Solvent-free efficient synthesis of 2,4,5-triarylimidazolines from aromatic aldehydes and hexamethyldisilazane

Article abstract of DOI:10.1055/s-2003-39396

A one-step preparation of 2,4,5-triarylimidazolines from aromatic aldehydes was accomplished by heating with hexamethyldisilazane under solvent-free conditions. This reaction provides a convenient preparative method for triarylimidazolines having a variety of aryl groups.

Full text of DOI:10.1055/s-2003-39396

1
236
PAPER
Solvent-Free Efficient Synthesis of 2,4,5-Triarylimidazolines from Aromatic
Aldehydes and Hexamethyldisilazane
a
a
b
a
a
S
H
ynthesis of 2,4,5-
i
Triaryl
t
imidaz
o
olines shi Uchida, Takashi Shimizu, Paidi Yella Reddy, Shuichi Nakamura, Takeshi Toru*
a
Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan
b
AISIN COSMOS R & D Co., Ltd., 5-50 Hachiken-cho, Kariya, Aichi, 448-8650, Japan
Fax +81(52)7355217; E-mail: toru@ach.nitech.ac.jp
Received 14 February 2003; revised 19 March 2003
lines without any catalysts under solvent-free reaction
conditions.
Abstract: A one-step preparation of 2,4,5-triarylimidazolines from
aromatic aldehydes was accomplished by heating with hexamethyl-
disilazane under solvent-free conditions. This reaction provides a We have now found that heating a mixture of benzalde-
convenient preparative method for triarylimidazolines having a va-
riety of aryl groups.
hyde 1a and 1.1 equivalents of HMDS at 120 °C without
a solvent resulted in the direct formation of 2,4,5-triphe-
Key words: imidazoline, hexamethyldisilazane, solvent-free, alde- nylimidazoline 3a in 89% yield (Method A) (Scheme 1
hyde, methanediamine
and Table 1, entry 1). The above reaction conditions are
peculiar when compared with the yields of the following
two-step reactions (Method B). First, a mixture of 1a and
2
,4,5-Triarylimidazolines are good precursors of HMDS was heated at 60 °C for 6 hours. At this stage the
1
imidazoles and 1,2-diarylethylenediamines, the latter can diimine 2a is formed almost quantitatively as we have
7
be used as chiral ligands in enantioselective reactions after previously reported, then the volatile components such as
2
optical resolution. Thermal treatment of aldehydes and HMDS, ammonia, trimethylsilyl ether, etc. formed during
ammonia gives the diimine compounds, N,N -bis(arylme- the course of the reaction were evaporated under vacuum.
thylidene)arylmethanediamines, a solution of which is The resulting mixture was heated at 120 °C for 8 hours, to
3
4
heated at high temperature or heated with a strong base
afford 3a in 59% yield (entry 2). Imidazoline 3b was also
to form triarylimidazolines. However, the reaction of al- formed in higher yields in the one-step procedure (Method
dehydes and ammonia requires heating over 1 week to A) than in the two-step preparation (Method B) (entries 1
5
form diimine compounds. The reaction of aldehydes and vs. 2 and 3 vs. 4). Table 1 shows the results of the reaction
hexamethyldisilazane (HMDS) in the presence of ZnCl2 of various aromatic aldehydes.
6
in CH Cl has been reported to form diimine compounds.
Previously, we have described a convenient synthesis of
diimine derivatives from aldehydes by heating with
HMDS without a solvent. A recent report on a one-step
synthesis of 2,4,5-triphenyl and 2,4,5-tri(2-pyridyl)imida-
zolines from the corresponding aldehydes and HMDS
with lithium bromide in DMSO prompts us to present our
results obtained in the one-pot synthesis of triarylimidazo-
2
2
Diimine compounds are known to cyclize to cis-imidazo-
lines in a disrotatory fashion on heating, the intermediary
5
carbanions (Scheme 1, A) presumably being formed. In
7
8
fact, cyclization of diimine compounds occurs readily in
5
the presence of a strong base such as phenyllithium, po-
4
9
tassium methoxide, sodium ethoxide, or ammonium hy-
10
droxide. Therefore, the cyclization to the imidazoline in
the present reaction was apparently accelerated in the
Ar
1
.1 equiv HMDS
Ar CHO
HN
Ar
N
Ar
_
120 °C
1
Method A
Ar
3
N
N
Method B
Ar
Ar
Ar
1.1 equiv HMDS
N
N
A
60 °C
120 °C
Ar
Ar
2
Scheme 1
Synthesis 2003, No. 8, Print: 05 06 2003.
Art Id.1437-210X,E;2003,0,08,1236,1240,ftx,en;F01203SS.pdf.
Georg Thieme Verlag Stuttgart · New York
©

PAPER
Synthesis of 2,4,5-Triarylimidazolines
1237
presence of bases such as HMDS and its derived amines was obtained by heating 1g in the presence of 0.2 equiva-
including ammonia in the one-step procedure. Indeed, lents of chlorotrimethylsilane at 120 °C for 60 hours, im-
when the isolated diimine 2a was heated at 120 °C for 8 idazoline 3h was not formed under similar reaction
hours with newly added HMDS (1 equiv), 3a was ob- conditions. These results indicate that the electron-donat-
tained in 85% yield.
ing group on the benzene ring apparently retards not only
the imine formation but also the cyclization to the imida-
zoline. The imidazolines obtained in the thermal reaction
were confirmed to have cis geometry by comparison of
As shown in Table 1, the reaction time apparently de-
pends on Hammet’s rule which is in good agreement with
the results of our diimine synthesis and other syntheses
7
1
1
1
the H NMR spectral data of imidazolines 3 and N-ben-
from aromatic aldehydes and ammonia. We have previ-
3
7
zoylated imidazolines with the reported data. In addition,
ously observed that p-anisaldehyde (1g) and p-(N,N-di-
the cis geometry of 3a was further confirmed by X-ray
crystallography.
methylamino)benzaldehyde (1h) give the respective
diimine derivatives by heating with HMDS in the pres-
ence of chlorotrimethylsilane. Although imidazoline 3g
1
2
Table 1 Reaction of Aromatic Aldehydes with HMDS to Form Imidazolines 3a
Entry
1
Method
A
Aldehyde
Temp 60 °C Reaction
Time (h)
Temp 120 °C Reaction Product
Time (h)
Yield (%)
89
1a
–
6
3a
O
O
2
3
4
5
6
7
B
A
B
A
A
A
1a
1b
1b
1c
1d
1e
6
–
9
–
–
–
8
6
3a
3b
3b
3c
3d
3e
59
92
47
77
79
82
Cl
Cl
O
O
O
O
6
32
40
Br
Me
1.5
O
O
8
9
A
1f
–
24
3f
82
S
O
A
A
A
A
A
1g
1h
1i
–
60b
48b
3g
3h
3i
58
0
MeO
O
1
1
1
1
0
1
2
3
–
(
Me) N
2
O
10c
–
–
87
78
47
N
O
1j
120
–
3j
N
O
1k
5c,d
3k
N
O
a
b
c
d
The reaction was performed in a sealed tube unless otherwise noted.
Chorotrimethylsilane (0.2 equiv) was added.
The reaction was performed in a vial equipped with a condenser.
MeOH was added after heating for 3 h and the mixture was heated for an additional 2 h.
Synthesis 2003, No. 8, 1236–1240 ISSN 1234-567-89 © Thieme Stuttgart · New York

1
238
H. Uchida et al.
PAPER
–
1
Three positional isomers of pyridinecarbaldehydes cm . Mass spectra were measured on a Hitachi M-2000S spectro-
meter. HMDS and aldehydes were distilled under argon before use.
showed completely different chemical behavior (entries
1
1–13). It was 2-pyridinecarbaldehyde (1i) that reacted
Imidazolines from Aldehydes; cis-2,4,5-Triphenylimidazoline
3a); Typical Procedure
A mixture of benzaldehyde (1a; 157 mg, 1.5 mmol) and HMDS
342 L, 260 mg, 1.6 mmol) was heated in a sealed tube at 120 °C
8
,10
most smoothly. Tri-2-pyridylimidazoline (3i) was ob-
tained in 87% yield, when 1i was heated with HMDS at
6
(
0 °C for 10 hours. It was necessary to heat 3-pyridine-
(
carbaldehyde (1j) with HMDS at 120 °C in order to obtain for 6 h. Yellowish-white precipitate appeared during the heating.
imidazoline 3j in high yield. On the other hand, tri-4-py- After cooling, the volatile compounds were removed under vacuum
to leave a residue which was purified by silica gel column chroma-
ridylimidazoline (3k) was not obtained under the normal
thermal conditions. Heating a mixture of 1k and HMDS at
tography using hexanes–EtOAc (9:1) as eluent to give 131 mg
4
(
1
89%) of 3a; mp 132.0–133.0 °C (toluene–n-hexane) (Lit. mp
27–128 °C, Lit. mp 129–130 °C).
7
0 °C for 12 hours afforded amide 5 in 43% yield. When
13
the reaction was interrupted after 3 hours, the main prod-
6
a,11
uct was pyridylmethanimine 4 (Scheme 2).
When this cis-2,4,5-Tris(p-chlorophenyl)imidazoline (3b)
4
mixture was further heated at 110 °C for an additional 2 Yield: 92%; mp 199.2–200.0 °C (toluene–n-hexane) (Lit. mp 176–
1
77 °C).
hours, amide 5 was formed in 54% yield, probably via the
,3-proton shift as shown in Scheme 2. No formation of
1
1
H NMR (CDCl ): = 5.43 (br s, 1 H), 5.44 (s, 2 H), 6.93 (d, 4 H,
3
imidazoline 3k was observed by these thermal treatments. J = 8.4 Hz), 7.05 (d, 4 H, J = 8.4 Hz), 7.47 (d, 2 H, J = 8.3 Hz),
7
.89 (d, 2 H, J = 8.3 Hz).
However, strangely, when 5 equivalents of methanol was
+
added to the mixture after heating at 70 °C for 3 hours and MS (EI): m/z (%) = 400 (M , 15), 261 (100), 226 (55).
the mixture was continuously heated for an additional 2
Anal. Calcd for C H Cl N : C, 62.79; H, 3.76; N, 6.97. Found: C,
2
1
15
3
2
hours, imidazoline 3k was formed in 47% yield. We as-
sumed that 3k was formed via methanolysis of 4 produc-
ing N,O-acetal 6 which reacted with the remaining 4 to cis-2,4,5-Tris(p-bromophenyl)imidazoline (3c)
6
2.76; H, 3.68; N, 6.93.
Yield: 77%; mp 188–189 °C (toluene–n-hexane).
afford the diimine 7 and thence imidazoline 3k
–
1
(
Scheme 2).
IR (KBr): 3129, 1612, 1486, 1406, 1011 cm .
1H NMR (CDCl3): = 5.41 (br s, 1 H), 5.41 (s, 2 H), 6.82 (d, 4 H,
J = 8.3 Hz), 7.20 (d, 4 H, J = 8.3 Hz), 7.63 (d, 2 H, J = 8.3 Hz), 7.81
In summary, we have demonstrated an efficient method
for the preparation of triarylimidazolines from the corre-
sponding aromatic aldehydes on treatment with HMDS at
(
d, 2 H, J = 8.3 Hz).
+
MS (EI): m/z (%) = 533 (M + 1, 14), 351 (78), 89 (100).
1
20 °C. We also showed the difference in reactivity of
three positional isomers of pyridinecarbaldehydes. The Anal. Calcd for C H Br N : C, 47.14; H, 2.83; N, 5.24. Found: C,
2
1
15
3
2
4
7.20; H, 2.55; N, 5.24.
present reactions are regarded as an environmentally
friendly preparative method performed without a solvent.
cis-2,4,5-Tris(p-tolyl)imidazoline (3d)
4
Yield: 79%; mp 127–128 °C (toluene–n-hexane) (Lit. mp 126–
NMR spectra were recorded on a Varian Gemini-200 spectrometer.
Microanalyses were performed on a Perkin-Elmer 24B elemental
analyzer. Melting points were measured on a Yanagimoto micro
melting point apparatus and are uncorrected. IR spectra were re-
corded on a Jasco FT/IR-200 spectrometer and are reported in
128 °C).
1H NMR (CDCl3): = 2.18 (s, 6 H), 2.43 (s, 3 H), 5.39 (s, 2 H), 6.85
(
s, 8 H), 6.86 (br s, 1 H), 7.28 (d, 2 H, J = 8.2 Hz), 7.86 (d, 2 H,
J = 8.2 Hz).
CHO
TMS
TMS
O
TMS
O
O
N
CHO
HMDS
OTMS
N
NH(TMS)
H
N
N
N
N
N
-TMSOH
O
1k
OR
1
,3-proton shift
N
N
N
H
N
N
N
N
N
4
5
R=H or TMS
N
N
TMS(Me)
NH2
O
4
3k
HN
OTMS(Me)
N
N
N
N
6
N
N
N
N
7
Scheme 2
Synthesis 2003, No. 8, 1236–1240 ISSN 1234-567-89 © Thieme Stuttgart · New York

PAPER
Synthesis of 2,4,5-Triarylimidazolines
1239
cis-2,4,5-Tris(2-furyl)imidazoline (3e)
N-[4-Pyridyl(trimethylsilyloxy)methyl]-4-pyridylmethanimine
(4)
4
Yield: 82%; mp 118.5–119.5 °C (toluene–n-hexane) (Lit. mp 115–
16 °C).
1
A mixture of 4-pyridinecarbaldehyde (100 mg, 0.93 mmol) and
HMDS (299 L 226 mg, 1.4 mmol) was heated at 70 °C for 3 h un-
der argon. After cooling, the mixture was purified by column chro-
matography on neutral alumina using hexane–EtOAc (7:3) as eluent
to give 4i as a colorless viscous oil. Because of its instability, only
its NMR spectra were measured.
1H NMR (CDCl3): = 8.70 (2 H, dd, J = 1.5, 4.4 Hz), 8.61 (2 H, dd,
J = 1.6, 4.6 Hz), 8.55 (2 H, d, J& nbsp;= 2.2 Hz), 7.64 (2 H, dd,
J = 1.6, 4.4 Hz), 7.39 (2 H, dd, J = 1.6, 4.5 Hz), 6.30 (1 H, d,
J = 2.0 Hz), 0.21 (9 H, s).
1H NMR (CDCl3): = 5.39 (br s, 1 H), 5.39 (s, 2 H), 6.05 (d, 2 H,
J = 3.0 Hz), 6.18 (m, 2 H), 6.54 (m, 1 H), 7.13 (d, 1 H, J = 3.6 Hz),
7
.20 (d, 2 H, J = 1.8 Hz), 7.53 (d, 1 H, J = 1.6 Hz).
cis-2,4,5-Tris(2-thienyl)imidazoline (3f)
Yield: 82%; mp 125–126 °C (toluene–n-hexane).
IR (KBr): 3270, 1595, 1519, 1466, 1375, 1291, 1245, 1096, 850
–
1
cm .
1H NMR (CDCl3): = 5.64 (s, 2 H), 6.75–6.83 (m, 4 H), 7.06–7.16
13
C NMR (CDCl ): = 155.0, 150.2, 149.9, 149.7, 122.1, 121.9,
3
(m, 3 H), 7.30 (br s, 1 H), 7.51 (d, 2 H, J = 4.4 Hz).
8
7.2, 0.4.
+
MS (EI): m/z (%) = 316 (M , 22), 204 (46), 78 (100).
N-(4-Pyridylmethyl)-4-pyridinecarboxamide (5)14
Anal. Calcd for C H N O : C, 74.20; H, 6.23; N, 7.21. Found: C,
2
4
24
2
3
A mixture of 4-pyridinecarbaldehyde (200 mg, 1.9 mmol) and
HMDS (598 L 452 mg, 2.8 mmol) was heated at 70 °C for 12 h un-
der argon. The mixture was evaporated under vacuum to give a
crude solid which was purified by silica gel column chromatogra-
phy using EtOAc–MeOH (9:1) as eluent to give 85 mg (43%) of 5i
as a colorless solid; mp 145–146 °C (pyridine).
7
4.20; H, 6.38; N,7.21.
cis-2,4,5-Tris(p-methoxyphenyl)imidazoline (3g)
Yield: 58%; mp 147–148 °C (toluene–n-hexane).
IR (KBr): 3381, 3006, 2935, 2835, 1608, 1510, 1448, 1242, 1172,
1
1H NMR: = 3.68 (s, 6 H), 3.88 (s, 3 H), 5.35 (s, 2 H), 6.59 (d, 4 H,
J = 8.8 Hz), 6.87 (d, 4 H, J = 8.8 Hz), 6.99 (d, 2 H, J = 8.9 Hz),
–
1
028 cm .
–
1
IR (KBr): 3290, 1646, 1600, 1542, 1318 cm .
1H NMR (CDCl3): = 8.76 (2 H, dd, J = 0.8, 4.4 Hz), 8.57 (2 H, dd,
J = 0.8, 4.4 Hz), 7.65 (2 H, dd, J = 0.8, 4.4 Hz), 7.24 (2 H, dd,
J = 0.8, 4.4 Hz), 6.86 (1 H, br), 4.67 (2 H, d, J = 6.0 Hz).
7
.91 (d, 2 H, J = 8.9 Hz).
+
MS (EI): m/z (%) = 388 (M , 27), 252 (100), 237 (37).
1
3
C NMR (CDCl ): = 166.0, 150.4, 149.7, 147.1, 141.1, 122.3,
3
Anal. Calcd for C H N O : C, 74.20; H, 6.23; N, 7.21. Found: C,
7
2
4
24
2
3
1
21.0, 42.7.
4.20; H, 6.38; N, 7.21.
+
MS (EI): m/z (%) = 213 (M , 100), 185 (3), 106 (46), 78 (30).
cis-2,4,5-Tris(2-pyridyl)imidazoline (3i)
Yield: 87%; mp 146.0–146.8 °C (washed with EtOAc) (Lit. mp
41–143 °C).
Anal. Calcd for C H N O: C, 67.61; H, 5.16; N, 19.72. Found: C,
1
2
11
3
1
0
6
7.41; H, 5.16; N, 19.57.
1
HRMS: m/z calcd for C H N O, 213.0903; found, 213.0908.
1
2
11
3
cis-2,4,5-Tris(3-pyridyl)imidazoline (3j)
Yield: 78%; mp (decomposed before melting).
Acknowledgment
–
1
IR (KBr): 3432, 1618, 1425, 1028, 710 cm .
This work was partly supported by a Grant-in-Aid for Scientific Re-
search on Priority Areas (A) ‘Exploitation of Multi-Element Cyclic
Molecules’ (No. 14044040) from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan.
1H NMR (CDCl3): = 5.60 (s, 2 H), 5.60 (br s, 1 H), 6.97–7.24 (m,
2
8
H), 7.25–7.29 (m, 3 H), 7.44–750 (dd, 1 H, J = 4.9, 7.8 Hz), 8.26–
.35 (m 4 H,), 8.74–8.82 (dd, 1 H, J = 1.4, 4.8 Hz), 9.15 (1 H, d,
J = 1.6 Hz).
+
MS (EI): m/z (%) = 301 (M , 30), 195 (100), 168 (33).
References
HRMS: m/z calcd for C H N , 301.1328; found, 301.1331.
1
8
15
5
(
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6
4
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5
1
7
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(
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(
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(
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Synthesis 2003, No. 8, 1236–1240 ISSN 1234-567-89 © Thieme Stuttgart · New York