Kinetics and mechanism of large rate enhancement in the alkaline hydrolysis of N′-morpholino-N-(2′-methoxyphenyl)phthalamide

Article abstract of DOI:10.1021/jo702695k

(Chemical Equation Presented) The apparent second-order rate constant (k_OH) for hydroxide-ion-catalyzed conversion of 1 to N-(2′-methoxyphenyl)phthalamate (4) is ～10³-fold larger than k_OH for alkaline hydrolysis of N-morpholinobenzamide (2). These results are explained in terms of the reaction scheme 1→ ^k1obs3→k^2obs4 where 3 represents N-(2′- methoxyphenyl)phthalimide and the values of k_2obs/k_1obs vary from 6.0 × 10² to 17 × 10² within [NaOH] range of 5.0 × 10^-3 to 2.0 M. Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of 1 decrease from 21.7 × 10^-3 to 15.6 × 10^-3 s^-1 with an increase in ionic strength (by NaCl) from 0.5 to 2.5 M at 0.5 M NaOH and 35°C. The values of k_obs, obtained for alkaline hydrolysis of 2 within [NaOH] range 1.0 × 10^-2 to 2.0 M at 35°C, follow the relationship k_obs = k_OH[HO^-] + k_OH′[HO ^-]² with least-squares calculated values of k_OH and k_OH′ as (6.38 ± 0.15) × 10^-5 and (4.59 ± 0.09) × 10^-5 M^-2 s^-1, respectively. A few kinetic runs for aqueous cleavage of 1, N′-morpholino- N-(2′-methoxyphenyl)-5-nitrophthalamide (5) and N′-morpholino-N- (2′-methoxyphenyl)-4-nitrophthalamide (6) at 35°C and 0.05 M NaOH as well as 0.05 M NaOD reveal the solvent deuterium kinetic isotope effect (= k_obs^H2O/k_obs^D2O) as 1.6 for 1, 1.9 for 5, and 1.8 for 6. Product characterization study on the cleavage of 5, 6, and N-(2′-methoxyphenyl)-4-nitrophthalimide (7) at 0.5 M NaOD in D ₂O solvent shows the imide-intermediate mechanism as the exclusive mechanism.

Full text of DOI:10.1021/jo702695k

Kinetics and Mechanism of Large Rate Enhancement in the
Alkaline Hydrolysis of
N′-Morpholino-N-(2′-methoxyphenyl)phthalamide
Yoke-Leng Sim, Azhar Ariffin, and M. Niyaz Khan*
Department of Chemistry, Faculty of Science, UniVersity of Malaya, 50603 Kuala Lumpur, Malaysia
niyaz@um.edu.my
ReceiVed December 18, 2007
The apparent second-order rate constant (kOH) for hydroxide-ion-catalyzed conversion of 1 to N-(2′-
methoxyphenyl)phthalamate (4) is ∼103-fold larger than kOH for alkaline hydrolysis of N-morpholinoben-
k1obs k2obs
zamide (2). These results are explained in terms of the reaction scheme 1 f 3 f 4 where 3 represents
N-(2′-methoxyphenyl)phthalimide and the values of k2obs/k1obs vary from 6.0 × 102 to 17 × 102 within
[NaOH] range of 5.0 × 10-3 to 2.0 M. Pseudo-first-order rate constants (kobs) for alkaline hydrolysis of
1 decrease from 21.7 × 10-3 to 15.6 × 10-3 s-1 with an increase in ionic strength (by NaCl) from 0.5
to 2.5 M at 0.5 M NaOH and 35 °C. The values of kobs, obtained for alkaline hydrolysis of 2 within
[NaOH] range 1.0 × 10-2 to 2.0 M at 35 °C, follow the relationship kobs ) kOH[HO-] + kOH′[HO-]2 with
least-squares calculated values of kOH and kOH′ as (6.38 ( 0.15) × 10-5 and (4.59 ( 0.09) × 10-5 M-2
s-1, respectively. A few kinetic runs for aqueous cleavage of 1, N′-morpholino-N-(2′-methoxyphenyl)-
5-nitrophthalamide (5) and N′-morpholino-N-(2′-methoxyphenyl)-4-nitrophthalamide (6) at 35 °C and
H2O
0.05 M NaOH as well as 0.05 M NaOD reveal the solvent deuterium kinetic isotope effect () kobs
/
kobsD O) as 1.6 for 1, 1.9 for 5, and 1.8 for 6. Product characterization study on the cleavage of 5, 6, and
N-(2′-methoxyphenyl)-4-nitrophthalimide (7) at 0.5 M NaOD in D2O solvent shows the imide-intermediate
mechanism as the exclusive mechanism.
2
Introduction
at 80 °C. The intrinsic reactivity toward oxygen nucleophiles
of formamide is found to be larger by more than 104-fold
compared to that of benzamide, N-alkyl-substituted benzamides,2–4
and benzanilide.5 Thus, the values of kw for benzamide,
benzanilide, and proteins should be significantly smaller than
10-9 s-1 at 35 °C (i.e., half-life of >22 years).
The study of the mechanistic aspects of the aqueous cleavage
of an amide bond under a variety of reaction conditions could
be considered of significant importance because the hydrolytic
degradation of protein involves apparently the hydrolysis of an
essentially N-alkylamide bond.1 The respective values of second-
order rate constants for hydroxide ion (kOH) and hydronium ion
catalyzed (kH) hydrolysis of an N-alkylamide bond are in the
order of 10-6 and 10-7 M-1 s-1 at 35 °C.2,3 There seems to be
only one report4 that describes a careful determination of the
pseudo first-order rate constant (kw) for the uncatalyzed reaction
of water with formamide and the value of kw is 8.4 × 10-8 s-1
The apparent specific base catalysis (i.e., kOH value) is
increased by ∼106-fold in the alkaline hydrolysis of phthalamide
(kOH ) 4.9 M-1 s-1)6 and N,N′-dimethylphthalamide (kOH
)
7.6 M-1 s-1)6 compared to that of benzamide and N-methyl-
benzamide due to occurrence of different catalytic reaction
mechanisms in the alkaline hydrolysis of phthalamide and
benzamide. Since the appearance of the classical paper of Shafer
(1) Fersht, A. R. Enzyme Structure and Mechanism; W. H. Freeman: San
Francisco, 1977.
(2) Bunton, C. A.; Noyak, B.; O’Connor, C. J. J. Org. Chem. 1968, 33, 572.
(3) Bunton, C. A.; Farbers, S. J.; Milbank, A. J. G.; O’Connor, C. J.; Turkey,
T. A. J. Chem. Soc., Perkin Trans. 2 1972, 1869.
(4) Hine, J.; King, R. S.-M.; Midden, R.; Sinha, A. J. Org. Chem. 1981, 46,
3186.
(5) Broxton, T. J.; Duddy, N. W. Aust. J. Chem. 1979, 32, 1717.
3730 J. Org. Chem. 2008, 73, 3730–3737
10.1021/jo702695k CCC: $40.75 2008 American Chemical Society
Published on Web 04/15/2008

Alkaline Hydrolysis of N′-Morpholino-N-(2′-methoxyphenyl)phthalamide
FIGURE 1. Plots showing the dependence of pseudo-first-order rate constants, kobs, versus [NaOH] for alkaline hydrolysis of 1 (9, with 103 kobs
and 5.0 × 10-4 M 1) and 2 (2, with 105 kobs and 6.0 × 10-4 M 2) at 35 °C. The solid lines are drawn through the least-squares calculated data points
using eq 1 (for 9) and eq 2 (for 2), respectively.
and Morawetz,6 the reaction mechanisms of intramolecular
carboxamido group assisted alkaline hydrolysis of another amide
group and related reactions have been extensively studied.7
Although many facets of these reactions are well understood,
no attempt seems to have been made to understand the effects
of steric crowding of the intramolecular nucleophilic site on
the intramolecular rate assistance in these reactions. The values
of kOH for the cleavage of amide and ester bonds of
2-H2NCOC6H4CO2Me, 2-H2NCH2C6H4CO2Me, 2-H2NCH2C6-
H4CONH2, and 2-HOCH2C6H4CONH2 are 3.1 × 103 (25.9 °C),6
7 × 103 (30 °C),8 0.16 (30 °C),9 and 0.15 M-1 s-1 (30 °C),10
respectively. These results show that the values of kOH are
weakly and highly dependent on pKa values of the conjugate
acids of intramolecular nucleophile and leaving groups, respec-
tively. The present study was initiated (i) to find out whether
the alkaline aqueous cleavage of amide bond of 1 involves the
intramolecular carboxamido group assistance and (ii) to discover
the effect of molecular crowding at the nucleophilic site on the
rate of intramolecular carboxamide-assisted cleavage of amide
bond. The results and their probable mechanistic explanations
are described.
Results and Discussion
Effects of [NaOH] on Pseudo-First-Order Rate Constants
(kobs) for the Amide Bond Cleavage of 1. A series of kinetic
runs was carried out for the alkaline aqueous cleavage of 1
within [NaOH] range 5.0 × 10-3 to 2.0 M at 35 °C where the
ionic strength was kept constant at 1.0 and 2.0 M (by NaCl)
for kinetic runs within [NaOH] range 5.0 × 10-3 to 1.0 M and
1.2-2.0 M, respectively. The observed data (kobs versus
[NaOH]) are shown graphically by Figure 1. The values of kobs
were found to fit reasonably well to the following empirical
equation:
R[HO-]
kobs
)
(1)
1 + ꢀ[HO-]
(6) Shafer, J. A.; Morawetz, H. J. Org. Chem. 1963, 28, 1899.
(7) (a) Chiong, N. G.; Lewis, S. D.; Shafer, J. A. J. Am. Chem. Soc. 1975,
97, 418. (b) Belke, C. J.; Su, S. C. K.; Shafer, J. A. J. Am. Chem. Soc. 1971, 93,
4552. (c) Fife, T. H.; Duddy, N. W. J. Am. Chem. Soc. 1983, 105, 74. (d) Fife,
T. H.; Chauffe, L. J. Org. Chem. 2000, 65, 3579. (e) Fife, T. H.; Singh, R.;
Bembi, R. J. Org. Chem. 2002, 67, 3179.
where R and ꢀ represent empirical constants. Nonlinear least-
squares calculated values of R and ꢀ are (6.32 ( 0.30) × 10-2
M-1 s-1 and 1.28 ( 0.10 M-1, respectively. Although the
change in ionic strength from 1.0 to 2.0 M seems to have
kinetically insignificant effect on kobs, the values of kobs obtained
at 1.0 M ionic strength only were also treated with eq 1. The
(8) Fife, T. H.; DeMark, B. R. J. Am. Chem. Soc. 1976, 98, 6978.
(9) Fife, T. H.; DeMark, B. R. J. Am. Chem. Soc. 1977, 99, 3075.
(10) Okuyama, T.; Schmir, G. L. J. Am. Chem. Soc. 1972, 94, 8805.
J. Org. Chem. Vol. 73, No. 10, 2008 3731

Sim et al.
SCHEME 1
SCHEME 3
SCHEME 2
out the occurrence of reaction mechanism shown in Scheme 1
because the pKa of morpholine may not be expected to be
significantly lower than that of N-methylaniline.
nonlinear least-squares calculated values of R and ꢀ are (5.89
( 0.18) × 10-2 M-1 s-1 and 1.10 ( 0.08 M-1, respectively.
These results show that the values of R and ꢀ decreased by 7%
and 14%, respectively, with the change in ionic strength from
2.0 to 1.0 M.
To discover the effects of ionic strength, µ, a few kinetic
runs were carried out at 0.5 M NaOH, 35 °C and within [NaCl]
range 0.0-2.0 M. The values of kobs revealed an almost linear
decrease with an intercept and slope of (23.5 ( 0.2) × 10-3
s-1 and -(3.12 ( 0.13) × 10-3 M-1 s-1, respectively. These
results show that the increase in µ from 1.0 to 2.0 M decreased
kobs by ∼16%.
Furthermore, these reported values of kOH for the cleavage
of amide and ester bonds predict that the value of kOH for the
alkaline hydrolysis of 2 should be comparable with kOH for 1 if
hydrolysis of both 1 and 2 involves a reaction mechanism similar
to that in Scheme 1 with Ki1 ) 0 for 2. To test this prediction
practically, a few kinetic runs were carried out for alkaline
hydrolysis of 2 within [NaOH] range 1.0 × 10-2 to 2.0 M at
35 °C, 2.0 M ionic strength (by NaCl), and 6.0 × 10-4 M 2.
The observed pseudo-first-order rate constants (kobs) are shown
graphically in Figure 1. These values of kobs were fitted to eq 2:
kobs ) kw + kOH[HO-] + kOH′[HO-]2
(2)
Effects of [NaOH] on kobs for Alkaline Hydrolysis of
N-Morpholinobenzamide (2). On the basis of product char-
acterization and the fact that the observed data (Aobs versus time,
t) of alkaline hydrolysis of 1 followed a strictly simple first-
order rate law, one might argue in the favor of the occurrence
of a reaction mechanism as shown in Scheme 1. However, the
calculated value of kOH (≡ R ) 5.89 × 10-2 M-1 s-1 at 35 °C)
for 1 is significantly larger than kOH for alkaline hydrolysis of
N-methylbenzanilide5 (kOH ) 1.10 × 10-4 M-1 s-1 at 65.5 °C)
and the values of kOH for benzamide, N-methylbenzamide, and
N,N-dimethylbenzamide at 100.4 °C are 15.7 × 10-4, 7.20 ×
10-4, and 15.2 × 10-4 M-1 s-1, respectively.2 The values of
kOH for hydrolysis of C6H5CO2Me,12 2-MeO2CC6H4CO2Me,11
2-Me2NCONHC6H4CO2Me,122-MeOC6H4CO2Me,13C6H5CONH2,2,14
and 2-MeOC6H4CONH214 are 0.125 (30 °C),12 0.130 (25 °C)11,
0.263 (30 °C),12 0.062 (35 °C),13 1.58 × 10-3 (100.4 °C),2
∼1.10 × 10-3 (100 °C),14 and 1.10 × 10-3 M-1 s-1 (100 °C),14
respectively. In view of these results, the >103-fold larger value
of kOH for 1 compared with that for N-methylbenzanilide rules
where kw, kOH, and kOH′ represent uncatalyzed first-order,
hydroxide ion catalyzed second-order, and third-order rate
constants, respectively. The least-squares calculated respective
values of kw, kOH, and kOH′ are (5.15 ( 7.48) × 10-7 s-1, (6.27
( 0.22) × 10-5 M-1 s-1, and (4.64 ( 0.11) × 10-5 M-2 s-1
.
The value of kw with a standard deviation of more than 100%
is considered to be unreliable. The respective contributions of
the kw term and the kOH′[HO-]2 term are e14% at g0.05 M
NaOH and e3% at e0.05 M NaOH. Thus, a relatively more
reliable value of kw may be expected to result if the data
treatment with eq 2 involves kobs values at e0.05 M NaOH.
The least-squares fit of kobs values (at e0.05 M NaOH) to eq 2
with kOH′ ) 0 gave kw and kOH as (-3.2 ( 0.6) × 10-7 s-1 and
(7.93 ( 0.18) × 10-5 M-1 s-1, respectively. The negative value
of kw shows that the kw value is not different from zero under
the experimental condition of the study.
The kobs values were also treated with eq 2 where kw was set
to zero, and the least-squares calculated values of kOH and kOH
′
turned out to be (6.38 ( 0.15) × 10-5 M-1 s-1 and (4.59 (
0.09) × 10-5 M-2 s-1, respectively. These calculated values of
kOH and kOH′ are changed by only e1.8% compared to those
obtained from eq 2 with kw ) 5.15 × 10-7 s-1. The extent of
(11) Khan, M. N. Indian J. Chem. 1986, 25A, 831.
(12) Hegarty, A. F.; Bruice, T. C. J. Am. Chem. Soc. 1970, 92, 6575.
(13) Khan, M. N.; Olagbemiro, T. O. J. Org. Chem. 1982, 47, 3695.
(14) Bruice, T. C.; Tanner, D. W. J. Org. Chem. 1965, 30, 1668.
3732 J. Org. Chem. Vol. 73, No. 10, 2008

Alkaline Hydrolysis of N′-Morpholino-N-(2′-methoxyphenyl)phthalamide
reliability of the observed data fit to eq 2 is evident from the
standard deviations (2%) associated with the calculated values
of kOH and kOH′ and from the plot of Figure 1 where a solid
line is drawn through the least-squares calculated data points.
The value of kOH (6.38 × 10-5 M-1 s-1) for 2 is only ∼1.7-
fold smaller than kOH value (11.0 × 10-5 M-1 s-1 at 65.5 °C)
for N-methylbenzanilide.5 However, the value of kOH for 2 is
∼103-fold smaller than kOH (≡ R ) 5.89 × 10-2 M-1 s-1 at 35
°C) for 1. Thus, it is almost certain that different reaction
mechanisms are involved in the alkaline hydrolysis of 1 and 2.
Mechanistic Discussion of Alkaline Hydrolysis of 2. The
kinetic term similar to kOH′[HO-]2 of eq 2 was the basis for
Biechler and Taft to propose for the first time in 1957 the
presence of a highly reactive oxy dianionic tetrahedral inter-
mediate, T, on the reaction path in the hydrolysis of acetanilide
under highly alkaline medium.15 The kinetic equation similar
to eq 2 with kw ) 0 has been used in the kinetic studies on
alkaline hydrolysis of ꢀ-sulfam16a and several amides and
imides.16 In view of these studies, a plausible reaction mech-
explained in terms of a plausible reaction mechanism as shown
in Scheme 3. Perhaps it is noteworthy that the rate law: rate )
kOH[1][H2O] (the corresponding reaction mechanism is shown
in Scheme 3) is kinetically indistinguishable from the rate law:
rate ) kOH′[1][HO-] (the corresponding reaction mechanism is
not shown). However, on the basis of arguments described
elsewhere,19 the preferred reaction mechanism of the present
3
reacting system is shown in Scheme 3, where the k2 -step is
considered rate-determining.
The value of the second-order rate constant (kOH) for
hydroxide ion catalyzed hydrolysis of 3 is 29.1 M-1 s-1 20 under
the present reaction conditions. Thus, the values of k33 vary from
14.6 × 10-2 to 58.2 s-1 within [NaOH] range 5.0 × 10-3 to
3
2.0 M of the study. Thus, the values of k3 /kobs vary from 6.0
× 102 to 17 × 102, and consequently 3 cannot be detected
directly by any conventional method under the present reaction
conditions. A skeptic might argue that nondetectability of 3 does
not essentially prove or disprove the validity of the reaction
mechanism of Scheme 3. Although it seems difficult to offer a
satisfactory and convincing explanation to this skepticism, it is
difficult also to ignore or reject the possibility of the formation
of 3 on the reaction path for two reasons: (i) We could not find
any other plausible reaction mechanism that could explain the
formation of 4 with a rate larger than the rate of alkaline
hydrolysis of 2 by a factor of ∼103-fold. (ii) Several reports on
closely related reactions revealed spectrophotometric evidence
fortheformationofphthalimideanditsN-substitutedderivatives.18,19
The observed rate law, rate ) kobs [1]T, and Scheme 3 can
lead to eq 4:
anism for hydrolysis of 2, under highly alkaline medium, may
2
2
be shown in Scheme 2 where the k2 -step and the k3 -step are
presumably rate-determining. The observed rate law: rate )
kobs[2]T and Scheme 2 can lead to eq 3:
3
3
k2 K1 K′i3[HO-]
kobs
)
(4)
1 + K′i3[HO-]
2
2
2
2
kobs ) k2 K1 [HO-] + k3 Ki2K1 [HO-]2
(3)
where K1 ) k1 /k-1 and Ki2 ) ki2/k-i2. Equation 3 is similar
where K′i3 ) Ki3/[H2O] ) Ka /Kw with Ka1 ) [1-][H+]/[1], and
1
2
2
2
3
3
K1 ) k1 /k-13. Comparison of eq 4 with eq 1 gives R )
to eq 2 with kw ) 0, kOH ) k2 K1 , and kOH′ ) k3 Ki2K1 . The
2
2
2
2
3
3
1
k2 K1 K′i3 and ꢀ ) K′i3 ) Ka /Kw. The significantly larger pKa
2
2
assumption that the k2 -step and the k3 -step are rate-determining
3
of the conjugate acid of the leaving group in the k2 -step
would be correct only if k-12 > k2 , k-12 > ki2[HO-], and k-i
2
2
compared with that in the k-13-step and the release of the five-
membered ring strain in the k-13-step cause k-13 . k23 and this
> k3 . The pKa of T1- may be estimated as 14.8 (see Appendix
in the Supporting Information, SI) and hence the ki2-step and
the k-i2-step involve proton transfer in the thermodynamically
favorable and unfavorable direction, respectively. Under such
2
3
inequality validates the assumption that the k2 -step is rate-
determining. The calculated value of ꢀ () Ka /Kw ) 1.10 M-1
)
1
gives pKa1 ) 13.6 with pKw ) 13.62.21 The value of pKa1 may
be compared with the reported pKa of benzamide of 14-1522
and σI for H and C6H5 as 0.0 and 0.12,23 respectively. The pKa
of benzamide is also expected to be reduced by the replacement
of o-H by o-CON(CH2CH2)2.
situation, the values of ki2 and k-i should be ∼1010 and ∼109
2
M-1 s-1, respectively.17 Since k-12 must be larger than ki2 [HO-]
and k22 because the k1 -step cannot be rate-determining and the
2
maximum value of ki2 [HO-] ) 2 × 1010 s-1 (maximum [HO-]
) 2.0 M), therefore k-1 should be larger than 2 × 1010 s-1
.
2
The experimentally determined second-order rate constants,
The presence of equilibrium between T1- and T22- (Scheme 2)
3
3
kOH, (≡ R ) k2 K1 K′i3) for hydroxide ion catalyzed cyclization
reactions of methyl o-carbamoylbenzoate, methyl o-aminom-
ethylbenzoate, phthalamide, N,N′-dimethylphthalamide, and
o-aminomethylbenzamide are 3.1 × 103 (25.9 °C),6 7 × 103
(30 °C),8 4.9 (25.9 °C),6 7.6 (25.9 °C),6 and 0.16 M-1 s-1 (30
°C),9 respectively. It appears from these results that the pKa of
the neutral nucleophile has essentially no effect on the kOH value,
whereas the pKa of the conjugate acid of the leaving group has
requires that ki2 [HO-] > k2 . Thus, the expected inequality of
2
k-1 > ki2 [HO-] > k2 shows that the k2 -step is rate-
determining.
2
2
2
Mechanistic Discussion of Alkaline Hydrolysis of 1. Phthal-
amide, N,N′-di-, and trisubstituted phthalamides are known to
undergo efficient hydroxide ion catalyzed intramolecular cy-
clization to form respective phthalimide and N-substituted
phthalimides in aqueous solution.18 The nearly 103-fold larger
rate of alkaline hydrolysis of 1 compared with that of 2 may be
(18) (a) Khan, M. N. J. Chem. Soc., Perkin Trans. 2 1990, 435. (b) Khan,
M. N. Indian J. Chem. 1991, 30A, 777. (c) Khan, M. N. Indian J. Chem. 1994,
33B, 646.
(15) Biechler, S. S.; Taft, R. W., Jr. J. Am. Chem. Soc. 1957, 79, 4927.
(16) (a) Baxter, N. J.; Rigoreau, L. J. M.; Laws, A. P.; Page, M. I. J. Am.
Chem. Soc. 2000, 122, 3375. (b) Khan, M. N. Micellar Catalysis. In Surfactant
Science Series; CRC Press, Taylor & Francis Group, LLC: Boca Raton, FL,
2006; Vol. 133,Chapters 2 and 7 and references therein.
(17) Fife, T. H.; Bambery, R. J.; DeMark, B. R. J. Am. Chem. Soc. 1978,
100, 5500. and references therein.
(19) Khan, M. N. J. Chem. Soc., Perkin Trans. 2 1988, 213.
(20) Sim, Y.-L.; Ariffin, A.; Khan, M. N. J. Org. Chem. 2007, 72, 2392.
(21) Ritchie, C. D.; Wright, D. J.; Huang, D.-S.; Kamego, A. A. J. Am. Chem.
Soc. 1975, 97, 1163.
(22) Barlin, G. B.; Perrin, D. D. Quart. ReV. Chem. Soc. 1966, 20, 75.
(23) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
J. Org. Chem. Vol. 73, No. 10, 2008 3733

Sim et al.
SCHEME 4
a significant effect on kOH values. These results are consistent
with the reaction mechanism shown in Scheme 3. The value of
kOH () 5.89 × 10-2 M-1 s-1) for 1 at 35 °C is nearly 40- and
20-fold smaller than kOH for phthalamide (at 25.9 °C)6 and
N-piperidinylphthalamide (at 35 °C),24 respectively. These
results may be explained in terms of larger steric crowding at
the intramolecular nucleophilic site of 1 than that of phthalamide
and N-piperidinylphthalamide.
The rate of alkaline aqueous cleavage of 5 was studied at 0.05
M NaOH and 0.05 M NaOD in the respective H2O and D2O
solvents containing 3% v/v CH3CN and 1.5 × 10-4 M 5 at 1.0 M
ionic strength (by NaCl). These kinetic runs yielded the values of
pseudo-first-order rate constants, kobsH O and kobsD O as (1.47 ( 0.04)
× 10-3 s-1 and (8.36 ( 0.34) × 10-4 s-1, respectively. Similar
kinetic runs, under almost identical conditions, were carried out
2
2
H2O
D2O
with 6 and gave 103kobs
) 2.10 ( 0.04 s-1 and 103kobs
)
Alkaline Aqueous Cleavage of Amide Bond of 5 and 6
at 35 °C. Evidence other than kinetics for intermediacy of imide
(3), based upon a synthetic approach, may be described as
follows. The idea behind these synthetic experiments is that the
symmetrical imide, such as 3, contains two identical electrophilic
carbonyl groups and as a consequence the nucleophilic attack
by HO-/H2O can occur equally well at these two identical
electrophilic centers, producing equimolar hydrolysis products
that are the same (4). With the addition of the nitro group at
the 4-position in the imide (7), the hydrolysis products 8 and 9
are distinguishable (Scheme 4). If the imide-intermediate
mechanism is the exclusive mechanism, then the same ratio of
8 and 9 would be formed for each isomer (5 and 6) as shown
in Scheme 4. If instead (i) amide anions were involved in an
intramolecular general base-assisted hydrolysis or (ii) mecha-
nism involving intermediacy of N-arylisophthalimide (10) (these
alternative mechanisms are not entirely and unambiguously
excluded by kinetic data), then only one product would be
observed from each isomer (5 and 6), i.e. only 8 from 5 and
only 9 from 6 (Scheme 4).
1.11 ( 0.02 s-1. In order to compare the solvent deuterium kinetic
isotope effects (dKIE) for the cleavage of 5 and 6 with that of 1,
two kinetic runs for the aqueous cleavage of 1 were carried out at
35 °C, 0.05 M NaOH and 0.05 M NaOD in the respective H2O
and D2O solvents containing 2% v/v CH3CN and 5.0 × 10-4
M
H2O
1 at 1.0 M ionic strength (by NaCl). These kinetic runs gave kobs
and kobsD O as (2.83 ( 0.03) × 10-3 s-1 and (1.72 ( 0.02) × 10-3
2
s-1, respectively. The value of dKIE ) kobsH O/kobsD O ) 1.6, which
2
2
H2O
may be compared with dKIE for 5 and 6. The value of kobs
/
D2O
kobs
(1.8) for 5 is similar to that for 6 (1.9). These values of
dKIE may be compared with dKIE of 1.2 reported for hydroxide
ion catalyzed cyclization of methyl 2-aminomethylbenzoate.8 These
results support for the occurrence of similar mechanism in the
aqueous cleavage of 5 and 6 and rule out the mechanism involving
intermediacy of 10. In view of eq 1 and Scheme 3, kobs ) k23K13Ki′3
H2O
because, at 0.05 M NaOH, 1 + Ki′3[HO-] ≈ 1 and hence kobs
/
kobsD O ) {(k23K13Ka1)H OKwD O}/{(k23K13Ka1)D OKwH O} where Ki′3
2
2
2
2
2
D2O
) Ka1/Kw and KwH O ) aHaOH. Using the values of KwH O/Kw
2
2
) 6.5,25 Ka1 H O/Ka1 D O ) 4.2,26 and kobsH O/kobs
) 1.6, one
D2O
2
2
2
(25) Bender, M. L.; Kezdy, F. J.; Zerner, B. J. Am. Chem. Soc. 1963, 85,
3017.
(24) Khan, M. N. Colloids Surfactants A 2001, 181, 99.
3734 J. Org. Chem. Vol. 73, No. 10, 2008

Alkaline Hydrolysis of N′-Morpholino-N-(2′-methoxyphenyl)phthalamide
FIGURE 2. 1H NMR peaks for 3-H (d), 6-H (d), and OMe (two overlapping singlet, s) of 8 and 9 and two sets of methylene protons (t) of
morpholine, O(CH2)2 and N(CH2)2, for hydrolysis products of 5, 6, and 7 (where equimolar morpholine was externally added to the hydrolysis
products of 7). The symbol xHy represents the H attached to the x-C of compound y.
gets (k2 K13)H O/(k2 K13)D O ) 2.5. The dKIE of magnitude of 2.5
is consistent with the proposal of k23 as the rate-determining step
in Scheme 3.
the m-position with respect to the electrophilic center). However,
different resonance effects of the NO2 group on nucleophilic
centers in 5 and 6 are not expected to have any significant effects
on kobs values because the reported results on closely related
reactions show that kobs values are almost independent and
highly dependent on the respective pKa of conjugate acids of
nucleophiles and leaving groups.6,8
3
3
2
2
The nearly 40% larger value of kobs for 6 (103 kobs ) 2.10
s-1) compared with that for 5 (103 kobs ) 1.47 s-1) is plausible
in view of the fact that the powerful electron-withdrawing
resonance effect of the NO2 group activates the electrophilic
center more strongly in 6 than in 5 (where the NO2 group is at
To characterize the alkaline aqueous degradation products
1
(26) Capon, B.; Ghosh, B. C. J. Chem. Soc. B 1966, 472.
of 5, 6, and 7 by the use of H NMR spectrometric technique,
J. Org. Chem. Vol. 73, No. 10, 2008 3735

Sim et al.
TABLE 1. Characteristic Peaks with Integration (In) of Different Chemical Shifts (δH) for 8, 9, and Morpholine Obtained from Alkaline
Aqueous Cleavage of 5, 6, and 7
8
9
8 + 9
δOMe
morpholine
δ3H
δ6H
δ3H
δ6H
δ
δ
O(CH2)2
N(CH2)2
a
b
c
d
e
f
g
h
substrate ppm In3H ppm In6H ppm In3H ppm In6H ppm InOMe ppm InOCH ppm InNCH R1
R2
R3
R4
R5
R6 R7
R8
5
7.77 1.00 8.43 0.79 8.47 1.37 7.72 1.63 3.84 7.81
7.66 1.00 8.33 0.80 8.36 1.30 7.62 1.61 3.74 7.94
7.68 1.00 8.35 0.83 8.38 1.47 7.63 1.65 3.76 7.87
3.67 11.47 2.78 11.10 1.73 1.63 0.91 0.94 0.92 0.75 0.95 0.78
3.57 10.36 2.68 11.51 1.63 1.61 1.02 0.92 1.01 0.81 0.91 0.73
3.59 10.80 2.70 10.80 1.77 1.65 0.97 0.97 0.98 0.85 0.98 0.85
6
7i
b
c
d
e
f
a R1 ) (In3H)9/(In6H)8. R2 ) (In6H)9/(In3H)8. R3 ) [4(InOMe 8+9]/[3(InOCH)]. R4 ) [4(InOMe 8+9]/[3(InNCH)]. R5 ) 4Y1/(InOCH). R6 ) 4Y2/(InOCH).
) )
g R7 ) 4Y1/(InNCH). h R8 ) 4Y2/(InNCH); Y1 ) (In3H)8 + (In6H)9 and Y2 ) (In6H)8 + (In3H)9. i Equimolar morpholine was externally added to the
product mixture.
these reactions were carried out in D2O under reaction conditions
similar to those for the kinetic runs (mentioned earlier) except
that the initial concentrations of 5, 6, and 7 were kept at 1.39
× 10-2 M and the hydroxide ion concentration at 0.50 M. The
reactions were allowed to progress for >10 half-lives. Then the
1H NMR spectra of the reaction products mixtures were
obtained. In order to compare the spectrum of the aqueous
alkaline hydrolysis products of 7 with the corresponding spectra
of alkaline aqueous degradation products of 5 and 6, a specific
amount of morpholine was added to the hydrolysis product
mixture of 7, which could produce the concentration of
morpholine into the product mixture equivalent to the initial
concentration of 7. The 1H NMR spectra of the reaction products
of 5 and 6 are identical to that of 7 containing an externally
added molar amount of morpholine identical to the initial molar
amount of 7 (Figures I-III in Supporting Information). A
specific segment of these spectra containing peaks of only 3-H
and 6-H of 8 and 9 and two sets of methylene protons of
morpholine (hydrolysis products of 8 and 9 as well as equimolar
externally added to the hydrolysis product of 7) is shown in
Figure 2. The chemical shifts (δH) and the integration (In) values
of these peaks are summarized in Table 1. The molar ratio of
the products, [9]/[8], formed from alkaline aqueous cleavage
of 5, 6, and 7 should be equal to R1 ) (In3H)9/(In6H)8 or R2 )
(In6H)9/(In3H)8. Similarly, molar ratio of the products, ([8] +
[9])/[morpholine], formed from alkaline aqueous cleavage of 5
Experimental Section
Materials. N-Morpholinobenzamide (2) was synthesized using
a literature procedure involving the reaction of benzoyl chloride
with morpholine, and the observed spectroscopic data are in
complete agreement with the corresponding reported data.27 All
common chemicals used were commercial products of highest
available purity. Standard solutions of 1 (2.5 × 10-2 M) and 2
(3.0 × 10-2 M) were prepared in acetonitrile. COSY, HMQC,
HMBC, and NOESY experiments were used to confirm the NMR
peak assignments in compounds 5 and 6.
Synthesis of N′-Morpholino-N-(2′-methoxyphenyl)phthala-
mide (1). Morpholine (1.00 mL, 11.5 mmol) in THF (3.0 mL) was
added slowly to N-(2′-methoxyphenyl)phthalimide (2) (3.35 g, 13.2
mmol) in THF (3.0 mL). The resulting solution was refluxed for
∼17 h. The solvent was removed under reduced pressure to give a
brown gel (3.57 g, 91%). Purification was carried out with column
chromatography using hexane/ethyl acetate (EtOAc) as eluent
starting from 1:0 to 0:1 of hexane/EtOAc. After removal of the
solvent, the resulting brownish gel was recrystallized from EtOAc
to give white crystals of 1 (2.03 g, 57%), mp 128-130 °C. δH
(400 MHz, CDCl3, TMS): 3.27 (broad, 2H), 3.64 (broad, 6H), 3.89
(s, 3H), 6.91 (d J ) 9.3, 1H), 6.98-7.02 (m, 1H), 7.07-7.11 (m,
1H), 7.35 (d J ) 8.8, 1H), 7.49-7.57 (m, 2H), 7.78 (d J ) 7.6,
1H), 8.44 (d J ) 7.8, 1H), 8.58 (s, 1H). δC (100 MHz, CDCl3):
42.3, 47.7, 55.8, 66.49, 66.53, 110.2, 120.1, 121.1, 124.3, 126.8,
127.6, 128.1, 129.4, 131.2, 134.3, 135.4, 148.4, 165.3, 169.7. νmax
(Nujol/cm-1): 3452, 3426 (νNRR′), 1656, 1633 (νCdO) cm-1. UV
(CH3CN): λmax) 282 nm, δ ) 6560 M-1 cm-1; Rf ) 0.13 (EtOAc/
hexane ) 1:1). 1H, 13C, and COSY NMR spectra of 1 are in
Supporting Information.
Synthesis of N′-Morpholino-N-(2′-methoxyphenyl)-5-nitro-
phthalamide (5) and N′-morpholino-N-(2′-methoxyphenyl)-4-
nitrophthalamide (6). Morpholine (0.16 g, 1.84 mmol) was added
with thorough mixing to N-(2′-methoxyphenyl)-4-nitrophthalimide
(7) (0.52 g, 1.76 mmol) in THF (10.0 mL). The resulting reaction
mixture was refluxed overnight (∼17 h), and the completion of
reaction was monitored by TLC. Two distinct spots on TLC
indicated the formation of two isomeric products. Separation and
purification of these two isomeric products (as fractions I and II)
was carried out with column chromatography using mixed EtOAc/
chloroform 1:2 as eluent. However, the two fractions of isomers (I
and II) still contained minor amounts of 7. With the addition of a
minimum amount of chloroform, a solid compound formed in
fraction I and a crystalline compound formed in fraction II. The
solid and crystalline compounds were washed quickly with diethyl
ether and acetone, filtered, and dried under high vacuum to produce
nice yellow products of both isomers: isomer A (solid compound)
59.3 mg and isomer B (crystalline compound) 101 mg. The
structural identification by NMR spectroscopy using HMQC,
HMBC, and NOESY experiments revealed isomer A as 5 and
isomer B as 6. Considerable low yield of isomeric products 5 and
and 6 should be equal to R3 ) [4(In
) [4(In
OMe 8+9]/[3(InNCH Mor], R5 ) 4Y1/(InOCH Mor
(InOCH Mor, R7 ) 4Y1/(InNCH Mor or R8 ) 4Y2/(InNCH Mor
)
OMe 8+9]/[3(InOCH Mor
, R6 ) 4Y2/
, where
)
], R4
)
)
)
)
)
)
Mor ) morpholine, OCH ) O(CH2)2, NCH ) N(CH2)2, Y1 )
(In3H)8 + (In6H)9 and Y2 ) (In6H)8 + (In3H)9. The calculated
values of R1, R2, R3, R4, R5, R6, R7, and R8 are summarized in
Table 1 which reveals that within the limits of experimental
errors: (i) the values of molar ratio of products 8 and 9 remained
the same, whether 5, 6, or 7 was the reactant, and (ii) the values
of the molar ratio of ([8] + [9])/[morpholine] remained almost
1 (Table 1). These observations support the imide-intermediate
mechanism (Scheme 3) as the exclusive mechanism for the
alkaline aqueous cleavage of 1.
It is perhaps noteworthy that the effects of the nitro group
on the rate of alkaline aqueous cleavage of 5 and 6, where the
nitro group is at the respective m- and p-position with respect
to the electrophilic reaction site may be expressed by the value
of (kobs)6/(kobs)5 ) 1.43 (where (kobs)6 ) 2.10 × 10-3 s-1 and
(kobs)5 ) 1.47 × 10-3 s-1 at 0.05 M NaOH). The nearly identical
value of [9]/[8] (1.67), whether 5, 6, or 7 was the reactant,
revealed the expected significantly larger effect of NO2 from
the p-position than from m-position to the electrophilic reaction
site (i.e., CO group) in the alkaline hydrolysis of 7.
(27) Lysen, M.; Kelleher, S.; Begtrup, M.; Kristensen, J. L. J. Org. Chem.
2005, 70, 5342.
3736 J. Org. Chem. Vol. 73, No. 10, 2008

Alkaline Hydrolysis of N′-Morpholino-N-(2′-methoxyphenyl)phthalamide
6 may be attributed to (i) both isomers 5 and 6 appeared to be
unstable in the contact with silica gel, and (ii) low solubility of the
crude products in EtOAc which could have caused precipitation in
the column during eluting process. Analytical and NMR spectral
data for 5: C19H19N3O6. δH (400 MHz, CDCl3, TMS): 3.27 (t J )
5.04, 2H, (Morp)CH2), 3.64 (m, 6H, (Morp)CH2), 3.89 (s, 3H, Ar-
OCH3), 6.90-6.92 (dd J ) 8.24 and 1.40, 1H, ArH11), 7.01 (t J )
7.80, 1H, ArH13), 7.10-7.15 (td J ) 7.80 and 1.36, 1H, ArH12),
7.94 (d J ) 8.68, 1H, ArH3), 8.20 (d J ) 2.28, 1H, ArH6), 8.34
(dd J ) 8.68 and 2.28, 1H, ArH4), 8.40 (dd J ) 7.80 and 1.36, 1H,
ArH14), 8.59 (s, 1H, NH8). δC (100 MHz, CDCl3): 42.5, 47.8
(2CH2(Morp)), 55.9 (Ar-OCH3), 66.4, 66.5 (2CH2(Morp)), 110.4
(ArC11), 120.3 (ArC14), 121.2 (ArC13), 122.2 (ArC6), 124.4 (ArC4),
125.2 (ArC12), 127.0 (ArC9), 129.8 (ArC3), 136.9 (ArC2), 139.9
(ArC7), 148.5 (ArC5), 149.0 (ArC10), 163.5 (C1), 167.2 (C15). Rf )
the kinetic procedure were the same as described elsewhere.18a The
observed absorbance (Aobs) at different reaction times (t) were found
to fit to eq 5:
Aobs ) δapp[R0] exp(-kobst) + A∞
(5)
where [R0] is the initial concentration of 1 or 2, δapp is the apparent
molar extinction coefficient of the reaction mixture, A∞ ) Aobs at t
) ∞, and kobs represents pseudo-first-order rate constant for alkaline
hydrolysis of 1 or 2. The rates of reactions were generally carried
out for the reaction period of g8 half-lives. Only for the reactions
at 0.01 and 0.02 M NaOH for hydrolysis of 2, the respective
observed maximum values of t correspond to 1.4 and 3.3 half-
lives. However, the more reliable values of kobs for these kinetic
runs were obtained from eq 5 by data analysis, which included a
est
est
data point at t g 7 half-lives with Aobs ) A∞ where A∞ values
were obtained from the sum of absorbance of expected hydrolytic
products benzoate ion and morpholine at this particular wavelength.
The satisfactory observed data fit to eq 5 is evident from the
1
0.46 (EtOAc/hexane ) 1:1). H, 13C, COSY, and HMQC NMR
spectra of 5 are in Supporting Information. Analytical and NMR
spectral data for 6: C19H19N3O6. δH (400 MHz, CDCl3, TMS): 3.24
(t J ) 5.04, 2H, (Morp)CH2), 3.71 (m, 6H, (Morp)CH2), 3.89 (s,
3H, Ar-OCH3), 6.91 (d J ) 8.24, 1H, ArH11), 7.00 (t J ) 7.80,
1H, ArH13), 7.12 (m, 1H, ArH12), 7.53 (d J ) 8.24, 1H, ArH6),
standard deviations of <8% for calculated kinetic parameters, kobs
,
δapp, and A∞. However, the standard deviations associated with these
8.37 (dd J ) 8.24 and 1.84, 1H, ArH5), 8.38 (d J ) 8.68, 1H,
ArH14), 8.60 (d J ) 1.84, 1H, ArH3), 8.61 (s, 1H, NH8). δC (100
MHz, CDCl3): 42.4, 47.7 (2CH2(Morp)), 55.9 (Ar-OCH3), 66.4,
66.5 (2CH2(Morp)), 110.3 (ArC11), 120.3 (ArC14), 121.2 (ArC13),
123.5 (ArC3), 125.1 (ArC12), 125.9 (ArC5), 127.0 (ArC9), 128.3
(ArC6), 136.1 (ArC2), 141.3 (ArC7), 148.0 (ArC4), 148.6 (ArC10),
163.0 (C1), 167.7 (C15). Rf ) 0.23 (EtOAc/hexane ) 1:1). 1H, 13C,
COSY, HMQC, HMBC, and NOESY NMR spectra of 6 are in
Supporting Information.
calculated kinetic parameters were <2% for 1 and 2 within [NaOH]
range 5.0 × 10-3 to 2.0 M and 5.0 × 10-2 to 2.0 M, respectively.
The rates of alkaline aqueous cleavage of 5 and 6 were studied
spectrophotometrically by monitoring the appearance of product
for 5 at 290 nm and disappearance of 6 at 265 nm. The observed
data (Aobs versus t) for 6 and 5 fit satisfactorily to eqs 5 and
Aobs ) δapp[R0][1 - exp(-kobst)] + A0
(6)
where A0 ) Aobs at t ) 0. Details of the data analysis have been
described elsewhere.16b
Product Characterization. Alkaline hydrolytic products for 1
were ascertained spectrophotometrically to be 4 and morpholine.
The calculated values of A∞ (from eq 5) at 310 nm and under
different reaction conditions gave the value of δ ) (1473 ( 29)
M-1 cm-1 for product, which is similar to the observed value of δ
) 1480 M-1 cm-1 obtained using authentic 4. The value of δ for
authentic 2-methoxyaniline at 310 nm is 80 M-1 cm-1. These
observations show nearly 100% conversion of 1 to 4 under the
present experimental conditions. The products in the alkaline
hydrolysis of 2 were confirmed by comparing the values of molar
extinction coefficients, δ, obtained from A∞ values (calculated from
eq 5) with the corresponding δ values of authentic samples of
benzoic acid and morpholine obtained under conditions of kinetic
runs. These observations showed that the 100% products were
benzoic acid and morpholine for 2.
It is perhaps worth mentioning that the attempt to separate the
isomeric product mixture obtained from the reaction of synthesized
N-(2′-methoxyphenyl)-4-methylphthalimide with morpholine was
unsuccessful.
Synthesis of N-(2′-Methoxyphenyl)-4-nitrophthalimide (7).
4-Nitrophthalic anhydride (1.08 g, 5.58 mmol) and 0.692 mL of
o-methoxyaniline (0.76 g, 6.14 mmol) were added into a 25.0 mL
round-bottom flask containing 10.0 mL of glacial acetic acid. A
precipitate formed immediately after addition of amine at room
condition. The reaction mixture was then refluxed for 2 h after
which TLC indicated the completion of the reaction. The reaction
mixture was allowed to cool to room temperature and then poured
into distilled water. The resulting yellow precipitates were filtered
and dried to give 1.58 g (95.0%) of crude product. Further
purification carried out by recrystallization with ethanol (95%)
afforded fine pale yellow needles of 7 (1.42 g, 85.5%). δH (400
MHz, CDCl3, TMS): 3.80 (s, 3H), 7.06-7.09 (m, 2H), 7.27 (dd J
) 7.56 and 1.6, 1H) 7.47 (dt J ) 8.24 and 1.8, 1H), 8.13 (d J )
8.24, 1H), 8.65 (dd J ) 8.24 and 2.28, 1H), 8.75 (d J ) 1.84, 1H).
δC (100 MHz, CDCl3): 55.9, 112.3, 119.2, 119.6, 121.1, 125.0,
129.4, 129.8, 131.3, 133.7, 136.7, 151.9, 155.2, 165.0, 165.3.
Kinetic Measurements. The rates of alkaline hydrolysis of 1
and 2 were studied spectrophotometrically by monitoring the
disappearance of 1 at 310 nm and 2 at 250 nm. The temperature
was kept constant at 35 °C and aqueous reaction mixture for each
kinetic run contained 2% v/v CH3CN for 1 and 2. The details of
Acknowledgment. The authors thank the Ministry of Science,
Technology and Innovation for ScienceFund (Project No. 14-
02-03-4014) and the University of Malaya for financial support.
The authors also thank to Ms. Norzalida Zakaria for her help
in carrying out the NMR (HMQC, HMBC, and NOESY) ex-
periments. The authors express their deep gratitude and thanks
to Professor Daniel A. Singleton of Texas A&M University for
suggesting experiments as an additional possible test for an
imide-intermediate mechanism.
1
Supporting Information Available: Appendix; H NMR
spectra for alkaline aqueous cleavage of 5, 6, and 7 (Figures
I-III); 1H, 13C, and COSY NMR spectra of 1; 1H, 13C, COSY,
1
and HMQC NMR spectra of 5; and H, 13C, COSY, HMQC,
HMBC, and NOESY NMR spectra of 6. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
JO702695K
J. Org. Chem. Vol. 73, No. 10, 2008 3737