1,3-Dibromo-5,5-dimethylhydantoin as a new imidazolidine dehydrogenating agent: Synthesis of 4,5-dihydro-1H-imidazolium salts

Article abstract of DOI:10.3987/COM-06-10667

A study about the scope of the method for the attainment of 4,5-dihydro-1H-imidazolium salts by dehydrogenation of N,N′-dibenzyl- and N-aryl-N′-benzylimidazolidines is presented. Employed dehydrogenating agents were N-bromoacetamide (NBA), N-bromosuccinim

Full text of DOI:10.3987/COM-06-10667

HETEROCYCLES, Vol. 68, No. 4, 2006
701
HETEROCYCLES, Vol. 68, No. 4, 2006, pp. 701 - 712. © The Japan Institute of Heterocyclic Chemistry
Received, 6th January, 2006, Accepted, 9th March, 2006, Published online, 10th March, 2006. COM-06-10667
1,3-DIBROMO-5,5-DIMETHYLHYDANTOIN AS A NEW
IMIDAZOLIDINE DEHYDROGENATING AGENT: SYNTHESIS OF
4,5-DIHYDRO-1H-IMIDAZOLIUM SALTS
María C. Caterina, María A. Figueroa, Isabel A. Perillo, and Alejandra
Salerno*
Department of Organic Chemistry, Faculty of Pharmacy and Biochemistry,
University of Buenos Aires. Junín 956 (1113) Buenos Aires, Argentina. E-mail:
asalerno@ffyb.uba.ar
Abstract – A study about the scope of the method for the attainment of
4,5-dihydro-1H-imidazolium salts by dehydrogenation of N,N´-dibenzyl- and
N-aryl-N´-benzylimidazolidines is presented. Employed dehydrogenating agents
were N-bromoacetamide (NBA), N-bromosuccinimide (NBS), carbon
tetrachloride and 1,3-dibromo-5,5-dimethylhydantoin (DBDMH). DBDMH was
the choice reagent due to the purity of the attained products, lowest reaction times
and highest yields.
INTRODUCTION
4,5-Dihydro-1H-imidazolium salts with different types of substitution have been used as suitable models of
the coenzyme N5,N10-methenyltetrahydrofolic acid, promoting biochemical transfer of one carbon unit at
the oxidation level of formic acid.1-3 Thus, in attempts to mimic the biological process chemically
reproducing the transfer of C-2, reactions of various N,N´-disubstituted salts with suitable nucleophilic
reagents have been studied.4 Furthemore, the value of 4,5-dihydro-1H-imidazolium salts as synthetic
precursors has been demonstrated.4c,4d,5-8 Additionally, 4,5-dihydro-1H-imidazolium salts have been
studied owing to their surfactant activity9 and employed in various metal-catalysed reactions10 and as
catalysts in the opening of epoxides.11
Attainment of 4,5-dihydro-1H-imidazolium salts has been developed by different synthetic routes,
depending on the required substitution pattern. Thus, N-alkyl substituted salts were classically synthesized
by quaternization of the corresponding 4,5-dihydro-1H-imidazole.8 The method is limited by the precursor
imidazole synthesis and the nature of the alkylating agent determined by the mechanism, a typically SN2

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HETEROCYCLES, Vol. 68, No. 4, 2006
pathway. Reaction of electron-rich alkenes (bisimidazolidinylidene derivatives) in acid media12 and the
N-formyl-N,N´-diarylethylenediamines cyclization,13 have been employed to the attainment of
2-unsubstituted 1,3-diaryl-4,5-dihydro-1H-imidazolium salts.
On the other hand, synthesis of
N,N´-disubstituted salts by cyclocondensation of N,N´-dialkylethylenediamines with carboxylic acids and
derivatives has been little studied.14,15
Synthesis of 4,5-dihydro-1H-imidazolium salts by dehydrogenation of imidazolidines results a highly
attractive method due to the easy preparation of precursor aminals by condensation of aldehydes with
properly
substituted
ethylenediamines.16
The
synthesis
of
1,3-diaryl-
and
1,2-diaryl-3-alkyldihydro-imidazolium salts with NBA, NBS and carbon tetrachloride among other
dehydrogenating agents has been previously studied by us.17
Continuing ongoing research on the synthesis and study of 4,5-dihydro-1H-imidazolium salts5-7,17 we
present in this work the synthesis of a series of N-benzyl- and N,N´-dibenzyl-4,5-dihydro-1H-imidazolium
salts (1) by dehydrogenation of the corresponding imidazolidines (2) (Scheme 1). In order to determine the
scope of application of the method, reactions of compounds (2a-u) with the reagents mentioned above as
well as with 1,3-dibromo-5,5-dimethylhydantoin (DBDMH, a N-haloamide closely related to NBS and
NBA) were assayed.
RESULTS AND DISCUSSION
Precursor imidazolidines (2) were easily obtained by condensation of N-aryl-N´-benzylethylenediamines (3,
R1=Ar, R3=Bn) and N,N´-dibenzylethylenediamines (3, R1=R3=Bn) with aldehydes in ethanol under reflux.
Synthesis of compounds (3) had been previously optimized by us by reaction of ethylenediamine or
N-arylethylenediamines with benzaldehydes promoted by microwave irradiation, and further reduction of
the formed imines.18
Scheme 1
R1
N
N
R3
R2
H
b
a
2
+
R1
N
N
R3
NHR3
3
O
X-
+
R2-C
R1NH
R2
1
H
a: Ethanol, b: NBA, NBS, DBDMH or CCl4
X= Br-, Cl-

HETEROCYCLES, Vol. 68, No. 4, 2006
703
Table 1: 4,5-Dihydro-1H-imidazolium salts (1), imidazolidines (2) and ethylenediamines (3)
Compounds
R1
R2
R3
1,2,3
a
b
c
C6H5CH2
4-CH3OC6H4CH2
4-ClC6H4CH2
3,4-Cl2C6H3CH2
4-CH3OC6H4CH2
C6H5CH2
H
H
C6H5CH2
4-CH3OC6H4CH2
4-ClC6H4CH2
3,4-Cl2C6H3CH2
4-CH3OC6H4CH2
C6H5CH2
H
d
e
H
CH3
f
C6H5
C6H5
C6H5
C6H5
4-ClC6H4
4-CH3OC6H4
3,4-Cl2C6H3
3-ClC6H4
H
g
h
i
4-CH3OC6H4CH2
4-ClC6H4CH2
3,4-Cl2C6H3CH2
4-ClC6H4CH2
4-ClC6H4CH2
4-ClC6H4CH2
4-ClC6H4CH2
4-CH3C6H4
4-CH3OC6H4CH2
4-ClC6H4CH2
3,4-Cl2C6H3CH2
4-ClC6H4CH2
4-ClC6H4CH2
4-ClC6H4CH2
4-ClC6H4CH2
C6H5CH2
j
k
l
m
n
o
p
q
r
4-ClC6H4
H
C6H5CH2
4-NO2C6H4
H
C6H5CH2
4-CH3OC6H4
4-CH3C6H4
H
C6H5CH2
C6H5
C6H5
C6H5
C6H5
C6H5CH2
s
4-ClC6H4
C6H5CH2
t
4-NO2C6H4
C6H5CH2
u
4-CH3OC6H4
C6H5CH2
Dehydrogenations of 2 with NBS, NBA and DBDMH were carried out at room temperature by stirring a
mixture of the imidazolidine and the oxidant in THF. In this medium, salts generally precipitated as long as
they were formed; otherwise precipitation was induced by adding ether. NBS and NBA promoted
dehydrogenation of 2-unsubstituted N,N´-dibenzylimidazolidines (2a-d) in times which varied between 1
and 3 h, with yields of 50-70%, being in general highest with NBA (Table 2). For the C-2 substituted
imidazolidines (2e-m) reaction times increased to 2-5 h and yields decreased in approximately a 10%.
For unsymmetrically N,N´-disubstituted imidazolidines (2n-u), reaction yields decreased with both
reagents to 30-40% and reaction times varied between 4.5-5 h for 2-unsubstituted compounds (2n-q) and
increased up to 6-6.5 h for 2-phenyl substituted compounds (2r-u).
Reactions with carbon tetrachloride were carried out by refluxing solution of imidazolidines. Reaction
times varied between 2-8 h. The prolonged heating promoted the formation of by-products and/or
decomposition of the desired compounds, so as product purity and reaction yields strongly diminished in
comparison with the above N-haloamides.

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HETEROCYCLES, Vol. 68, No. 4, 2006
Reactions with DBDMH were carried out in the same conditions as those with NBS and NBA. In all cases,
dehydrogenation of imidazolidines (2a-u) with DBDMH occurred in a much faster rate than with the other
reagents and requires under 5 min for imidazolidines (2a-m) (1,3-dibenzyl) and under 15 min for the rest of
compounds. Reaction yields increased to 60-70 % in the synthesis of salts (1n-u) and to 70-90% for the
synthesis of N,N´-symmetrically substituted compounds (2a-m).
Table 2: Summary of dehydrogenations (2Æ1) using, NBA, NBS, DBDMH and CCl4
substrate
NBA
Time
NBS
DBDMH
Time
CCl4
Time
Yield
(%)
68
70
69
65
60
59
55
58
57
52
59
55
59
32
40
38
39
37
39
37
36
Time
(h)
2.5
2
Yield
(%)
57
55
52
50
45
50
47
43
49
40
41
50
45
30
35
31
31
32
34
30
30
Yield
(%)
89
86
78
77
85
87
78
85
88
85
86
79
87
65
63
69
68
69
64
66
65
Yield
(%)
38
38
32
21
30
35
25
25
30
32
30
29
29
22
22
19
20
25
20
23
21
(h)
1.5
1.5
1
(min)
2
(h)
2
2a
2b
2c
2d
2e
2f
2
2.5
3
2
1
2
3
1
4
3
4
4
7
2.5
2.5
3.5
3.5
4.2
3.5
3
3
3
6
2g
2h
2i
2.5
4
4
6
3
7.5
5.5
8
4.5
5
2
2j
4
2k
2l
4
3
6.5
4
4.5
4
2
2m
2n
2o
2p
2q
2r
2s
3
2
6.5
6.5
6
4
4.5
5
8
4.5
4.5
4.5
5.5
6
7
5
12
10
10
15
8
6.5
5
5
6
7
6.6
6.5
6.5
7.5
6.5
6.5
2t
6.5
6
2u
10
Although dehydrogenation process could theoretically continue until reaching the aromatic imidazolium
salt, no secondary product with these features was observed in the studied cases.
In order to determine if reaction with DBDMH occurs by a ionic or radical mechanism, as it was proposed
for other N-haloamides, reactions were repeated in the same conditions but with a radical promoting
reagent (benzoyl peroxide) and a radical inhibitor (BHT). In both cases yields and reaction times did not

HETEROCYCLES, Vol. 68, No. 4, 2006
705
vary. Thus, a ionic mechanism is proposed, which probably involves bromination of imidazolidine
nitrogen followed by deprotonation and displacement of a bromide anion.
Scheme 2
O
O
+
Br
R1
N
N
R3
- N
N
Br
+
Br
N
N
Br
R1
N
N
R3
R2
R2
H
H
O
O
O
+
+
R1
N
N
R3
H
N
N
Br
Br-
R2
O
Melting points and spectroscopic data of compounds (1a-u) are given in Table 3. As it arose from
structural and physical features N-aryl-N’-benzyldihydroimidazolium salts (1n-u), belong to a new family
of ionic liquids.19
Table 3: Data for products (1a-u) (X=Br)
1H NMR (DMSO-d6/TMS). δ, J (Hz)
Product
mp (ºC)
163-165
1a
8.95 (s, 1 H, NCHN), 7.45-7.35 (m, 10 H, aromatics), 4.70 (s, 4 H, CH2), 3.89
(s, 4 H, CH2-CH2)
1b
1c
172-174
190-192
158-160
10.17 (s, 1 H, NCHN), 7.30 (d, 4 H, aromatics, J=8.6), 6.89 (d, 4 H, aromatics,
J=8.6), 4.76 (s, 4 H, CH2), 3.78 (s, 6 H, CH3), 3.68 (s, 4 H, CH2-CH2)
8.79 (s, 1H, NCHN), 7.45 (d, 4 H, aromatics, J=8.1), 7.40 (d, 4 H, aromatics.,
J=8.1),4.61 (s, 4 H, CH2), 3.70 (s, 4H, CH2-CH2)
1d
1e
8.70 (s, 1 H, NCHN), 7.70-7.85 (m, 4 H, aromatics), 7.45-7.50 (m, 2 H,
aromatics), 4.66 (s, 4 H, CH2), 3.72 (s, 4 H, CH2-CH2)
hygroscopic 7.26 (d, 4 H, aromatics, J=8.5), 6.90 (d, 4 H, aromatics, J=8.5), 4,72 (s, 4 H,
CH2), 3.90 (s, 4 H, CH2-CH2), 3.80 (s, 6 H, OCH3), 2.60 (s, 3 H, CH3)
1f20
1g
165-167
7.72-7.60 (m, 5 H, aromatics), 7.31-7.39 (m, 10 H, aromatics), 4.40 (s, 4 H,
CH2), 3.89 (s, 4 H, CH2-CH2)
hygroscopic 7.90 (dd, 2 H, aromatics., J1=7.2, J2=2.0), 7.63-7.65 (m, 3 H, aromatics), 7.10
(d, 4 H, aromatics, J=8.1), 6.81 (d, 4 H, aromatics, J=8.1), 4.42 (s, 4 H, CH2),
4.02 (s, 4 H, CH2-CH2), 3.73 (s, 6 H, CH3)
1h
180-182
7.80-7.69 (m, 5 H, aromatics), 7.47 (d, 4 H, aromatics, J=8.2), 7.35 (d, 4 H,
aromatics, J=8.2), 4.38 (s, 4 H, CH2), 3.87 (s, 4 H, CH2-CH2)

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HETEROCYCLES, Vol. 68, No. 4, 2006
1i
1j
198-199
173-174
7.70-7.60 (m, 9 H, aromatics), 7.40 (s, 2 H, aromatics), 4.42 (s, 4 H, CH2), 3.92
(s, 4H, CH2-CH2)
7.79 (d, 2 H, aromatics, J=8.9), 7.72 (d, 2 H, aromatics, J=8.9), 7.46 (d, 4 H,
aromatics, J=8.3), 7.38 (d, 4 H, aromatics, J=8.3), 4.40 (s, 4 H, CH2), 3.88 (4 H,
CH2-CH2)
1k
1l
167-169
192-194
7.61 (d, 2 H, aromatics, J=9.1), 7.46 (d, 4H, aromatics, J=8.5), 7.36 (d, 4H,
aromatics, J=8.5), 7.21 (d, 2 H, aromatics, J=9.1), 4.45 (s, 4 H, CH2), 3.87 (s, 4
H, CH2-CH2), 3.81 (s, 3 H, CH3)
7.90-8.10 (m, 2 H, aromatics), 7.60-7.70 (m, 1 H, aromatics), 7.45 (d, 4 H,
aromatics, J=9.1), 7.34 (d, 4 H, aromatics, J=9.1), 4.41 (s, 4 H, CH2), 3.87 (s, 4
H, CH2-CH2)
1m
1n
1o
119-120
oil
7.90-7.60 (m, 4 H, aromatics), 7.37 (d, 4 H, aromatics., J=8.3), 7.19 (d, 4 H,
aromatics, J=8.3), 4.60 (s, 4 H, CH2), 4.15-4.00 (m, 4 H, CH2-CH2)
9.75 (s, 1 H, NCHN), 7.60-7.30 (m, 9 H, aromatics), 5.22 (s, 2 H, CH2C6H5),
4.42-4.30 (m, 2 H, CH2N), 3.85-3.70 (m, 2 H, CH2N), 2.10 (s, 3 H, CH3)
11.3 (s, 1 H, NCHN), 7.60-7.20 (m, 9 H, aromatics), 5.20 (s, 2 H, CH2C6H5),
4.35 (bs, 2 H, CH2N), 4.10 (bs, 2 H, CH2N)
210
1p
oil
9.85 (s, 1 H, NCHN), 8.39 (d, 2 H, aromatics, J=9.1), 7.56 (d, 2 H, aromatics,
J=9.1), 7.50-7.30 (m, 5 H, aromatics), 4.88 (s, 2 H, CH2C6H5), 4.40 (t, 2 H,
CH2N, J=9.0), 3.90 (t, 2 H, CH2N, J=9.0)
1q
1r
oil
oil
9.70 (s, 1 H, NCHN), 7.60-7.30 (m, 9 H, aromatics), 5.00 (s, 2 H, CH2C6H5),
4.32 (m, 2 H, CH2N), 3.80 (m, 2 H, CH2N), 3.64 (s, 3 H, OCH3)
7.65-7.50 (m, 4 H, aromatics), 7.48-7.30 (m, 6 H, aromatics), 7.20-7.05 (m, 4
H, aromatics), 4.52 (s, 2 H, CH2C6H5), 4.44 (t, 2 H, J=9.8, CH2N), 4.04 (t, 2 H,
J=9.8, CH2N), 2.20 (s, 3 H, CH3)
1s
1t
oil
oil
oil
7.65-7.20 (m, 14 H aromatics), 4.75 (s, 2 H, CH2C6H5), 4.55( t, 2 H, CH2N, J=
10.0), 4.27( t, 2 H, CH2N, J= 10.0)
8.10-7.20 (m, 14 H aromatics), 4.75 (s, 2 H, CH2C6H5), 4.65 (t, 2 H, CH2N, J=
9.3), 4.37 (t, 2 H, CH2N, J= 9.3)
1u
7.70-7.52 (m, 5 H aromatics), 7.47-7.32 (m, 5 H aromatics), 7.26 (dd, 2 H
aromatics., J1=6.9, J2= 1.8), 6.88 (dd, 2 H aromatics., J1=6.9, J2= 1.8), 4.53 (s,
2 H, CH2C6H5), 4.41 (t, 2 H, CH2N, J=9.7), 4.04 (t, 2 H, CH2N, J=9.7), 3.67 (s,
3 H, OCH3)
In summary, DBDMH was the choice reagent for N-benzyl-N´-aryl- and N,N´-dibenzylimidazolidine
dehydrogenation. The method is operationally simple, with very low reaction times, easy work-up
procedure and high reaction yields, also being the reagent a cheap commercially available chemical.

HETEROCYCLES, Vol. 68, No. 4, 2006
707
EXPERIMENTAL
1
Melting points were determined with a Büchi capillary apparatus and are uncorrected. H NMR spectra
were recorded on a Bruker MSL 300 MHz spectrometer using DMSO-d6. Standard Concentration of the
samples was 20 mg/mL. Chemical shifts are reported in ppm (δ) relative to TMS as an internal standard.
D2O was employed to confirm exchangeable protons (ex). MS (EI) were recorded with a GC-MS Shimadzu
QP-1000 spectrometer operating at 20 eV. TLC analyses were carried out on aluminium sheets silica gel 60
F254 using benzene-methanol (9:1) as the solvent. Column chromatography was performed on silica gel 60
(0.063-0.200 mesh) with typically 30-50 g of stationary phase per gram substance.
N,N´-Disubstituted ethylenediamines (3).
Compounds (3a,b,c,o,p,q18 and 3d)21 were prepared following literature procedure.18 The physical data and
elemental analyses of the new compounds are as follows:
N-Benzyl-N´-4(methylphenyl)ethylenediamine (3n)
1
Yield: 60%. H NMR: δ= 7.50-7.20 (m, 9 H, aromatics), 3.81 (s, 2 H, CH2Ar), 3.25 (t, J=6.1 Hz, 2 H,
CH2NAr,), 2.85 (t, J=6.1 Hz, 2 H, CH2NBn,), 2.30 (s, 3 H, CH3), 1.90 (bs, 1 H, NH). MS: m/z= 240 (M+.).
Anal. Calcd. for C16H20N2: C, 79.96; H, 8.39, N, 11.66. Found: C, 79.89; H, 8.37; N, 11.68.
Imidazolidines (2).
Compounds (2) were obtained by reaction of the corresponding N,N’-disubstituted ethylenediamines (3)
and aldehydes in ethanol.16 Compounds (2a,22 2b,g,e,16 2f,23 2h,i.j,24 2m,18 2o,25 2p,25 2s,26 2t,18) were
previously described in the literature. The physical data and elemental analyses of the new compounds are
as follows.
1,3-Di-(4-chlorobenzyl)imidazolidine (2c)
Yield: 81%. mp: 173-175ºC (ethanol). 1H NMR: δ= 7.15 (d, J= 6.7 Hz, 4 H, aromatics), 7.10 (d, J=6.7 Hz,
4 H, aromatics), 3.64 (s, 4 H, CH2Ar), 3.36 (s, 2 H, NCH2N), 2.80 (s, 4 H, CH2N). MS: m/z= 320 (32%),
M+.; 322 (20%), (M+2)+.. Anal. Calcd for C17H18N2Cl2: C; 63.56, H; 5.62, N; 8.72. Found: C; 63.62, H; 5.60,
N; 8.70.
1,3-Di-(3,4-dichlorobenzyl)imidazolidine (2d)
Yield: 83%. mp: 158-160ºC (ethanol). 1H NMR: δ= 7.50 (s, 2 H, aromatics), 7.34-7.45 (m, 2 H, aromatics),
7.15-7.18 (dd, J1= 8.0 Hz, J2= 1.9 Hz, 2 H, aromatics), 3.64 (s, 4 H, CH2Ar), 3.37 (s, 2 H, NCH2N), 2.82 (s,
4 H, CH2N). MS: m/z= 388 (42%), M+.; 390 (56%), (M+2)+.; 392 (27%), (M+4)+.. Anal. Calcd for
C17H16N2Cl4: C; 52.34, H; 4.13, N; 7.18. Found: C; 52.30, H; 4.15, N; 7.16.

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HETEROCYCLES, Vol. 68, No. 4, 2006
1,3-Di-(3,4-dichlorobenzyl)-2-phenylimidazolidine (2i)
1
Yield: 78%. mp: 81-83ºC (ethanol). H NMR: δ= 7.59-7.56 (m, 2 H, aromatics), 7.43-7.25 (m, 7 H,
aromatics), 7.06 (dd, J1= 8.1 Hz, J2= 1.6 Hz, 2 H, aromatics), 3.81 (s, 1 H, NCHN), 3.68 (d, J=13.3 Hz, 2 H,
CH2Ar,), 3.21-3.10 (m, 4 H, CH2Ar and CH2N), 2.49-2.44 (m, 2 H, CH2N). MS: m/z= 464 (35%), M+.; 466
(44%), (M+2)+.; 468 (23%), (M+4)+.. Anal. Calcd for C23H20N2Cl4: C, 59.25; H, 4.32; N, 6.01. Found: C;
59.32, H; 4.30, N; 5.99.
1,3-Di-(4-chlorobenzyl)-2-(4-metoxyphenyl)imidazolidine (2k)
1
Yield: 83%. mp: 110-111ºC (ethanol). H NMR: δ= 7.5 (dd, J1=6.7 Hz, J2=2.1 Hz, 2 H, aromatics),
7.30-7.15 (m, 8 H, aromatics), 6.91 (dd, J1=6.7 Hz, J2=2.1 Hz , 2 H, aromatics), 3.89 (s, 1 H, NCHN), 3.82
(s, 3 H, OCH3), 3.89 (d, J=13.8, 2 H, CHHAr), 3.20-3.05 (m, 4 H, CHHAr, CH2N), 2.43 (m, 2 H, CH2N).
MS: m/z=426 (38%), M+.; 428 (25%), (M+2)+.. Anal. Calcd for C24H24N2OCl2: C; 67.45, H; 5.66, N; 6.55.
Found: C; 67.56, H; 5.68, N; 6.56.
1-Benzyl-3-(4-methylphenyl)imidazolidine (2n)
Yield: 74%. mp: 67-69ºC (ethanol). 1H NMR: δ= 7.41-7.32 (m, 5 H, aromatics), 7.02 (dd, J1= 6.6 Hz, J2=
2.0 Hz, 2 H, aromatics), 6.40 (dd, J1= 6.6 Hz, J2= 2.0 Hz, 2 H, aromatics), 3.99 (s, 2 H, NCH2N), 3.77 (s, 2
H, CH2Ar), 3.42 (t, J=6.4 Hz, 2 H CH2NAr,), 3.02 (t, J= 6.4 Hz, 2 H, CH2NBn), 2.25 (s, 3 H, CH3). MS:
m/z= 252 (M+.). Anal. Calcd for C17H20N2: C; 80.91, H; 7.99, N; 11.10. Found: C; 80.86, H; 8.01, N; 11.14.
1-Benzyl-3-(4-methoxyphenyl)imidazolidine (2q)
1
Yield: 74%. mp: 78-80ºC (ethanol). H NMR: δ= 7.32-7.28 (m, 5 H, aromatics), 6.90 (d, J=8.6 Hz, 2 H,
aromatics), 6.43 (d, J=8.6 Hz, 2 H, aromatics), 3.80 (s, 2 H, NCH2N), 3.78 (s, 3 H, OCH3), 3.67 (s, 2 H,
CH2Ar), 3.40 (t, J= 6.2 Hz, 2 H CH2NAr,), 3.01 (t, J=6.2 Hz, 2 H, CH2NBn). MS: m/z= 268 (M+.). Anal.
Calcd for C17H20N2O: C; 76.09, H; 7.51, N; 10.44. Found: C; 76.15, H; 7.53, N; 10.47.
1-Benzyl-3-(4-methylphenyl)-2-phenylimidazolidine (2r)
Yield: 72%. mp: 113-114ºC (ethanol). 1H NMR: δ= 7.36-7.23 (m, 10 H, aromatics), 6.94 (d, J= 8.6 Hz, 2 H,
aromatics), 6.41 (d, J= 8.6 Hz, 2 H, aromatics), 5.01 (s, 1 H, NCHN), 3.75 (d, 1 H, J= 12.9 Hz, CHHAr),
3.52 (d, 1 H, J= 12.9 Hz, CHHAr), 3.22-3.14 (m, 2 H, CH2NAr), 2.91-2.85 (m, 2 H, CH2NBn), 2.20 (s, 3 H,
CH3). MS: m/z= 328 (M+.). Anal. Calcd for C23H24N2: C; 84.11, H; 7.36, N; 8.53. Found: C; 84.20, H; 7.34,
N; 8.56.
1-Benzyl-3-(4-methoxyphenyl)-2-phenylimidazolidine (2u)

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Yield: 78%. mp: 101-103ºC (ethanol). 1H NMR: δ= 7.40-7.20 (m, 10 H, aromatics), 6.74 (dd, J1=6.9 Hz,
J2= 2.3 Hz, 2 H, aromatics), 6.44 (dd, J1=6.9 Hz, J2=2.3 Hz, 2 H, aromatics), 4.95 (s, 1 H, NCHN), 3.73 (d,
J= 12.8 Hz, 1 H, CHHAr), 3.70 (s, 3 H, OCH3), 3.48 (d, J=12.8 Hz, 1 H, CHHAr), 3.22-3.14 (m, 2 H,
CH2N), 2.89-2.82 (m, 2 H, CH2N). MS: m/z= 344 (M+.). Anal. Calcd for C23H24N2O: C; 80.20, H; 7.02, N;
8.13. Found: C; 80.29, H; 7.04, N; 8.10.
Synthesis of 1H-4,5-Dihydroimidazolium Salts (1).
Reaction of Imidazolidines (2) with N-Bromosuccinimide, N-Bromoacetamide or
1,3-Dibromo-5,5-dimethylhydantoin. General Procedure.
To a stirred solution of compounds (2) (10 mmol) in THF (30 mL), the corresponding dehydrogenating
agent (12 mmol) was added in portions while the reaction was monitoried by TLC. After complete
disappearance of starting material, salts precipitate in variable times (Table 2); otherwise precipitation was
induced by adding ethyl ether. The solid products were collected and recrystallized from anhydrous
methanol and the oils were purified by chromatographyc method using chloroform-methanol (8:2) as the
solvent.
Melting points and 1H-NMR data are given in Table 3. Elemental analyses of the new compounds are as
follows.
1,3-Dibenzyl-4,5-dihydro-1H-imidazolium bromide (1a)
Anal. Calcd for C17H19N2Br: C; 61.64, H; 5.78, N; 8.40. Found: C; 61.73, H; 5.80, N; 8.38.
1,3-Di-(4-methoxybenzyl)-4,5-dihydro-1H-imidazolium bromide. (1b)
Anal. Calcd for C19H23N2O2Br: C; 58.32, H; 5.92, N; 7.16. Found: C; 58.42, H; 5.91, N; 7.18.
1,3-Di-(4-chlorobenzyl)-4,5-dihydro-1H-imidazolium bromide (1c)
Anal. Calcd for C17H17N2BrCl2: C; 51.03, H; 4.28, N; 7.00. Found: C; 51.10, H; 4.26, N; 7.02.
1,3-Di-(3,4-dichlorobenzyl)-4,5dihydro-1H-imidazolium bromide (1d)
Anal. Calcd for C17H15N2BrCl4: C; 43.53, H; 3.22, N; 5.97. Found: C; 43.48, H; 3.23, N; 5.99.
1,3-Di-(4-methoxybenzyl)-2-methyl-4,5-dihydro-1H-imidazolium bromide (1e)
Anal. Calcd for C20H25N2OBr2: C; 59.26, H; 6.22, N; 6.91. Found: C; 59.37, H; 6.21, N; 6.93.

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1,3-Di-(4-methoxybenzyl)-2-phenyl-4,5-dihydro-1H-imidazolium bromide. (1g)
Anal. Calcd for C25H27N2O2Br: C; 64.24, H; 5.82, N; 5.99. Found: C; 64.15, H; 5.83, N; 6.01.
1,3-Di-(4-chlorobenzyl)-2-phenyl-4,5-dihydro-1H-imidazolium bromide (1h)
Anal. Calcd for C23H21N2BrCl2: C; 58.01, H; 4.44, N; 5.88. Found: C; 58.11, H; 4.45, N; 5.86.
1,3-Di-(3,4-dichlorobenzyl)-2-phenyl-4,5dihydro-1H-imidazolium bromide (1i)
Anal. Calcd for C23H19N2BrCl4: C; 50.68, H; 3.51, N; 5.14. Found: C; 50.59, H; 3.52, N; 5.15.
1,3-Di-(4-chlorobenzyl)-2-(4-chlorophenyl) -4,5-dihydro-1H-imidazolium bromide (1j)
Anal. Calcd for C23H20N2BrCl3: C; 54.09, H; 3.95, N; 5.49. Found: C; 54.20, H; 3.96, N; 5.47.
1,3-Di-(4-chlorobenzyl)-2-(4-methoxyphenyl)-4,5-dihydro-1H-imidazolium bromide (1k)
Anal. Calcd for C24H23N2OBrCl2: C; 56.94, H; 4.58, N; 5.53. Found: C; 57.03, H; 4.60, N; 5.51.
1,3-Di-(4-chlorobenzyl)-2-(3,4-dichlorophenyl)-4,5-dihydro-1H-imidazolium bromide (1l)
Anal. Calcd for C23H19N2BrCl4: C; 50.68, H; 3.51, N; 5.14. Found: C; 50.77, H; 3.49, N; 5.13.
1,3-Di-(4-chlorobenzyl)-2-(3-chlorophenyl)-4,5-dihydro-1H-imidazolium bromide (1m)
Anal. Calcd for C23H20N2BrCl3: C; 54.09, H; 3.95, N; 5.49. Found: C; 53.98, H; 3.97, N; 5.51.
1-Benzyl-3-(4-methylphenyl)-4,5-dihydro-1H-imidazolium bromide (1n)
Anal. Calcd for C17H19N2Br: C; 61.64, H; 5.78, N; 8.46. Found: C; 61.71, H; 5.76, N; 8.45.
1-Benzyl-3-(4-chlorophenyl)-4,5-dihydro-1H-imidazolium bromide (1o)
Anal. Calcd for C16H16N2BrCl: C, 54.65; H, 4.59; N, 7.97. Found: C; 54.72, H; 4.57, N; 7.96.
1-Benzyl-3-(4-nitrophenyl)-4,5-dihydro-1H-imidazolium bromide (1p)
Anal. Calcd for C16H16N3O2Br: C; 53.05, H; 4.45, N; 11.60. Found: C; 53.15, H; 4.44, N; 11.59.
1-Benzyl-3-(4-methoxyphenyl)-4,5-dihydro-1H-imidazolium bromide (1q)
Anal. Calcd for C17H19N2OBr: C; 58.80, H; 5.51, N; 8.07. Found: C; 58.71, H; 5.53, N; 8.08.
1-Benzyl-3-(4-methylphenyl)-2-phenyl-4,5-dihydro-1H-imidazolium bromide (1r)

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Anal. Calcd for C23H23N2Br: C; 67.82, H; 5.69, N; 6.88. Found: C; 67.91, H; 5.67, N; 6.87.
1-Benzyl-3-(4-chlorophenyl)-2-phenyl-4,5-dihydro-1H-imidazolium bromide (1s)
Anal. Calcd for C22H20N2BrCl: C; 61.77, H; 4.71, N; 6.55. Found: C; 61.83, H; 4.70, N; 6.54.
1-Benzyl-3-(4-nitrophenyl)-2-phenyl-4,5-dihydro-1H-imidazolium bromide (1t)
Anal. Calcd for C22H20N3O2Br: C; 60.28, H; 4.60, N; 9.59. Found: C; 60.38, H; 4.59, N; 9.57.
1-Benzyl-3-(4-methoxyphenyl)-2-phenyl-4,5-dihydro-1H-imidazolium bromide (1u)
Anal. Calcd for C23H23N2OBr: C; 65.25, H; 5.48, N; 6.62. Found: C; 65.19, H; 5.50, N; 6.64.
Reaction of Imidazolidines (2) with Carbon Tetrachloride. General Procedure.
A solution of imidazolidine (0.5 mmol) in carbon tetrachloride (100 mL) was refluxed, and the reaction was
monitored by TLC, until disappearance of the starting material (1-6 h). The resulting solids were filtered
and recrystallized from methanol.
ACKNOWLEDGEMENTS
This work was financially supported by the University de Buenos Aires. The authors thank J. Duran and M.
Agman for technical assistance.
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