Kinetics and mechanism of the phenolysis of asymmetric diaryl carbonates

Article abstract of DOI:10.1021/jo0255532

The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in aqueous solution (25.0°C, ionic strength 0.2 M (KCI)). Under an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order kinetics and are first order in phenoxide. The Broensted-type plots for the nucleophilic rate constants (kN) are linear, with slopes β = 0.48 (MPNPC), 0.67 (C1PNPC), 0.41 (MPDNPC), and 0.32 (C1PDNPC). The magnitude of these slopes and the absence of a curvature in the Broensted plot at pK_a = 7.1 for the C1PNPC reactions are consistent with concerted mechanisms (one step). The carbonates MPDNPC and C1PDNPC are more reactive than MPNPC and C1PNPC, respectively, toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding 4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than 4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is deduced for log k_N in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving group. This equation shows that for these reactions, the sensitivity of log k_N to the basicity of the nonleaving moiety (β_nlg = -0.27) is very similar to that of the nucleofuge (β_lg = -0.25).

Full text of DOI:10.1021/jo0255532

4494
J . Org. Chem. 2002, 67, 4494-4497
Kin etics a n d Mech a n ism of th e P h en olysis of Asym m etr ic Dia r yl
Ca r bon a tes
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 24, 2002
The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl
carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl
2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are
subjected to a kinetic investigation in aqueous solution (25.0 °C, ionic strength 0.2 M (KCI)). Under
an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order
kinetics and are first order in phenoxide. The Bro¨nsted-type plots for the nucleophilic rate constants
(kN) are linear, with slopes â ) 0.48 (MPNPC), 0.67 (ClPNPC), 0.41 (MPDNPC), and 0.32
(ClPDNPC). The magnitude of these slopes and the absence of a curvature in the Bro¨nsted plot at
pKa ) 7.1 for the CIPNPC reactions are consistent with concerted mechanisms (one step). The
carbonates MPDNPC and ClPDNPC are more reactive than MPNPC and CIPNPC, respectively,
toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in
the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively
charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving
group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding
4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than
4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted
phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same
substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic
tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is
deduced for log kN in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving
group. This equation shows that for these reactions, the sensitivity of log kN to the basicity of the
nonleaving moiety (ânlg ) -0.27) is very similar to that of the nucleofuge (âlg ) -0.25).
In tr od u ction
sted-type plots were obtained and interpreted through a
concerted reaction path.5
Although the kinetics and mechanisms of the phenoly-
sis of esters and thioesters have been the subject of many
reports,1-4 the mechanisms of the phenolysis of carbon-
ates and thiocarbonates have received little attention.5,6
We have studied kinetically the phenolysis of S-(4-
nitrophenyl), S-(2,4-dinitrophenyl), and S-(2,4,6-trinitro-
phenyl) ethyl thiocarbonates.5 In this work, linear Bro¨n-
Lately, we have also performed mechanistic studies of
the phenolyses of 4-nitrophenyl, 2,4-dinitrophenyl, and
2,4,6-trinitrophenyl methyl carbonates (1, 2, and 3,
respectively)6a and also of phenyl 4-nitrophenyl and 2,4-
dinitrophenyl phenyl carbonates (4 and 5, respectively).6b
In all these investigations, concerted mechanisms were
proposed on the basis of the Bro¨nsted-type plots ob-
tained.6
(1) (a) Hupe, D. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 451.
(b) Pohl, E. R.; Hupe, D. J . J . Am. Chem. Soc. 1980, 102, 2763. (c)
Guanti, G.; Cevasco, G.; Thea, S.; Dell’ Erba, C.; Petrillo, G. J . Chem.
Soc., Perkin Trans. 2 1981, 327. (d) Buncel, E.; Um, I. H.; Hoz, S. J .
Am. Chem. Soc. 1989, 111, 971.
Recently, we have investigated the phenolysis of bis-
(4-nitrophenyl) carbonate (6), bis(4-nitrophenyl) thiono-
carbonate, and methyl 4-nitrophenyl thionocarbonate.6c
A linear Bro¨nsted plot of slope â ) 0.66 found for the
former reactions is consistent with a concerted mecha-
nism. The biphasic Bro¨nsted plots with a sharp curvature
(with slopes from â ) 0.3-0.4 to â ) 1.3-1.6) obtained
for the two latter reactions indicate that these reactions
are stepwise, through an anionic tetrahedral inter-
mediate.6c
On the other hand, we have also examined kinetically
the reactions of secondary alicyclic amines with 4-nitro-
phenyl and 2,4-dinitrophenyl 4-methylphenyl carbonates
(MPNPC and MPDNPC, respectively).7 On the basis of
the sharp curvature of the Bro¨nsted-type plot obtained
(2) (a) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc.
1987, 109, 6362. (b) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am.
Chem. Soc. 1989, 111, 2647. (c) Ba-Saif, S. A.; Colthurst, M. J .; Waring,
M. A.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1991, 1901. (d)
Hengge, A. J . Am. Chem. Soc. 1992, 114, 6575. (e) Colthurst, M. J .;
Nanni, M.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1996, 2285. (f)
Maude, A. B.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1997, 179.
(3) Kwon, D.-S.; Lee, G.-J .; Um, I.-H. B. Korean. Chem. Soc. 1990,
11, 262. Park, Y. S.; Kim, C. K.; Lee, B. S.; Lee, I.; Lim, W. M.; Kim,
W. K. J . Phys. Org. Chem. 1995, 8, 32. Um, I.-H.; Yoon, H.-W.; Lee,
J .-S.; Moon, H.-J .; Kwon, D.-S. J . Org. Chem. 1997, 62, 5939.
(4) Stefanidis, D.; Cho, S.; Dhe-Paganon, S.; J encks, W. P. J . Am.
Chem. Soc. 1993, 115, 1650.
(5) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 1999, 64,
2310.
(6) (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.; Angel, M.; Arellano,
D.; Santos, J . G. J . Org. Chem. 2001, 66, 6571.
(7) Castro, E. A.; Andu´jar, M.; Campodo´nico, P.; Santos, J . G. Int.
J . Chem. Kinet. 2002, 34, 309.
10.1021/jo0255532 CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/15/2002

Phenolysis of Asymmetric Diaryl Carbonates
Ta ble 1. Va lu es of p Ka of P h en ols a n d kN for Rea ction s of P h en oxid es w ith Dia r yl Ca r bon a tesa
kN/s-1M-1
J . Org. Chem., Vol. 67, No. 13, 2002 4495
phenoxide substituent
pKa of phenol
MPNPC
ClPNPC
MPDNPC
ClPDNPC
4-methoxy
none
3-chloro
3-cyano
4-cyano
2,6-difluoro
pentafluoro
10.3
9.9
9.0
8.6
7.8
7.1
5.3
12.7 ( 0.3
5.3 ( 0.1
2.8 ( 0.03
26.0 ( 0.8
12.6 ( 0.5
6.25 ( 016
17.5 ( 0.6
23.7 ( 0.6
6.4 ( 0.4
2.85 ( 0.1
12.4 ( 0.06
15.7 ( 0.9
5.95 ( 0.2
4.49 ( 0.15
0.40 ( 0.02
0.64 ( 0.02
1.59 ( 0.04
0.202 ( 0.006
a
Both the pKa and kN values were determined in aqueous solutions, at 25.0 °C and an ionic strength of 0.2 M (KCl).
for MPNPC, we concluded that these reactions are
stepwise, through the formation of a zwitterionic tetra-
hedral intermediate (T(). On the other hand, the slight
curvature found for MPDNPC indicates that the reac-
tions of these amines with MPDNPC are concerted.7
To extend our mechanistic investigations on diaryl
carbonates, in the present work, we studied kinetically
the phenolysis of 4-methylphenyl 4-nitrophenyl carbonate
(MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIP-
NPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MP-
DNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate
(CIPDNPC).
The object of this work is threefold: (i) to verify the
destabilizing effect of a phenoxy substituent in a tetra-
hedral intermediate relative to a secondary alicyclic
amino group, (ii) to examine the influence of the leaving
and nonleaving groups of the substrate on the kinetics
and mechanisms by comparing the reactions of the
present investigation among them and with the phenoly-
ses of 4-6, and (iii) to assess the above effects through a
multiparametric equation involving the basicity of the
nucleophile, the nucleofuge, and the nonleaving group.
F igu r e 1. Bro¨nsted-type plots for the phenolysis of 4-meth-
ylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-methylphe-
nyl 2,4-dinitrophenyl carbonate (MPDNPC), in aqueous solu-
tion (25.0 °C, ionic strength 0.2 M). Slopes are â ) 0.48 and
0.41, respectively.
reactions. The preparation of the kinetic samples and the pH
adjustments were carried out as reported.6b Pseudo-first-order
rate coefficients (kobsd) were found in all cases. The experi-
mental conditions of the reactions and the kobsd values obtained
are shown in Tables S1-S4 (pages S2-S5 in Supporting
Information).
P r od u ct Stu d ies. 4-Nitrophenoxide or 2,4-dinitrophenox-
ide anions were identified as one of the products of the
corresponding title reactions by comparison of the UV-vis
spectra after completion of some of these reactions with those
of authentic samples of 4-nitrophenol or 2,4-dinitrophenol
under the same experimental conditions.
Exp er im en ta l Section
Ma ter ia ls. The phenols were purified by distillation or
recrystallization. The carbonates MPNPC and MPDNPC were
synthesized as published elsewere.7 The substrates CIPNPC
and CIPDNPC were synthesized by a modification of a
standard procedure,8 as reported.6b
The crystallized (diethyl ether) CIPNPC melted at 113.4-
114.4 °C and was identified as follows: 1H NMR (200 MHz,
CDCI3) δ 7.34 (d, 2H, J ) 9.0 Hz), 7.45 (m, 4H), 8.32 (d, 2H,
J ) 9.0 Hz); 13C NMR (50 MHz, CDCl3) δ 121.70 (C-2′/6′),
122.13 (C-2/6), 125.45 (C-3/5), 129.82 (C-3′/5′), 132.24 (C-4′),
145.72 (C-4), 149.13 (C-1′), 150.80 (C-1), 155.13 (CdO). Anal.
Calcd for C13H8NO5Cl: C, 53.17; H, 2.75; N, 4.77. Found: C,
53.38; H, 2.52; N, 4.67.
Resu lts a n d Discu ssion
The crystallized (diethyl ether) CIPDNPC melted at 103-
104 °C and was identified as follows: 1H NMR (200 MHz,
CDCI3) δ 7.25 (sd, 2H, J ) 9.1 Hz), 7.42 (sd, 2H, J ) 9.1 Hz),
7.67 (d, 1H, J ) 8.9 Hz), 8.60 (dd, 1H, J ) 8.9, 2.7 Hz), 9.07
(d, 1 H, J ) 2.7 Hz); 13C NMR (50 MHz, CDCI3) δ 121.21 (C-
6′), 122.13 (C-2/6), 126.24 (C-3′), 129.67 (C-5′), 129.91 (C-3/5),
132.55 (C-4), 141.10 (C-2′), 145.59 (C-4′), 148.07 (C-1), 149.10
(C-1′), 149.96 (CdO). Anal. Calcd for C13H7N2O7Cl: C, 46.15;
H, 2.07; N, 8.28. Found: C, 46.03; H, 1.86; N, 8.23.
The kinetic law found for all the reactions studied is
given by eq 1, where kobsd is the pseudo-first-order rate
constant, k0 and kN are the rate coefficients for hydrolysis
and phenolysis of the substrates, respectively, and ArO-
represents the substituted phenoxide nucleophile.
kobsd ) k0 + kN[ArO-]
(1)
Kin etic Mea su r em en ts. These were performed spectro-
photometrically by following the production of 4-nitrophenox-
ide or 2,4-dinitrophenoxide ions at 400 or 360 nm, respectively,
using a diode array instrument. 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 with respect to the substrate was employed. The initial
concentration of the substrates was 3 × 10-5 M in all the
The values of k0 at ca. pH 10 are 102k0/s-1 ) 0.2, 1.0,
0.3, and 1.5 for the hydrolysis of MPNPC, ClPNPC,
MPDNPC, and ClPDNPC, respectively.
Plots of kobsd vs [ArO-] at a constant pH were linear
for the reactions of both substrates, with slopes (kN)
independent of the pH. The values of kN are shown in
Table 1. With these values and the pKa of the phenols,
the Bro¨nsted-type plots (shown in Figures 1 and 2) were
obtained.
(8) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
6970.

4496 J . Org. Chem., Vol. 67, No. 13, 2002
Castro et al.
substitution of an amino moiety in T( by a methyl group
greatly destabilizes this intermediate.
The magnitude of the slope of the Bro¨nsted plot alone
does not prove that the reactions are concerted. One must
be sure that there is no Bro¨nsted break at the pKa
expected for a change in the rate-determining step for a
hypothetical stepwise reaction.12 If the phenolyses of
MPNPC and ClPNPC were stepwise, the Bro¨nsted plot
break for both reactions would be at pKa ) 7.1, the pKa
of 4-nitrophenol. The lack of break in the Bro¨nsted plot
for the phenolysis of ClPNPC (Figure 2) indicates that
this reaction is concerted. For the phenolysis of MPNPC,
the values of kN found are very similar to those obtained
in the phenolysis of 4.6b Since the latter reaction is
concerted, it is very likely that the phenolysis of MPNPC
is also driven by a concerted mechanism, in view of the
similar pKa values of phenol and 4-methylphenol.13
The hypothetical Bro¨nsted break for the phenolysis of
MPDNPC and ClPDNPC for a stepwise reaction would
be at pKa ) 4.1 (the pKa of 2,4-dinitrophenol). This
breakpoint lies beyond the pKa range of the phenols used
in this work. However, if the phenolyses of MPNPC and
ClPNPC are concerted, it is reasonable that the phenoly-
ses of MPDNPC and ClPDNPC would also be concerted.
This is because of the better nucleofuge involved in the
latter intermediate, which would destabilize it even more
compared to the intermediate in the mononitro deriva-
tives.
F igu r e 2. Bro¨nsted-type plots for the phenolysis of 4-chlo-
rophenyl 4-nitrophenyl carbonate (ClPNPC) and 4-chlorophe-
nyl 2,4-dinitrophenyl carbonate (ClPDNPC), in aqueous solu-
tion (25.0 °C, ionic strength 0.2 M). Slopes are â ) 0.67 and
0.32, respectively.
The Bro¨nsted plots are linear with the following slope
(â) values: â ) 0.48, 0.41, 0.67, and 0.32 for the reactions
of MPNPC, MPDNPC, ClPNPC, and ClPDNPC, respec-
tively. The magnitudes of these â values suggest that
these reactions are concerted. Similar â values have been
found in the concerted phenolyses of aryl methyl carbon-
ates 1, 2, and 3 (â ) 0.67, 0.48, and 0.52, respectively),6a
of aryl phenyl carbonates 4 and 5 (â ) 0.61 and 0.49,
respectively),6b and for the symmetrical diaryl carbonate
6 (â ) 0.66).6c
The concerted aminolyses of related substrates show
â values similar to those above. Linear Bro¨nsted plots
with slopes â ) 0.56 and 0.48 are exhibited in the
reactions of secondary alicyclic amines with S-(2,4-
dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiol-
carbonates.9 In the reactions of the same amines with
carbonates 310 and MPDNPC,7 the â values are found to
be 0.36 and 0.55, respectively. Also, methoxycarbonyl
transfer between isoquinoline and pyridines shows a
linear Bro¨nsted plot of slope â ) 0.58.11
The anionic tetrahedral intermediates formed in the
phenolysis of diaryl carbonates must be very unstable
due to the three O-aryl groups attached to the central
carbon of the intermediate. Williams has argued that two
O-aryl groups are sufficient to destabilize anionic tetra-
hedral intermediates (T-), for instance, those formed in
the phenolysis of aryl acetates.2a Therefore, it is reason-
able that the “intermediates” formed in the reactions of
the present work would be so unstable as to prevent their
existence, enforcing a concerted mechanism.12
On the other hand, we have found that the aminolysis
(secondary alicyclic amines) of MPNPC is stepwise, i.e.,
the zwitterionic tetrahedral intermediate (T() formed in
these reactions exists.7 The fact that the anionic “inter-
mediate” T- formed in the phenolysis of aryl acetates is
either nonexistent or highly unstable2a means that
The phenolysis of MPNPC is concerted (this work),
whereas the aminolysis (secondary alicyclic amines) of
this compound is stepwise.7 This means that substitution
of a secondary alicyclic amino moiety in a zwitterionic
tetrahedral intermediate by a phenoxy group destabilizes
the intermediate. This result is in agreement with the
fact that the phenolyses of ethyl S-(4-nitrophenyl) thiol-
carbonate5 and carbonate 16a are concerted, in contrast
to their aminolyses, which are stepwise.14,15
The greater reactivity toward phenoxides of MPDNPC
and ClPDNPC than that of the corresponding mononitro
substrates, MPNPC and ClPNPC (Figures 1 and 2),
should be due to the presence of the second nitro group
in the dinitro substrates, which leaves their carbonyl
carbon more positive and makes them better electro-
philes. Moreover, since the second nitro group is in the
nucleofuge, this should be expelled faster from MPDNPC
and ClPDNPC than 4-nitrophenoxide anion from MP-
NPC and ClPNPC, respectively.
On the other hand, the greater reactivity of ClPDNPC
than MPDNPC and of ClPNPC than MPNPC toward
phenoxides (Table 1) could be explained by the greater
electron-withdrawing effect of 4-chloro than 4-methyl.
To evaluate the influence of the nonleaving moiety on
the kinetics of these reactions, the Bro¨nsted plots for the
phenolyses of MPNPC, ClPNPC, 6,6c and 46b are com-
pared in Figure 3. In this figure, pKa (nlg) is the pKa of
the conjugate acid of the “nonleaving” group and each
line corresponds to a different phenoxide nucleophile. As
seen in Figure 3, the plots are linear; the slopes vary from
-0.24 to -0.33. The negative slopes are due to the fact
(9) 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.
(10) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 2001,
66, 6000.
(11) Chrystiuk, E.; Williams, A. J . Am. Chem. Soc. 1987, 109, 3040.
(12) Williams, A. Chem. Soc. Rev. 1994, 23, 93.
(13) Albert, A.; Serjeant, E. P. The Determination of Ionization
Constants; Chapman and Hall: London, 1971; p 87. J encks, W. P.;
Gilchrist, M. J . Am. Chem. Soc. 1968, 90, 2622.
(14) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1994,
59, 3572.
(15) Unpublished results from this laboratory.

Phenolysis of Asymmetric Diaryl Carbonates
J . Org. Chem., Vol. 67, No. 13, 2002 4497
F igu r e 3. Bro¨nsted-type plots (for the nonleaving group) for
the phenolysis of aryl 4-nitrophenyl carbonates, in aqueous
solution (25.0 °C, ionic strength 0.2 M). Substituents in the
phenoxide nucleophiles are 4-MeO, H, 3-Cl, and 4-CN.
F igu r e 4. Logarithmic plot of experimental vs calculated
(through eq 2) kN for the reactions of a series of phenoxide
ions with a series of diaryl carbonates, in aqueous solution
(25.0 °C, ionic strength 0.2 (KCl)). Carbonates are phenyl,
4-methylphenyl, and 4-chlorophenyl 2,4-dinitrophenyl carbon-
ates for 5, MPDNPC, and ClPDNPC, respectively, and phenyl,
4-methylphenyl, 4-chlorophenyl, and 4-nitrophenyl 4-nitro-
phenyl carbonates for 4, MPNPC, ClPNPC, and 6, respectively.
that nucleophilic attack is hindered by electron donation
from the nonleaving group.
The Bro¨nsted plots for the nonleaving group (not
shown) obtained for the phenolysis of MPDNPC, ClPD-
NPC, and 56b are also linear with slopes similar to those
found for the phenolysis of the mononitro derivatives
(Figure 3). Unfortunately, the Bro¨nsted plots for the
dinitro compounds contain only three points for each
nucleophile and the pKa span is smaller than that for the
mononitro derivatives.
With the kN values found in the present reactions and
those for the phenolysis of 4-6, together with the pKa
values of the nucleophiles (Table 1), the leaving groups
(7.1 and 4.1 for 4-nitro- and 2,4-dinitrophenols, respec-
tively),13 and the nonleaving groups (10.1, 9.9, 9.4, and
7.1 for 4-methyl, H, 4-chloro-, and 4-nitrophenols, re-
spectively),5,13 eq 2 can be deduced by regression analysis
(n ) 37, R2 ) 0.952). In this expression, N, nlg, and lg
refer to the nucleophile and the nonleaving and leaving
groups, respectively; the pKa coefficients (â) are subjected
to an error of (0.1.
carbonates,8 and O-ethyl dithiocarbonates.17 In all of
these works, the low âlg value (between 0 and -0.3) has
been associated with a stepwise mechanism where the
formation of the zwitterionic tetrahedral intermediate is
the rate-determining step. For the aminolysis of the less
reactive aryl thiolcarbonates, where the breakdown to
products of the tetrahedral intermediate is the rate-
determining step, the values of -âlg are in the 0.7-0.8
range.18
The similar sensitivity of log kN to the basicity of both
the nonleaving and leaving groups is probably due to the
fact that both are phenoxy groups. On the other hand, it
is noteworthy that the sum of sensitivities to the nucleo-
phile (âN) and leaving (âlg) and nonleaving (ânlg) groups
is approximately zero (see eq 2). This means that for the
phenolysis of bis(aryl) carbonates with the same aryl
group in the nucleophile and the leaving group, the rate
constants must be the same, independent of the aryl
group. Namely, the rate constant for the reaction of
phenoxide anion with bis(phenyl) carbonate should be
approximately equal to that of 4-nitrophenoxide anion
with bis(4-nitrophenyl) carbonate. Unfortunately, this is
not possible to prove experimentally since these reactions
are indistinguishable. Nevertheless, it seems reasonable
that a strong nucleophile will have a great difficulty in
displacing a strongly basic leaving group because a good
nucleophile is a bad nucleofuge.
log kN ) -0.038 + 0.54pKa(N) -
0.25pKa(lg) - 0.27pKa(nlg) (2)
A logarithmic plot of the experimental kN vs the
calculated one through eq 2 is shown in Figure 4; the
slope is unity.
The sensitivity of log kN to the nucleophile pKa (âN
)
0.54) is in agreement with those exhibited in the con-
certed phenolysis of aryl methyl carbonates6a and in the
concerted aminolysis of carbonyl compounds, as men-
tioned above.7,9-11
Ack n ow led gm en t. We thank FONDECYT of Chile
(Project 2000051) for financial assistance. P.P. thanks
CONICYT for a scholarship.
On the other hand, the sensitivity of log kN to the
leaving group basicity (âlg ) -0.25) is similar to that
found in the phenolysis of aryl methyl carbonates;6a but
its absolute value is greater than that exhibited by the
Su p p or tin g In for m a tion Ava ila ble: Tables S1-S4 con-
taining the values of kobsd and experimental conditions for the
reactions. This material is available free of charge via the
Internet at http://pubs.acs.org.
concerted aminolysis ethyl S-aryl thiolcarbonates (âlg
)
-0.19)9 and lower than the corresponding ones for the
concerted phenolysis of aryl acetates (from -0.5 to -0.6)
and aryl formates (from -0.3 to -0.9).4
J O0255532
However, the âlg value in eq 2 is also similar to those
found in stepwise aminolyses of acetate esters,16 diaryl
(17) Castro, E. A.; Cubillos, M.; Iban˜ez, F.; Moraga, I.; Santos, J . G.
J . Org. Chem. 1993, 58, 5400.
(16) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
7018.
(18) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1999,
64, 6342.