Hydrolysis of 4-imino-imidazolidin-2-ones in acid and the mechanism of cyclization of hydantoic acid amides

Article abstract of DOI:10.1039/b708397a

The hydrolysis of iminohydantoins generates the same tetrahedral intermediate T as that obtained in the cyclization of hydantoic acid amides to hydantoins. The ratio of the products of imine hydrolysis under kinetic control is determined by the relative height of the barriers of the breakdown of T to amide or to hydantoin. Thus the partitioning of products unequivocally proves which is the rate determining step in the cyclization reaction - formation or breakdown of T. UV and ¹H NMR monitoring of the acid catalyzed hydrolysis of four 5-substituted 4-imino-1-methyl-3-(4-nitrophenyl)imidazolidin- 2-ones 1 found hydantoins 3 as the only products. The kinetics of hydrolysis of imines were measured in 0.001-1 M HCl. Contrary to the remaining imines, 1,5-dimethyl-4-imino-3-(4-nitrophenyl)imidazolidin-2-one 1b is readily oxidized as stock solution in THF containing peroxides to 1,5-dimethyl-5-hydroxy-4-imino- 3-(4-nitrophenyl)imidazolidin-2-one 1d. In all cases, hydrolysis was found to be zero order with respect to [H⁺]. As imines are fully protonated under the acidity studied, this is evidence of a transition state of a single positive charge. Comparison of imine hydrolysis rates with previous data on rates of cyclization of the corresponding amides of hydantoic acids allowed conditions (acid concentration, substitution pattern - gem-dimethyl effect) to be found that guaranteed kinetic control of the products obtained. Thus it was unequivocally proven that formation of the tetrahedral intermediate is rate determining in the cyclization of hydantoic acid amides. The small steric effects upon methyl substitution at 5-C and a solvent kinetic isotope effect k_H/k_D of 1.72 favour a mechanism for imine hydrolysis whereby the rate is limited by water attack on the protonated imine concerted with proton transfer from attacking water to a second water molecule. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708397a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Hydrolysis of 4-imino-imidazolidin-2-ones in acid and the mechanism of
cyclization of hydantoic acid amides†
Violina T. Angelova,a Nikolay G. Vassilev,b Asen H. Koedjikovb and Ivan G. Pojarlieff*b
Received 4th June 2007, Accepted 10th July 2007
First published as an Advance Article on the web 31st July 2007
DOI: 10.1039/b708397a
The hydrolysis of iminohydantoins generates the same tetrahedral intermediate T as that obtained in
the cyclization of hydantoic acid amides to hydantoins. The ratio of the products of imine hydrolysis
under kinetic control is determined by the relative height of the barriers of the breakdown of T to
amide or to hydantoin. Thus the partitioning of products unequivocally proves which is the rate
determining step in the cyclization reaction—formation or breakdown of T. UV and 1H NMR
monitoring of the acid catalyzed hydrolysis of four 5-substituted 4-imino-1-methyl-3-
(4-nitrophenyl)imidazolidin-2-ones 1 found hydantoins 3 as the only products. The kinetics of
hydrolysis of imines were measured in 0.001–1 M HCl. Contrary to the remaining imines,
1,5-dimethyl-4-imino-3-(4-nitrophenyl)imidazolidin-2-one 1b is readily oxidized as stock solution in
THF containing peroxides to 1,5-dimethyl-5-hydroxy-4-imino-3-(4-nitrophenyl)imidazolidin-2-one 1d.
In all cases, hydrolysis was found to be zero order with respect to [H+]. As imines are fully protonated
under the acidity studied, this is evidence of a transition state of a single positive charge. Comparison of
imine hydrolysis rates with previous data on rates of cyclization of the corresponding amides of
hydantoic acids allowed conditions (acid concentration, substitution pattern—gem-dimethyl effect) to
be found that guaranteed kinetic control of the products obtained. Thus it was unequivocally proven
that formation of the tetrahedral intermediate is rate determining in the cyclization of hydantoic acid
amides. The small steric effects upon methyl substitution at 5-C and a solvent kinetic isotope effect
kH/kD of 1.72 favour a mechanism for imine hydrolysis whereby the rate is limited by water attack on
the protonated imine concerted with proton transfer from attacking water to a second water molecule.
compared to acid hydrolysis of amides, which has been shown8
conclusively to proceed through rate determining attack of water,
As an integral part of the Edman degradation procedure for
which is equivalent to attack of the ureido group.
Introduction
sequencing of peptides, the acid catalysed cyclization of N-
thiocarbamoyl amino acid amides and peptides has been exten-
We now report a solution to this problem by studying the
hydrolysis of imines 1 shown in Scheme 1.
sively studied.1 In the case of the oxygen analogues, the hydantoic
acid amides, the mechanism of cyclization under acid catalysis
is less well understood2–4 as opposed to the more extensively
studied base catalyzed cyclization.3–5 In moderately concentrated
acids above Ho of −1 (the Hammett acidity function), Kava´lek
et al.2 observed saturation kinetics due to protonation of the
ureido group, which eliminates its nucleophilicity, and also due
to considerable protonation of the amide moiety. These authors
assume rate determining attack of the ureido group on the
protonated amide function to give a tetrahedral intermediate
(k−2 and T of Scheme 2), however, without the support of any
Scheme 1
experimental proof. Our study of the cyclization of hydantoic acid
N-methylamides3 and amides4 when R = p-nitrophenyl showed
acid catalysis below pH 2–3. No definite answer could be given
As demonstrated in Scheme 2, the first step of the reaction is
as to which is the r.d.s. although according to estimates from
fractionation factors6,7 deuterium solvent kinetic isotope effects
fitted better with a rate determining leaving of the amino group
from TH+. The cyclization of hydantoic acid amides can be
to generate the tetrahedral intermediate T shown in the neutral
form because the O-protonated form obtained upon addition
is a stronger acid than H3O+. This same intermediate lies on
the reaction path between amide and hydantoins. The products
obtained from partitioning of T under kinetic control provide an
unequivocal answer concerning the relative heights of the energy
barriers determining k2 and k3 and thus, respectively, which is the
rate determining step of ureido amide 2 cyclization.
aFaculty of Pharmacy, Medical University, ul. Dunav 2, Sofia, Bulgaria
bInstitute of Organic Chemistry, Bulgarian Academy of Sciences, ul. Acad.
G. Bonchev block 9, Sofia, Bulgaria. E-mail: ipojarli@orgchm.bas.bg
† In memoriam to Professor Keith Yates.
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order rate constants, kobs, were obtained by non-linear regression
t
curve-fitting to the equation: At = Aoe−k + A∞, where At, Ao
obs
and A are the absorbances at time, t, zero and infinity, respectively.
Multiple scan spectra taken during the course of the hydrolysis
showed good isosbestic points and the infinity spectra (10s1/2
)
were found to be identical with the spectra of the respective 3-
(4-nitrophenyl)hydantoins at the same concentration. A “clean”
reaction was also supported by well-behaved first order kinetics.
The time dependent NMR studies were recorded at room
temperature (22–23 ◦C) in solutions made up from 2 mg imine,
0.3 ml of DMSO-d6 and 0.3 ml of 0.11 M DCl yielding ca. 0.03 M
free DCl in the sample. Series of proton spectra (25–51) were
measured at intervals of 5 to 30 min until the hydrolysis of imine
was completed (8–16 h). Proton chemical shifts were calibrated
according to the signal of DMSO-d6. The NMR spectra indicated
no side reactions.
The Fourier transform spectra were carefully phase corrected
and then integrated using the multiint au program, which inte-
grates a series of spectra within selected integration regions using
the same scaling factor. The obtained integrals for the peaks for
methyl groups (in the intervals of 2.87–2.91 and 2.91–2.71 ppm
(1a), 1.33–1.43 and 1.52–1.66 ppm (1b), 1.24–1.49 and 1.50–
1.75 ppm (1c)) were then fitted simultaneously by the program
DynaFit11 according to the following reaction mechanism of
pseudo-first order reaction:
Scheme 2
Experimental
k
obs
UV spectra were measured on a Specord UV–Vis spectrophotome-
ter. NMR spectra were recorded on a Bruker Avance DRX 250
spectrometer, operating at 250.13 MHz for protons. A 5 mm dual
1H–13C probe head was used at room temperature. Chemical shifts
are quoted in p.p.m. as d values against TMS and couplings in Hz.
1 −→ 3
The changes in concentrations of monitored compounds over
time were computed by solving an initial value problem described
by the following system of differential equations:
d[1]/dt = −kobs [1]
Materials
d[3]/dt = +kobs [3]
Inorganic reagents were of analytical grade and used without
further purification. HCl solutions were prepared with CO2-free
distilled water. D2O, 99 atom % and deuterium chloride, DCl,
22 wt % were from Aldrich. The preparations have been described
previously of 4-imino-1-methyl-3-(4-nitrophenyl)imidazolidin-
2-one, 1,5-dimethyl-4-imino-3-(4-nitrophenyl)imidazolidin-2-one,
4-imino-3-(4-nitrophenyl)-1,5,5-trimethylimidazolidin-2-one,9 1a,
1b and 1c respectively, 1,5-dimethyl-5-hydroxy-4-imino-3-(4-
nitrophenyl)imidazolidin-2-one10 1d, 1-methyl-3-(4-nitrophenyl)-
imidazolidin-2,4-dione,1,5-dimethyl-3-(4-nitrophenyl)imidazolidin-
2,4-dione and 1,5,5-trimethyl-3-(4-nitrophenyl)imidazolidin-2,4-
dione,3 3a, 3b, 3c and 1,5-dimethyl-5-hydroxy-3-(4-nitrophenyl)-
imidazolidin-2,4-dione10 3d.
Results and discussion
On a preparative scale, iminohydantoins have long been known to
give hydantoins in acid.12 However, for mechanistic conclusions it
is important to know whether this is the result of kinetic as opposed
to thermodynamic control and further, if any alternate products
are formed in significant amounts. The change with time of the
UV spectra of iminohydantoins 1 in 0.001–1 M HCl indicates
direct conversion into hydantoins 3: the peaks around 250 nm
due to protonated imines deplete with time, accompanied by the
formation of a peak at 280 nm due to the respective hydantoins,
the process being characterized by a clean isosbestic point. If
amides 2 were formed, these would give rise to absorption at
330 nm, characteristic of the p-nitrophenylureido group.3 UV-
Spectrophotometry is of course insensitive to small amounts of
impurities so product analysis was further carried out by means of
1H NMR. The reaction was followed in 0.03 M DCl in 1 : 1 DMSO-
d6–D2O; organic solvent was added to overcome poor solubility in
water. The scans, taken over periods of time, demonstrate a clean
conversion of protonated imine into hydantoins, illustrated for the
case of 1b in Fig. 1.
Kinetic measurements
The kinetics of hydrolysis of imines 1a-1d were measured in
aqueous HCl (0.001–1 M) by means of UV spectroscopy and in
DMSO-d6–DCl by means of 1H NMR.
The UV experiments were carried out in the thermostatted cell
of a Specord UV–Vis spectrophotometer at 25.0 0.1 ◦C. The
rate was followed by monitoring the decrease of absorbance at
250 nm (kmax of the protonated imines; product hydantoins absorb
at kmax 280 nm). The reaction was initiated by injecting 30 ll of
0.01 mol dm−1 stock solution (THF or acetonitrile, see Results) of
the corresponding imine to 2.80 cm3 of HCl solution. Pseudo-first-
1H NMR monitoring of the hydrolysis of the remaining imines
also showed hydantoin 3 as practically a single product with no
significant quantities of amides or other side products. However,
crucial for deductions regarding the slow step of hydantoinamide
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Fig. 2 Log–log plot of the pseudo-first-order rates of hydrolysis of imines
1 against [HCl] at ionic strength of 1 M (KCl): open squares 1a, full squares
1b (stock solution in acetonitrile), full triangles 1c, open circles 1d (1b
oxidized as stock solution in THF), full circles 1d (stock solution from
isolated 1d).
Fig. 1 1H NMR scans over time intervals showing the conversion of 1b
into 3b in 0.03 M DCl (1 : 1 DMSO-d6–D2O). The N-methyl singlet at
2.92 and the C-methyl doublet at 1.59 transform into signals at 2.87 and
1.38 ppm, respectively.
In THF, the batch used was shown to contain peroxides, 1b
oxidized readily and completely, more so because of the relatively
small substrate concentration of 0.01 M. When the 5-hydroxy
compound 1d itself was used as a substrate, practically the same
rate constant was observed as that of presumably 1b (Fig. 1). When
the stock solution of 1b was prepared in acetonitrile containing
no peroxides, a four times smaller constant was observed, which
is listed in Table 1.
cyclizations is selectivity to be kinetically determined. In the
present case, if the rate of cyclization of the ureido amides to
hydantoins is much faster than the hydrolysis of imine, the amide,
if formed, will not accumulate. Thus information on the rates of
imine hydrolysis and ureido amides was needed. The kinetics of
cyclization of the latter in the case 2b and 2c was available from
previous work.4
We now report the rates of hydrolysis of imines 1 in 0.001–1 M
HCl. The rate profiles obtained by measuring the rates by means
of UV are presented in Fig. 2
The plots in Fig. 1 show zero order of the reaction towards
[H+]. Observed rate constants, presented in Table 1, are averaged
values of all experiments. Table 1 also contains the rate constants
obtained by means of 1H NMR in a DMSO-d6–D2O (1 : 1) solution
9
+
of DCl. According to the pKBH -values of the substrates (Table 1),
Initially, only the rates of hydrolysis of compounds 1a,b,c were
measured by means of UV by preparing 1 × 10−2 M stock solutions
of the substrates in THF. Surprisingly, imine 1b with a single
methyl group at position 5 of the 4-imino-imidazolidin-2-one ring
reacted much faster than the compound without 5-methyl groups
1a and the compound with two methyl groups 1c. This did not
fit with reactivities observed upon methyl substitution in similar
systems, e.g., those observed in alkaline hydrolysis of hydantoins13
where each methyl group brings about a 5-fold decrease in rate.
However, a very different rate of hydrolysis of 1b in DMSO–
DCl was found, which was only slightly greater than that of
1a. The discrepancy between the rates was solved when it was
established by means of NMR that the mono substituted (at 5-
C) iminohydantoin 1b autoxidized in solution into the hydroxy
derivative 1d. In DMSO or AcCN, the process was complete upon
standing for about a week at room temperature.10 On the other
hand, 1a and 1c were found to be stable under the same conditions.
these are completely protonated at the acidities studied. Thus the
zero order against [H+] corresponds to a singly positively charged
transition state.
Contrarily, in the 0–3 pH range, the rate of cyclization of ureido
amides 2 to hydantoin is mainly first order in [H+] and is described
by the equation:
+
kobs = kHaH + kw
where kH is the rate constant for catalysis by H+ and kw for
catalysis by water. Values for amides 2b and 2c are available from
ref. 4.‡ Thus because of the acid catalysis, product analysis will
be least distorted by the conversion of amide into hydantoins at
the lowest acidity. For this reason, in Table 1 observed rates of
cyclization in 0.001 M are presented. The values listed in Table 1
‡ In these rate comparisons, the difference between rate coefficients based
on concentrations and those on activities were ignored.
Table 1 Pseudo-first-order rate constants, 104 kobs s−1, of acid catalyzed hydrolysis of 4-iminoimidazolidin-2-one 1 and cyclization rates of hydantoic
acid amides 2 at 25 ◦C and I = 1 M (KCl)
Hydrolysis of imines 1 in H2Oa
Hydrolysis of imines 1 in DMSO–D2Ob
Cyclization of amides 2 in 0.001 M HClc
pKNH of imines 1d
+
2
a
b
c
0.979
1.81
0.324
7.77
1.19
1.49
0.311
5.38
5.02
5.13
0.065
0.81
d
a Averages from UV data in 0.001–1 M HCl, I = 1 M (KCl) 25.0 ◦C. b Data from NMR monitoring in 0.011 M DCl in DMSO–D2O (1 : 1) at 22–23 ◦C.
c Calculated from rate equation and data from ref. 4 (see text). d Ref. 9.
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demonstrate that in the most dilute acid solution, cyclization of 2b
is almost 30 times more slow than hydrolysis of 1b; however, with
1c the ratio is inversed, the cyclization being three times faster.
This is a manifestation of the gem-dimethyl effect manifested in
acceleration of cyclizations or deceleration of ring-opening upon
substitution in the chain.14
The gem-dimethyl effect offers another possibility to manipulate
the rate ratio of hydrolysis versus cyclization. The former, as
already discussed, is generally slowed down by methyl substitution
in position 5 while cyclizations are strongly enhanced in hydantoic
systems.14
N-methylamides3 suggest that 2a should cyclize at least 10 times
more slowly than 2b. Thus contrarily to amide 2c, the cyclization of
2a to hydantoin will have practically no effect on the composition
of the products of hydrolysis of imine 1a. As already mentioned,
monitoring by means of NMR revealed only hydantoin as the
product (Fig. 4) giving fully reliable evidence for the slow stage
amide cyclization, i.e., formation of the tetrahedral intermediate.
The extent of such possible interference is best illustrated by an
analysis of the changes in concentration with time in the extreme
case of amide as the only kinetic product of imine hydrolysis. Then
the following consecutive reactions would be observed:
1 → 2 → 3
Fig. 3 shows the calculated conversions in 0.01 M HCl—a
concentration close to that of the NMR experiments. Even under
this more unfavourable HCl concentration, the mol fraction of
the 5-methylhydantoin 3b is less than 10% for a 90% depletion of
initial imine. Due to the strong acceleration of cyclization caused
by the additional methyl group in 2c, the picture with compounds
c is completely different and under these conditions [2c] attains a
maximum in its concentration, which is less than 10%. Thus the
experiments with compound 1c have no diagnostic power with
respect to the r.d.s. in amide cyclization.
Fig. 4 Change of the N-Me signal of 1a with time in 0.03 M DCl (1 : 1
DMSO-d6–D2O).
Since no amide formation could be detected by the meth-
ods of analysis used, the barrier to the leaving of the x-(p-
nitrophenyl)ureido group from T should be much higher than
that of the ammonium cation. This finding defines the highest
barrier in the amide cyclization reaction; this means, however,
that this barrier does not participate in the imine hydrolysis and
characteristics of the latter can shed no further mechanistic detail
on amide cyclization.
Scheme 2 depicts the first step of imine hydrolysis as addition
of a water molecule to protonated imine 1H+ producing TH+
(O)
with a◦protonated OH group which will rapidly shed a proton to
give T because it is a stronger acid than H3O+. Acid catalysis by
protonation on the p-nitroanilino nitrogen leads to amide 2 while
protonation of the amino group gives, alternatively, hydantoin 3.
Scheme 2 yields the relationship:
V = kobs[1H+] = k2[T][H+] + k3[T][H+]
As argued above, k2 << k3; then application of the steady-state
approximation gives
k1k3
k−1 + k3
V/[1H+] = kobs
=
Results from acid amide hydrolysis8 and ester aminolysis15
favour k1 as the r.d.s. in acid catalyzed hydrolysis of iminohydan-
toins because the protonated amino group shows the lower barrier
upon leaving from tetrahedral intermediates. In basic media where
protons become scarce, even proton transfer to the amino group
in T becomes rate limiting,3,4 the splitting off of the amino group
being a faster reaction. Recently, the same process was also shown
to be a viable reaction pathway in a heterocyclic rearrangement
Fig. 3 Calculated plots of the change of concentration in 0.01 M HCl
with time for compounds b and c in the hypothetical case when only the
amide forms under kinetic control (see text).
Data on the rate of acid catalyzed cyclization of ureido amide
2a without methyl groups at 5-C are not available—results on the
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involving attack of the amino group of an isothiourea on an amide
function.16
were predicted for the formation of T transition state TS1 and
breakdown of T–TS2:
The reactivities in the series of imines studied support such
an assignment. The small rate decrease of only 3–4 times upon
introducing two methyl groups at 5-C in 1c agrees better with a
loose transition state of the addition of water than the second step
which involves k1/k−1 equilibrium ([T]/[1]) where steric strains will
be fully expressed. The faster reaction of the hydroxy derivative 1d
is apparently related to destabilization of charge of the ground state
dispersing in the transition state. In 0.015 M DCl, a solvent kinetic
isotope effect kH/kD of 1.72 (experiment with 1a) was measured,
which is evidence of a slow proton transfer. Most likely this is a
proton from the attacking molecule being transferred to a second
water molecule as depicted in the formula:
The cyclization of amide 2c in 0.95 M and 0.3 M HCl and DCl
gave SKIE kH/kD equal to 0.67 and 0.79 respectively, substantially
lower effects than those observed for amide hydrolysis of slightly
above 1 by Brown who calculated for a TS similar to TS1, SKIE =
1.1. Thus, TS2 for the cyclization reaction appeared the better
alternative, bearing in mind that water and the ureido group do not
differ much in their acid–base properties. Since the contradicting
evidence above, from partitioning of the tetrahedral intermediate
generated from imine hydrolysis, is unequivocal, we can iterate
that determination of the rate determining step should be based
on solid kinetic evidence.
The transition state for the alternative possible r.d.s., k3, is
unlikely to involve a slow proton transfer because the tetrahedral
Conclusions
intermediate protonated on the amino group TH+ is a more
(N)
stable structure than TH+(O) and should allow equilibrium whereby
a SKIE < 1 is expected.
1
UV and H NMR monitoring of the acid catalyzed hydrolysis
of 5-substituted 4-imino-1-methyl-3-(4-nitrophenyl)imidazolidin-
2-ones showed hydantoins as the only products. In 0.001–1 M HCl,
the reaction is zero order with respect to [H+]. Comparison with
previous data on rates of cyclization of the corresponding amides
of hydantoic acids allowed conditions (acid concentration, substi-
tution pattern—gem-dimethyl effect) to be found that guaranteed
kinetic control of the products obtained. Thus, unequivocal proof
was obtained that the formation of the tetrahedral intermediate
is rate determining in the cyclization of hydantoic acid amides
and the alternative previous suggestion based only on calculated
solvent isotope effects was refuted. The small steric effects upon
methyl substitution at 5-C and a solvent kinetic isotope effect
kH/kD of 1.72 favour a mechanism for imine hydrolysis whereby the
rate is limited by a water attack on the protonated imine concerted
with proton transfer from the attacking water to a second water
molecule.
Finally, returning to the problem of the rate determining
step in acid catalyzed cyclization of amides of hydantoic acids,
the evidence presented above, that the imine hydrolysis yields
hydantoins as products under kinetic control, is unequivocal proof
that the r.d.s. is the formation of the tetrahedral intermediate by
attack of the ureido group on the protonated amide (Scheme 3).
Acknowledgements
We thank the National Science Foundation of Bulgaria (Grant X-
1408) for financial support and Dr Iva Blagoeva of the Bulgarian
Academy of Sciences for useful discussions and cooperation.
Scheme 3
Previous suggestion3 that breakdown of the tetrahedral inter-
mediate could be the r.d.s. based only on the size of an inverse
solvent kinetic isotope effect can be discarded and shows how
insufficient such evidence can be. The suggestion stemmed from
comparison with the mechanism of acid catalyzed amide hydrol-
ysis studied extensively by Brown et al.8 who convincingly proved
the participation of a water molecule in the proton transfers. Part
of his evidence is based on the difference in solvent isotope effects
calculated from fractionation factors7 and reasonable assumptions
on the reaction coordinate for the alternative r.d.s., formation and
breakdown of the tetrahedral intermediate. In the cyclization of
the hydantoic acid amides studied before,3 the following SKIE
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