Facile synthesis of triarylmethanimine promoted by a lewis acidbase pair: Theoretical and experimental studies

Article abstract of DOI:10.1071/CH12090

An efficient method for triarylmethanimine synthesis promoted by a Lewis acidbase pair (AlCl3Et3N) was designed using mechanistic analysis with the aid of density functional theory. A series of triarylmethanimines were successfully prepared under mild con

Full text of DOI:10.1071/CH12090

CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 1390–1395
Full Paper
http://dx.doi.org/10.1071/CH12090
Facile Synthesis of Triarylmethanimine Promoted
by a Lewis Acid Base Pair: Theoretical and
]
Experimental Studies
A
A
A
A B
,
Yan Liu, Qiwu Yang, Dongling Hao, and Wenqin Zhang
A
Department of Chemistry, Tianjin University, Tianjin 300072, P. R. China.
Corresponding author. Email: zhangwenqin@tju.edu.cn
B
An efficient method for triarylmethanimine synthesis promoted by a Lewis acid–base pair (AlCl3–Et3N) was designed
using mechanistic analysis with the aid of density functional theory. A series of triarylmethanimines were successfully
prepared under mild conditions in good to excellent yields with a simple work-up procedure. The promoter, the Lewis
acid–base pair (AlCl3–Et3N), is inexpensive, efficient, and shows good functional group tolerance. The experimental
results show that the electronic effect played a significant role, i.e. the reactions proceeded smoothly when electron-
sufficient arylamines and electron-deficient ketones were used as substrates.
Manuscript received: 11 February 2012.
Manuscript accepted: 27 April 2012.
Published online: 29 June 2012.
Introduction
reaction times, high temperatures, expensive substrates, toxic
reagents, poor functional group tolerance, or limited scopes.
AlCl3 acting as a promoter for the imine synthesis from
ammonia and acetophenone was first reported by Strain.[13]
However, high temperature and a long reaction time are needed
and the yield was very low (less than 40 %). Based on Strain’s
report,[13] a computational design[28–30] and our own theoretical
analysis, herein an improved synthesis for triarylmethanimines
promoted by an AlCl3–Et3N pair, is reported (Scheme 1).
Schiff bases[1] are an important class of biologically active
compounds.[2,3] They play an important role in the biosynthesis
of amino acids[4] and aza-aromatic heterocycles.[5] Moreover,
Schiff bases have proved to be attractive building blocks for
creating new active and selective sites in coordination chemis-
try. Nowadays Schiff bases are still considered ‘privileged
ligands’ by modern chemists.[6,7]
Unlike the condensation of alkylarylketones (such as aceto-
phenone) and arylamines,[8–10] the direct condensation of
diarylketones with arylamines is very difficult and a high
temperature and/or catalysts are required.[11,12] Brønsted or
Lewis acids, such as AlCl[313] ZnCl2,[14] HBr,[14] POCl3,[15]
BF3,[16] PhN(MgBr)2,[17] EtAlCl2,[18] TiCl4,[19–21] or
Pd(OAc)[222] have been used to promote the reaction by enhanc-
ing the activity of the electrophile. It is also essential to add
either the dehydration agent, tetraethyl orthosilicate
(Si(OEt)4),[23] or activated molecular sieves[24,25] in order to
facilitate the reaction. Other methods, such as the hydroamina-
tion of alkynes have also been reported.[26] Metathesis of oximes
with arylboronic acids is another alternative method.[27] In spite
of their potential usefulness, the methods mentioned above all
suffer from one or more drawbacks such as low yields, long
Results and Discussion
Theoretical Results
The density functional theory (DFT) results show that the
atomic polar tensor (APT) charge on the carbonyl carbon in
Ph2C¼O is þ1.142 eꢀ and the carbonyl carbon becomes more
positive (þ1.565 eꢀ) in the presence of AlCl3. The charge
analysis shows that AlCl3 acts as a promoter to enhance the
electrophilicity of the carbonyl carbon.
In terms of the mechanism, the reaction proceeds by a
nucleophilic addition giving a hemiaminal –C(OH)(NHR)–
intermediate, followed by an elimination of water to yield the
imine. There are two possible transition states (TS) through
Ar
N
O
AICI3/Et3N
r.t.
NH2
ϩ
Ar
R1
R2
R1
R2
1
2
3
Ar ϭ Ph, p-NO2-C6H5, o-X-C6H5, m-X-C6H4, p-X-C6H5, 3, 5-di-NO2-C6H4, 1-naphthyl,
R1, R2 ϭ H, F, MeO, NO2
Scheme 1
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

Synthesis of triaryl imines promoted by AlCl3–Et3N pair
1391
which the hemiaminal intermediate can be obtained, a) the
hydrogen on the amine directly migrates to the oxygen (four-
membered ring transition state) and b) the H migrates with a
transfer (six-membered ring transition state). The results are
depicted in Fig. 1. The energy of the barrier for the nucleophilic
addition step, which occurs through a four-membered TS, is
54.2 kcal molꢀ1 (Fig. 1a) and is 15.3 kcal molꢀ1 higher than that
promoted by AlCl3 (Fig. 1b). The high barrier of the four-
membered TS can be attributed to ring strain. In order to reduce
the calculation resource, ammonia was used in place of aryla-
mine to act as a proton transfer agent. It was found that ammonia
is a better transfer agent than water in the six-membered ring
transition state (Fig. 1c, d). In the case of ammonia, the energy
barrier of the TS is at 38.1 kcal molꢀ1, which is 4.3 kcal molꢀ1
lower than that of water. Therefore, in the following study, a six-
membered transition state with ammonia as the transfer agent
was chosen as the optimized transition state.
six-membered TS is 106.4 J molꢀ1 ꢁ Kꢀ1 higher than that of the
four-membered TS, the enthalpy barrier of the six-membered TS
is only 6.0 kcal molꢀ1 compared with the 33.1 kcal molꢀ1 of the
enthalpy barrier of the four-membered TS. As a result, the
AlCl3-promoted process through the six-membered TS is still
feasible. The hydroxy leaving barrier is only 8.5 kcal molꢀ1
which is 8.3 kcal molꢀ1 lower than the hemiaminal formation
step, i.e. the first step is the rate determining step, in which the
hemiaminal intermediate (INT-1) forms after a nucleophilic
attack and there is a H transfer with the aid of another molecule
of amine.
HO–AlCl–3 then leaves through TS-2 followed by Et3N
capturing a hydrogen on the nitrogen atom which has a
low barrier; the whole process is an exergonic process
(ꢀ16.5 kcal molꢀ1). Finally the strongly exergonic reaction
between Et3NHþ and HOAlClꢀ3 occurs to form very stable
Et3NHCl and HOAlCl2, and this makes the whole process
irreversible (see Supplementary Material Fig. S11). The low
activation energy indicates that the reaction occurs at low
temperature.
The total energy profile of the reaction is shown in Fig. 2.
The activation energy of the AlCl3-promoted hemiaminal inter-
mediate formation step is 16.8 kcal molꢀ1 (Fig. 2, TS-1), which
is 21.3 kcal molꢀ1 lower than that without the Lewis acid
(Fig. 1d) and 19.5 kcal molꢀ1 higher than the barrier of the
four-membered TS. Although the entropy change through a
The solvent effect of CHCl3 was also taken into
consideration. The energy profile of the process showing the
solvent effect is shown in Fig. S12 (Supplementary Material).
38.1
(12.8)
42.4
(18.8)
54.2
36.3
(44.0)
(33.1)
H
O
H H
N
H
N
H
O
H
H
H
O
H
N
H
CI3Al
H
0.0
(0.0)
0.0
(0.0)
O
N
0.0
(0.0)
O
N
H
0.0
(0.0)
H
H2O
NH3
H
N
NH2
NH2
NH2
H
O
AIC3
O
O
O
a
b
c
d
Fig. 1. Energy profiles of the comparative study with different mechanisms. The relative free energies and electronic energies (in parentheses) are given in
kcal molꢀ1
.
2.2
(Ϫ40.8)
2.2
(Ϫ26.3)
0.0
(0.0)
TS-2
TS-1
Cl3Al
Reagent
Ϫ6.3
(Ϫ49.3)
H
N
O
NH2
H
NH2
Ϫ14.6
INT-1
OH
AlCl3
(Ϫ46.8)
ϩ
Ϫ16.5
HN
H
O
(Ϫ42.7)
NH3
H
complex
H–NH2
AlCl3
O
ϩ
ϩ
HN
AlCl3
Et3N
N
Et3N
O
H
H
ϩ
N
AlCl3
ϩ
ϩ
HOAlCl2
Cl
NH3
Et3N
Et3N
Et3N–H
Fig. 2. Energy profile of the whole reaction process. The relative free energies and electronic energies (in parentheses) are given in kcal molꢀ1
.

1392
Entry
Y. Liu et al.
Table 1. Effect of solvent
EDGs at the ortho-positions were prolonged to more than 10 h
and the yields were low (Table 3). Among these, 2,6-dimethy-
laniline gave the lowest yield. In the case of 2-nitroaniline, no
reaction product was isolated (Table 3). This suggests that both
the steric effect and the electron withdrawing character of the
nitro group hinder the condensation.
Next, the effect of substituents on the diarylketones was
studied and the results are listed in Table 4. The electronic effect
of the diarylketones was opposite to that of the arylamine, i.e. an
electron-deficient diarylketone facilitated the reaction (Table 4)
whereas an electron-sufficient one hindered the reaction
(Table 4). Excellent yields were obtained for 9-fluorenone and
9,10-anthraquinone and this may be attributed to the instability
of both substrates which is caused by the anti-aromatic cyclo-
pentadienone and quinone moieties (Table 4). In conclusion, the
condensation is facilitated by EDGs and halogen on arylamines,
and EWGs on diarylketones.
Ph2CO/PhNH2/AlCl3/Et3N ¼ 1 : 2 : 1.7 : 5.1, room temperature
Solvent
Time [h]
Yield [%]
1
2
3
4
5
CH2Cl2
20
5
83.9
92.3
59.9
70.9
85.4
CHCl3
CCl4
24
24
8
Benzene
Chlorobenzene
Table 2. Optimization of the reaction conditions at room temperature
Entry
Diarylketone
[equiv.]
Arylamine
[equiv.]
AlCl3
Et3N
Yield
[%]
[equiv.]
[equiv.]
1
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.2
1.4
1.6
1.6
1.8
2.0
1.6
1.6
1.6
1.6
1.6
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.0
1.35
2.05
2.4
0
5.1
5.1
5.1
5.1
5.1
5.1
5.1
3.0
4.05
6.15
7.2
0
53.3
89.7
90.6
98.7
98.2
95.4
92.3
74.3
77.1
97.3
95.5
2
3
Conclusions
4
5A
Using Strain’s report as a basis, an improved method to syn-
thesize triarylmethanimines was theoretically designed and then
the method was performed experimentally. The reaction
between arylamine and diarylketone was promoted by a Lewis
acid–base pair, AlCl3–Et3N, and the preferred solvent was
chloroform. Electron-sufficient arylamines and electron-
deficient diarylketones facilitated the reaction whereas the steric
effect hindered the reaction.
6
7
8
9
10
11
12
B
–
AReaction was refluxed for 5 h. BNo reaction.
Experimental
Computational Details
The CHCl3 solvent effect lowered the barrier of the whole
process from 16.8 to 13.6 kcal molꢀ1
Molecular geometries of model complexes were optimized
using the Restricted Becke3LYP (rB3LYP)[31,32] functional.
Frequency calculations at the same level of theory were also
performed to identify all the stationary points as minima (zero
imaginary frequencies) or transition states (one imaginary fre-
quency). Intrinsic reaction coordinates (IRC)[33,34] were calcu-
lated for the transition states to confirm that such structures
indeed connect two relevant minima. The 6-31G(d) basis set was
used for C, N, and O atoms, and 6-31G(d, p) was used for H. The
effective core potentials (ECPs) of Hay and Wadt with a double-
x basis set (LanL2DZ)[35–37] were used for Al and Cl atoms.
Polarization functions were added for Cl (xd ¼ 0.514).[38] The
effect of solvent was examined by performing single-point self-
consistent reaction field (SCRF) calculations based on the
polarizable continuum model (PCM)[39,40] for all of the gas-
phase-optimized species. The solvent calculations were carried
out by using the SCFVAC keyword to obtain the free energies of
solvation then adding the relevant thermal corrections RTln(RT/
P), which is followed by adding the energies to gas phase
energies.[41] CHCl3 was chosen as the solvent, using the default
.
In conclusion, an imine formation reaction promoted by the
AlCl3–Et3N pair was verified through the DFT method. The
computational results show that the Lewis acid AlCl3 lowers
the barrier of the process, and an additional base Et3N makes the
whole process irreversible. The thermodynamic viability of the
assumed process was also confirmed by the theoretical method.
Experimental Results
Based on the calculated results, a series of experiments were
performed. The reaction between benzophenone and aniline was
selected as the model reaction. Experiments to optimize the
solvent for the condensation of benzophenone and aniline were
conducted and the results are shown in Table 1. Chloroform was
found to be the best solvent which gives an excellent yield of
92.3 %. The mole ratio of the reactants was also optimized, and
the results are given in Table 2. The best ratio of
Ph2C¼O/PhNH2/AlCl3/Et3N was 1 : 1.6 : 1.7 : 5.1 (5 h gave a
yield of 98.7 %). When the temperature was elevated to reflux
for 5 h, no further enhancement of the yield was obtained
(Table 2). Obviously, room temperature is the best selection.
In order to broaden the substrate scope, a series of arylamines
were selected and the results are listed in Table 3. Arylamines
with electron donating groups (EDGs) and halogens gave
excellent yields under the optimized conditions (Table 3),
whereas those with electron withdrawing groups (EWGs) gave
unsatisfactory results (Table 3). For example, an aniline with
MeO or F, Cl or Br at the para-position shortened the reaction
time, whereas a NO2 or CF3 group prolonged it. Owing to the
electron withdrawing character of thiazolyl, 2-aminothiazole
also gave a low yield (Table 3). In addition, the steric effect
plays a significant role. The reaction times for substrates with
˚
parameter in Gaussian03, e ¼ 4.9, solvent radius ¼ 2.48 A, and
the atomic radii are taken from the UFF. To reduce computa-
tional costs, the PhNH2 which acts as a H transfer medium was
replaced by NH3. All calculations were performed with the
Gaussian 03 software package.[42]
Experimental Details
General
Melting points were determined by an X-6 micro-melting
point apparatus and were uncorrected. High resolution mass
spectra (HRMS) were obtained on a micrOTOF-Q II 10204
instrument. 1H NMR spectra were obtained in CDCl3 on a

Synthesis of triaryl imines promoted by AlCl3–Et3N pair
1393
Table 3. Scope of arylamine subtrates
Ar
O
N
3
AlCl3/Et3N
r.t.
Ar NH2
1
ϩ
Ph
Ph
Ph
Ph
2
Yield of
ArN(MgBr)[212] [%]
Yield of ZnCl[29] or
HBr[9] [%]
Yield of
POCl[310] [%]
Entry
ArNH2
Product
Time [h]
Yield [%]
m.p. [8C]
NH2
1
3a
3b
3c
3d
3e
3f
5
4
98.2
96.2
97.3
96.4
96.7
77.5
68.0
72.0
–
81
78.5
80
–
86.0
104–106
70–72
MeO
Cl
NH2
2
–
–
–
–
–
NH2
3
3
88–89
Br
NH2
NH2
NH2
4
3
–
82–83
F
5
4
–
–
102–104
152–153
6A
20
–
–
O2N
F3C
NH2
7
3g
18
69.1
–
–
–
115–116
F3C
OEt
NH2
8
3h
3i
16
10
10
14
94.1
–
–
–
–
–
–
–
–
–
–
–
–
–
Me
NH2
9A
94.2
50–51
93–95
100–101
Cl
F
10A
3j
93.3B
92.8B
NH2
11A
3k
NH2
NH2
Me
Me
12
13
3l
20
20
38.0
–
–
–
–
–
–
90–92
NO2
NH2
3m
Trace
–
NH2
14A
3n
3o
16
24
59.7
38.2
–
–
74.0
–
–
–
130–132
83–84
N
15A
NH2
S
A Reactions under reflux. B Yield obtained by HPLC.

1394
Y. Liu et al.
Table 4. Scope of diarylketone substrates
Ph
O
N
AlCl3/Et3N
ϩ
Ar1
Ar2
Ph NH2
Ar1
Ar2
r.t.
1
2
3
Yield of
ArN(MgBr)[212] [%]
Yield of ZnCl[29]
or HBr[9] [%]
Entry
Diarylketones
Product
Time [h]
Yield [%]
98.2
m.p. [8C]
O
1
3a
3p
3q
5
–
–
–
–
–
–
104–106
O
2
3
4
4
82.4
91.8
118–120
125–126
F
O
O2N
O
4
3r
6
89.7
39.0
66–67
MeO
O
5
3s
3t
4
8
98.0
78.0
81.1B
–
–
–
–
76.0
–
158–160
105–106
123–125
O2N
NO2
O
O
6
MeO
OMe
7A
3u
24
Me2N
NMe2
O
8
3v
2.5
98.4
97.2
82.0D
75.0
80–82
O
O
9C
3w
2
–
–
192–194
AReaction under reflux. BYield obtained by HPLC. CReaction conditions: anthraquinone/aniline/AlCl3/Et3N ¼ 1 : 3.2 : 3.4 : 10.2, room temperature. DThe
amine is 4-methoxyaniline in the literature.
Varian 400 spectrometer. If not otherwise noted, chemical shifts
are reported as values in ppm relative to tetramethylsilane
(TMS) using the residual CDCl3 peak in the CDCl3 solution
as the internal standard (d ¼ 7.26 relative to TMS).
mixture was stirred for the desired time at room temperature
or at reflux. After completion of the reaction (monitored by
TLC), the mixture was treated with 4 M NaOH solution and then
extracted three times with CH2Cl2. The combined organic
extracts were dried over Na2SO4 and concentrated under
vacuum. The crude product was further purified by column
chromatographyand the yieldwascalculatedbased ontheweight
of pure product. Selected data for typical compounds follow.
Compound 3a. dH (400 MHz, CDCl3) 7.77–7.71 (m, 2H),
7.50–7.43 (m, 1H), 7.40 (t, J 7.3, 2H), 7.24 (s, 2H), 7.17–7.08
Procedure
A flask was charged with a substituted benzophenone
(1, 2.0 mmol), AlCl3 (3.4 mmol), and 20 mL chloroform. A
solution of arylamines (2, 3.2 mmol) in Et3N (10.2 mmol) was
then added dropwise with stirring. The resulting reaction

Synthesis of triaryl imines promoted by AlCl3–Et3N pair
1395
(m, 5H), 6.91 (t, J 7.3, 1H), 6.72 (d, J 7.6, 2H). m/z (HRMS)
Calc. for C19H16Nþ ([M þ H]þ): 258.1277. Found: 258.1278.
Compound 3b. dH (400 MHz, CDCl3) 7.73 (d, J 7.3, 2H),
7.51 – 7.33 (m, 3H), 7.30 – 7.23 (m, 3H), 7.12 (d, J 5.2, 2H), 6.68
(s, 4H), 3.72 (s, 3H).
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Compound 3f. m/z (HRMS) Calc. for C19H15N2Oþ2 ([M þ
H]þ): 303.1128. Found: 303.1126.
65, 5680. doi:10.1021/JO000509C
Compound 3i. dH (400 MHz, CDCl3) 7.79 (d, J 7.3, 2H),
7.49–7.45 (m, 1H), 7.41 (t, J 7.3, 2H), 7.24 (d, J 5.8, 3H), 7.12–
7.04 (m, 3H), 6.91 (t, J 7.4, 1H), 6.83 (t, J 7.3, 1H), 6.43 (d, J 7.6,
1H), 2.18 (s, 3H).
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JO00072A051
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Compound 3j. dH (400 MHz, CDCl3) 7.73 (d, J 7.3, 2H),
7.48 (t, J 7.2, 1H), 7.40 (t, 2H), 7.35–7.16 (m, 5H), 7.10 (d, J 6.4,
2H), 6.59 (d, J 8.5, 2H).
Compound 3o. dH (400 MHz, CDCl3) 7.78 (s, 2H), 7.55–
7.27 (m, 7H), 7.20 (s, 2H), 6.90 (d, J 3.2, 1H).
Soc. 1999, 121, 9550. doi:10.1021/JA992130H
Compound 3u.
dH (400 MHz, CDCl3) 7.57 (d, J 8.5, 2H),
[25] J. S. M. Samec, J. E. Ba¨ckvall, Chemistry 2002, 8, 2955. doi:10.1002/
1521-3765(20020703)8:13,2955::AID-CHEM2955.3.0.CO;2-Q
[26] A. Corma, C. Gonzalez-Arellano, M. Iglesias, M. T. Navarro,
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OL070561W
7.07 (t, J 7.5, 2H), 6.91 (d, J 8.3, 2H), 6.80 (t, J 7.2, 1H), 6.69 (d,
J 7.6, 2H), 6.56 (d, J 8.5, 2H), 6.38 (d, J 8.4, 2H), 3.33 (q, J 6.8,
4H), 3.23 (q, J 6.9, 4H), 1.11 (t, J 6.9, 6H), 1.05 (t, J 6.9, 6H). m/z
(HRMS) Calc. for C27H34Nþ3 ([M þ H]þ): 400.2747. Found:
400.2753.
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Compound 3v. dH (400 MHz, CDCl3) 7.92 (d, J 7.3, 1H),
7.57 (d, J 6.9, 2H), 7.49–7.16 (m, 6H), 6.99 (d, J 7.3, 2H), 6.90
(t, J 7.6, 1H), 6.56 (d, J 7.5, 1H).
Supplementary Material
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NMR spectra, HRMS spectra, and computational Cartesian
coordinates and energies are available on the Journal’s website.
Acknowledgements
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