Oxidative bromination of imidazoheterocycle bromohydrates

Article abstract of DOI:10.1007/s10593-012-0903-8

The behavior of imidazo[1,2-a]pyridinium, imidazo[1,2-a]pyrimidinium, and imidazo[2,1-b]thiazolium bromides derivatives in oxidative bromination reactions has been studied. It has been established that reaction products structures and their yields depend on the properties of the substituents in the bicycle and the oxidant concentration. 0009-3122/12/4710-12802012 Springer Science+Business Media, Inc.

Full text of DOI:10.1007/s10593-012-0903-8

Chemistry of Heterocyclic Compounds, Vol. 47, No. 10, January, 2012 (Russian Original Vol. 47, No. 10, October, 2011)
OXIDATIVE BROMINATION OF IMIDAZOHETEROCYCLE
BROMOHYDRATES*
N. V. Kovalenko1***, A. Yu. Cherepakha1, G. P. Kutrov1**,
O. V. Shishkin2, and S. V. Shishkina2
The behavior of imidazo[1,2-a]pyridinium, imidazo[1,2-a]pyrimidinium, and imidazo[2,1-b]thiazolium
bromides derivatives in oxidative bromination reactions has been studied. It has been established that
reaction products structures and their yields depend on the properties of the substituents in the bicycle
and the oxidant concentration.
Keywords: bromohydrates, imidazoheterocycles, oxidative bromination.
It is known [1,2] that the attack of electrophilic agents predominantly proceeds on the imidazole ring of
imidazoheterocycles*. Among such reactions there is considerable interest in bromination because bromine-
substituted imidazo[1,2-a]pyridines, imidazo[1,2-a]pyrimidines, and imidazo[2,1-b]thiazoles have found use as
physiologically active substances [3]
Most frequently bromination of the starting bases is carried out by bromine in chloroform [4], in acetic
acid [5] or with the help of such brominating agents as N-bromosuccinimide and N-bromoacetamide in ethanol
or carbon tetrachloride [6]. Interesting is the method of synthesis of bromo-substituted imidazoheterocycles by
treating bromohydrates of the starting bases with an oxidizing agent. Note that carrying out this type of reaction
in dimethyl sulfoxide, the solvent also acts as oxidant [7]. We have used hydrogen peroxide as oxidant in our
work on the synthesis of bromo-substituted imidazo[1,2-a]pyridines [8, 9] and also sodium nitrite in glacial
AcOH (in particular in the synthesis of 5-bromo-6-(bromomethyl)imidazo[2,1-b]thiazole [10]).
Earlier [8, 11] we have synthesized bromides of imidazo[1,2-a]pyridinium, imidazo[1,2-a]pyrimi-
dinium, and imidazo[2,1-b]thiazolium derivatives by interaction of the corresponding 2-aminoheterocycles with
mono- or dihalo ketones. The reaction products were isolated as the salts 1a-g and converted into the dehydrated
salts 2a-g.
_______
*The term imidazoheterocycles means bicyclic heterosystems, one of the rings of which is imidazole (for
example, imidazopyridine, imidazopyrimidine, and imidazothiazole).
_______
*Dedicated to the illustrious life of Academician M.O. Lozinskii.
**Deceased.
***To whom correspondence should be addressed, e-mail: kutrov@ukr.net.
1Taras Shevchenko Kyiv National University, 64 Volodymyrska St., Kiev 01601, Ukraine.
2Institute of Monocrystals of National Academy of Sciences of Ukraine, 60 Lenin Ave., Kharkiv 310001,
Ukraine.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No.10, 1548-1554, October, 2011. Original
article submitted March 12, 2011.
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0009-3122/12/4710-1280©2012 Springer Science+Business Media, Inc.

In the present work a study of the behavior of compounds 1 and 2 in the oxidative bromination reaction
is described. For a more complete study of the reaction we have also synthesized salts containing aryl
substituents in position 2 of the imidazo[1,2-a]pyridinium ring. On quaternization of 2-aminopyridine with
substituted phenacyl bromides the salt 1 was isolated only in the case of interaction with p-bromophenacyl
bromide. In reaction with p-methyl- and p-methoxy-substituted phenacyl bromides, mixtures of the salts 1 and 2
were formed, from which the dehydrated salts 2h-j were isolated after short boiling in glacial AcOH.
Compounds 2h-j were described earlier [12] however oxidative bromination with them was not carried
out.
Bromo-substituted imidazo[1,2-a]pyridines 3a-d,g and imidazo[1,2-a]pyrimidines 3e,f were obtained by
reaction of hydrogen peroxide with bromides 1a-g or salts 2a-g.
The reaction proceeded in high yield and the end product was crystallized from solution in analytically
pure state as colorless crystals.
R3
R2
R3
R2
H
N
R3
R2
H
X
X
X
N
N
OH
H2O2
H2O2
R
R
N
N+
N+
Br –
R
3 days
3 days
Br –
R1
R1
R1
Br
1a–h
2a–j
3a–j
Br
NH2
H2O2
3–4 weeks
1a,b or 2a,b
N
Br
R1
4a,b
1–3 a–f R = BrCH2, a–c X = CH, a R1 = R2 = R3 = H, b R1 = Me, R2 = R3 = H, c R1 = R3 = H, R2 = Cl,
d X = CBr, R1 = R3 = H, R2 = Br, e,f X = N, e R1 = R2 = R3 = H, f R1 = R3 = Me, R2 = H; 1–3 g R = Me,
X = CBr, R1 = R3 = H, R2 = Br; 1–3 h X = CH, R = 4-BrC6H4, R1= R2 = R3 = H; 2, 3 i,j X = CH,
R1 = R2 = R3 = H, i R = 4-MeC6H4, j R = 4-MeOC6H4; 4a R1 = H, b R1 = Me
The high yields (91-93%) of products 3a-d were found both when using salts of the type 1a-d (15%
H2O2) and when using dehydrated salts 2a-d (30% H2O2). Products 3h-j (yields 50-70%) were obtained from
compounds 1h, 2h-j and from mixture of salts 1i and 2i, 1j and 2j. This fact may be explained by steric
hindrance caused by substituents for bromine atom to attack the ring.
Solid product began to precipitate from the reaction mixture already after a few hours but the reaction
was completed in 3 days. A spot of an alternative product appeared on a chromatogram on increasing time. For
example, after separation of 3a or 3b, 2-amino-3,5-dibromopyridines 4a and 4b were isolated in 5 and 6% yield
respectively from the reaction mixture after 3-4 weeks. These compounds have been synthesized previously by
other methods [13]. Use of other oxidizing agents, such as sodium nitrite in glacial AcOH, potassium
permanganate in weakly acidic media, or cerium(IV) salts also led to compounds 3 [8]. However, in contrast to
hydrogen peroxide, the yields of final products were lower, the reaction times were longer, and additional
purification was required. We suggest that salts 2 in solution exist in equilibrium with small amount of the free
base and hydrobromic acid, the latter of which reacts with the oxidizing agent to produce free bromine which
then reacts with the base. The characteristic signals of the 3-CH2 protons and also those of the NH and OH
groups of salts 1 or the CH and NH groups of salts 2 are absent in the 1H NMR spectra of compounds 3. In the
13C NMR spectra of compounds 3, the signal of the CBr group carbon was observed in the 90-96 ppm region.
Signals ascribed to the carbon of the CH2Br group (compounds 3a-f) were observed in the 24.8-26.1 ppm range.
The signals of the remaining protons and carbon nuclei of the heterocyclic ring agree with the proposed
structure.
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The course of oxidative bromination of imidazo[2,1-b]thiazole derivatives 5a-d and structures of the
products obtained 6a-d depend on the nature of substituents on the starting compounds [8-11, 14]. Thus,
bromination of compounds 5a-c with a methyl group in position 3 or 6 proceeds with 30% H2O2 at 10-15°C with
yields of ~70%. Structures of the obtained products correspond to 5-bromo-6-methylimidazo[2,1-b]thiazole (6a)
and 5-bromo-3,6-dimethylimidazo[2,1-b]thiazole (6b) which have been synthesized earlier by other methods [14].
In the case of bromide 5c, 5-bromo-6-bromomethyl-3-methylimidazo[2,1-b]thiazole (6c) should be separated
immediately as it appeared because of the formation of a solid precipitate which prevented the reaction in the whole
volume. Oxidative bromination of compound 5d occurred twice with the formation of 5,5-dibromoimidazo[2,1-b]-
thiazol-6(5H)-one (7), the structure of which was confirmed by X-ray structural analysis.
H
N
H
H2O2
Br
Br –
S
S
N
S
N
R
N+
R
N+
R1 Br –
5a–c
N
R1
5d
Br
6a–c
H2O2
N
S
N
H2O2
– Br2
H2O2, Br2
– HBr
S
N
Br
S
O
OH
N
N
N
Br
Br
Br
Br
9
8
7
5, 6 a,b R = Me, a R1 = H, b R1 = Me; c R = CH2Br, R1 = Me
Structure of compound 7 by X-ray crystallographic data.
It may be suggested that the reaction proceeds via a stage of compound 8 formation, which is not
precipitated from solution but is further oxidized. This was confirmed when compound 8, synthesized earlier by
another method [10], after suspending in 15% hydrogen peroxide at a temperature of 10-15°C was dissolved
slowly and after a day yellow needles of the imidazothiazolone 7 were formed. It may be suggested that the
excess hydrogen peroxide oxidizes the CH2Br group into a carboxyl group to form the N-oxide and more
bromine. The carboxyl group is easily decarboxylated, as can be seen by formation of carbon dioxide bubbles,
while the N-oxide is isomerized to form the 2-hydroxy derivative 9. The C=C bond in the imidazole ring adds
bromine and immediately loses HBr to give the product 7 which is readily crystallized. Other products of
oxidation and bromination of unestablished structure remained in solution.
Thus it has been shown that imidazo[1,2-a]pyridinium and imidazo[1,2-a]pyrimidinium bromides are
converted in high yields under the influence of hydrogen peroxide to the corresponding 3-bromo-2-bromo-
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methylimidazo[1,2-a]pyridines or 3-bromo-2-bromomethylimidazo[1,2-a]pyrimidines. This reaction is of a
general nature in the synthesis of this class of products. The behavior of 3R-6R'-7H-imidazo[2,1-b]thiazolium
bromohydrates is different in the oxidative bromination reaction. While the 3- and 6-methyl derivatives gave the
expected 5-bromoimidazo[2,1-b]thiazoles, the compounds unsubstituted in positions 3 and 6 unexpectedly
formed 5,5-dibromoimidazo[2,1-b]thiazol-6(5H)-one.
EXPERIMENTAL
1H and 13C NMR spectra were recorded in 1:1 DMSO-d6–CCl4 solution with TMS as internal standard
with a Varian Mercury 400 spectrometer (400 and 100 MHz respectively). Elemental analysis was performed
with a Vario MICRO Cube analyzer. A Boetius PHMK apparatus (VEB Analytic, Dresden, Germany) was used
to determine the melting points (uncorrected). The course of reactions and the purity of synthesized products
were monitored by TLC on Silufol UV-254 plates with 9:1 chloroform–methanol solvents system.
2-(4-Bromophenyl)-2-hydroxy-2,3-dihydro-1H-imidazo[1,2-a]pyridinium Bromide (1h). p-Bromo-
phenacyl bromide (2.78 g, 0.01 mol) in AcOEt (15 ml) was added with stirring to a solution of 2-aminopyridine
(0.94 g, 0.01 mol) in AcOEt (10 ml). The mixture was kept at room temperature for 2 days. The precipitate was
filtered off, washed with acetone and ether, and dried in air. Yield 82%. The melting point was not determined – the
1
2
compound was readily dehydrated. H NMR spectrum, δ, ppm (J, Hz): 4.65 (1H, d, J = 15.0) and 4.85 (1H, d,
2J = 15.0, 3-CH2); 7.12 (1H, m, H-6); 7.55 (1H, br. s, NH); 7.65 (2H, d, 3J = 7.6, H Ar); 7.68 (1H, d, 3J = 8.6, H-8);
8.04 (2H, d, 3J = 7.6, H Ar); 8.11 (1H, dd, 3J = 6.7, 3J = 8.6, H-7); 8.43 (1H, d, 3J = 6.5, H-5); 10.21 (1H, s, OH).
Found, %: C 42.07; Br 43.07; N 7.57. C13H12Br2N2O. Calculated, %: C 41.97; Br 42.95; N 7.53.
Compounds 3a-g (General Method). Salts 1a-h [8, 11] or 2a-h [8, 11] (0.01 mol) were dissolved in
hydrogen peroxide (5 ml, 15 or 30% respectively) and kept at room temperature. After three days a colorless
precipitate was filtered off, washed with water, dried and recrystallized from ethanol.
3-Bromo-2-bromomethylimidazo[1,2-a]pyridine (3a). Yield 93%; mp 146-147°C (mp 146°C
1
[8]). H NMR spectrum, δ, ppm (J, Hz): 4.67 (2H, s, CH2Br); 6.93 (1H, t, 3J = 7.1, H-6); 7.28 (1H, dd, 3J = 9.2,
3
3
3J = 7.6, H-7); 7.59 (1H, d, J = 9.2, H-8); 8.06 (1H, d, J = 6.8, H-5). 13C NMR spectrum, δ, ppm: 25.5
(CH2Br); 94.6 (C-3); 113.9 (C-6); 117.6 (C-8); 124.6 (C-7); 125.9 (C-5); 140.7 (C-2); 145.0 (C-8a). Found, %:
C 33.29; H 2.06; N 9.58; Br 55.03. C8H6Br2N2. Calculated, %: C 33.14; H 2.09; N 9.66; Br 55.11.
3-Bromo-2-bromomethyl-5-methylimidazo[1,2-a]pyridine (3b). Yield 92%; mp 132-133°C (mp 132°C
[8]). 1H NMR spectrum, δ, ppm (J, Hz): 2.95 (3H, s, 5-CH3); 4.70 (2H, s, CH2Br); 6.75 (1H, d, 3J = 7.2, H-6); 7.28
3
3
3
(1H, dd, J = 8.5, J = 7.2, H-7); 7.45 (1H, d, J = 8.5, H-8). 13C NMR spectrum, δ, ppm: 20.8 (5-CH3); 26.1
(CH2Br); 93.3 (C-3); 114.9 (C-8); 116.3 (C-6); 125.8 (C-7); 136.6 (C-2); 141.9 (C-5); 143.4 (C-8a). Found, %:
C 35.59; H 2.58; N 9.28; Br 52.61. C9H8Br2N2. Calculated, %: C 35.56; H 2.65; N 9.22; Br 52.57.
3-Bromo-2-bromomethyl-6-chloroimidazo[1,2-a]pyridine (3c). Yield 91%; mp 162-163°C (mp
1
3
162°C [11]). H NMR spectrum, δ, ppm (J, Hz): 4.72 (2H, s, CH2Br); 7.39 (1H, d, J = 8.0, H-7); 7.65 (1H, d,
3J = 8.0, H-8); 8.45 (1H, d, J = 1.9, H-5). C NMR spectrum, δ, ppm: 25.1 (CH2Br); 95.6 (C-3); 118.5 (C-8);
121.6 (C-6); 122.5 (C-7); 127.1 (C-5); 141.8 (C-2); 143.4 (C-8a). Found, %: C 29.78; H 1.61; Br 49.14;
Cl 11.03; N 8.96. C8H5Br2ClN2. Calculated, %: C 29.62; H 1.55; Br 49.26; Cl 10.93; N 8.64.
4
13
3,6,8-Tribromo-2-bromomethylimidazo[1,2-a]pyridine (3d). Yield 92%; mp 192°C (decomp.). 1H NMR
spectrum, δ, ppm (J, Hz): 4.69 (2H, s, CH2Br); 7.79 (1H, s, H-7); 8.47 (1H, s, H-5). 13C NMR spectrum, δ, ppm:
24.7 (CH2Br); 97.5 (C-3); 107.4 (C-8); 112.0 (C-6); 124.4 (C-7); 130.8 (C-5); 141.4 (C-2); 142.3 (C-8a). Found,
%: C 21.72; H 1.00; Br 71.69; N 6.52. C8H4Br4N2. Calculated, %: C 21.46; H 0.90; Br 71.38; N 6.26.
3-Bromo-2-bromomethylimidazo[1,2-a]pyrimidine (3e). Yield 56%; mp 210°C (mp 210°C [8]).
3
3
1H NMR spectrum, δ, ppm (J, Hz): 4.74 (2H, s, CH2Br); 7.04 (1H, dd, J = 4.4, J = 6.4, H-6); 8.56 (1H, d,
3
4
3J = 4.4, H-7); 8.88 (1H, dd, J = 6.4, J = 1.8, H-5). 13C NMR spectrum, δ, ppm: 25.0 (CH2Br); 93.7 (C-3);
110.6 (C-6); 133.2 (C-2); 142.1 (C-5); 147.7 (C-7); 151.4 (C-8a). Found, %: C 29.03; H 1.84; N 14.53;
Br 54.98. C7H5Br2N3. Calculated, %: C 28.90; H 1.73; N 14.44; Br 54.93.
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3-Bromo-2-bromomethyl-5,7-dimethylimidazo[1,2-a]pyrimidine (3f). Yield 89%; mp 194-195°C
1
(mp 194-195°C [9]). H NMR spectrum, δ, ppm (J, Hz): 2.57 (3H, s, CH3); 2.66 (3H, s, CH3); 4.67 (2H, s,
13
CH2Br); 6.83 (1H, s, H-6). C NMR spectrum, δ, ppm: 19.9 (5-CH3); 24.4 (7-CH3); 25.9 (CH2Br); 91.3 (C-3);
112.0 (C-6); 142.3 (C-2); 145.4 (C-5); 148.6 (C-7); 160.6 (C-8a). Found, %: C 34.00; H 2.88; N 13.12;
Br 50.21. C9H2Br2N3. Calculated, %: C 33.89; H 2.84; N 13.17; Br 50.10.
3,6,8-Tribromo-2-methylimidazo[1,2-a]pyridine (3g). Yield 89%; mp 117-119°C. 1H NMR spectrum,
δ, ppm (J, Hz): 2.40 (3H, s, 2-CH3); 7.68 (1H, s, H-5); 8.36 (1H, s, H-7). 13C NMR spectrum, δ, ppm: 13.7
(2-CH3); 95.4 (C-3); 106.3 (C-8); 111.2 (C-6); 124.7 (C-7); 129.4 (C-5); 141.1 C-2); 143.2 (C-8a). Found, %:
C 26.27; H 1.50; Br 64.69; N 7.52. C8H5Br3N3. Calculated, %: C 26.05; H 1.37; Br 64.99; N 7.56.
Compounds 3h-j (General Method). 15% Hydrogen peroxide (5 ml) was added to salt (1h, 2h-j) or a
mixture of salts (1i and 2i, or 1j and 2j) (0.01 mol) and the mixture was kept for 3 days at room temperature.
The colorless precipitate was filtered off, washed with water, and dried. Compound 3d was recrystallized,
compounds 3i and 3j were sublimed.
3-Bromo-2-(4-bromophenyl)imidazo[1,2-a]pyridine (3h). Yield 70%; mp 156-157°C (Et2O) (mp
156-157°C [12]). 1H NMR spectrum, δ, ppm (J, Hz): 7.09 (1H, m, H-6); 7.36 (1H, t, 3J = 7.1, H-7); 7.62 (3H, m,
H Ar, H-8); 8.04 (2H, d, 3J = 8.0, H Ar); 8.33 (1H, d, 3J = 7.6, H-5).
1
3-Bromo-2-(4-methylphenyl)imidazo[1,2-a]pyridine (3i). Yield 57%; mp 109-110°C (subl.). H NMR
3
spectrum, δ, ppm (J, Hz): 2.40 (3H, s, CH3); 7.03 (1H, t, J = 7.0, H-6); 7.25 (2H, m, H Ar); 7.32 (1H, dd,
3
3
3
3
3J = 8.0, J = 7.1, H-7); 7.57 (1H, d, J = 8.0, H-8); 7.97 (2H, d, J = 8.0, H Ar); 8.28 (1H, d, J = 7.6, H-5).
Found, %: C 58.64; H 3.97; Br 27.92; N 9.83. C14H11BrN2. Calculated, %: C 58.56; H 3.86; Br 27.83; N 9.76.
3-Bromo-2-(4-methoxyphenyl)imidazo[1,2-a]pyridine (3j). Yield 53%; mp 102-103°C (subl.) (mp
101-103°C [12]). 1H NMR spectrum, δ, ppm (J, Hz): 3.84 (3H, s, OCH3); 7.00 (2H, m, H Ar); 7.04 (1H, m, H-6);
7.32 (1H, dd, 3J = 8.0, 3J = 7.1, H-7); 7.57 (1H, d, 3J = 8.0, H-8); 8.02 (2H, d, 3J = 8.0. H Ar); 8.29 (1H, d, 3J = 7.6,
H-5). Found, %: C 55.89; H 3.72; Br 26.66; N 9.31. C14H11BrN2O. Calculated, %: C 55.47; H 3.66; Br 26.36;
N 9.24.
Compounds 4a,b (General Method). After isolation of product 3a or 3b, the solution was kept for 3
weeks. After 3 weeks the precipitate was filtered off, dried, and recrystallized.
1
3,5-Dibromopyridin-2-amine (4a). Yield 5%; mp 105°C (75% EtOH) (mp 105°C [13]). H NMR
spectrum, δ, ppm (J, Hz): 6.21 (2H, s, NH2); 7.77 (1H, s, H-6); 7.87 (1H, s, H-4).
3,5-Dibromo-6-methylpyridin-2-amine (4b). Yield 6%; mp 144°C (EtOH) (mp 144°C [13]). 1H NMR
spectrum, δ, ppm (J, Hz): 2.35 (3H, s, CH3); 6.06 (2H, br. s, NH2); 7.60 (1H, s, H-4).
Compounds 6a,b (General Method). 15% Hydrogen peroxide (5 ml) was added to salt 5a [9, 14]
(0.01 mol) or salt 5b [11] (0.01 mol) and kept at 10-15°C. The precipitate formed over a day was filtered off,
washed, and recrystallized.
1
5-Bromo-6-methylimidazo[2,1-b]thiazole (6a). Yield 86%; mp 93°C (xylene) (mp 93°C [14]). H
NMR spectrum, δ, ppm (J, Hz): 2.22 (3H, s, CH3); 7.24 (1H, d, 3J = 4.5, H-2); 7.60 (1H, d, 3J = 4.5, H-3).
5-Bromo-3,6-dimethylimidazo[2,1-b]thiazole (6b). Yield 79%; mp 95-96°C (subl.). 1H NMR
13
spectrum, δ, ppm (J, Hz): 2.19 (3H, s, CH3); 2.59 (3H, s, CH3); 6.78 (1H, s, H-2). C NMR spectrum, δ, ppm:
13.5 (6-CH3); 14.6 (3-CH3); 91.0 (C-5); 108.3 (C-2); 129.2 (C-3); 142.0 (C-6); 148.8 (C-7a). Found, %:
Br 34.86; N 12.78; S 14.36. C7H7BrN2S. Calculated, %: Br 34.57; N 12.12; S 13.87.
5-Bromo-6-bromomethyl-3-methylimidazo[1,2-b]thiazole (6c). 30% Hydrogen peroxide (5 ml) was
added to salt 5c [9] (3.13 g, 0.01 mol) at room temperature. The precipitate was filtered off, dried, and
recrystallized. Yield 78%; mp 118-119°C (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 2.58 (3H, s, 3-CH3); 4.60
(2H, s, CH2Br); 7.00 (1H, s, H-2). 13C NMR spectrum, δ, ppm: 14.5 (3-CH3); 26.3 (CH2Br); 93.6 (C-5); 109.9
(C-2); 129.2 (C-3); 141.8 (C-6); 149.9 (C-7a). Found, %: C 27.38; H 2.05; Br 51.62; N 8.86; S 10.04.
C7H6Br2N2S. Calculated, %: C 27.12; H 1.95; Br 51.55; N 9.04; S 10.34.
5,5-Dibromoimidazo[2,1-b]thiazol-6(5H)-one (7). 15% Hydrogen peroxide (15 ml) was added to salt
5d [9] (2.98 g, 0.01 mol) and the mixture was kept at 10-15°C. After a day the precipitate was filtered off, dried,
1284

and recrystallized. Yield 34%; mp 160-161°C (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 7.22 (1H, d, 3J =4.5,
3
H-2); 8.15 (1H, d, J = 4.5, H-3). 13C NMR spectrum, δ, ppm (J, Hz): 50.4 (C-5); 112.2 (C-2); 123.5 (C-3);
169.0 (C-6); 153.7 (C-7a). Found, %: C 20.16; H 0.65; Br 53.48; N 9.48; S 10.44. C5H2Br2N2OS. Calculated, %:
C 20.16; H 0.60; Br 53.63; N 9.40; S 10.76.
Crystallographic Analysis of Compound 7. Rhombic crystals, C5H2Br2N2OS, at 20°C a = 16.356(7),
b = 7.046(2), c = 7.023(3) Å, V = 809.4(5) Å3, Mr = 297.97, Z = 4, space group Pnma, dcalc = 2.445 g/cm3,
μ(MoKα) 10.212 mm-1, F(000) = 560. The elemental cell parameters and the intensities of 2332 reflections
(1228 independent, Rint = 0.060) were measured on an automatic four-circle Siemens P3/PC diffractometer
(MoKα radiation, graphite monochromator, θ/2θ scanning, 2θmax = 60°). The structure was determined by direct
methods using the SHELXTL of program suite [15]. The hydrogen positions were determined from difference
syntheses of electron densities and refined in the isotropic approximation. The absorptions were refined semi-
empirically from Ψ-scanning data (Tmin = 0.0380, Tmax = 0.04415). The structure was refined with respect to F2
in full-matrix least-squares analysis in the anisotropic approximation for non-hydrogen atoms to wR2 = 0.0797
for 1228 reflections (R1 = 0.048 for 831 reflexions with F>4σ(F), S = 0.933). Complete data for the structure
have been deposited in the Cambridge Crystallographic Data Center (CCDC 148666).
The authors thank Professor M. Yu. Kornilov for consultation in preparing the paper.
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