Desulfurization of thioureas, benzimidazoline-2-thiones and 1,3-dihydro-1,3-diaryl-2-thioxopyrimidine-4,6(2H,5H)-diones with nickel boride at ambient temperature

Article abstract of DOI:10.1039/b206398k

Nickel boride is reported to bring about desulfurization and reductive cleavage of N,N′-diarylthioureas to give corresponding anilines andN-methylanilines while N,N′-dialkylthioureas have been observed to undergo desulfurization to give formamidines; benzimidazoline-2-thiones are reported to undergo desulfurization to benzimidazoles and 1,3-dihydro-1,3-diaryl-2-thioxopyrimidine-4,6(2H,5H)-diones have been observed to yield corresponding hexahydropyrimidine-4,6-diones in high yields in dry methanol at ambient temperature.

Full text of DOI:10.1039/b206398k

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Desulfurization of thioureas, benzimidazoline-2-thiones and 1,3-
dihydro-1,3-diaryl-2-thioxopyrimidine-4,6(2H,5H)-diones with
nickel boride at ambient temperature1
1
Jitender M. Khurana,* Gagan Kukreja and Geeti Bansal
Department of Chemistry, University of Delhi, Delhi-110007, India.
E-mail: jmkhurana@chemistry.du.ac.in; Fax: ϩ91 11 7666605
Received (in Cambridge) 2nd July 2002, Accepted 3rd September 2002
First published as an Advance Article on the web 15th October 2002
Nickel boride is reported to bring about desulfurization and reductive cleavage of N,NЈ-diarylthioureas to give
corresponding anilines and N-methylanilines while N,NЈ-dialkylthioureas have been observed to undergo
desulfurization to give formamidines; benzimidazoline-2-thiones are reported to undergo desulfurization to
benzimidazoles and 1,3-dihydro-1,3-diaryl-2-thioxopyrimidine-4,6(2H,5H )-diones have been observed to
yield corresponding hexahydropyrimidine-4,6-diones in high yields in dry methanol at ambient temperature.
(Va–c), benzimidazolinethiones (VIIa–c), and 2-thiobar-
biturates (IXa–m) with nickel boride in dry methanol at
ambient temperature. The nickel boride was prepared in situ
from anhydrous nickel chloride and sodium borohydride. The
diarylthioureas (Ia–h) underwent complete reductive desul-
furization with concomitant cleavage of carbon–nitrogen bond
with nickel boride using a 1:3:9 molar ratio of substrate:
NiCl2:NaBH4except Ib. The reactions were complete in ∼30–60
min as monitored by TLC, and gave the corresponding anilines
(II) and N-methylanilines (III) in nearly equal ratios (eqn. 1).
Introduction
Reductive desulfurization is an important step in organic syn-
thesis for the removal of sulfur so as to manipulate the structure
of the substrate. A variety of reagents have been used for the
desulfurization of different classes of organic compounds.
Desulfurization of thioureas with reagents such as MeSO2Cl–
Et3N/DMAP,2 NBS,3 thionyl chloride,4 di(2-pyridyl) sulfite5
and di-2-pyridylthionocarbonate–DMAP6 is reported to give
mono- and diaryl carbodimides. Raney nickel has also been
reported as a dethiating agent for thioureas, but the results are
not consistent, e.g., unsubstituted thioureas gave formamidine
hydrochloride in the presence of ammonium chloride7 or a
mixture of methane, ammonia and methylamine;8 N-(o-tolyl)-
thiourea gave o-toluidine;9 N,NЈ-di- and N,N,NЈ-trisubstituted
thioureas have been reported to yield corresponding form-
amidines, while N-monosubstituted thioureas gave N,NЈ-
disubstituted formamidines.10
(1)
The synthesis of hexahydropyrimidine-4,6-diones (some
of which are of pharmaceutical importance11) by reductive
desulfurization of 1,3-dihydro-1,3-diaryl-2-thioxopyrimidine-
4,6(2H,5H )-diones (2-thiobarbiturates) has not received wide
attention. Though sodium amalgam,12 Zn-HCOOH12 and
Raney nickel12 have been reported as dethiating agents for 2-
thiobarbiturates, the yields range from low to moderate. Other
factors associated with Raney nickel e.g., storage, high reaction
temperatures, dependence on solvent and type of Raney nickel
(W-1 to W-8) also restrict its applications.12 The alternative
approach for the synthesis of hexahydropyrimidine-4,6-diones
by the condensation of malonic acid derivatives with amidines,
esters, acid chlorides etc. also suffers from certain disadvan-
tages.13 Therefore in our opinion a better synthesis of hexa-
hydropyrimidine-4,6-diones is required. In recent years nickel
boride14 has been reported as a convenient and an efficient
reagent for the desulfurization of mercaptans, sulfides, sulf-
oxides, sulfones and thioketals.15 It has also been reported as a
chemoselective reagent for deoxygenation of sulfoxides and
selenoxides.16 In view of its versatility and ease of handling, we
decided to explore its application for the desulfurization of
thioureas, benzimidazolinethiones and 2-thiobarbiturates.
The reactions carried out in ethanol, DMF and THF were
sluggish and showed the formation of a complex mixture of
products, unlike the reactions in dry methanol. Therefore dry
methanol was the solvent of choice in these reactions.
Reactions of Ia and Ib when carried out with hydrated nickel
chloride and ordinary methanol were very slow. Diarylthio-
ureas (Ia and Ib) were recovered unchanged when treated with
nickel chloride or sodium borohydride independently under
identical conditions, thus proving the involvement of in situ
generated nickel boride in the reactions. The reaction of N,NЈ-
di(p-chlorophenyl)thiourea (If ) with nickel boride yielded
p-chloroaniline and N-methyl-p-chloroaniline (run 10), unlike
the reactions of N,NЈ-di(p-bromophenyl) and N,NЈ-di(p-iodo-
phenyl)thiourea which yielded a mixture of four components
(runs 11,12). The mixture consisted of halogenated and de-
halogenated anilines— the latter being formed by competitive
dehalogenation. However, complete dehalogenation could not
be achieved in either case despite using very high molar ratios
of substrate to nickel chloride to sodium borohydride (1:15:45).
The reactions carried out at 0 ЊC also showed the presence of
dehalogenated products.
Monoarylthioureas (IVa,b) also underwent reductive de-
sulfurization with concomitant carbon–nitrogen bond cleavage
(eqn. 2) under the above reaction conditions. The correspond-
ing anilines (II) were obtained in higher yields as compared to
Results and discussion
In this paper, we report reductive desulfurization of diaryl-
thioureas (Ia–h), monoarylthioureas (IVa–b), dialkylthioureas
2520
J. Chem. Soc., Perkin Trans. 1, 2002, 2520–2524
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b206398k

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N-methylanilines (III) (runs 13,14). This suggests that reductive
cleavage of the carbon–nitrogen bond adjacent to the aryl
group is preferred over the C–NH2 bond. This encouraged us
to investigate the fate of dialkylthioureas (Va–c) with nickel
boride under similar reaction conditions. The dialkylthiourea
(Va) did not react as fast as diarylthioureas (I) and showed the
formation of a mixture of products. After modifying various
reaction conditions, it was observed that N,NЈ-di(n-butyl)-
thiourea (Va) underwent complete reaction with nickel boride
using a 1:5:5 molar ratio of substrate:NiCl2:NaBH4 in dry
methanol at ambient temperature. The desulfurized product
obtained was identified to be N,NЈ-di(n-butyl)formamidine
(VIa),17 unlike the reductive cleavage products in the reactions
of di- and monoarylthioureas. Subsequently, reactions of
N,NЈ-dicyclohexylthiourea (Vb) and N,NЈ-di(n-propyl)thio-
urea (Vc) with nickel boride were also found to be complete and
gave the corresponding formamidines (eqn. 3).
(5)
desulfurization is dependent on the molar ratio of substrate to
nickel chloride to sodium borohydride (Table 2). The reaction
of IXd with nickel boride gave a mixture of brominated (Xd)
and debrominated (Xa) hexahydropyrimidine-4,6-diones (run
28). Completely debrominated product Xa was obtained
using higher molar ratios of substrate:NiCl2:NaBH4 (run 29),
whereas no dechlorinated product was isolated in the reactions
of IXe even on using a higher molar ratio of substrate to nickel
boride (run 31). These results are in agreement with previous
observation on competing dehalogenation in the case of
diarylthioureas (runs 10–12). This method has been utilized to
prepare 5-ethyl-5-phenylhexahydropyrimidine-4,6-dione (Xl),
a known antiepileptic agent (marketed under the trade name of
‘Mysoline’), in 81% yield compared to its reported synthesis
using Raney nickel in 30–45%.12 Similarly, 1,3-diphenylhexa-
hydropyrimidine-4,6-dione (Xa) could be prepared in 85%
yield, compared to the reported yield of 30% using Raney
nickel.18
Reactions of 5-monosubstituted-2-thiobarbiturates (IXj and
IXk) also yielded the corresponding hexahydropyrimidine-4,6-
diones (Xj and Xk) with nickel boride in high yields. However,
reactions of 5-monosubstituted-2-thiobarbiturates with Raney
nickel have been reported to afford corresponding 4,6-di-
hydroxypyrimidines.19 Thus it can be inferred that the substitu-
tion pattern in the pyrimidine ring at C-5 IXa–m does not play a
decisive role in the final outcome of the desulfurized product
with nickel boride unlike Raney nickel.19 The desulfurization of
5-phenyl-1,3-diphenyl-2-thiobarbituric acid (IXh) required a
very high molar ratio of the reagent probably due to the steric
crowding of the three phenyl rings (run 35). We have further
observed that nickel boride loses its activity with time since no
desulfurization was observed in the reactions of IXa and IXb
when treated with preformed nickel boride after 72 h.
(2)
(3)
The formation of formamidines as the desulfurized product
in the reactions of dialkylthioureas prompted us to further
investigate the reactions of cyclic thioureas. Thus reaction of
benzimidazoline-2-thione (VIIa) was carried out with nickel
boride using a 1:3:9 molar ratio (substrate:NiCl2:NaBH4) in
dry methanol at ambient temperature. The starting material
disappeared completely after 5 min (run 18) and the isolated
product was identified to be benzimidazole (VIIIa). Subse-
quently, reactions of N-methylbenzimidazoline-2-thione (VIIb)
and N-phenylbenzimidazoline-2-thione (VIIc) were carried out
with nickel boride under identical conditions which yielded N-
methyl- and N-phenylbenzimidazole (VIIIb and VIIIc), respec-
tively (eqn. 4). All these results are tabulated in Table 1. In view
of the successful reductive desulfurization of diarylthioureas
(I), monoarylthioureas (IV), dialkylthioureas (V) and benz-
imidazoline-2-thiones (VII), we decided to investigate the
desulfurization of 2-thiobarbiturates with nickel boride and
optimize the conditions, as this would lead to a new method for
the preparation of hexahydropyrimidine-4,6-diones.
Therefore we conclude that nickel boride is an efficient
reagent for reductive desulfurization of diaryl (I) and mono-
arylthioureas (IV) to give anilines and N-methylanilines;
dialkylthioureas (V) to give formamidines and 2(1H)-benz-
imidazolinethiones (VII) to give benzimidazoles. It also pro-
vides a new procedure for the efficient conversion of 2-thio-
barbiturates (IX) to corresponding hexahydropyrimidine-4,6-
diones at ambient temperature.
(4)
We also report herein a simple and convenient procedure for
the reductive desulfurization of 2-thiobarbiturates (IXa–m)
with nickel boride at ambient temperature to the corresponding
hexahydropyrimidine-4,6-diones (Xa–m) in high yields (eqn. 5).
Methanol was also the solvent of choice for quantitative reduc-
tive desulfurization of 2-thiobarbiturates. The reactions of IXa
and IXb were not complete with sodium borohydride alone
even after 48 h using a molar ratio of 1:10 (substrate:NaBH4),
unlike the quantitative desulfurization of IXa with nickel
boride in ∼1 h (run 23). These results are listed in Table 2.
The reagent shows high selectivity towards desulfurization
since it does not affect the carbonyl groups. The rate of
Experimental
All the melting points were recorded on a labequip apparatus
and are uncorrected. Elemental analysis was carried out on a
Perkin–Elmer 2400 Series II CHNS/O analyzer. IR spectra were
recorded on a Perkin–Elmer FT-IR Spectrum-2000. NMR
spectra were recorded on FT-NMR model R-600 Hitachi (60
MHz) with TMS as the internal standard. Mass spectra were
recorded on a JEOL JMS D-300 Spectrometer using the elec-
tron impact method (70 eV). The products were identified by
co-TLC, mp, mixed mp (wherever available), IR, NMR and
mass spectra.
J. Chem. Soc., Perkin Trans. 1, 2002, 2520–2524
2521

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Table 1 Reactions of thioureas and benzimidazoline-2-thiones with nickel boride in dry MeOHa at ambient temperature
Run
Substrate (S)
Molar ratio S:NiCl2:NaBH4
Timeb/min
Products
% Isolated yield
1.
2.
3.
4.
5.
6.
7.
8.
Ia
1:3:9
1:3:0
1:0:9
1:5:15
1:3:0
1:0:9
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:3:9
1:5:5
1:5:5
1:10:10
1:3:9
1:2:6
1:3:9
30
120
120
45
120
120
60
60
30
60
30
30
30
15
30
30
10
5
IIa ؉ IIIa
—
—
IIb ؉ IIIb
—
44 ϩ 48
c
Ia
—
—
c
Ia
Ib
44 ϩ 40
c
Ib
—
—
c
Ib
—
Ic
IIc ؉ IIIc
IId ؉ IIId
IIe ؉ IIIe
IIf ؉ IIIf
44 ϩ 48
47 ϩ 46
49 ϩ 41
49 ϩ 48
—
Id
9.
Ie
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
If
d
Ig
—
—
d
Ih
—
IVa
IVb
Va
Vb
Vc
VIIa
VIIb
VIIc
IIa ؉ IIIa
IIb ؉ IIIb
VIa
VIb
VIc
VIIIa
VIIIb
VIIIc
57 ϩ 17
58 ϩ 19
91
95
94
84
86
79
25
30
a 50 mL of the solvent was used per g of the substrate. b Time for disappearance of starting material. c The starting material was recovered
unchanged. d The reaction mixture showed the presence of at least four components including dehalogenated products even after stirring the reaction
mixture for 24 h.
Table 2 Reactions of 2-thiobarbiturates with nickel boride in dry MeOHe at ambient temperature.
Run
Substrate (S)
Molar ratio S:NiCl2:NaBH4
Timeb/h
Product
% Isolated yield
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
IXa
IXa
IXa
IXb
IXb
IXc
IXc
IXd
IXd
IXe
IXe
IXf
IXf
IXg
IXh
IXi
1:5:5
1:7:7
1:10:10
1:7:7
8f
1.5
1
1.25
0.5
5
7
2.5
1
—
—
80
85
87
89
59
79
Xa
Xa
Xb
Xb
Xc
Xc
—
Xa
Xe
Xe
Xf
Xf
Xg
Xh
Xi
Xi
—
Xj
Xk
Xl
Xm
1:10:10
1:10:10
1:5:15
1:10:10
1:15:15
1:5:5
1:10:10
1:7:7
1:10:10
1:10:10
1:25:25
1:3:3
1:5:5
1:5:5
1:10:10
1:10:10
1:10:10
1:5:5
g
—
76h
80
86
79
82
78
85
87
90
—
82
86
81
82
3
0.5
14
2.5
0.5
5.25
48
4
IXi
IXj
IXj
IXk
IXl
IXm
8f
4
3.5
5.75
2.5
b Time for disappearance of starting material. e 30 mL of solvent was used per g of the substrate. f Reaction was incomplete but showed the formation
of a new product. g Reaction was complete and TLC showed the presence of a mixture of products. h The corresponding debrominated product
(1,3-diphenylhexahydropyrimidine-4,6-dione) was isolated.
thesized by the condensation of N,NЈ-diarylthioureas with
Starting materials
malonic acid in the presence of acetyl chloride.27 C-5 sub-
Dry MeOH was prepared by the known procedure.20
stituted 2-thiobarbiturates were obtained by the condensation
Anhydrous NiCl2 (Qualigens) was prepared by heating NiCl2ؒ
6H2O in a nickel crucible at 200 ЊC until golden yellow. NaBH4
(E. Merck) was used in all the reactions. Diarylthioureas were
of thiourea with substituted diethyl malonate in the presence of
sodium ethoxide.19,28
prepared from corresponding anilines and carbon disulfide
according to the reported procedure.21 Monoarylthioureas were
prepared from the corresponding anilines and ammonium thio-
Reactions of diarylthioureas (I) and
monoarylthioureas (IV)
cyanate as reported.22 Aliphatic amines and CS2 were used for
the preparation of dialkylthiourea.23 2(1H)-Benzimidazoline-
thione was prepared from o-phenylenediamine and CS2 by
the known procedure.24 Methylation of 2(1H)-benzimidazoline-
thione with methyl iodide gave N-methylbenzimidazoline-
2-thione.25 N-Phenylbenzimidazoline-2-thione was prepared
from o-chloronitrobenzene, aniline and CS2 according to the
reported procedure.26 1,3-Diaryl-2-thiobarbiturates were syn-
In a typical experiment, 1 g (4.38 mmol) of N,NЈ-diphenyl-
thiourea (Ia) was dissolved in 50 mL of dry methanol in a 100
mL round-bottomed flask, mounted over a magnetic stirrer.
Anhydrous NiCl2 1.70 g (13.5 mmol) was added, followed by
careful addition of NaBH4 (1.49 g, 39.5 mmol) while stirring
the solution vigorously. TLC was monitored using benzene as
eluent. The reaction was found to be complete in 30 min. The
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J. Chem. Soc., Perkin Trans. 1, 2002, 2520–2524

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reaction mixture was filtered through a Celite pad (∼1 inch) and
washed with methanol (2 × 10 mL). The combined filtrate was
diluted with water (∼150 mL) and was extracted with ethyl
acetate (3 × 10 mL). The combined extract was dried over
anhydrous MgSO4, filtered and concentrated on a rotary
vacuum evaporator under reduced pressure. The mixture was
subjected to column chromatography over silica gel (100–200
mesh) and eluted with benzene:ethyl acetate. Eluate was con-
centrated under reduced pressure to afford aniline (0.178 g,
44%) and N-methylaniline (0.22 g, 48%) which were analysed by
co-TLC, superimposable IR and NMR spectra. Substituted
anilines and N-methylanilines, obtained from substituted diaryl
and monoarylthioureas, were also identified by co-TLC, IR and
NMR spectra.
tion was monitored by TLC using ethyl acetate-methanol as
eluent. After disappearance of the starting material, the
reaction mixture was filtered through a Celite pad (∼ 1 inch)
and washed with methanol (2 × 10 mL). The combined filtrate
was diluted with water (∼ 150 mL) extracted with ethyl acetate
(3 × 10 mL). The combined extract was dried over anhydrous
MgSO4, filtered and concentrated on
a rotary vacuum
evaporator under reduced pressure to afford hexahydropyr-
imidine-4,6-diones (X).
In the case of IXi, the combined filtrate obtained after filtra-
tion of the reaction mixture over a Celite pad, was concentrated
under reduced pressure. The precipitate was macerated with dry
acetone (2 × 10 mL), filtered and washed with dry acetone
(2 × 2 mL). The combined filtrate was concentrated on a rotary
vacuum evaporator to yield hexahydropyrimidine-4,6-dione
(Xi).
The pure samples were obtained by crystallization from ethyl
acetate:petroleum ether (60–80 ЊC) (Xa–g) or ethanol (Xh–m),
and were analysed by mp, IR, NMR and mass spectra. Thus,
1,3-diphenylhexahydropyrimidine-4,6-dione (Xa) mp 176–178
ЊC (lit.,18 178 ЊC), hexahydropyrimidine-4,6-dione (Xi) mp
239–240 ЊC (decomp.) (lit.,31 240–243 ЊC decomp.). 5-Phenyl-
hexahydropyrimidine-4,6-dione (Xj) mp 208–210 ЊC (lit.,32 206–
210 ЊC), 5-ethyl-5-phenylhexahydropyrimidine-4,6-dione (Xl)
mp 280–281 ЊC (lit.,12 281–282 ЊC) and 5-ethyl-5-isoamyl-
hexahydropyrimidine-4,6-dione (Xm) mp 268–270 ЊC (lit.,19
271 ЊC) were obtained as white solids from the corresponding
2-thiobarbiturates as described above. The spectroscopic data
of newly synthesized hexahydropyrimidine-4,6-diones is listed
below.
Xb: Mp 165–166 ЊC (white solid); anal. calc. (found) for
C18H18N2O2: C, 73.45 (73.36); H, 6.16 (6.20); N, 9.52 (9.64)%.
νmax (KBr)/cmϪ1: 1690; δH (CDCl3, 60 MHz): 7.25 (s, 8H, Ar–
H), 5.40 (s, 2H, C2–H), 3.70 (s, 2H, C5–H), 2.40 (s, 6H, –CH3);
m/z (EI): 294 (Mϩ, 55%), 224 (5), 175 (15), 119 (100), 133 (95),
91 (100).
Xc: Mp 160–161 ЊC (white solid); anal. calc. (found) for
C18H18N2O2: C, 73.45 (73.54); H, 6.16 (6.24); N, 9.52 (9.61)%.
νmax (Nujol)/cmϪ1: 1673; δH (CDCl3, 60 MHz): 7.25 (s, 8H, Ar–
H), 5.35 (s, 2H, C2–H), 3.70 (s, 2H, C5–H), 2.35 (s, 6H, –CH3);
m/z (EI): 294 (Mϩ, 50%), 224 (10), 175 (15), 119 (100), 133 (80),
91 (100).
Xe: Mp 149–150 ЊC (white solid); anal. calc. (found) for
C16H12Cl2N2O2: C, 57.33 (57.26); H, 3.61 (3.59); N, 8.36
(8.47)%. νmax (KBr)/cmϪ1: 1699; δH (CDCl3, 60 MHz): 7.45 (s,
8H, Ar–H), 5.60 (s, 2H, C2–H), 3.75 (s, 2H, C5–H); m/z (EI):
335 (Mϩ, 10%), 300 (20), 266 (10), 195 (5), 153 (25), 139 (75),
105 (100).
Xf: Mp 181–182 ЊC (white solid); anal. calc. (found) for
C18H18N2O4: C, 66.25 (66.49); H, 5.56 (5.60); N, 8.58 (8.40)%.
νmax (KBr)/cmϪ1: 1683, 1668; δH (CDCl3, 60 MHz): 7.0–7.4
(m, 8H, Ar–H), 5.35 (s, 2H, C2–H), 3.85 (s, 6H, –OCH3), 3.70
(s, 2H, C5–H); m/z (EI): 326 (Mϩ, 80%), 191 (15), 149 (100), 135
(30).
Xg: Mp 192–193 ЊC (white solid); anal. calc. (found) for
C18H18N2O4: C, 66.25 (66.39); H, 5.56 (5.44); N, 8.58 (8.45)%.
νmax (KBr)/cmϪ1: 1680, 1660; δH (CDCl3, 60 MHz): 6.90–7.35
(m, 8H, Ar–H), 5.35 (s, 2H, C2–H), 3.80 (s, 6H, –OCH3), 3.65
(s, 2H, C5–H); m/z (EI): 326 (Mϩ, 82%), 191 (13), 149 (100), 135
(27).
Xh: Mp 210–213 ЊC (white solid); anal. calc. (found) for
C22H18N2O2: C, 77.17 (77.09); H, 5.30 (5.42); N, 8.18 (8.40)%.
νmax (KBr)/cmϪ1: 1711, 1676; δH (CDCl3, 60 MHz): 7.20–7.60
(m, 15H, Ar–H), 5.25 (d, 2H, J = 4.5Hz, C2–H), 5.0 (s, 1H, C5–
H); m/z (EI): 342 (Mϩ, 20%), 223 (15), 119 (100), 105 (17), 77
(20).
Reactions of dialkylthioureas (V)
In a 100 mL round-bottomed flask was placed a mixture of
N,NЈ-di(n-butyl)thiourea (Va) (1 g, 5.32 mmol), dry methanol
(50 mL) and anhydrous NiCl2 (3.45 g, 26.6 mmol). 1 g
(26.6 mmol) of NaBH4 was added with continuous stirring.
Disappearance of starting material was monitored by TLC
using benzene:ethyl acetate (95:5) as eluent. The reaction was
complete after 30 min. The reaction mixture was filtered
through a Celite pad (∼ 1 inch) and washed with methanol (2 ×
10 mL). The combined filtrate was concentrated over a Buchi
rotavapour and the product was triturated with dry solvent
ether (3 × 10 mL) from the residue. The ether was removed from
the combined extract and the isolated oil was identified to be
N,NЈ-di(n-butyl)formamidine17 (VIa) (0.75 g, 91%) by NMR
and mass spectra. Similarly, N,NЈ-di(n-cyclohexyl)formamidine
(VIb) was obtained as a white solid, mp 104–105 ЊC (lit.,29
106 ЊC) and N,NЈ-di(n-propyl)formamidine (VIc) was obtained
as a yellow oil17 from di(n-cyclohexyl)thiourea (Vb) and di-
(n-propyl)thiourea (Vc), respectively. All the formamidines
showed characteristic peak corresponding to –N–CH᎐N– at
᎐
δ 7.4–7.5.
Reactions of benzimidazoline-2-thiones (VII)
A mixture of benzimidazoline-2-thione (1 g, 6.67 mmol), dry
methanol (50 mL) and anhydrous NiCl2 (2.6 g, 20.0 mmol) was
placed in a 100 mL round-bottomed flask, fitted with a reflux
condenser and a guard tube. NaBH4 (2.27 g, 60.0 mmol) was
carefully added with continuous stirring. The progress of the
reaction was monitored using benzene:ethyl acetate (90:10) as
an eluent. The starting material disappeared completely after
5 min. The reaction mixture was filtered through a Celite pad
(∼ 1 inch) and washed with methanol (2 × 10 mL). The com-
bined filtrate was diluted with water (∼ 150 mL) extracted with
ethyl acetate (3 × 10 mL). The combined extract was dried
over anhydrous MgSO4, filtered and concentrated on a rotary
vacuum evaporator under reduced pressure. The solid obtained
was recrystallized using ethyl acetate:petroleum ether (60–80
ЊC) and was identified as benzimidazole (VIIIa) (0.66 g, 84%)
by mp 171 ЊC (lit.,30 172–174 ЊC), co-TLC, mixed mp and NMR
spectra. Similarly, N-methylbenzimidazole (VIIb), mp 58–60 ЊC
(lit.,30 59–62 ЊC) and N-phenylbenzimidazole (VIIIc), mp 95–96
ЊC (lit.,30 97 ЊC) were obtained as white solids from N-methyl-
and N-phenylbenzimidazoline-2-thiones (VIIb,c), respectively.
Reactions of 2-thiobarbiturates (IX)
2-Thiobarbiturates (IX) (1 g, n mmol), anhydrous NiCl2-
(according to Table 2) and dry methanol (30 mL) were placed in
a 100 mL conical flask fitted with a condenser and a mercury
trap. The flask was mounted over a magnetic stirrer. Sodium
borohydride (according to Table 2) was added very cautiously
while stirring the solution vigorously. The progress of the reac-
Xk: Mp 230–231 ЊC (white solid); anal. calc. (found) for
C11H12N2O2: C, 64.70 (64.62); H, 5.92 (5.87); N, 13.72 (13.85)%.
νmax (KBr)/cmϪ1: 3363, 3172, 1659; δH (DMSO-d6, 60 MHz): 9.0
(s broad, 2H, –NH), 7.30 (s, 5H, Ar–H), 5.40 (s, 2H, C2–H),
J. Chem. Soc., Perkin Trans. 1, 2002, 2520–2524
2523

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BP 734 511/1955 (Chem. Abstr., 1956, 50, 7881); (c) W.R. Boon and
3.90 (t, 1H, C5–H), 3.50 (d, 2H, –CH2Ph); m/z (EI): 204 (Mϩ,
C.H. Vasey, USP 2 666 056/1954 (Chem. Abstr., 1955, 49, 9045f ).
14 J. M. Khurana and A. Gogia, Org. Prep. Proced. Int., 1997, 29, 1.
15 (a) W. E. Truce and F. M. Perry, J. Org. Chem., 1965, 30, 1316;
(b) T. G. Back, K. Yang and H. R. Krouse, J. Org. Chem., 1992, 57,
1986; (c) T. G. Back and K. Yang, J. Chem. Soc., Chem. Commun.,
1990, 819; (d ) D. H. R. Barton and W. Liu, Tetrahedron Lett., 1997,
2431.
16 J. M. Khurana, A. Ray and S. Singh, Tetrahedron Lett., 1998, 3829.
17 C. T. Taylor and W. A. Ehrhart, J. Org. Chem., 1963, 28, 1108 and
ref. 10.
70%), 119 (10), 84 (75), 56 (100), 44 (10).
Acknowledgements
We are pleased to acknowledge the financial support for this
work by CSIR, New Delhi, India, Project No. 01(1567)/99/
EMR-II. The authors GK and GB also wish to thank CSIR for
providing SRF.
18 J. Büchi, H. Braunschweiger, M. Kira and H. Lauener, Helv. Chim.
Acta., 1966, 49, 2337.
19 W. B. Whalley, E. L. Anderson, F. Dugan, J. W. Wilson and
G. E. Ullyot, J. Am. Chem. Soc., 1955, 77, 745.
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