Eqn. (2) is formally analogous to eqn. (1), commonly used for
the mechanism depicted in Scheme 1. Fig. 2 illustrates the influence
of glyme concentration on the a and b terms of eqn. (2). As can be
observed, the a term exhibits a linear dependence on [Glyme]. This
result is consistent with the mechanism generally accepted for the
catalysis by crown ethers and glymes of the aminolysis of
carboxylic esters.
Eqn. (3) accounts for the intercepts in the plots of the influence
of [Glyme] on the b term (Fig. 2), which correspond to the base-
¡
catalyzed decomposition of the intermediate, T . Application of
eqn. (3) leads to the values of k for the butylaminolysis of
B
nitrophenyl acetate and caprate respectively. The rate constants
can be determined for this new reaction pathway which shows base
and glyme catalysis simultaneously. The values of k
butylaminolysis of nitrophenyl acetate and caprate respectively can
be obtained from eqn. (3) and Fig. 2. Values of k , k and k are
D
for the
The mechanism shown in Scheme 1 predicts that the b term
should be independent of the glyme concentration. However, the
results reported in Fig. 2 for the butylaminolysis of 4-nitrophenyl
caprate and 4-nitrophenyl acetate indicate that this term shows a
linear dependence on glyme concentration.
C
B
D
shown in Table 1. A tentative explanation on how a butylamine
¡
molecule can attach G?T to help the reaction is that one of the
two nitrogen–hydrogen bonds breaks in all cases, but frequently it
¡
reverses and reforms the G?T complex (since the entities are still
The mechanism presented in Scheme 1 needs to be further
developed in order to explain the observed kinetic data. The
presence of a kinetic term showing a second-order dependence on
butylamine concentration and a first-order dependence on glyme
concentration may be accounted for by the possibility that the
bonded together). In turn decomplexation can be assisted
(enforced) by transfer of the proton (removed by the glyme from
the nitrogen) to a butylamine molecule, then followed by
decomplexation as the favoured pathway and regeneration of
the carbonyl group and loss of the aryloxide group, in preference
to loss of the butyl amino group, giving the aminolysis product.
From the reaction scheme illustrated in Scheme 2, the following
microscopic rate equation can be obtained in terms of the
¡
G?T complex may decompose by a base-catalyzed pathway. The
proposed mechanism is shown in Scheme 2 along with the new
reaction pathway. From this scheme, the following rate equation
can be obtained. Previous studies have investigated the influence of
glymes or crown ethers on the butylaminolysis rate constant
keeping constant the butylamine concentration. In this way only a
term that is first order in glyme or crown ether concentration can
be detected.
¡
¡
equilibrium formation constants of T and the G?T complex.
1
T TG
2
obs 5 kGK K [Glyme][BuNH ] +
k
(
4)
T
2
T
TG
2
(
k2K + k K K [Glyme])[BuNH2]
G
2
k
obs 5 k
C
[Glyme][BuNH
2
] + (k
B
+ k
D
[Glyme])[BuNH
2
]
(3)
This expression is analogous to eqn. (3). Therefore, it is possible
to estimate the microscopic meaning of the constants k , k and
B
C
Rate constants, k , for the glyme-catalyzed butylaminolysis of
C
T
K , k
1
G
T
5 k K K and k
TG
2
G
T
5 K K K . From the
TG
k : k
D B
5 k
2
C
D
nitrophenyl acetate and nitrophenyl caprate, can be determined
quotient between the values of k
C
and k , the relationship between
D
from the influence of glyme concentration on the a term (Fig. 2).
the spontaneous and base-catalyzed rate constants for the
¡
decomposition of G?T can be obtained: k /kD 5 k /k . In all
1
G
2
G
C
cases, the butylaminolysis rate of 4-nitrophenyl acetate is observed
to be faster than that for 4-nitrophenyl caprate. The rate of the
¡
butylamine-catalyzed decomposition of the intermediate T , k , is
B
3
times faster for acetate than it is for caprate. Likewise, the rate of
¡
spontaneous decomposition of the G?T complex, k , is also 3.6
C
times faster for acetate than for caprate. This difference in
reactivity decreases when considering the butylamine-catalyzed
¡
decomposition pathway of the G?T complex, k , showing that
D
the rate constant is 1.5 times larger for acetate than it is for
caprate. This result reveals that in both catalytic pathways
complexation of the phase transfer agent takes place on the
ammonium moiety.
The values obtained for the butylaminolysis of 4-nitrophenyl
1
2
acetate, k
C D
G G
/k 5 k /K 5 3.68 M, and 4-nitrophenyl caprate,
1
2
k
C D
/k 5 k /k 5 1.56 M, suggest that the decomposition
G
G
Fig. 2 Influence of glyme concentration on the a and b terms (eqn. (2))
for the butylaminolysis of 4-nitrophenyl caprate (a: #, b: $) and
4-nitrophenyl acetate (a: %, b: &).
pathway of the glyme-intermediate complex to spontaneously give
the reaction products is favored against the pathway in which this
complex is assisted by an amine molecule to give the corresponding
amide.
Table 1 Rate constants for the glyme-catalyzed butylaminolysis of
p-nitrophenyl acetate and p-nitrophenyl caprate
2
/M
2
21
s
22 21
s
23 21
s
k
C
k
B
/M
k
D
/M
2
1
22
22
22
NPA
NPC
3.10 6 10
8.59 6 10
6.94 6 10
2.29 6 10
8.43 6 10
5.51 6 10
22
22
Scheme 2
3
818 | Chem. Commun., 2005, 3817–3819
This journal is ß The Royal Society of Chemistry 2005