Kinetics and mechanism of the aminolysis of p-nitrophenyl N-phenylcarbamates

Article abstract of DOI:10.1002/(SICI)1099-1395(199710)10:10<725::AID-POC943>3.0.CO;2-X

Kinetic studies of the reactions of p-nitrophenyl N-phenylcarbamates with benzylamines were carried out in acetonitrile at 25·0°C. Second-order (k₂) and third-order (k₃) rate constants were observed for all the Y-substituted carbamates except for Y=m Cl. The relatively large magnitude of ρ_x (for X-substituted benzylamines) and ρY together with a positive cross-interaction constant ρ_xy supports a stepwise mechanism involving rate-limiting breakdown of the zwitterionic tetrahedral intermediate T^±. Kinetic isotope effect studies with deuterated benzylamine (XC₆H₄CH₂ND₂) indicate that in the base-catalyzed path, k₃, rate-limiting deprotonation occurs at the amino group of benzylamine within the T^± intermediate. The low ΔH^≠ and ΔS^≠ values for the k₃ process are in accord with the proposed mechanism.

Full text of DOI:10.1002/(SICI)1099-1395(199710)10:10<725::AID-POC943>3.0.CO;2-X

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 725–730 (1997)
KINETICS AND MECHANISM OF THE AMINOLYSIS OF
p-NITROPHENYL N-PHENYLCARBAMATES
HAN JOONG KOH,1 OK SUN KIM,2 HAI WHANG LEE2 AND IKCHOON LEE2*
1 Department of Science Education, Chonju National University of Education, Chonju 560-757, Korea
2 Department of Chemistry, Inha University, Inchon 402-751, Korea
Kinetic studies of the reactions of p-nitrophenyl N-phenylcarbamates with benzylamines were carried out in acetonitrile
at 25·0 °C. Second-order (k2 ) and third-order (k3 ) rate constants were observed for all the Y-substituted carbamates
except for Y=mϪCl. The relatively large magnitude of ␳ (for X-substituted benzylamines) and ␳ together with a
X
Y
positive cross-interaction constant ␳ supports a stepwise mechanism involving rate-limiting breakdown of the
XY
zwitterionic tetrahedral intermediate T± . Kinetic isotope effect studies with deuterated benzylamine (XC6H4CH2ND2 )
indicate that in the base-catalyzed path, k3 , rate-limiting deprotonation occurs at the amino group of benzylamine within
the T± intermediate. The low ⌬H≠ and ⌬S≠ values for the k3 process are in accord with the proposed mechanism. © 1997
John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 10, 725–730 (1997) No. of Figures: 1 No. of Tables: 5 No. of References: 16
Keywords: p-nitrophenyl N-phenylcarbamates; stepwise mechanism; rate-limiting breakdown of T± .
Received 6 February 1997; revised 7 March 1997; accepted 7 April 1997
INTRODUCTION
mechanism, an isocyanate is formed instead of a zwitter-
ionic tetrahedral intermediate T± (Scheme 2). We extend
here our series of kinetic studies on the effects of acyl group
on the mechanism of the aminolysis of carbonyl compounds
to the reactions of p-nitrophenyl N-phenylcarbamates with
benzylamines in acetonitrile [equation (1)].
Although the mechanism of the aminolysis of aryl esters1
(1) and carbonates2 (2) have been investigated extensively,
reports on the aminolysis mechanism of N-phenylcarba-
mates (3) are scarce. These three classes of carbonyl
compounds differ only in the acyl part , R, RO and RNH,
where R is alkyl or aryl, with a similar phenoxy leaving
group, OAr. The aminolysis mechanism of the carbamates is
therefore expected to be similar to the relatively well known
aminolysis mechanisms of the esters 1 and carbonates 2.
Shawali et al.3 proposed a stepwise mechanism with rate-
limiting breakdown of a tetrahedral intermediate, T± , for the
reactions of aryl N-arylcarbamates, R=Ar in 3, with n-
butylamine in dioxane. Their kinetic results were
compatible with the stepwise mechanism involving two
reaction pathways, an overall second-order, k2 , and an
overall third-order, k3 , process (Scheme 1), which is similar
to the aminolysis mechanisms proposed for some of 1 and 2,
but is in contrast to an ElcB mechanism proposed by
Menger and Glass4 for the reaction of p-nitrophenyl N-
phenylcarbamate with diethylamine in toluene. In this latter
(1)
In this work, we apply the mechanistic criteria based on
the cross-interaction constants, ␳ in equations (2), where i
ij
and j denote the substituents in the nucleophile (X),
substrate (Y) and leaving group (Z).5 The simple second-
order expression
log(kij /kHH )=␳ ␴i+␳ ␴j+␳ ␴ ␴
j
(2a)
i
j
ij
i
is obtained by a Taylor series expansion of log kij around
␴i=␴j=0 and neglecting pure second-order (␳ and ␳jj ) and
ii
higher-order (␳iij , etc.) terms. The cross-interaction con-
* Correspondence to: Ikchoon Lee.
stant, ␳ij , can be alternatively defined as5
© 1997 John Wiley & Sons, Ltd.
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726
H. J. KOH ET AL.
Scheme 1
Scheme 1, where Z=p-NO2 and R=XC6H4CH2 . The k3 step
Ѩ␳ Ѩ␳
j
i
␳ij=
=
(2b)
is deprotonation by benzylamine of the tetrahedral inter-
mediate, T± , to yield the anionic intermediate TϪ . The k3
step is the rate-determining step for the right-hand side path
of Scheme 1 since breakdown of T± to products should be
fast.6 The proton transfer is fast in water but not in MeCN.
As a result, the k3 step may become competitive with k2 .1k, l
The Hammett (␳X and ␳Y ) and Brønsted (␤X ) coefficients
determined are given in Table 1 for the k2 step. The
exceptionally large magnitudes of these selectivity parame-
ters are consistent with the stepwise mechanism with
rate-limiting breakdown of T± ;1, 2 both ␳ and ␳ should be
Ѩ␴ Ѩ␴
i
j
RESULTS AND DISCUSSION
The reactions of p-nitrophenyl N-phenylcarbamates (PCB)
with benzylamines (BA) in acetonitrile at 25·0 °C obey a
third-order rate law:
rate=kobs[PCB]
obs =k2[BA]+k3[BA]2
(3)
(4)
X
Y
k
except for Y=m-Cl, for which k3=0. The second and third
order-rate constants, k2 and k3 , are obtained from a straight
line plot of kobs /[BA] vs [BA] (Figure 1) as the intercept and
slope, respectively. For the case of Y=m-Cl, the slope of a
plot of kobs vs [BA] yielded k2 . The values of k2 and k3 are
summarized in Tables 1 and 2. The k3 values are in general
greater than the corresponding k2 values as noted for the
similar reaction of esters1a (1), halides,1h carbonates6 (2) and
carbamates3 (3).
An isocynate intermediate proposed by Menger and
Glass4 was not found in the analysis of the infrared
spectrum (2275–2240 cmϪ1 ). The product studies and the
third-order kinetics [equation (4)] observed lead us to a
likely mechanism for the present reactions as shown in
Figure 1. The plot of kobs /[C6H5CH2NH2 ] vs [benzylamine] for the
reaction of p-nitrophenyl N-phenylcarbamate with benzylamine in
acetonitrile at 25·0 °C
Scheme 2
© 1997 John Wiley & Sons, Ltd.
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AMINOLYSIS OFp-NITROPHENYL N-PHENYLCARBAMATES
727
(5)
Table 1. Second-order rate constants, k2 (dm3 molϪ1 sϪ1 ), for the
reactions of p-nitrophenyl N-(Y)phenylcarbamates with X-benzyla-
(correlation coefficient=0·974):
mines in acetonitrile at 25·0 °C
log(kXY /kHH )=Ϫ1·62␴X +1·94␴Y +1·10␴ ␴
X
Y
Y
indicates that ␳ is positive and relatively large, which is
XY
again in line with the proposed stepwise mechanism; it has
been shown both experimentally and theoretically that the
X
p-CH3
H
p-Cl
m-Cl
p-CH3
0·403a
0·953
2·28b
3·57a
8·49
sign of ␳XY should be positive in contrast to the negative ␳
XY
1·23
3·26
for the SN2 processes.5, 11 In the bond-making step, a
stronger electron acceptor substituent in the substrate
(␦␴Y >0) invariably leads to a tighter bond with the
20·2b
5·81
H
p-Cl
m-Cl
0·528
0·201
0·0471a
0·112
0·655
0·268
1·89
0·894
3·20
nucleophile in the TS (␦␳X <0) so that ␳ (=␦␳X /␦␴Y ) is
XY
0·962a
2·29
negative.5, 11 The values of ␳ (Ϫ1·62) and ␳Y(1·94) are in
X
0·161
0·458
good agreement with the corresponding values in Table 1
determined independently using the simple (first-order)
Hammett equation (␳X =Ϫ1·63; ␳Y =1·92).
0·267b
Ϫ1·73
1·75
5·45b
Ϫ1·06
1·07
c
␳
Ϫ1·63
1·65
Ϫ1·55
1·56
X
d
␤
X
For the base-catalyzed process, k3 , a similar multiple
regression analysis yielded a much worse correlation,
probably due to complex effects of the substituents. For
example, the effects of substituent in nucleophile, X, on the
rate of deprotonation is complex since the effect of the
catalyst benzylamine should be opposite to that of benzyl-
amine within T± .
a At 15·0 °C.
b At 35·0 °C.
c The ␴ values were taken from Ref. 7. Correlation coefficients were better
than 0·997 in all cases.
d The pKa (H2O, 25·0 °C) values were taken from Ref. 8. Correlation
coefficients were better than 0·997 in all cases.
Kinetic deuterium isotope effects involving amine pro-
tons on the second- (k2 ) and third-order (k3 ) rate constants
are studied incorporating deuterium isotopes (i) in the
substrate amino group, YC6H4NH(D)COOC6H4NO2 , and
(ii) in the nucleophile amino group, XC6H4CH2NH2(D2 ).
The results are summarized in Tables 3 and 4. Reference to
Table 3 reveals that the isotope effects involving substrate
amine protons are small (kH /kD ≈1·10) for k2 and negligible
(ca 1·00) for k3 . The small kinetic isotope effects for k2
probably arise from the same origin as those found for the
deuterated nucleophiles in the stepwise mechanisms of the
esters and carbonates; the kH /kD values are normally slightly
greater than unity for such processes, in contrast to inverse
effects (kH /kD <1·0) observed for the rate-limiting attack
process, i.e. for SN2 or rate-limiting formation of T ± .5c The
negligible effects for k3 suggest that the substrate amine
proton is not involved in the deprotonation step, k3 . This is
in contrast to the kinetic deuterium isotope effects observed
with the deuterated nucleophiles in Table 4. Again for k2 ,
the kH /kD values are similar to those for the deuterated
substrate amine in Table 3. However, for the k3 step, we
observe strong primary isotope effects (kH /kD ≈1·7–1·8),
which can be taken as clear evidence that in the base-
catalyzed process, k3 , deprotonation takes place at the
amino group of the nucleophile in the TS. This supports our
proposed mechanism presented in Scheme 1 for the
reactions of p-nitrophenyl N-phenylcarbamates with benzyl-
amines in acetonitrile.
compared with other corresponding values after taking into
account of a non-conjugating intervening group, CH2 and
NH, present in the benzylamine nucleophile and in the
N-phenylcarbamate substrate respectively, which are known
to reduce the ␳ values by a factor of ca 2·8.9 The large ␤X
values are also in line with the proposed mechanism,1, 2 but
they are less reliable since the pKa values used are those in
water, not in acetonitrile. However, it is well known that
although the absolute pKa values are different in H2O and
MeCN, the ⌬pKa =(pKa )MeCN Ϫ(pKa )H O values for the
2
structurally similar amines are nearly the same.10 Thus the
␤
values should be nearly the same in both H2O and
X
MeCN.10c
The 16 k2 values in Table 1 are subjected to multiple
regression using equation (2a) with i, j=X, Y. The result
Table 2. Third-order rate constants, k3 (dm6 molϪ2 sϪ1 ), for the
reactions of p-nitrophenyl N-(Y)phenylcarbamates with X-benzyl-
amines in acetonitrile at 25·0 °C
Y
X
p-CH3
H
p-Cl
p-CH3
8·59a
9·45
10·4b
5·61
10·2
6·37
2·50
32·3
27·5
7·04
H
p-Cl
m-Cl
1·51
A relatively strong acceptor in the substrate (Y=m-Cl)
seems to stabilize the TS (k2 ) for the uncatalyzed pathway
so that the k2 path becomes predominant over the base-
catalyzed process, k3 . This is reasonable since in the TS (k2 )
negative charge develops (␳Y >0) at the carbonyl carbon,
which can be delocalized by a stronger acceptor Y.
1·14a
1·28
1·43b
1·38
4·98
a At 15·0 °C.
b At 35·0 °C.
© 1997 John Wiley & Sons, Ltd.
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728
H. J. KOH ET AL.
Table 3. Kinetic isotope effects on the second- (k2 ) and the third-order rate
constants (k3 ) for the reactions of deuterated p-nitrophenyl N-(Y)phenylcar-
bamates [YC6H4NH(D)C(O)OC6H4-p-NO2 ] with X-benzylamines in
acetonitrile at 25·0 °C
X
Y
k
2(H) (dm3 molϪ1 sϪ1
)
)
k
2(D) (dm3 molϪ1 sϪ1
)
kH/kD
H
H
m-Cl
n-Cl
H
m-Cl
H
0·655±0·002a
5·80±0·03
0·161±0·004
2·29±0·02
0·585±0·003
5·60±0·02
0·460±0·004
2·14±0·04
1·12±0·03b
1·04±0·02
1·10±0·05
1·07±0·03
m-Cl
X
Y
k
3(H) (dm6 molϪ2 sϪ1
k
3(D) (dm6 molϪ2 sϪ1
)
kH /kD
H
m-Cl
H
H
6·37±0·03a
1·38±0·02
6·34±0·06
1·16±0·02
1·01±0·04b
1·19±0·03
a Standard deviation.
b Standard error.
the aminolysis of the esters, halides and carbonates.1e,h, 3, 13
We conclude that the aminolysis of p-nitrophenyl N-
phenylcarbamates in acetonitrile proceeds by a stepwise
mechanism with rate-limiting breakdown of the zwitterionic
tetrahedral intermediate, T± . The breakdown step can be
either base-catalyzed (k3 ) or uncatalyzed (k2 ).
EXPERIMENTAL
From the k2 and k3 values at three temperatures, we
determined the activation parameters, ⌬H≠ and ⌬S≠, for
each pathways. The results in Table 5 indicate that the ⌬H≠
and Ϫ⌬S≠ values are relatively small for the k2 processes,
but the ⌬H≠ values are very small and the Ϫ⌬S≠ values are
fairly large for the k3 processes. These are in good accord
with the trends found for the corresponding activation
parameters for the uncatalyzed and catalyzed stepwise
breakdown processes of the tetrahedral intermediate, T± , in
Materials. The solvent, acetonitrile, was of HPLC grade
(Aldrich), and was further distilled over phosphorus pent-
oxide. The benzylamine nucleophiles, of GR grade
(Aldrich), were used without further purification. Prepara-
tions of deuterated benzylamines were as described
previously.14 The analysis (NMR and GC–mass spectrome-
try) of the deuterated benzylamines showed more than a
99% deuterium content, so no corrections to kinetic effects
for incomplete deuterium content were made.15
Table 4. Kinetic isotope effects on the second- (k2 ) and the third-order rate
constants (k3 ) for the reactions of p-nitrophenyl N-(Y)phenylcarbamates with
deuterated X-benzylamines [XC6H4CH2NH2(D2 )] in acetonitrile at 25·0 °C
X
Y
k
2(H) (dm3 molϪ1
)
k
2(D) (dm3 molϪ1
)
kH /kD
p-CH3
p-CH3
m-Cl
p-CH3
m-Cl
p-CH3
m-Cl
0·958±0·007a
8·49±0·06
0·112±0·004
2·29±0·02
0·855±0·008
7·19±0·05
0·102±0·004
2·01±0·04
1·12±0·03b
1·18±0·02
1·10±0·05
1·14±0·04
m-Cl
X
Y
k
3(H) (dm6 molϪ2 sϪ1
)
k
3(D) (dm6 molϪ2 sϪ1
)
kH /kD
p-CH3
p-CH3
m-Cl
p-CH3
p-Cl
p-CH3
p-Cl
9·45±0·06a
32·3±0·8
5·59±0·06
19·9±0·6
0·703±0·007
2·80±0·06
1·69±0·03b
1·62±0·04
1·82±0·04
1·78±0·03
1·28±0·04a
4·98±0·08
m-Cl
a Standard deviation.
b Standard error.
© 1997 John Wiley & Sons, Ltd.
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AMINOLYSIS OFp-NITROPHENYL N-PHENYLCARBAMATES
729
Table 5. Activation parametersa for the reactions of p-nitrophenyl
(N-(Y)phenylcarbamates with X-benzylamines in acetonitrile
again over MgSO4 . After expulsion of solvent, the analysis
(GC–mass spectrometry) of dried deuterated p-nitrophenol
had >99% deuterium content.
⌬H ≠
Ϫ⌬S ≠
X
Y
Reaction (kcal molϪ1
)
(cal molϪ1 KϪ1
)
C6H5NDC(=O)OC6H4-p-NO2: a solution of deuterated
p-nitrophenol (0·01 mol) in dry benzene (10 ml) was added
to a solution of phenyl isocyanate (0·01 mol). A catalytic
quantity (0·5 mol) of pyridine was added and the solution
was refluxed for 1 h. On evaporation of the solvent in
vacuo, the carbamate precipitated and was recrystallized
from chloroform–pentane. The deuterated substrate synthe-
sized was confirmed by spectral and mass spectrometric
analysis as follows. C6H5NDC(=O)OC6H4-p-NO2: m.p.
p-CH3 p-CH3
p-CH3 m-Cl
k2
k2
k2
k2
k3
k3
14·3
14·2
14·8
14·2
1·1
11
9
15
15
53
53
m-Cl
m-Cl
p-CH3
m-Cl
p-CH3 p-CH3
m-Cl p-CH3
1·4
a Calculated values at 25·0 °C.
146–148 °C; ␦H (CDCl3 ), 7·2–8·3 (9H, m, phenyl); ␯
max
(KBr), 2800 (CH, aromatic), 2300 (ND), 1720 (C=O); m/z
Substrates. p-Nitrophenyl N-phenylcarbamate. A
solution of p-nitrophenol (0·01 mol) in dry benzene (10 ml)
was added to a solution of phenyl isocyanate (0·01 mol). A
catalytic quantity (0·5 ml) of pyridine was added and the
solution refluxed for 1 h. On evaporation of the solvent in
vacuo, the carbamate precipitated and was recrystallized
from chloroform–pentane. The other substituted phenyl N-
phenylcarbamates were prepared in an analogous manner
and recrystallized from chloroform–pentane. The substrates
synthesized were confirmed by spectral and elemental
analysis as follows.
259 (M+ ).
Kinetic procedures. Rates were measured conductimet-
rically in acetonitrile. The conductivity bridge used in this
work was a laboratory-made computer aromatic A/D
converter conductivity bridge. Pseudo-first-order rate con-
stants, kobs , were determined by the Guggenheim method16
with
a
large excess of benzylamine; [carbamate-
]=2·0ϫ10Ϫ4 mol dmϪ3 and [benzylamine=0·02–0·25 mol-
dmϪ3. The rate constants k2 and k3 were obtained by
plotting kobs /[benzylamine] vs [benzylamine] and determin-
ing the intercept and slope of the straight line. The k2 and k3
values in Tables 1 and 2 are the averages of more than
triplicate runs and were reproducible to within ±3%.
p-CH3C6H4NHC(؍O)OC6H4-p-NO2: m.p. 135–136 °C;
␦H (CDCl3 ), 7·2–8·3 (8H, m, phenyl), 6·9 (1H, s, NH), 2·3
(3H, s, CH3 ); ␯max (KBr), 3400 (NH), 2800 (CH, aromatic),
1720 (C=O); m/z 272 (M+ ). Calc. for C14H12N2O4: C, 61·8;
H, 4·4. Found: C, 61·8; H, 4·3%.
Product analysis. p-Nitrophenyl N-phenylcarbamate
was reacted with excess p-methylbenzyl amine with stirring
for more than 15 half-lives at 25·0 °C in acetonitrile and the
products were isolated by evaporating the solvent under
reduced pressure. The product mixture was subjected to
column chromatography (silica gel, 20% ethyl acetate–n-
hexane). Analysis of the product gave the following
results.
C6H5NHC(=O)OC6H4-p-NO2: m.p. 146–148 °C; ␦H
(CDCl3 ), 7·2–8·3 (9H, m, phenyl), 6·8 (1H, s, NH); ␯
max
(KBr), 3400 (NH), 2800 (CH, aromatic), 1720 (C=O); m/z
258 (M+ ). Calc. for C13H10N2O4: C, 60·5; H, 3·9. Found: C,
60·4; H, 3·8%.
p-ClC6H4NHC(=O)OC6H4-p-NO2: m.p. 111–112 °C;
␦H (CDCl3 ), 7·2–8·3 (8H, m, phenyl), 6·9 (1H, s, NH); ␯
max
C6H5NHC(=O)NHCH2C6H4-p-CH3: RF =0·33 (20%
ethyl acetate–n-hexane); m.p. 182–184 °C; ␦H(CDCl3 ),
7·2–8·2 (9H, m, C6H5 , C6H4 ), 6·9 (1H, s, NH), 4·2 (2H, d,
(KBr), 3400 (NH), 2800 (CH, aromatic), 1720 (C=O); m/z
292 (M+ ). Calc. for C13H9N2O4Cl: C, 53·4; H, 3·1. Found:
C, 53·3; H, 3·1%.
CH2 ), 2·1 (3H, s, CH3 );
␯
(KBr), 3220 (NH),
max
m-ClC6H4NHC(=O)OC6H4-p-NO2: m.p. 111–112 °C;
3050–3100(CH), 2950 (CH, aromatic), 1710 (C=O); m/z
285 (M+ ). Calc. for C15H16N2O: C, 63·2; H, 5·6. Found: C,
63·1; H, 5·5%.
␦H (CDCl3 ), 7·2–8·3 (8H, m, phenyl), 6·9 (1H, s, NH); ␯
max
(KBr), 3400 (NH), 2800 (CH, aromatic), 1720 (C=O); m/z
292 (M+ ). Calc. for C13H9N2O4Cl: C, 53·4; H, 3·1. Found:
C, 53·3; H, 3·1%.
ACKNOWLEDGEMENTS
We thank the Ministry of Education for a Basic Science
Research Grant and Inha University.
Deuterated p-nitrophenyl N-phenyl carbamate
[C6H5NDC(=O)OC6H4-p-NO2 ]. DOC6H4-p-NO2: a solu-
tion of potassium hydroxide in methanol was added to a
solution of p-nitrophenol. The reaction mixtures were kept
for 2 h and the solvent, methanol, was removed. The salt,
KOC6H4-p-NO2 , was dissolved in excess D2O under a
nitrogen atmosphere and left over 12 h at 25·0 °C. The
solution was neutralized with DCl. The deuterated p-
nitrophenol was extracted with dry diethyl ether and dried
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© 1997 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 725–730 (1997)