Effect of Solvents on Acid-Catalyzed Claisen Amino Rearrangement in N-(1-Methyl-2-butenyl)aniline

Article abstract of DOI:10.1134/S0036024418120026

Abstract: The effect solvents have on the processes of rearrangement and elimination in N-(1-methyl-2-butenyl)aniline (I) in the presence of HCl is studied. It is shown that the dependence of the rearrangement and elimination rate constants of (I) · HCl on the nature of solvents are described perfectly by the Koppel–Palm equation, which considers both nonspecific and specific solvation. The inhibitory effect of solvent nucleophilicity is explained by the complexation between (I) · HCl and solvent molecules. Analysis of the (I) · HCl conversion products obtained in a mixed solvent (m-toluidine + nitrobenzene) demonstrates the intermolecular transfer of the allyl moiety, confirming the formation of allyl cations in the Claisen amino rearrangement.

Full text of DOI:10.1134/S0036024418120026

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 1, pp. 23–27. © Pleiades Publishing, Ltd., 2019.
Russian Text © I.B. Abdrakhmanov, V.M. Sharafutdinov, A.G. Mustafin, Yu.S. Zimin, R.R. Daminev, 2019, published in Zhurnal Fizicheskoi Khimii, 2019, Vol. 93, No. 1, pp. 23–27.
CHEMICAL KINETICS
AND CATALYSIS
Effect of Solvents on Acid-Catalyzed Claisen Amino Rearrangement
in N-(1-Methyl-2-butenyl)aniline
I. B. Abdrakhmanova, V. M. Sharafutdinovb, A. G. Mustafinc, Yu. S. Ziminc,*, and R. R. Daminevb
aUfa Institute of Chemistry, Russian Academy of Sciences,
Ufa, 450054 Republic of Bashkortostan, Russia
bUfa State University of Petroleum Technology, Ufa, 450062 Republic of Bashkortostan, Russia
cBashkir State University, Ufa, 450076 Republic of Bashkortostan, Russia
*е-mail: ZiminYuS@mail.ru
Received February 13, 2018
Abstract—The effect solvents have on the processes of rearrangement and elimination in N-(1-methyl-2-
butenyl)aniline (I) in the presence of HCl is studied. It is shown that the dependence of the rearrangement
and elimination rate constants of (I) · HCl on the nature of solvents are described perfectly by the Koppel–
Palm equation, which considers both nonspecific and specific solvation. The inhibitory effect of solvent
nucleophilicity is explained by the complexation between (I) · HCl and solvent molecules. Analysis of the
(I) · HCl conversion products obtained in a mixed solvent (m-toluidine + nitrobenzene) demonstrates the
intermolecular transfer of the allyl moiety, confirming the formation of allyl cations in the Claisen amino
rearrangement.
Keywords: N-(1-methyl-2-butenyl)aniline, Claisen rearrangement, acidic catalysis, effect of solvents
DOI: 10.1134/S0036024418120026
INTRODUCTION
The kinetics of N-(1-methyl-2-butenyl)aniline
conversion was studied using the experiment-point
approach. Experiments were performed at 126°C in
The acid-catalyzed Claisen rearrangement in the
series of N-allylanilines is a convenient way of synthe-
sizing ortho-substituted arylamines [1]. A number of sealed glass ampoules (reaction mixture volume,
researchers [2–4] believe this reaction proceeds
according to the intramolecular 3,3-sigmatropic
mechanism. In [5, 6], we showed the mechanism of
rearrangement in N-(1-methyl-2-butenyl)aniline was
different than commonly believed. Relatively low acti-
vation enthalpy, positive activation entropy [5], and
the formation of para-products and cross rearrange-
ment products during the simultaneous conversion of
~1 mL). The catalyst (HCl) was introduced into the
reaction mixture in the form of hydrochloride of
compound I. The freshly prepared reaction mixture
was loaded in the ampules, which were then sealed
and placed into a thermostat. The temperature was
maintained with a precision of 0.2°C. The first
sample (the zero point) was taken after 2 min of heat-
two substrates [6] testify to the more complicated ing. The ampoule was cooled to 20°C and opened,
mechanism of this process.
after which excess triethylamine was added to convert
the hydrochlorides of aromatic amines into free
bases. The reaction mixture was subsequently ana-
lyzed via gas–liquid chromatography, using pentade-
cane as an internal standard. The other ampoules
were taken at specific time intervals and analyzed in
the same way.
It is known that concerted reactions depend weakly
on the nature of solvents [7]. To study the rearrange-
ment in N-(1-methyl-2-butenyl)aniline, we studied
the kinetic patterns of this reaction in different sol-
vents in the presence of HCl.
EXPERIMENTAL
N-(1-Methyl-2-butenyl)aniline (I) and 2-(1-
methyl-2-butenyl)aniline (II) were synthesized
according to [8] and [9], respectively. The physico-
chemical characteristics of the synthesized com-
pounds corresponded to the literature data. Freshly
distilled solvents were used in all the experiments.
RESULTS AND DISCUSSION
In [5], it was shown that the conversion of N-(1-
methyl-2-butenyl)aniline in the presence of HCl
obeys the laws of the parallel first-order reaction
23

24
ABDRAKHMANOV et al.
reaction to the effect of nonspecific (Y, Р) and specific
NH2 · HCl
(Е, В) solvation. Parameter Y is expressed by Kirk-
wood function Y = (ε − 1)/(2ε + 1), where ε is the
dielectric permittivity of the solvent. Parameter Р is
determined by the formula Р = (n2 − 1)/(n2 + 2),
where n is the refractive index of the solvent.
k1
NH · HCl
(I) · HCl
(II) · HCl
Using the Koppel–Palm equation to describe the
kinetic results in the solvents listed in Table 1 produces
the expressions for rearrangement,
NH2 · HCl
k2
+
log k1 = –11.17 + 3.93Y + 13.52Р + 0.47Е
– 3.04 ×10–3В (r = 0.994)
1
where k1 is the rate constant of rearrangement, k2 is the
rate constant of elimination, and k = k1 + k2 is the rate
constant of (I) · HCl consumption in both processes.
We established that the directions of (I) · HCl conver-
sion remained the same after the nature of solvents was
altered.
The results from studying the effect solvents have
on the rate constants of (I) · HCl rearrangement and
elimination are presented in Table 1. The data show
the highest rate of rearrangement was observed in
nitrobenzene, while the highest rate of elimination was
for a mixture of two solvents (chlorobenzene + nitro-
benzene). The lowest rates of both processes were
observed in N,N-dimethylalanine. It should be noted
that the nature of the solvents had a stronger effect on
rearrangement (k1 changed by a factor of 30) than
elimination (k2 changed by a factor of 18) (Table 1).
and elimination,
log k2 = –9.26 – 0.85Y + 13.68Р + 0.62Е
– 3.89 ×10–3В (r2 = 0.996),
where r1 and r2 are the correlation coefficients.
According to our calculations, the rate constants of
rearrangement and elimination in the gas phase are
5−8 orders of magnitude lower than in solvents. It
seems the H+ proton is bound more strongly to the Cl−
anion than amine in the gas phase due to there being
no solvation, reducing the catalytic effect Н+ has on
the conversion of (I).
The contributions from individual solvent parame-
ters to the reactivity of the initial reagent can be esti-
mated from the data of Tables 2 and 3.
The accelerating effect polarizability has on log k1
and log k2 indicates that the transitional states of both
reactions are of a polar character [11]. The different
signs of polarity coefficient у for rearrangement and
elimination apparently show the rate-controlling
stages of these reactions are different for these reac-
tions.
The effect the solvents had on the rate constants of
rearrangement and elimination are described perfectly
by the Koppel–Palm equation, which considers both
nonspecific and specific solvation [10]:
log ki = log k0i + yY + рР + еЕ + bВ,
where ki is the reaction rate constant in the solvent; k0i
is the reaction rate constant in the gas phase; Y is the
The growth of k1 as the polarity of a solvent
polarity of the solvent; Р is the polarizability of the sol- increases testifies to the stronger separation of charges
vent; Е and В are the empirical electrophilicity and in the activated complex, relative to the initial reagent,
nucleophilicity parameters of the solvent; and y, р, е, the total charge of which is more compact [12]. This is
and b are coefficients characterizing the sensitivity of a likely possible at the stage of the formation of a σ com-
Table 1. Rate constants of the rearrangement (k1) and elimination (k2) of N-(1-methyl-2-butenyl)aniline, depending on
the nature of solvents at 126°C*
k1 × 105, s−1
k2 × 105, s−1
Solvent
Y
Р
Е
В
Chlorobenzene
3.49 0.05
7.76 0.59
12.4 1.6
4.90 0.49
5.66 0.23
5.45 0.24
3.21 0.14
3.61 0.70
2.90 0.45
0.31 0.03
0.3775
0.4285
0.4788
0.4581
0.4690
0.4798
0.3641
0.3968
0.4058
0.4147
0.4041
0.3728
0.3414
0.4166
0
0
0
38.0
52.6
Chlorobenzene + nitrobenzene
Nitrobenzene
67.0
Acetophenone
7.17 0.13
4.41 0.27
5.10 0.27
0.41 0.01
0.70
1.65
2.60
0
202.0
246.5
291.0
422.0
N,N-Dimethylformamide + acetophenone
N,N-Dimethylformamide
N,N-Dimethylaniline
* Parameters of the solvents were taken from [9].
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EFFECT OF SOLVENTS ON ACID-CATALYZED CLAISEN AMINO REARRANGEMENT
25
Table 2. Contribution from nonspecific and specific solvent solvations to log k1 at 126°C
Solvent
уY
рР
еЕ
bВ
Chlorobenzene
1.4850
1.6856
1.8835
1.8020
1.8449
1.8874
1.4323
5.3631
5.4848
5.6051
5.4618
5.0388
4.6144
5.6308
0
0
0
–0.1156
–0.1599
–0.2037
–0.6143
–0.7496
–0.8850
–1.2833
Chlorobenzene + nitrobenzene
Nitrobenzene
Acetophenone
0.3263
0.7691
0.2119
0
N,N-Dimethylformamide + acetophenone
N,N-Dimethylformamide
N,N-Dimethylaniline
Table 3. Contribution from nonspecific and specific solvent solvations to log k2 at 126°C
Solvent
уY
рР
еЕ
bВ
Chlorobenzene
–0.3195
–0.3627
–0.4053
–0.3877
–0.3970
–0.4061
–0.3082
5.4282
5.5313
5.6731
5.5281
5.0999
4.6703
5.6991
0
–0.1479
–0.2046
–0.2607
–0.7860
–0.9591
–1.1323
–1.6420
Chlorobenzene+ nitrobenzene
Nitrobenzene
0
0
Acetophenone
0.4340
1.0230
1.6920
0
N,N-Dimethylformamide + acetophenone
N,N-Dimethylformamide
N,N-Dimethylaniline
plex whose positive charge is stabilized as a result of an inert solvent; as with all aromatic amines, it is
solvation.
monomerized according to the data in [14], while the
associative state of amines in strongly base solvents
depends on the difference between the basicities of the
amine and the solvent.
The decelerating effect of solvent nucleophilicity is
probably associated with the complexation between
(I) · HCl and solvent molecules at a ratio of [(I) · HCl] :
[solvent] = 1 : 1, leading to a drop in the amount of
protonated (I) and thus the catalytic effect of HCl.
Analysis of the products of (I) conversion obtained
in a mixed solvent (m-toluidine + nitrobenzene)
Excess free base (I) has an inhibitory effect on both
rearrangement and elimination. It is likely that an
inactive complex with a ratio of [amine] : [HCl] = 2 :
1 is in this case formed:
1.6
1.2
0.8
0.4
Ar
Ar
Ar NH···HCl···HN
Ar NH · HCl + HN
R
R
R
R
where R is an allyl moiety. The constant of this equi-
librium varies from 50 to 104 for different amines [13].
Similar complexation processes are also possible in
any other solvent with appreciable nucleophilicity.
To determine the effect of specific solvation, we
studied the acid-catalyzed rearrangement of com-
pound (I) in a mixed m-toluidine + nitrobenzene sol-
vent. The rate constant of total (I) · HCl consumption
falls as the molar content of m-toluidine rises (Fig. 1).
0
0.2
0.4
0.6
0.8
1.0
x
This nonlinear dependence indicates that one of
the mixed solvent system’s components is inert, while
the other is capable of the specific solvation of
reagents. In the considered system, nitrobenzene was
Fig. 1. log k versus m-toluidine molar content x for the
acid-catalyzed conversion of compound (I) in a mixed
m-toluidine + nitrobenzene solvent at 132°C. See the text
for denotations.
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26
ABDRAKHMANOV et al.
10
8
10
8
6
6
II
III
IV
III
4
4
V
I
2
2
II
V
IV
I
0
2
4
6
8
10
t, h
0
2
4
6
8
10
t, h
Fig. 3. Kinetic curves of the change in the concentrations
of compound (I) conversion products obtained in a mixed
m-toluidine + nitrobenzene solvent at 132°C and [m-tolu-
Fig. 2. Kinetic curves of the change in the concentrations
of compound (I) conversion products obtained in a mixed
m-toluidine + nitrobenzene solvent at 132°C and [m-tolu-
idine] = 6.0 mol/L. See the text for denotations.
idine] = 1.54 mol/L. See the text for denotations.
0
0
At high m-toluidine concentrations (6.0 mol/L),
an allyl cation is transferred from amine (I) to the
nitrogen atom of m-toluidine, resulting in high yields
of aniline (III) and N-(1-methyl-2-butenyl)-3-meth-
ylaniline (IV) (path 1–4–5):
revealed the intermolecular transfer of allyl moieties,
providing further evidence of the formation of allyl
cations during Claisen amino rearrangement. The
kinetic curves of the accumulation of products of
amine (I) isomerization are shown in Figs. 2 and 3.
NH2
NH2
(III)
NH2
NH · HCl
1
4
5
(I) · HCl
+ +
+
+
NH2
(IV) · HCl
+
+
NH3
NH2 · HCl
NH3
2
3
6
7
(II) · HCl
+
+
Rearrangement in (IV) (path 5–6) is hindered, due rangement of N-(1-methyl-2-butenyl)aniline in dif-
to the formation of an inactive complex in excess ferent solvents.
m-toluidine. Rearrangement (paths 1–2 and 5–6) is
also possible at lower concentrations of it (1.54 mol/L),
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Translated by E. Glushachenkova
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 1 2019