
ylphenyl and 4-chlorophenyl N-phenyl carbamates and 1-(4-
chlorophenoxycarbonyl)morpholine, respectively. This was
concluded by comparison of a sample at the end of the reactions
with an authentic sample of the corresponding product, by
HPLC with column Eurospher C-18 (10 cm, 7µm), eluant
acetonitrile/water ) 70/30, isocratic mode 0.5 mL/min.
All reactions were studied under excess of the amine over
the substrate. The initial substrate concentration was 5 × 10-5
Resu lts a n d Discu ssion
(5) Castro, E. A.; Iban˜ez, F.; Saitu´a, A. M.; Santos, J . G. J . Chem.
Res. (M) 1993, 317.
(6) Castro, E. A.; Ureta, C. J . Org. Chem. 1989, 54, 2153.
(7) (a) Castro, E. A.; Angel, M.; Pavez, P.; Santos, J . G. J . Chem.
Soc., Perkin Trans. 2 2001, 2351. (b) Castro, E. A.; Pavez, P.; Santos,
J . G. J . Org. Chem. 2002, 67, 4494.
(8) Dannley, R. L.; Lukin, M.; Shapiro, J . J . Org. Chem. 1955, 20,
92. Broxton, T. J .; Deady, L. W.; Lim, R. H. K. Aust. J . Chem. 1981,
34, 1993.
The kinetic law obtained under the reaction conditions
is that described by eq 1, where P is 2,4-dinitrophenoxide
anion, S is the substrate, and kobsd is the pseudo-first-
order rate coefficient (excess of amine was used through-
out).
(9) Castro, E. A.; Leandro, L.; Millan, P.; Santos, J . G. J . Org. Chem.
1999, 64, 1953.
(10) Williams, A. J . Chem. Soc., Perkin Trans. 2 1972, 808.
(11) Union Chim. Belge, Belg. P. 532057, 1954; Beilstein 27/1, p 292,
1983.
(12) Albert, A.; Serjeant, E. P. The Determination of Ionization
Constants; Chapman and Hall: London, 1971; p 44.
d[P]
dt
) kobsd[S]
(1)
Plots of kobsd against [amine] at constant pH were linear
in accordance with eq 2, where k0 and kN are the rate
J . Org. Chem, Vol. 68, No. 15, 2003 5931

Castro et al.
TABLE 4. Va lu es of p Ka for Con ju ga te Acid s of
Secon d a r y Alicyclic Am in es a n d kN Va lu es for Rea ction s
of Th ese Am in es w ith 4-Ch lor op h en yl 2,4-Din itr op h en yl
Ca r bon a te (ClP DNP C)a
TABLE 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
Rea ction s of An ilin es w ith 4-Ch lor op h en yl
2,4-Din itr op h en yl Ca r bon a te (ClP DNP C)a
c
103 [N]tot
(M)
103 kobsd
(s-1
no. of
runs
amine
pHb
)
amine
pKa
kN (s-1 M-1
)
4-methoxyaniline
7.0 1.25-15.0
2.63-26.8
7
6
6
7
7
7
7
7
6
7
6
6
6
7
6
8
7
6
9
7
7
piperidine
piperazine
1-(2-hydroxyethyl)piperazine
morpholine
1-formylpiperazine
1-(2-hydroxyethyl)piperazinium ion
piperazinium ion
10.82
9.71
9.09
8.48
7.63
5.60
5.37
541 ( 13
259 ( 8
81 ( 3
7.2 0.625-12.5 1.44-22.1
7.5 0.625-12.5 1.46-23.1
4-methylaniline
aniline
7.0 0.65-12.5
7.2 0.65-12.5
7.5 0.65-12.5
7.0 5.0-60.0
7.2 5.0-60.0
7.5 5.0-60.0
7.0 0.50-60.0
7.2 0.50-16.0
7.5 1.00-12.0
0.633-7.42
0.752-8.42
0.724-8.25
2.23-22.9
2.16-20.9
2.24-22.7
0.389-1.48
0.370-3.69
0.451-2.67
0.439-2.17
0.507-2.57
0.438-1.76
0.324-1.18
0.342-1.05
0.355-0.962
0.264-0.371
0.254-0.567
0.296-0.741
77 ( 3
21.6 ( 0.8
3.2 ( 0.1
2.7 ( 0.1
a
Both the pKa and kN values were determined in 44 wt %
aqueous ethanol, at 25.0 °C, ionic strength 0.2 (KCl).
3-methoxyaniline
3-aminoacetophenone 7.0 2.50-30.0
7.2 2.50-30.0
7.5 2.50-25.0
3-aminobenzonitrile
7.0 6.50-100
7.2 6.50-78.0
7.5 6.50-78.0
4-aminoacetophenone 7.0 1.10-17.6
7.2 2.20-52.0
7.5 6.50-78.0
a
In 44 wt % ethanol-water, at 25.0 °C and an ionic strength
b
of 0.2 (KCl). pH maintained with phosphate buffer 0.01 M.
c Concentration of total amine (free base plus protonated forms).
TABLE 3. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
Rea ction s of An ilin es w ith 4-Meth ylp h en yl
2,4-Din itr op h en yl Ca r bon a te (MP DNP C)a
c
103 [N]tot
(M)
103 kobsd
(s-1
no. of
runs
F IGURE 1. Brønsted-type plot (statistically corrected) ob-
tained in the reactions of SA amines with 4-chlorophenyl 2,4-
dinitrophenyl carbonate (ClPDNPC) in 44 wt % ethanol-
water, at 25.0 °C and an ionic strength of 0.2 M. The slope (â)
is 0.44.
amine
pHb
)
4-methoxyaniline
7.0 1.25-15.0 0.820-8.92
7.2 1.25-15.0 0.774-8.08
7.5 1.25-15.0 0.831-8.33
7.0 0.65-12.5 0.288-3.00
7.2 0.65-12.5 0.248-3.06
7.5 0.65-12.5 0.244-3.35
7.0 5.00-60.0 0.675-6.57
7.2 5.00-60.0 0.681-6.64
7.5 5.00-60.0 0.780-7.13
7.0 1.00-16.0 0.148-1.23
7.2 1.00-16.0 0.149-1.23
7.5 1.00-16.0 0.150-1.21
6
7
7
7
7
7
7
7
7
7
7
7
6
6
7
7
7
7
6
7
7
4-methylaniline
aniline
5.7), and 3 × 10-5 (pH 4.3-4.9).
kobsd ) k0 + kN[amine]
(2)
3-methoxyaniline
For the reactions of SA amines with ClPDNPC the
values of kN (obtained as the slopes of plots of eq 2) and
pKa are summarized in Table 4. These values, as well as
those of the pKa of the conjugate acids of the SA amines,
were statistically corrected with q ) 2 for piperazine and
p ) 2 for the conjugate acids of the SA amines, except
that for piperazinium ion with p ) 4.1f,4-6,13 With these
corrected values the Brønsted-type plot (shown in Figure
1) was obtained. The slope of this linear plot is â ) 0.44.
The values of kN (obtained as the slopes of plots of eq
2) for the reactions of anilines with MPDNPC and
ClPDNPC are shown in Table 5, together with the
experimentally determined pKa values of the anilinium
ions. With these kN values and those of the statistically
corrected pKa of the conjugate acids of the anilines (with
q ) 1 and p ) 3),5,9,13 the Brønsted-type plots (shown in
Figure 2) were obtained. These linear plots show slopes
â ) 0.68 and 0.66, respectively.
3-aminoacetophenone 7.0 2.50-30.0 0.184-0.787
7.2 1.25-25.0 0.104-0.587
7.5 2.50-30.0 0.175-0.838
3-aminobenzonitrile
7.0 6.50-100 0.106-0.424
7.2 6.50-100 0.109-0.431
7.5 6.50-100 0.0987-0.351
4-aminoacetophenone 7.0 13.0-78.0 0.0865-0.199
7.2 6.50-78.0 0.0846-0.190
7.5 6.50-78.0 0.0867-0.198
a
In 44 wt % ethanol-water, at 25.0 °C and an ionic strength
b
of 0.2 (KCl). pH maintained with phosphate buffer 0.01 M.
c Concentration of total amine (free base plus protonated forms).
coefficients for solvolysis and aminolysis of the sub-
strates, respectively. The values of k0 and kN were
obtained as the intercept and slope, respectively, of plots
of kobsd against [amine]. These values were pH indepen-
dent, within the pH range employed for the reactions of
each amine. For the solvolysis of MPDNPC and ClPD-
NPC in the 7.0-7.5 pH range, the values of k0 are 8 ×
10-5 and 2.8 × 10-4 s-1, respectively. For the solvolysis
of ClPDNPC, the k0 (s-1) values at other pH are 1 × 10-3
(pH 9-10), 6 × 10-4 (pH 8.2-8.8), 6 × 10-5 (pH 5.1-
The value of â ) 0.44 found for the reactions of SA
amines with ClPDNPC (Figure 1) is in accord with a
concerted process where the structure of the transition
state remains constant with the variation of the nucleo-
(13) Bell, R. P. The Proton in Chemistry; Methuen: London, 1959;
p 159.
5932 J . Org. Chem., Vol. 68, No. 15, 2003

Aminolysis of Diaryl Carbonates
TABLE 5. Va lu es of p Ka for Con ju ga te Acid s of An ilin es
a n d kN Va lu es for Rea ction s of Th ese Am in es w ith
4-Meth ylp h en yl 2,4-Din itr op h en yl Ca r bon a te (MP DNP C)
a n d 4-Ch lor op h en yl 2,4-Din itr op h en yl Ca r bon a te
(ClP DNP C)a
102 kN (s-1 M-1
)
amine
pKa
MPDNPC
ClPDNPC
4-methoxyaniline
4-methylaniline
aniline
3-methoxyaniline
3-aminoacetophenone
3-aminobenzonitrile
4-aminoacetophenone
5.29
4.90
4.46
4.26
3.10
2.12
1.73
55 ( 1
175 ( 2
24.4 ( 0.6
11.1 ( 0.2
7.2 ( 0.03
2.3 ( 0.1
61 ( 1
36 ( 1
20.9 ( 0.2
6.7 ( 0.3
0.93 ( 0.002
0.62 ( 0.01
0.35 ( 0.01
0.149 ( 0.006
a
Both the pKa and kN values were determined in 44 wt %
aqueous ethanol, at 25.0 °C and an ionic strength of 0.2 (KCl)
F IGURE 3. Logarithmic plot of experimental kN/q vs calcu-
lated kN/q (through eq 3) for the reactions of SA amines with
ClPDNPC (b, this work) and MPDNPC (O, ref 4a) in 44 wt %
ethanol-water, at 25.0 °C and an ionic strength of 0.2 (KCl).
larger than those obtained in the same aminolysis of
MPDNPC in the same solvent.4a This can be explained
by the stronger electron withdrawal from 4-Cl in ClPD-
NPC than 4-Me in MPDNPC, which leaves the former
carbonate more prone to nucleophilic attack by the
amine.16
With the kN values (statistically corrected) found in the
reactions of SA amines with ClPDNPC, together with
those for the same aminolysis of MPDNPC,4a and the pKa
values of both the nucleophiles (statistically corrected)
and the nonleaving groups (10.1 and 9.4 for 4-methylphen-
ol and 4-chlorophenol in water, respectively), eq 3 can
be deduced by regression analysis (n ) 14, R2 ) 0.976).
In this expression N, and nlg refer to the nucleophile and
the nonleaving group, respectively; the pKa coefficients
(â) are subjected to an error of ( 0.1.
F IGURE 2. Brønsted-type plots (statistically corrected) ob-
tained in the reactions of anilines with ClPDNPC (b) and
MPDNPC (O) in 44 wt % ethanol-water, at 25.0 °C and an
ionic strength of 0.2 M. The slopes (â) are 0.68 and 0.66,
respectively.
phile basicity. This â value is in agreement with those
found in the following concerted reactions in water: SA
amines with S-(2,4-dinitrophenyl) and S-(2,4,6-trinitro-
phenyl) O-ethyl thiocarbonates (â ) 0.56 and 0.48,
respectively),14 quinuclidines with these two substrates
(â ) 0.54 and 0.47, respectively),15 anilines with the latter
compound (â ) 0.54),9 and SA amines with 2,4,6-
trinitrophenyl acetate and 2,4,6-trinitrophenyl methyl
carbonate (â ) 0.41 and 0.36, respectively).1f
For the reactions of SA amines with MPDNPC in 44
wt % ethanol-water a slightly curved Brønsted-type plot
was found, which was explained by a concerted mechan-
ism.4a If 4-Me as substituent in the nonleaving group of
this carbonate yields a concerted process, it is reasonable
that the substitution of this group by 4-Cl should also
lead to a concerted mechanism. This is because the latter
substituent, being more electron-withdrawing than Me,
should destabilize even more the hypothetical tetrahedral
intermediate (in a hypothetical stepwise process) com-
pared to that with 4-Me.
kN
log
) 3.81 + 0.41pK (N)
- 0.6pKa(nlg) (3)
corr
(
)
a
q
Figure 3 shows a plot of experimental log(kN/q) against
calculated log(kN/q) (through eq 3). The slope is unity,
and the intercept is zero.
The greater reactivity of ClPDNPC compared with that
of MPDNPC is a result of the negative value of ânlg (-0.6)
and the larger value of the pKa of 4-methylphenol
compared with that of 4-chlorophenol (10.1 and 9.4 in
water, respectively).
For the reactions of anilines with MPDNPC and
ClPDNPC the magnitude of the Brønsted slopes, â ) 0.68
and 0.66, respectively (Figure 2), are near the lower limit
of the Brønsted slopes found in stepwise aminolyses of
similar substrates when the breakdown to products of
the zwitterionic tetrahedral intermediate (T() is the rate-
determining step.17,18 For instance, Brønsted slopes of 0.7
have been found in the stepwise reactions of SA amines
with phenyl and 4-chlorophenyl ethyl thiolcarbonates in
water.17 Similarly, Brønsted slope values of 0.8 or lower
The kN values found for the reactions of SA amines
with ClPDNPC in 44 wt % ethanol-water (Table 4) are
(14) (a) Castro, E. A.; Iban˜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.
(15) Castro, E. A.; Mun˜oz, P.; Santos, J . G. J . Org. Chem. 1999, 64,
8298.
(16) Neuvonen, H.; Neuvonen, K.; Koch, A.; Kleinpeter, E.; Pasanen,
P. J . Org. Chem. 2002, 67, 6995.
(17) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1999,
64, 6342.
(18) J encks, W. P.; Gilchrist, M. J . Am. Chem. Soc. 1968, 90, 2622.
J . Org. Chem, Vol. 68, No. 15, 2003 5933

Castro et al.
SCHEME 1
SCHEME 2
F IGURE 4. Logarithmic plot of experimental kN vs calculated
kN (through eq 4) for the reactions of a series of anilines with
ClPDNPC (b) and MPDNPC (O), in 44 wt % ethanol-water
solution, 25.0 °C, ionic strength 0.2 (KCl).
have been reported for the stepwise aminolysis of 2,4-
dinitrophenyl acetate and 1-acetoxy-4-methoxypyridini-
um ion.18
On the other hand, linear Brønsted plots with slope
values of ca. 0.6 have been found for the concerted
reactions of pyridines with N-methoxycarbonylisoquino-
linium ion19 and SA amines with ethyl 2,4-dinitrophenyl
thiolcarbonate.14a Furthermore, linear Brønsted plots
with slopes greater than 0.6 have been obtained for the
concerted phenolyses of 4-chloro-2-nitrophenyl acetate (â
) 0.64),20 3-nitrophenyl formate (â ) 0.64),21 3-nitrophen-
yl acetate (â ) 0.66),21 4-nitrophenyl and 4-formylphenyl
acetates (â ) 0.75 and 0.79, respectively),20 and ClPNPC
(â ) 0.67).7b
Therefore, judging only by the magnitudes of the
Brønsted slopes (â ) 0.66-0.68) found for the reactions
of anilines with MPDNPC and ClPDNPC it is very
difficult to decide which is the mechanism (stepwise or
concerted) of these reactions. Nevertheless, the following
discussion allows us to be more inclined to the concerted
process.
With the kN values found in the present reactions of
anilines, together with the pKa values of the conjugate
acids of the nucleophiles (statistically corrected) and
nonleaving groups (the latter pKa are 10.1 and 9.4 for
4-methylphenol and 4-chlorophenol in water, respec-
tively), eq 4 can be deduced by regression analysis (n )
14, R2 ) 0.986). In this expression N and nlg refer to the
nucleophile and the nonleaving group, respectively; the
pKa coefficients (â) are subjected to an error of ( 0.1.
the same substrates (ânlg ) -0.6 in eq 3), which suggests
that both reaction series are ruled by the same (con-
certed) mechanism. This contrasts with the ânlg values
exhibited by the SA aminolysis and quinuclidinolysis of
MPNPC and ClPNPC (ânlg ) -0.3 and -0.2, respectively),
which were shown to be stepwise reactions where the
breakdown of the zwitterionic tetrahedral intermediate
(T() to products is the rate-determining step.4c
Therefore, the mechanism that we propose for the
reactions of anilines with MPDNPC and ClPDNPC is
shown in Scheme 1, where Ar′ is a substituted phenyl
group (4-chlorophenyl or 4-methylphenyl) and Ar-NH2 is
a substituted aniline. Nevertheless, in view of the
borderline âN values shown by these reactions it is also
possible that the mechanism is stepwise, through a T(
intermediate, with rate-limiting decomposition of T( to
products (Scheme 2). It is also possible that both mech-
anisms occur simultaneously if the transition states
involved are energetically similar.
As seen in Figure 2, ClPDNPC is more reactive (larger
kN values) than the methyl derivative toward anilines.
This is reasonable taking into account that 4-Cl is more
electron withdrawing than 4-Me.
A plot (not shown) of logkN for the anilinolysis of
ClPDNPC against logkN for the same aminolysis of
MPDNPC is linear, according to eq 5 (n ) 7; R2 ) 0.995).
This equation shows that the reactivity of ClPDNPC is
ca. 2.8-fold greater than that of MPDNPC toward a given
amine.
log kN ) 2.82 + 0.68pKa(N)corr - 0.69pKa(nlg) (4)
log kN(ClPDNPC) ) 0.98 log kN(MPDNPC) + 0.45
A logarithmic plot of the experimental kN vs the
calculated one through eq 4 is shown in Figure 4; the
slope is unity and the intercept is zero.
(5)
It has been found that the reactions of anilines with
ethyl S-(2,4-dinitrophenyl) thiolcarbonates in water are
stepwise9 whereas those of SA amines with the same
substrate in the same solvent are concerted.14a This
means that the zwitterionic tetrahedral intermediate
formed with a given aniline is greatly destabilized by
substitution of this amine by an isobasic SA amine. This
has been attributed to the superior leaving ability of SA
For the reactions of anilines the sensitivity of log kN
to the nonleaving group basicity (ânlg ca. -0.7 in eq 4) is
similar to that shown by the reactions of SA amines with
(19) Chrystiuk, E.; Williams, A. J . Am. Chem. Soc. 1987, 109, 3040.
(20) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1989,
111, 2647.
(21) Stefanidis, D.; Cho, S.; Dhe-Paganon, S.; J encks, W. P. J . Am.
Chem. Soc. 1993, 115, 1650.
5934 J . Org. Chem., Vol. 68, No. 15, 2003

Aminolysis of Diaryl Carbonates
amines from a (hypothetical) intermediate compared with
isobasic anilines.9
indicate that the latter solvent destabilizes the tetrahe-
dral intermediate that is formed in the reactions in water.
On the other hand, the aminolysis (SA) of ethyl 4-nitro-
phenyl thionocarbonate in water is stepwise,24a whereas
the same aminolysis of bis(4-nitrophenyl) thionocarbon-
ate in water is concerted.24b This indicates that the
change of EtO to 4-nitrophenoxy destabilizes the tetra-
hedral intermediate to the point of a mechanistic change.
Therefore, according to the above results, the change
in mechanism from stepwise for the reactions of anilines
with methyl 2,4-dinitrophenyl carbonate in water1e to
concerted for those of the same amines with MPDNPC
and ClPDNPC in aqueous ethanol (this work) should be
due to the destabilization caused to the tetrahedral
intermediate by the change of both the solvent (from
water to aqueous ethanol) and the nonleaving group
(from MeO to ArO).
If the reactions of MPDNPC and ClPDNPC with
anilines are concerted, as are those of these carbonates
with SA amines,4a this means that the substitution of a
SA amino group in a hypothetical tetrahedral intermedi-
ate by an isobasic anilino group does not bring enough
stabilization to the hypothetical intermediate as to
change the mechanism from concerted to stepwise. It is
possible that for the reactions of SA amines the mecha-
nism is enforced concerted (the “intermediate” is too
unstable to exist) and for the reactions of anilines the
intermediate does exist but is so unstable that the
concerted path is favored as the result of a smaller free
energy of activation compared to the stepwise process.22
The reactions of anilines with methyl 2,4-dinitrophenyl
carbonate in water are stepwise.1e If the reactions of the
same amines with MPDNPC and ClPDNPC in aqueous
ethanol are concerted (this work), it means that the
change of both the solvent (from water to aqueous
ethanol) and the nonleaving group (from MeO to ArO)
destabilize the zwitterionic tetrahedral intermediate. The
following precedents support this conclusion.
Ack n ow led gm en t. We thank A. Hormazabal for the
synthesis of the substrates. The financial assistance
provided by FONDECYT (Chile), projects 2010081 and
1020538, is gratefully acknowledged.
The reactions of SA amines with O-ethyl S-(2,4,6-
trinitrophenyl) dithiocarbonate in water are driven by a
stepwise mechanism,23a in contrast to the same reactions
in aqueous ethanol, which are concerted.23b These results
J O034385Q
(23) (a) Castro, E. A.; Iban˜ez, F.; Salas, M.; Santos, J . G.; Sepulveda,
P. J . Org. Chem. 1993, 58, 459. (b) Castro, E. A.; Cubillos, M.; Mun˜oz,
G.; Santos, J . G. Int. J . Chem. Kinet. 1994, 26, 571.
(22) J encks, W. P. Acc. Chem. Res. 1980, 13, 161. J encks, W. P.
Chem. Soc. Rev. 1981, 10, 345. Williams, A. Chem. Soc. Rev. 1994, 23,
93.
(24) (a) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1996,
61, 3501. (b) Castro, E. A.; Santos, J . G.; Tellez, J .; Uman˜a, M. I. J .
Org. Chem. 1997, 62, 6568.
J . Org. Chem, Vol. 68, No. 15, 2003 5935