Kinetics and mechanism of the aminolysis of phenyl and 4-nitrophenyl chloroformates in aqueous solution

Article abstract of DOI:10.1021/jo990146k

The reactions of secondary alicyclic amines with phenyl and 4- nitrophenyl chloroformates (PClF and NPClF, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 °C, ionic strength 0.2 (KCl). The reactions are followed spectropho

Full text of DOI:10.1021/jo990146k

J . Org. Chem. 1999, 64, 4817-4820
4817
Kin etics a n d Mech a n ism of th e Am in olysis of P h en yl a n d
4
-Nitr op h en yl Ch lor ofor m a tes in Aqu eou s Solu tion
Enrique A. Castro,* Maria G. Ruiz, Sandra Salinas, and J os e´ G. Santos*
Facultad de Qu ı´ mica, Pontificia Universidad Cat o´ lica de Chile, Casilla 306, Santiago 22, Chile
Received J anuary 26, 1999
The reactions of secondary alicyclic amines with phenyl and 4-nitrophenyl chloroformates (PClF
and NPClF, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 °C, ionic
strength 0.2 (KCl). The reactions are followed spectrophotometrically at 210-270 nm (PClF) and
at 310-400 nm (NPClF). Under amine excess, pseudo-first-order rate coefficients (kobsd) are obtained.
From linear plots of kobsd vs free amine concentration, the second-order rate coefficients (k
aminolysis are obtained. For the aminolysis of both substrates, linear Br o¨ nsted-type plots (log k
vs amine pK ) of slopes 0.23 (PClF) and 0.26 (NPClF) are found. The values of the slopes are
consistent with stepwise mechanisms where the formation of a zwitterionic tetrahedral intermediate
N
) for
N
a
(
(
1
T ) is the rate-determining step (k step). In contrast, the aminolysis (anilines) of the same
(
substrates in acetonitrile are concerted, which is attributed to destabilization of T in the latter
(
solvent due to a faster expulsion of the amine from T in acetonitrile compared to water. The values
1
of k are larger for the title reactions compared to the same aminolysis of the corresponding
thionochloroformates, and this is attributed to the relatively hard character of these amines which
prefer to bind to the harder carbonyl group (relative to thiocarbonyl). There is no change in
-
-
(
mechanism by the change of S by O in T , which should destabilize this intermediate. By
comparison with the stepwise pyridinolysis of methyl chloroformate, it is concluded that the changes
(
of methoxy by phenoxy (or 4-nitrophenoxy) and a pyridine by an alicyclic amine in T do not greatly
affect the stability of these intermediates.
In tr od u ction
solvent are stepwise.6b On the other hand, the aminolysis
secondary alicyclic amines) of aryl chlorothionoformates
(
Although much attention has been drawn to the
kinetics and mechanism of the aminolysis of carboxylic
in water is a two-stage reaction, whereby the formation
of T( is the rate-determining step.7
1
,2
3,4
acid derivatives such as esters and carbonates, the
aminolysis reactions of alkyl and aryl chloroformates5
have been less studied kinetically.
To shed more light on the mechanism of the aminolysis
of chloroformates and with the aim of clarifying the
influence of the amine nature, the solvent, and the
electrophilic center of the substrate (CO vs CS) on the
mechanism, in the present work we undergo a kinetic
study of the reactions of secondary alicyclic amines with
phenyl and 4-nitrophenyl chloroformates in water. We
will compare our results with those obtained in (i) the
,6
It has been found (through a biphasic Br o¨ nsted-type
plot) that the pyridinolysis of methyl chloroformate in
water is stepwise, through the formation of a tetrahedral
(
5b
intermediate (T ). In contrast, Lee and co-workers
found that the aminolysis of aryl chloroformates in
acetonitrile is concerted,6 whereas the pyridinolysis of
these substrates and methyl chloroformate in the same
a
5b
pyridinolysis of methyl chloroformate in water, in order
to investigate the effect of the amine nature; (ii) the
6
aminolysis of the same substrates in acetonitrile, to
assess the influence of the solvent; and (iii) the aminolysis
(
1) J ohnson, S. L. Adv. Phys. Org. Chem. 1967, 5, 237 and references
therein. Kirsch, J . F.; Kline, A. J . Am. Chem. Soc. 1969, 91, 1841.
Menger, F. M.; Smith, J . H. J . Am. Chem. Soc. 1972, 94, 3824. O’Leary,
M. H.; Marlier, J . F. J . Am. Chem. Soc. 1979, 101, 3300. Bell, K. H.
Aust. J . Chem. 1987, 40, 1723. Koh, H. J .; Lee, H. C.; Lee, H. W.; Lee,
I. Bull. Korean Chem. Soc. 1995, 16, 839. Cho, B. R.; Kim, Y. K.; Yoon,
C. M. J . Am. Chem. Soc. 1997, 119, 691. Maude, A. B.; Williams, A. J .
Chem. Soc., Perkin Trans. 2 1997, 179.
of the corresponding aryl chlorothionoformates (ArO-
7
CS-Cl) in water, to evaluate the effect of the electro-
philic center of the substrate.
Exp er im en ta l Section
(
2) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
Ma ter ia ls. Phenyl (PClF) and 4-nitrophenyl chloroformates
7
6
018.
(3) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
(NPClF) were from Aldrich (99% and 97%, respectively) and
8
970.
used as purchased. Amines were purified as described.
(4) (a) Bond, P. M.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2
Syn th esis of P r od u cts. The O-phenyl carbamates of
piperidine and morpholine (PiPC and MoPC) and the O-4-
nitrophenyl carbamates of piperidine and morpholine (PiNPC
and MoNPC) were synthesized as follows: To a solution of 391
mg (2.5 mmol) of PClF (or 506 mg, 2.5 mmol, of NPClF)
dissolved in acetonitrile (10 mL) was added slowly a solution
of 425 mg (5 mmol) of piperidine (or 415 mg, 5 mmol, of
1
7
(
976, 679. (b) Castro, E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99,
611. (c) Castro, E. A.; Freudenberg, M. J . Org. Chem. 1980, 45, 906.
d) Castro, E. A.; Iba n˜ ez, F.; Lagos, S.; Schick, M.; Santos, J . G. J .
Org. Chem. 1992, 57, 2691. (e) Castro, E. A.; Iba n˜ ez, F.; Saitua, A. M.;
Santos, J . G. J . Chem. Res., Synop. 1993, 56.
(5) (a) Hall, H. K., J r. J . Am. Chem. Soc. 1957, 79, 5439. (b) Bond,
P. M.; Castro, E. A.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2 1976,
8. (c) King, J . F.; Guo, Z. R.; Klassen, D. F. J . Org. Chem. 1994, 59,
095.
6
1
(
6) (a) Yew, K. H.; Koh, H. J .; Lee, H. W.; Lee. I. J . Chem. Soc.,
(7) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1997,
62, 4395.
(8) Castro, E. A.; Ureta, C. J . Org. Chem. 1989, 54, 2153.
Perkin Trans. 2 1995, 2263. (b) Koh, H. J .; Han, K. L.; Lee, H. W.;
Lee, I. J . Org. Chem. 1998, 63, 9834.
1
0.1021/jo990146k CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/06/1999

4
818 J . Org. Chem., Vol. 64, No. 13, 1999
Castro et al.
Ta ble 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
Sch em e 1
th e Am in olysis of P h en yl Ch lor ofor m a te (P ClF )a
1
04[N]tot,b
M
10 kobsd,
2
no. of
runs
-
1
amine
piperidinec
pH
6.2
s
30-300
100-500
30-500
2.0-4.0
2.0-4.5
2.0-5.0
2.0-5.0
2.0-5.0
2.0-4.0
2.0-4.0
2.0-5.0
2.0-5.0
2.0-4.5
2.0-5.0
2.0-5.0
1.63-2.38
2.38-4.05
1.85-9.24
2.74-4.07
2.83-4.79
2.93-5.47
3.67-7.58
4.48-8.68
4.96-9.08
6.15-12.5
2.04-2.54
2.29-3.67
4.03-7.38
1.47-2.62
1.91-3.21
7
5
7
5
6
7
4
4
3
3
6
7
7
4
4
6
7
.5
.0
1
-(2-hydroxyethyl)
6.0
6.3
c
piperazine
6
7
7
7
7
.5
.0
.3
.5
.8
morpholinec
6.2
6
7
.5
.0
1
-(2-hydroxyethyl)-
piperazinium ion
4.5
4.8
a
In aqueous solution at 25.0 °C, ionic strength is 0.2 M (KCl).
b
c
Concentration of total amine (free base plus protonated forms).
In the presence of 5 × 10 M phosphate buffer.
-3
Ta ble 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Am in olysis of 4-Nitr op h en yl Ch lor ofor m a te (NP ClF )a
P r od u ct Stu d ies. For the reactions of both substrates with
piperidine and morpholine, one of the products was identified
as the corresponding carbamate, by comparison of the UV
spectra at the end of the reactions with that of an authentic
sample of the corresponding carbamate under the same
experimental conditions.
In the reactions of NPClF, 4-nitrophenoxide ion was identi-
fied as another product (of the parallel hydrolysis); this was
achieved by comparison of the UV-vis spectra after completion
of the reactions with that of an authentic sample of 4-nitro-
phenol under the same conditions.
1
04[N]tot,b
M
10 kobsd,
2
no. of
runs
-
1
amine
pH
6.2
s
piperidinec
30-300
70-500
30-200
2.0-5.0
2.0-5.0
2.0-5.0
2.0-5.0
2.0-5.0
2.0-4.0
2.0-4.0
2.5-4.5
2.0-5.0
2.0-4.5
2.0-6.0
2.0-6.0
2.0-5.0
2.0-5.0
7.85-10.8
4.98-14.9
9.15-18.7
11.5-17.4
14.0-19.1
12.6-18.5
13.5-24.6
14.9-28.3
15.4-35.6
21.2-41.5
9.29-11.0
10.2-14.1
13.3-20.9
18.0-49.0
24.8-62.4
8.56-11.1
9.13-11.9
6
5
5
4
4
4
4
4
3
3
5
6
6
5
5
4
4
6
7
6.0
6.3
6
7
7
7
7
.5
.0
1
-(2-hydroxyethyl)-
piperazine
c
.5
.0
.3
.5
.8
Resu lts a n d Discu ssion
morpholinec
6.2
6
7
The general rate law obtained in the present reactions
is given by eqs 1 and 2, where Ar is phenyl or 4-nitro-
phenyl, k and k are the rate constants for hydrolysis
o N
and aminolysis of the substrates, respectively, and NH
represents a secondary alicyclic amine free base.
.5
.0
piperazinium ion
5.4
.5
5
1
-(2-hydroxyethyl)-
piperazinium ion
4.5
4.8
a
d[ArS-]
In aqueous solution at 25.0 °C, ionic strength is 0.2 M (KCl).
b
c
Concentration of total amine (free base plus protonated forms).
In the presence of 5 × 10 M phosphate buffer.
) kobsd[ClCOOAr]
(1)
(2)
-3
dt
kobsd ) k + k [NH]
o
N
morpholine) in acetonitrile (10 mL). The mixture was left 30
min at ambient temperature. Chloroform (15 mL) was added
to this mixture and the solution washed with 10% HCl and
water. The organic layer was dried with MgSO and filtered
4
under vacuum and the solvent evaporated off. The crystallized
N
The second-order rate coefficients for aminolysis (k )
were obtained as the slopes of plots of eq 2 at constant
pH and were pH independent in the reactions of piperi-
dine, morpholine, piperazinium ion, and 1-(2-hydroxy-
ethyl)piperazinium ion. Nevertheless, in the reactions of
products showed the following mp: MoPC 55.1-55.3 °C, PiPC
1
°
38.3-138.6 °C, MoNPC 74.9-75.1 °C, and PiNPC 98.2-98.3
1 13
C. The IR, H NMR, and C NMR spectra were consistent
1
-(2-hydroxyethyl)piperazine in the pH range 6.2-7.8,
with their structures.
different straight lines were obtained for each pH, due
to the existence of two nucleophiles (species 1 and 2 in
Scheme 1) which can react with the substrates. In this
case the reaction law obtained is given by eq 3, where
Kin etic Mea su r em en ts. These were carried out by means
of a Hewlett-Packard 8453 diode array spectrophotometer in
water, at 25.0 ( 0.1 °C, ionic strength 0.2 M (KCl), and, in
some cases, phosphate buffer 0.005 M.
The reactions of PClF were followed in the 210-270 nm
range, while the reactions of NPClF were studied at 400 nm
k
N1 and kN2 are the second-order rate constants for the
reactions of 1 and 2, respectively, F and F are the
1
2
(
formation of the 4-nitrophenoxide anion), except the reactions
corresponding molar fractions, and [NH]tot is the concen-
tration of total amine.9
with piperazinium and 1-(2-hydroxyethyl)piperazinium ions,
which were measured at 310 nm (following the appearance of
4
-nitrophenol).
The initial substrate concentration was (2-5) × 10 M. All
reactions were studied under an excess of amine over the
substrate (see below). Pseudo-first-order rate coefficients (kobsd
kobsd ) k + (k F + k F )[NH]
tot
(3)
o
N1
1
N2
2
-5
The slopes of the linear plots of kobsd vs [NH]tot at
constant pH correspond to kNobsd of eq 4. Rearrangement
of this equation yields eq 5. The kN1 and kN2 values were
)
were found for all reactions; the kobsd values were determined
by means of the “infinity” method. The experimental conditions
and the kobsd values for these reactions are shown in Tables 1
and 2.
(9) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991, 63.

Aminolysis of Phenyl Chloroformates in Aqueous Solution
J . Org. Chem., Vol. 64, No. 13, 1999 4819
Ta ble 3. Va lu es of th e p Ka of th e Con ju ga te Acid s of
Secon d a r y Alicyclic Am in es a n d kN for th e Am in olysis of
P h en yl Ch lor ofor m a te (P ClF ) a n d 4-Nitr op h en yl
a
Ch lor ofor m a te (NP ClF )
1
0-4kN, s-1 M-1
PCIF NPCIF
11.24 2.61 ( 0.06 8.94 ( 0.3
amine
pKa
piperidine
1
-(2-hydroxyethyl)piperizine 9.38 0.87 ( 0.04 2.67 ( 0.2b
morpholine
-(2-hydroxyethyl)-
piperazinium ion
piperazinium ion
b
8.78 0.77 ( 0.01 1.68 ( 0.06b
b
1
5.90 0.13 ( 0.01 0.34 ( 0.01c
c
0.16 ( 0.01 0.43 ( 0.04
5.81
0.271 ( 0.0001
a
Values of both pKa and kN in aqueous solution, at 25.0 °C;
b
ionic strength is 0.2 M (KCl). Values determined at pH 6.0-7.8
c
by means of eqs 3-5. Values determined at pH 4.5 and 4.8 by
means of eq 2.
obtained as slope and intercept, respectively of linear
9
2 1 2
plots of kNobsd/F vs F /F .
kNobsd ) kN2F + kN1F1
(4)
(5)
2
F igu r e 1. Br o¨ nsted-type plots (statistically corrected) ob-
tained in the reactions of secondary alicyclic amines with
kNobsd/F ) k + kN1F /F2
2
N2
1
4
-nitrophenyl (O) and phenyl (b) chloroformates in water, 25.0
C, ionic strength 0.2 (KCl). The slope values are 0.26 and 0.23,
respectively.
°
o
The hydrolysis rate constants (k ) for NPClF and PClF
-
1
are 0.078 ( 0.003 and 0.015 ( 0.002 s , respectively, at
the pH range studied. The latter value is in agreement
with that found in the direct hydrolysis of PClF (0.013
Sch em e 2
-
1
10
s
). We also measured the hydrolysis reactions in the
absence of amine, and the k values obtained were
consistent with those found in the aminolysis. In some
reactions, the contribution of k to the total observed rate
constant was more important than the aminolysis term,
o
o
k
N
[NH] in eq 2.
Although neither phenoxide or 4-nitrophenoxide ion
are products of the aminolysis reactions, their formation
through the parallel hydrolysis reaction allows the de-
termination of the kobsd values and, therefore, the ami-
nolysis rate constants through eq 2 or eqs 3-5.
In some cases the reactions were studied with only a
small excess of total amine over the substrate, but due
to the presence of the important parallel hydrolysis
reaction the pseudo-order condition was maintained, as
shown by the good correlation coefficients obtained
anism shown in Scheme 2 (X ) NO2 or H) for the
reactions under scrutiny. AppIication of the steady-state
(
treatment to T gives k ) k k /(k + k ); since the first
N
1
2
-1
2
step is limiting, it follows that in this scheme k . k
and therefore kN ) k1.
2
-1
1
As shown in Table 3, the k values for the aminolysis
of NPClF are greater than those of PClF. This is
reasonable due to the presence of the powerful electron-
withdrawing 4-nitro group in NPClF that leaves the
carbonyl group of this substrate more electrophilic than
that of PClF.
(greater than 0.999).
The second-order rate coefficients for aminolysis, ob-
tained either by eq 2 or eq 5, together with the pK
a
values
of the conjugate acids of the amines, are shown in Table
9
,11
3
. With these data (statistically corrected)
the Br o¨ n-
In contrast to the stepwise aminolysis of aryl chloro-
formates obtained in this work, Lee and co-workers found
that the reactions of anilines with the same substrates
sted-type plots of Figure 1 were obtained. The slopes of
the lines (â) are 0.23 and 0.26 for the aminolysis of PClF
and NPClF, respectively. The magnitude of â is in
agreement with the values of Br o¨ nsted slopes found in
stepwise mechanisms of similar reactions when the
formation of the zwitterionic tetrahedral intermediate
6a
in acetonitrile are concerted. This conclusion was
achieved on the basis of a negative value of the cross-
correlation coefficient, FXY (X and Y are substituents in
the amine and the “acyl” group, respectively), and an
(
2-4,7-9
(
T ) is the rate-determining step.
For the concerted
inverse secondary kinetic isotope effect, k
involving deuterated aniline nucleophiles. We have
H
/k
D
6a
≈ 0.7-0.9,
aminolysis of similar compunds, the values of â are
usually in the region 0.5-0.6.12
shown that anilines stabilize a tetrahedral intermediate
According to the rate law obtained, the analysis of
products, and the Br o¨ nsted plots, we propose the mech-
4e,13
relative to isobasic secondary alicyclic amines.
There-
fore, it is likely that the reactions of aryl chloroformates
with anilines in water be stepwise through the formation
(
10) Queen, A. Can. J . Chem. 1967, 45, 1619. Kevill, D. N.; D′Souza,
M. J . J . Chem. Soc., Perkin Trans. 2 1997, 1721.
11) Bell, R. P. The Proton in Chemistry; Methuen: London, 1959;
p 159.
(
of a tetrahedral intermediate (T ). The destabilization
(
of T in CH
3
CN relative to water is in line with the
(
results found in the aminolysis of S-aryl O-ethyl dithio-
(12) (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.; Hormaz a´ bal, A.; Santos, J .
G. Int. J . Chem. Kinet. 1998, 30, 267.
(13) Castro, E. A.; Leandro, L.; Mill a´ n, P.; Santos, J . G. J . Org.
Chem. 1999, 64, 1953.

4
820 J . Org. Chem., Vol. 64, No. 13, 1999
Castro et al.
carbonates: there is a change in mechanism from step-
tive to aryl chlorothionoformates) as well as a possible
concerted mechanism; nevertheless our results show that
this destabilizing effect is not great enough as to prevent
the existence of the intermediate.
wise (biphasic Br o¨ nsted plots and nonlinear plots of kobsd
1
4
vs amine concentration) to concerted (â ) 0.4-0.7 and
F
XY ) - 0.56)15 by the change from water to acetonitrile
as solvent. Similarly, the aminolysis of 2,4,6-trinitro-
phenyl O-ethyl dithiocarbonate is stepwise (biphasic
Br o¨ nsted plot) in water1 and concerted (â ) 0.53) in a
On the other hand, the pyridinolysis of methyl chlo-
roformate in water is stepwise.5b The change of methoxy
4b
(
by phenoxy or 4-nitrophenoxy as the “acyl” group of T
should stabilize this intermediate;
1
6
20,21
less polar solvent (44 wt % aqueous ethanol). This was
attributed to the fact that expulsion of the amine from
in contrast, sub-
stitution of a pyridine by a secondary alicyclic amine in
(
(
3,4e,9,13
T (k-1 in Scheme 2) should be enhanced in the less polar
T should destabilize it.
Therefore, it is reasonable
solvent, while the nucleofugality of the leaving group (k
2
)
that these opposing factors partially cancel each other
and, as a result, it is likely that these intermediates T
be of similar stability and therefore both reactions be
stepwise.
(
should not change significantly with the solvent nature,
rending the intermediate more unstable kinetically in the
less polar solvent.3
,7,16
The k values obtained in this work are ca. 50-100
1
The pyridinolysis of aryl chloroformates in acetonitrile
times greater than those of the same aminolysis of the
(
is stepwise, with the formation of T rate determining,
7
corresponding thionoformates, showing the greater re-
6b
as shown by a value of â ≈ 0.3. The fact that the title
activity of the carbonyl than the thiocarbonyl group of
the substrate toward alicyclic amines. This is consistent
with the faster attack of these amines to phenyl acetate
compared to phenyl thionoacetate.17 This result has been
explained by the harder nature of the carbonyl group
relative to thiocarbonyl and the fact that alicyclic amines
are considered relatively hard bases.1
reactions in water are governed by the same mechanism
and limiting step means that the larger value of k-1 (see
Scheme 2) brought about by the change of a pyridine by
(3,4e,9,13
a secondary amine in T
is counterbalanced by a
smaller k-1 value resulting from the change of acetonitrile
3,7,16
to water.
2
Since k (Scheme 2) should not be signifi-
7,18
cantly affected either by the amine nature or the sol-
It is known that the change of O for S- in the
-
3,7,16
vent,
it is reasonable that for both type of reactions
zwitterionic tetrahedral intermediate makes it less stable
k
T
2
. k-1 and the rate-limiting step is the formation of
in both cases.
-
-
due to the superior ability of O (relative to S ) to form
a double bond and expel faster both its leaving and amino
groups.19 From this point of view, a destabilized T could
be expected in the reaction of aryl chloroformates (rela-
(
(
Ack n ow led gm en t. The financial assistance given
to this work by Fondo Nacional de Investigaciones
Cient ´ı ficas y Tecnol o´ gicas (FONDECYT) of Chile is
gratefully acknowledged.
(14) (a) Cabrera, M.; Castro, E. A.; Salas, M.; Santos, J . G.;
Sep u´ lveda, P. J . Org. Chem. 1991, 56, 5324. (b) Castro, E. A.; Iba n˜ ez,
F.; Salas, M.; Santos, J . G.; Sep u´ lveda, P. J . Org. Chem. 1993, 58, 459.
J O990146K
(
c) Castro, E. A.; Cubillos, M.; Iba n˜ ez, F.; Moraga, I.; Santos, J . G. J .
Org. Chem. 1993, 58, 5400.
15) Oh, H. K.; Lee, J . Y.; Yun, J . H.; Park, Y. S.; Lee, I. Int. J .
Chem. Kinet. 1998, 30, 419.
16) Castro, E. A.; Cubillos, M.; Mu n˜ oz, G.; Santos, J . G. Int. J .
Chem. Kinet. 1994, 26, 571.
17) Castro, E. A.; Iba n˜ ez, F.; Santos, J . G.; Ureta, C. J . Org. Chem.
993, 58, 4908.
18) Pearson. R. G. J . Chem. Educ. 1968, 45, 581. Pearson, R. G. J .
Org. Chem. 1989, 54, 1423.
19) Castro, E. A.; Iba n˜ ez, F.; Santos, J . G.; Ureta, C. J . Chem. Soc.,
(
2
(20) It is expected that the values of both k-1 and k be smaller for
the T( formed in the aminolysis of PClF than that in the same
(
aminolysis of methyl chloroformate since the push exerted by PhO from
(
to expel both the amine and Cl- should be weaker than that by
T
(
MeO from the similar intermediate. This is due to the fact that
21
1
R R
σ (PhO) ) -0.40 and σ (MeO) ) -0.56, which means that PhO
(
(
should be less electron donating than MeO from T and therefore the
push provided by the former group should be weaker than that by the
(
latter. The same should apply to 4-nitrophenoxy as the “acyl” group of
in view of the stronger electron-withdrawing effect of this group
compared to PhO.
(
Perkin Trans. 2 1991, 1919. Hill, S. V.; Thea, S.; Williams, A. J . Chem.
Soc., Perkin Trans. 2 1983, 437. Kwon, D. S.; Park, H. S.; Um, I. H.
Bull. Korean Chem. Soc. 1991, 12, 93.
T
(21) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.