Convenient one-step synthesis of cis-2,4,5-triarylimidazolines from aromatic aldehydes with urea

Article abstract of DOI:10.1080/00397911.2010.533802

A simple and efficient method has been developed for the synthesis of cis-2,4,5-triarylimidazolines based on a one-step procedure of aldehydes and urea in the presence of cesium carbonate. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.533802

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Convenient One-Step Synthesis of
cis-2,4,5-Triarylimidazolines from
Aromatic Aldehydes with Urea
a
a
a
Fei Wang , Qian Liao & Chanjuan Xi
a
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical
Biology (Ministry of Education), Department of Chemistry , Tsinghua
University , Beijing , China
Accepted author version posted online: 30 Aug 2011.Published
online: 11 Nov 2011.
To cite this article: Fei Wang , Qian Liao & Chanjuan Xi (2012) Convenient One-Step Synthesis of
cis-2,4,5-Triarylimidazolines from Aromatic Aldehydes with Urea, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:6, 905-913, DOI:
10.1080/00397911.2010.533802
To link to this article: http://dx.doi.org/10.1080/00397911.2010.533802
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1
Synthetic Communications , 42: 905–913, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.533802
CONVENIENT ONE-STEP SYNTHESIS OF cis-
,4,5-TRIARYLIMIDAZOLINES FROM AROMATIC
ALDEHYDES WITH UREA
2
Fei Wang, Qian Liao, and Chanjuan Xi
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology
(Ministry of Education), Department of Chemistry, Tsinghua University,
Beijing, China
GRAPHICAL ABSTRACT
Abstract A simple and efficient method has been developed for the synthesis of cis-
2,4,5-triarylimidazolines based on a one-step procedure of aldehydes and urea in the
presence of cesium carbonate.
Keywords Aldehyde; cesium carbonate; imidazoline; urea
INTRODUCTION
,4,5-Triarylimidazolines are good precursors for the synthesis of
diarylethylenediamines, which have been employed as chiral ligands in enantioselec-
2
[
1–11]
tive reactions after optical resolution.
synthesis of 2,4,5-triarylimidazolines is the reaction of aromatic aldehydes with
The most important method for the
0
ammonia to give the diimine compounds, N,N -bis(arylmethylidene)arylmethanedia-
mines, followed by heating with a strong base such as sodium amide or phenyl-
lithium or heating at high temperature to form triarylimidazoline,
[
12–16]
which
often occurs in poor yield. Other methods have been reported for the synthesis of
diimines using aldehydes and hexamethyldisilazane (HMDS) in the presence of
Lewis acids such as ZnCl or without any catalyst under solvent-free conditions
2
[17–19]
using microwave irradiation.
Recently, one-step syntheses of 2,4,5-triarylimida-
zolines either from the corresponding aldehydes and HMDS with lithium bromide in
[20–22]
dimethyl sulfoxide (DMSO) or from aldehydes with a mixture of alumina with
ammonium acetate under solvent-free conditions using microwave irradiation have
Received March 11, 2010.
Address correspondence to Chanjuan Xi, Key Laboratory of Bioorganic Phosphorus Chemistry
and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing
100084, China. E-mail: cjxi@tsinghua.edu.cn
9
05

906
F. WANG, Q. LIAO, AND C. XI
Scheme 1. Reaction of aromatic aldehydes with urea.
[23]
been reported.
Although these protocols provided access to triarylimidazolines,
the development of simple and efficient methods is still strongly desirable. Herein,
we report a one-step synthesis of cis-2,4,5-triarylimidazolines based on the reaction
of aromatic aldehydes with urea and Cs CO in N,N-dimethylformamide (DMF)
2
3
(Scheme 1).
RESULTS AND DISCUSSION
The reaction of benzaldehyde 1a, chosen as a model compound, was studied in
the presence of several bases at solvents at different temperatures. Initially, we car-
ried out the reaction of benzaldehyde 1a (1.0 mmol) with urea (4.0 mmol) in the pres-
ꢀ
ence of cesium carbonate (2.0 mmol) in DMF at 100 C (Table 1, entry 1). However,
only 26 % yield of cis-2,4,5,-triphenylimidazoline 2a was obtained as determined by
NMR. Then we raised the temperature and prolonged the reaction time; the corre-
sponding yields increased (entries 2 and 4). Various ratios of benzaldehyde 1a and
urea as well as Cs CO were checked, and the best yield was obtained when the ratio
2
3
of 1a=urea=Cs CO was 1:4:2. The yield decreased when the amount of urea or
3
2
a
Table 1 Screen the reactions of benzaldehyde and urea under different conditions
b
Entry
Base
Solvent
Temp.
Time
Yield of 2a (%)
ꢀ
ꢀ
ꢀ
1
2
3
4
5
6
7
8
9
Cs
Cs
2
CO
CO
3
DMF
DMF
DMF
100 C
120 C
24 h
24 h
24 h
36 h
36 h
36 h
36 h
36 h
36 h
36 h
26
46
9
2
3
—
120 C
120 C
ꢀ
Cs
2
CO
CO
PO
3
DMF
DMF
78
56
64
23
51
30
26
ꢀ
K
K
2
3
120 C
120 C
ꢀ
3
4
DMF
DMF
ꢀ
AcONa
120 C
120 C
ꢀ
Cs
2
Cs
2
Cs
2
CO
CO
CO
3
3
3
NMP
Toluene
1,4-Dioxane
ꢀ
120 C
120 C
ꢀ
1
0
a
The reactions were performed with 1.0 mmol aldehyde 1a, 4.0 mmol urea, and 2.0 mmol base in 2 mL
solvent.
b
1
2 2
Determined by H NMR using CH Cl as internal standard.

SYNTHESIS OF cis-2,4,5-TRIARYLIMIDAZOLINES
907
Table 2. Reactions of aldehyde 1 and urea to synthesize imidazoline 2
a
Entry
Substrate 1
Product
Yield (%)
1
78(73)
79(70)
82(78)
72(65)
82(79)
2
3
4
5
6
7
8
62(56)
50(42)
60(52)
(
Continued )

908
F. WANG, Q. LIAO, AND C. XI
Table 2. Continued
a
Entry
Substrate 1
Product
Yield (%)
9
47(40)
a
NMR yields, isolated yields are given in parentheses.
Cs CO was reduced. In addition, when the amount of urea was increased to 6
2
3
equivalents, the yield of 2a also decreased. It is important to note that only a trace
of the desired product was obtained in the absence of Cs CO (entry 3). Next, the
2
3
applicability of other bases and solvents was also evaluated. Clearly, Cs CO proved
2
3
to be superior to K PO and K CO (entries 4–6). NaOAc gave poor yield (entry 7).
3
3
4
2
The solvent did have a great effect on the reaction. Besides DMF, 1-methyl-2-
pyrrolidinone (NMP) was also effective for the reaction (entry 8). When toluene
and dioxane were used as solvents, the reactions gave poor yields, which were
supposed to be because urea does not dissolve in such solvents (entries 9 and 10).
When DMSO was used as a solvent, we did not observe any of the desired product
[
24]
2
a; only the (E)-(2-(methylsulfinyl)vinyl)benzene
was obtained in 86 % yield,
which demonstrated the aldo-type reaction between aldehyde and DMSO in the
presence of Cs CO .
2
3
Under the optimized condition, a study on the substrate scope was carried out,
and the results are summarized in Table 2. The reaction of aldehyde 1 with urea
exhibited a broad scope. As shown in Table 2 (entries 1–5), the reactions using vari-
ous aromatic aldehydes having electron-donating groups (entries 2–6) proceeded
well to give the corresponding imidazolines in good yields. The reaction of thiophene
aldehyde with urea gave the corresponding imidazolines in good yield (entry 7). The
Figure 1. Configuration of 2 h in crystalline form.

SYNTHESIS OF cis-2,4,5-TRIARYLIMIDAZOLINES
909
reactions of 2-bromobenzaldehyde or 4-fluorobenzaldehyde with urea afforded the
corresponding imidazolines in moderated yields (entries 8 and 9). It should be noted
that if using p-chlorobenzaldehyde and p-bromobenzaldehyde as substrates, almost
no imidazoline product was obtained but other uncharacterized products formed in
the reaction. The imidazolines obtained in the reaction were confirmed to have cis
1
geometry by comparison of the H NMR spectral data of imidazolines 2 with the
In addition, the cis geometry of 2h was further confirmed by
[9,15,21]
reported data.
x-ray crystallography (Fig. 1).
In summary, we have described a general one-pot method for the convenient
synthesis of imidazoline derivatives via cooperative Cs CO cyclization of aldehydes
2
3
and urea. The yields are essentially moderate to good, and the configuration of
products was always 100 % favoring cis fashion. The application and further study
are in progress in this laboratory.
EXPERIMENTAL
All the reactions were carried out in predried a screw-capped test tube with a
Teflon-lined septum. DMF, toluene, 1,4-dioxane, and DMSO were freshly distilled.
1
13
H NMR and C NMR spectra were recorded on a Jeol 300 NMR spectrometer at
ambient temperature with CDCl as the solvent and TMS as internal standard. The
3
melting points were measured on an X-4 digital melting-point apparatus and are
uncorrected. Electrospray ionization–mass spectrometry (ESI-MS) were obtained
on a Bruker Esquire-Lc=MSn spectrameter. Flash column chromatography was
performed using silica gel (200–300 mesh).
Typical Procedure for Preparation of cis-2,4,5-Tris(o-tolyl)-
imidazoline (2c)
2-Methylbenzaldehyde 1c (1.0 mmol, 0.12 g), urea (4.0 mmol, 0.24 g), and
Cs CO (2.0 mmol, 0.65 g) were dissolved in DMF (2 mL) in a sealed tube. The mix-
2
3
ꢀ
ture was stirred at room temperature for 10 min and then heated at 120 C for 36 h.
ꢀ
The mixture was worked up with water (5 mL) at 0 C, extracted by ethyl ether,
and then dried with anhydrous sodium sulfate. The obtained organic solution was
concentrated under vacuum to leave a residue, which was purified by silica-gel column
chromatography (hexane=EtOAc 3=1) to afford 88 mg (78 %) of cis-2c. White solid,
ꢀ
ꢁ1
1
mp 159–160 C; IR (KBr): 3561, 3218, 2145, 1637, 1504, 1438, 1246, 1016 cm ; H
NMR (300 MHz, CDCl =TMS) d 2.20 (s, 6H, CH ), 2.69 (s, 3H, CH ), 5.03 (s, 1H,
3
3
3
HNCH), 5.67 (s, 2H, CHAr), 6.82–6.96 (m, 6H, H-Ar), 7.06–7.11 (m, 2H, H-Ar),
1
3
7
.24–7.38 (m, 3H, H-Ar), 7.70 (d, J ¼ 7.6 Hz, 1H, H-Ar); C NMR (75 MHz, CDCl3)
d 19.54 (s, CH ), 21.92 (s, CH ), 70.89 (br, s, NCHAr), 125.39, 125.91, 126.88, 128.58,
3
3
1
29.72, 129.93, 130.82, 131.39, 135.10, 137.20, 137.45 (Ar), 165.51 (NHCN); ESI-MS:
þ
m=z ¼ 341.3 ([M þ H] ). HRMS calcd. for C H N , 340.1939; found 340.1943.
2
4
24
2
[9]
cis-2,4,5,-Triphenyl-imidazoline (2a)
ꢀ
ꢁ1
;
Yield 73 %; white solid, mp 160–162 C; IR (KBr): 3717, 3027, 1668, 695 cm
1
H NMR (300 MHz, CDCl =TMS) d 5.45 (s, 2H, CHPh), 6.93–7.03 (m, 10 H, H-Ar),
3

910
F. WANG, Q. LIAO, AND C. XI
1
3
7
.47–7.50 (m, 3 H, H-Ar), 7.97 (d, J ¼ 7.9 Hz, 2 H, H-Ar); C NMR (75 MHz,
CDCl ) d 71.15 (br, s, NCHPh), 126.91, 127.37, 127.62, 127.75, 128.78, 130.16,
31.20, 139.10 (Ph), 164.59 (NHCN); ESI-MS: m=z ¼ 299.2 ([M þ H] ).
3
þ
1
[21]
cis-2,4,5-Tris(p-tolyl)imidazoline (2b)
ꢀ
Yield 70 %; white solid, mp 178–180 C; IR (KBr): 3616, 3168, 1607, 1564,
ꢁ
1 1
512, 1465, 1109 cm ; H NMR (300 MHz, CDCl =TMS) d 2.18 (s, 6H, CH ),
3 3
1
2
.43 (s, 3H, CH ), 5.40 (s, 2H, CHAr), 6.85 (s, 8H, H-Ar), 7.29 (d, J ¼ 7.9 Hz, 2H,
3
1
3
H-Ar), 7.86 (d, J ¼ 7.9 Hz, 2H, H-Ar); C NMR (75 MHz, CDCl ) d 21.13 (s,
3
CH ), 21.60 (s, CH ), 70.11 (br, s, NCHAr), 127.25, 127.54, 128.40, 129.36,
3
3
þ
1
36.20, 136.25, 141.28 (Ar), 164.37 (NHCN); ESI-MS: m=z ¼ 341.2 ([M þ H] ).
cis-2,4,5-Tris(m-tolyl)imidazoline (2d)
ꢀ
Yield 65 %; light yellow solid, mp 156–158 C; IR (KBr): 3581, 3227, 1637,
ꢁ
1 1
481, 693 cm ; H NMR (300 MHz, CDCl =TMS) d 2.13 (s, 6H, CH ), 2.43 (s,
3 3
1
3
6
H, CH ), 5.32 (s, 2H, CHAr), 5.53 (br, s, 1H, NHC), 6.74–6.81 (m, 6H, H-Ar),
3
.89–6.95 (m, 2H, H-Ar), 7.34–7.39 (m, 2H, H-Ar), 7.73 (d, J ¼ 6.5 Hz, 1H,
1
3
H-Ar), 7.86 (s, 1H, H-Ar); C NMR (75 MHz, CDCl ) d 21.31 (s, CH ), 21.46 (s,
3
3
CH ), 124.27, 124.71, 127.44, 128.14, 128.32, 128.57, 130.19, 131.83, 137.06,
3
þ
1
calcd. for C H N , 340.1939; found 340.1941.
38.53, 139.11 (Ar), 164.54 (NHCN); ESI-MS: m=z ¼ 341.3 ([M þ H] ). HRMS
2
4
24
2
[21]
cis-2,4,5-Tris(p-methoxyphenyl)imidazoline (2e)
ꢀ
Yield 79 %; white solid, mp 162–164 C; IR (KBr): 3618, 3205, 1627, 1509,
ꢁ
1
257, 994 cm ; H NMR (300 MHz, CDCl =TMS): 3.65 (s, 6H, CH ), 3.84 (s,
1
1
3
4
3
3
H, CH ), 5.28 (s, 2H, CHAr), 6.57 (d, J ¼ 8.6 Hz, 4H, H-Ar), 6.84 (d, J ¼ 8.6 Hz,
3
H, H-Ar), 6.93 (d, J ¼ 8.9 Hz, 2H, H-Ar), 7.88 (d, J ¼ 8.9 Hz, 2H, H-Ar);
1
3
C NMR (75 MHz, CDCl ): 55.12 (s, OCH ), 55.43 (s, OCH ), 69.93 (br, s,
3
3
3
NCHAr), 113.07, 113.89, 122.74, 128.62, 128.85, 131.57, 158.31, 161.75 (Ar), 163.91
þ
(NHCN); ESI-MS: m=z ¼ 389.2 ([M þ H] ).
cis-2,4,5-Tris(p-phenylphenyl)imidazoline (2f)
ꢀ
Yield 56 %; light yellow solid, mp 130–132 C; IR (KBr): 3593, 3251, 1638,
ꢁ
1 1
484, 842, 794, 732, 695 cm ; H NMR (300 MHz, CDCl =TMS) d 5.54 (s, 2H,
3
1
CHAr), 7.06 (d, J ¼ 8.2 Hz, 3H, H-Ar), 7.25–7.48 (m, 18H, H-Ar), 7.64–7.74 (m,
1
3
4
1
1
H, H-Ar), 8.08 (d, J ¼ 7.9 Hz, 2H, H-Ar); C NMR (75 MHz, CDCl ) d 126.54,
3
27.04, 127.18, 127.32, 127.47, 127.93, 128.10, 128.75, 129.07, 131.05, 138.20,
39.76, 140.29, 140.93, 144.07 (Ar), 164.43 (NHCN); ESI-MS: m=z ¼ 527.3
þ
(
[M þ H] ). HRMS calcd. for C H N , 526.2409; found 526.2413.
3
9
30
2
[21]
cis-2,4,5-Tris(2-thienyl)imidazoline (2g)
ꢀ
Yield 42 %; light yellow solid, mp 138–140 C; IR (KBr): 3679, 3216, 1627,
ꢁ
1 1
479, 1138 cm ; H NMR (300 MHz, CDCl =TMS) d 5.40 (s, 1H, NHCPh), 5.63
3
1

SYNTHESIS OF cis-2,4,5-TRIARYLIMIDAZOLINES
911
(
s, 2H, CHAr), 6.76–6.83 (m, 4H, H-Ar), 7.06–7.14 (m, 3H, H-Ar), 7.51 (d,
1
3
J ¼ 4.5 Hz, 2H, H-Ar); C NMR (75 MHz, CDCl ) d 125.03, 125.52, 126.46,
3
1
(
27.75, 128.23, 129.73, 133.11, 142.11 (Ar), 159.52 (NHCN); ESI-MS: m=z ¼ 317.1
þ
[M þ H] ).
cis-2,4,5-Tris(o-bromophenyl)imidazoline (2h)
ꢀ
Yield: 52 %; light yellow solid, mp 195–197 C; IR (KBr): 3618, 3213, 1732,
ꢁ
645, 1584, 1491, 1465, 1025 cm ; H NMR (300 MHz, CDCl =TMS) d 5.62
3
1
1
1
(br, s, 1H, HNCHAr), 6.03 (s, 2H, CHAr), 6.89–6.94 (m, 2H, H-Ar), 7.01–7.06
(
m, 2H, H-Ar), 7.26–7.44 (m, 6H, H-Ar), 7.67 (dd, J ¼ 7.9 Hz, J ¼ .1.0 Hz, 1H,
1
3
H-Ar), 7.89 (dd, J ¼ 7.5 Hz, J ¼ 1.7 Hz, 1H, H-Ar); C NMR (75 MHz, CDCl ) d
3
6
1
(
9.10 (br, s, NCHAr), 120.97, 124.18, 126.66, 127.79, 128.80, 130.34, 131.80,
32.10, 132.41, 133.84, 137.73 (Ar), 164.31 (NHCN); ESI-MS: m=z ¼ 536.8
þ
[M þ H] ). HRMS calcd. for C H Br N , 531.8785; found 531.8782.
2
1
15
3
2
[15]
cis-2,4,5-Tris(p-flurophenyl)imidazoline (2i)
ꢀ
Yield 40 %; light yellow solid, mp 102–104 C; IR (KBr): 3687, 2952, 1888,
1 1
ꢁ
1
618, 1605, 1508, 1231, 849, 812 cm ; H NMR (300 MHz, CDCl =TMS) d 5.39
3
(
s, 2H, CHAr), 6.72–6.78 (m, 4H, H-Ar), 6.86–6.91 (m, 4H, H-Ar), 7.12–7.18 (m,
1
3
2
NCHAr), 114.75, 115.87, 126.13, 128.98, 129.49, 134.63, 161.61, 163.58 (Ar),
1
H, H-Ar), 7.92–7.97 (m, 2H, H-Ar); C NMR (75 MHz, CDCl ) d 69.73 (br, s,
3
þ
66.32 (NHCN); ESI-MS: m=z ¼ 353.2 ([M þ H] ).
[24]
(
E)-(2-(Methylsulfinyl)vinyl)benzene
1
H NMR (300 MHz, CDCl =TMS) d 2.63 (s, 3H, CH ), 6.83 (d, J ¼ 15.4 Hz,
3
3
1
7
H, HCPh), 7.18 (d, J ¼ 15.4 Hz, 1H, HCSOCH ), 7.27–7.34 (m, 3H, Ph),
3 3
3
1
3
.38–7.41 (m, 2H, Ph); C NMR (75 MHz, CDCl ) d 41.03 (CH ), 127.74
(
CHSOCH ), 133.77(CHPh), 129.04, 129.85, 132.25, 136.46 (Ph); ESI-MS:
þ
3
m=z ¼ 167.1 ([M þ H] ).
Compound 2h
C H Br N , crystallized in monoclinic, space group P2(1)=n with cell
4
4
2
30
6
3
˚
parameters: a ¼ 11.633 (2), b ¼ 17.196 (3), c ¼ 20.112 (4)A , a ¼ 90.00, b ¼ 93.68
ꢀ
˚
3
(
3), c ¼ 90.00 , V ¼ 4015.1 (14) A , Z ¼ 4. CCDC 757397.
ACKNOWLEDGMENT
This work was supported by the National Natural Science Foundation of
China (20972085 and 21032004).

912
F. WANG, Q. LIAO, AND C. XI
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