Ab initio calculations of effect of (thio)xanthenyl and dibenzosuberenyl substituents on dehydrogenation of N-Benzylanilines

Article abstract of DOI:10.1134/S1070363213040166

Higher values of the energies of the highest occupied molecular orbitals for N-benzyl-4-[9-(thio)-xanthenyl]aniline, N-benzyl-4-(5-dibenzosuberenyl) aniline molecules and their zwitterions and the electron density increase on the reaction center (carbon a

Full text of DOI:10.1134/S1070363213040166

ISSN 1070-3632, Russian Journal of General Chemistry, 2013, Vol. 83, No. 4, pp. 703–707. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © L.P. Yunnikova, V.P. Feshin, T.A. Akent’eva, 2013, published in Zhurnal Obshchei Khimii, 2013, Vol. 83, No. 4, pp. 618–623.
Ab Initio Calculations of Effect of (Thio)xanthenyl
and Dibenzosuberenyl Substituents on Dehydrogenation
of N-Benzylanilines
L. P. Yunnikovaa, V. P. Feshinb, and T. A. Akent’evaa
a Pryanishnikov Perm State Agricultural Academy, ul. Petropavlovskaya 23, Perm, 614000 Russia
e-mail: yunnikova@yahoo.com
b Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, Perm, Russia
Received March 23, 2012
Abstract—Higher values of the energies of the highest occupied molecular orbitals for N-benzyl-4-[9-(thio)-
xanthenyl]aniline, N-benzyl-4-(5-dibenzosuberenyl)aniline molecules and their zwitterions and the electron
density increase on the reaction center (carbon atom in the CH2-group) for the latter were obtained in the
quantum-chemical calculations by RHF/6-31G(d) method. This explains the possibility and the ease of
dehydrogenation of N-benzyl-4-[9-(thio)xanthenyl]aniline with imines in trifluoroacetic acid due to effect of
the substituents. The calculated data suggest the same possibility for N-benzyl-4-(5-dibenzosuberenyl)aniline: it
is dehydrogenated already in the synthesis from N-benzylaniline and dibenzosuberenol in acetic acid.
DOI: 10.1134/S1070363213040166
Previously [1] to explain partial recovery of a
number of heterocyclic N-nucleophiles in the reaction
dehydrogenation of СН2–NH group of N-arylmethyl-
anilines by the quantum-chemical calculations using
the RHF/6-31G(d) method. This is due to that the
reaction of imines with dibenzopyran (xanthene) [2] or
dibenzothiopyran (thioxanthene) [3] leads to
dehydrogenation of heterocyclic group С9Н, while
using in the same reaction the heterocycle substituted
at the carbon atom C9, e. g., N-phenyl-methyl-4-[9-
(thio)xanthenyl]aniline I or II the СН2–NH group is
subjected to dehydro-genation instead of С9Н moiety.
with the tritylium cation (N-methylacridane
>
thioxanthene > 1,3-benzodithiol > xanthene) or pro-
tonated imines (N-methylacridane > xanthene > thio-
xanthene > 1,3-benzodithiol) a mechanism of the
dehydrogenation of imines, including the intermediate
formation of ylides isomeric to the heterocycles has
been suggested. Based on the results of quantum-
chemical calculations performed by RHF/6-31G(d)
method, the energies of the highest occupied molecular
orbitals (ЕHOMO) and the electron density on the
reaction center C9 for ylides are significantly higher
than the corresponding values for these heterocycles.
In combination with the kinetic data this fact confirms
the proposed reaction mechanism. The tentative
threshold value of the energy gap |ΔЕ| between ЕHOMO
of heterocycle ylide and ЕLUMO of the protonated imine
is 2.737 eV for the reactions with the “ylide”
mechanism. Below this value the interaction of imines
with heterocyclic H-nucleophiles by this mechanism is
possible and above it, impossible.
H
H
H
H
O
C9
X
C9
(S)
NH CH Ph
X = O (I), S (II).
The aim of this work is to clarify the reasons for
changing the reaction center from С9Н moiety to СН2–
NH group and to substantiate the unusual ease of
dehydrogenation of (thio)xanthenyl-containing amines
I and II. The secondary aromatic amines are known to
be dehydrogenated under rigid conditions. For
example, N-benzylaniline is dehydrogenated to N-
In continuation of these studies, in this work we
evaluated the effect of xanthenyl, thioxanthenyl, and
dibenzosuberenyl substituents on the process of
703

704
YUNNIKOVA et al.
Scheme 1.
CF3COOH
I or II
Ph
+ PhCH2NHPh
O (S)
PhCH
CH
N
+
NPh
benzylideneaniline with use of industrial vanadium
catalyst B-3 at 300°C [3].
control experiment showed that under these conditions
N-benzyl-4-methoxyaniline containing p-electron-donor
methoxy group is not dehydrogenated with N-benzyl-
ideneaniline. Moreover, this reaction is pH-dependent,
since it occurs only in trifluoroacetic acid, but not in
ethanol or benzene. Probably, the role of trifluoro-
acetic acid is the formation of an amine complex A (or
A1) with the acid and the conversion of the latter into a
more reactive intermediate, zwitterion (Scheme 2, 3).
The products of reaction of imines with xanthene
[2] or thioxanthene [3], for example, N-phenylmethyl-
4-[9-(thio)xanthenyl]anilines, are readily converted to
the corresponding N-phenylmethylene-4-[9-(thio)xanthe-
nyl]anilines on heating with N-benzylideneanilines in
trifluoroacetic acid (Scheme 1).
It is presumable that the heterocyclic substituents in
these molecules are strong electron-donors and thus
contribute to the process of dehydrogenation of the
Based on this assumption, we performed the quan-
tum-chemical calculation at the RHF/6-31G(d) level
СН2–NH group of the secondary amines. However, the using Gaussian 03W program [4] of N-benzyl-4-(9-
Scheme 2.
6
H
C14
H
N
4
5
1
13
12
11
10
3
8
X
2
9
H
H
7
I, II
6
H
4
5
1
13
12
11
10
−
C14
N
+
3
8
X
X
9
2
H
H
H
7
H
C
H
H
N+
HY
B, C
X
Y−
H
6
H
H
4
5
1
13
12
11
А
C14 N+
3
8
−
10
9
2
H
H
7
D, E
X = O (I, B, D), S (II, C, E); Y = CF3COOH.
H
C14
H
N
H
H
N+
C14
11
8
11
8
OCH3
OCH3
−
H
H
III
F
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 4 2013

Ab Initio CALCULATIONS OF EFFECT OF (THIO)XANTHENYL
705
Scheme 3.
6
H
C14
H
N
H
H
C
H
5
4
3
2
13
HY
12
11
10
+
8
N
H
9
1
H
H
15
Y−
7
А1
IV
6
H
H
5
4
3
2
13
12
11
10
C14 N+
8
−
9
1
H
H
7
G
Y = CF3COO.
molecule of V compared to the energy difference of
the frontier orbitals of amines I, II, IV and the
molecule V. The obtained values of the energy gaps
are significantly lower than the previously found
threshold value |ΔЕ| for the reaction of the protonated
imine with ylides of N-methylacridane, xanthene and
thioxanthene [1], which indirectly confirms the high
reactivity of zwitterions D, E, and G corresponding to
amines I, II and IV.
xanthenyl)aniline I and N-benzyl-4-(9-thioxanthenyl)-
aniline II molecules as well as the corresponding two
types of most likely zwitterions: (a) B and C, in which
the negative charge is formally located on the carbon
atom of the C–N group, and (b) D and E, in which the
negative charge is located on the heterocycle. For
comparison, the quantum-chemical calculations were
performed also for the secondary amine N-benzyl-4-
methoxyaniline III, containing the p-electron-donor
methoxy group in the para-position of aniline ring
instead of the heterocycle, and for the corresponding
zwitterion F. Furthermore, the molecules of N-benzyl-
4-(5-dibenzosuberenyl)aniline IV, containing the struc-
turally analogous dibenzosuberenyl substituent, and
the corresponding zwitterion G were calculated. The
zwitterions formation is described in Schemes 2 and 3.
The charges on the carbon atoms of С14–N group
for zwitterions D, E, and G increase compared with
Table 1. Energies of the highest occupied molecular orbitals
(EHOMO) of structures I–IV and energy gaps |ΔЕ| = ЕHOMO
(I–IV, B–G) – ЕLUMO (V)a
The calculated data are shown in Tables 1 and 2.
The ЕHOMO values (Table 1) and charges on the carbon
atoms of the С–N group in the zwitterions B, C and F
(Table 2) are comparable. Therefore, it can be assumed
that these zwitterions are not capable to be
dehydrogenated, since N-benzyl-4-methoxyaniline III
is not dehydrogenated under the above conditions. The
Structure
ЕHOMO, eV
–7.594
–5.930
–4.455
–7.551
–5.908
–4.627
–7.320
–5.896
–7.469
–4.530
|ΔЕ|, eV
4.664
3.001
1.526
4.622
2.979
1.698
4.391
2.967
4.540
1.601
I
B
D
II
C
E
Е
HOMO values of the zwitterions D, E, and G are higher
than that for B, C, and F and significantly higher than
for the structures I–IV.
III
F
The calculated values of the energy gaps |ΔЕ|
(Table 1) for the reaction of zwitterions D, E, and G
with the protonated N-benzylideneaniline [PhCH=NHPh]+
V indicate a significant convergence of the HOMO
energy level of these zwitterions and LUMO of the
IV
G
a ЕLUMO [PhCH=NHPh]+ (V) = –2.929 eV [1].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 4 2013

706
YUNNIKOVA et al.
Table 2. Charges on atoms (q) in structures I–IV and B–G
q, e
Structure
С1
С2
С3
С4
С5
С6
С7
С8
С9
С10
С11
С12
С13
С14
I
0.390 –0.032 –0.202 –0.033 0.390 –0.194 –0.195 0.035 –0.189 –0.254 0.346 –0.265 –0.221 –0.138
0.393 –0.031 –0.222 –0.033 0.391 –0.197 –0.197 0.079 –0.203 –0.225 0.185 –0.180 –0.244 –0.254
0.373 –0.013 –0.123 –0.013 0.372 –0.240 –0.241 0.102 –0.282 –0.179 0.028 –0.169 –0.295 –0.199
–0.205 0.071 –0.293 0.068 –0.207 –0.198 –0.200 0.050 –0.205 –0.255 0.347 –0.264 –0.219 –0.138
–0.204 0.066 –0.298 0.062 –0.207 –0.198 –0.200 0.085 –0.216 –0.225 0.186 –0.180 –0.242 –0.254
–0.215 0.070 –0.203 0.070 –0.215 –0.223 –0.224 0.106 –0.283 –0.178 0.028 –0.166 –0.295 –0.199
B
D
II
C
E
III
F
0.409
0.447
0.319
0.171
–0.141
–0.255
IV
G
0.028 0.003 –0.216 0.033 0.028 –0.230 –0.230 –0.013 –0.212 –0.257 0.344 –0.263 –0.207 –0.136
0.007 0.009 –0.191 0.007 0.007 –0.215 –0.213 0.110 –0.297 –0.163 0.016 –0.173 –0.285 –0.196
the charges for the molecules I–IV (Table 2). This
explains the easy dehydrogenation of the secondary
aromatic amines I and II containing heterocyclic
moiety. The partial negative charges on the carbon
atom C3 of the structures D, E, and G are significantly
lower than that for the same carbon atom in the struc-
tures I–III (Table 2). This explains the deactivation of
C3H group and also the transfer of the reaction center
from this group to the СН2–NH fragment.
resulted from the partial delocalization of the charge of
the atom C3. The electron density on the carbon atoms
C1 and C5 and the oxygen atoms in the zwitterion and
amine I varies slightly. The charges on the atoms C10
and C12 (q –0.179, –0.169 e) in the zwitterion are
lower as compared with the respective charges in the
amine I (q –0.254, –0.265 e). Apparently, such charges
redistribution is due to an increase in the electron
density on the atom C11 in the zwitterion.
The electron density on the carbon atoms C9, C13,
and C11 in zwitterions D, E, and G increases as
compared with those atoms in the structures I–III.
Apparently, such redistribution of the electron density
in the zwitterions demonstrates the significant contribu-
tion of quinoid structure H, thereby increasing the
electron density on the carbon atom C14 and explains
the easy dehydrogenation of the secondary aromatic
amines I and II containing heterocyclic moiety.
Thus, on the basis of the calculations performed it
is possible to predict the ability of N-benzyl-4-(5-
dibenzosuberenyl)aniline IV to the dehydrogenation.
This conclusion is indirectly supported by the
experimental fact that the interaction of N-benzyl-
aniline VI with dibenzosuberenol VII in acetic acid is
accompanied by the formation of N-phenylmethyl-4-
(5-dibenzo[а,d]cycloheptatrienyl)aniline VIII as a main
product and N-phenylmethylene-4-(5-dibenzo[a,d]cyclo-
heptatrienyl)aniline IX as a side product. The latter
forms due to the dehydrogenation of СН2–NH group
(Scheme 4).
+
Ph
CH2
N
H
X
EXPERIMENTAL
The 1H NMR spectra were registered on a
spectrometer Mercury 300 (300 MHz) with internal
reference HMDS. The GC-MS spectra were obtained
on a Agilent Technologies 6890N/5975V instrument
[column HP-5ms (30 × 0.25 mm, 0.25 μm), carrier gas
helium, electron impact ionization (70 eV), the tem-
perature of the oven 100°C, evaporator temperatures
H
X = O, S.
The partial negative charges on the carbon atoms
C6 and C7 of the heterocyclic moiety in the structures
D and E increase as compared with the charges on the
same atoms in the structures I and II. Apparently, this
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 4 2013

Ab Initio CALCULATIONS OF EFFECT OF (THIO)XANTHENYL
707
Scheme 4.
H
CH3COOH
Ph
+
Ph
CH2
NH
CH2
NH
HO
VI
VII
VIII
+
Ph
CH
N
−
IX
2. Yunnikova, L.P., Zh. Org. Khim., 1995, vol. 31, no. 1,
260, 270°C. Dibenzosuberenol is a commercial
product.
p. 76.
3. Yunnikova, L.P., Khim. Geterotsikl. Soed., 1995, no. 7,
N-Phenylmethyl-4-(5-dibenzo[a,d]cycloheptatri-
enyl)aniline (VIII). a. A mixture of 0.24 g (1.3 mmol)
of N-benzylaniline VI, 0.21 g (1 mmol) of dibenzo-
suberenol VII, and 4 ml of acetic acid was kept at
room temperature for 3 h. The reaction mixture was
diluted with water. The crystals were separated,
washed several times with water, and dried. Yield 0.22 g
p. 1003.
4. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E.,
Robb, M.A., Cheeseman, J.R., Montgomery, J.A., Jr.,
Vreven, T., Kudin, K.N., Burant, J.C., Millam, J.M.,
Iyengar, S.S., Tomasi, J., Barone, V., Mennucci, B.,
Cossi, M., Scalmani, G., Rega, N., Petersson, G.A.,
Nakatsuji, H., Hada, M., Ehara, M., Toyota, K., Fukuda, R.,
Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y.,
Kitao, O., Nakai, H., Klene, M., Li, X., Knox, J.E.,
Hratchian, H.P., Cross, J.B., Bakken, V., Adamo, C.,
Jaramillo, J., Gomperts, R., Stratmann, R.E., Yazyev, O.,
Austin, A.J., Cammi, R., Pomelli, C., Ochterski, J.W.,
Ayala, P.Y., Morokuma, K., Voth, G.A., Salvador, P.,
Dannenberg, J.J., Zakrzewski, V.G., Dapprich, S.,
Daniels, A.D., Strain, M.C., Farkas, O., Malick, D.K.,
Rabuck, A.D., Raghavachari, K., Foresman, J.B., Ortiz, J.V.,
Cui, Q., Baboul, A.G., Clifford, S., Cioslowski, J.,
Stefanov, B.B., Liu, G., Liashenko, A., Piskorz, P.,
Komaromi, I., Martin, R.L., Fox, D.J., Keith, T., Al-
Laham, M.A., Peng, C.Y., Nanayakkara, A., Сhalla-
combe, M., Gill, P.M.W., Johnson, B., Chen, W.,
Wong, M.W., Gonzalez, C., and Pople, J.A.,
GAUSSIAN 03, Rev. D.1, Gaussian, Inc., Wallingford,
CT, 2005.
1
(61.3%), mp 128–129°C (hexane). H NMR (CDCl3),
δ, ppm: 3.85 s (1H, NH), 4.18 s (2Н, CH2), 5.27 s (1Н,
С5Н), 6.31–7.43 m (19Н, АrН + С10Н, С11Н). Mass
spectrum, m/z (Irel, %): 373 [М]+(100); 282(15); 191
(19); 91(43); 77(3).
b. A mixture of 0.09 g (0.5 mmol) of N-benzyl-
aniline VI, 0.21 g (1 mmol) of dibenzosuberenol VII,
and 4 ml of acetic acid was kept at room temperature
for 4 h. The reaction mixture was diluted with water.
The crystals were separated, washed with water, dried,
and extracted with diethyl ether. The precipitated
compound IX was separated. Yield 0.006 g (3.2%), mp
186–187°C (benzene). From ether extract amine VIII
was isolated in a yield of 59%.
REFERENCES
1. Yunnikova, L.P., Makhova, T.V., Yunnikov, A.L., and
Gorokhov, V.Yu., Zh. Org. Khim., 2009, vol. 45, no. 5,
p. 749.
5. Kozlov, N.S., Pak, V.D., and Yunnikova, L.P., Zh. Org.
Khim., 1973, vol. 9, no. 4, p. 847.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 4 2013