A Sandmeyer type reaction for bromination of 2-mercapto-1-methyl- imidazoline (N2C4H6S) into 2-bromo-1-methyl- imidazole (N2C4H5Br) in presence of copper(i) bromide

Article abstract of DOI:10.1039/c1dt11327e

2-Mercapto-1-methyl-imidazoline (N₂C₄H₆S) is converted at room temperature into 2-bromo-1-methyl-imidazole (N ₂C₄H₅Br) in presence of copper(i) bromide in acetonitrile-chloroform mixture via extrusion of sulfur as sulfate and oxidation of Cu^I into Cu^II. 2-Bromo-1-methyl-imidazole was isolated as its self assembled tetranuclear Cu^II cluster, [Cu ₄(η¹-N-(N₂C₄H ₅Br)₄(μ₄-O)(μ-Br)₆] 1 {η¹-N-(N₂C₄H₅Br) = 2-bromo-1-methyl-imidazole}.

Full text of DOI:10.1039/c1dt11327e

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COMMUNICATION
A Sandmeyer type reaction for bromination of 2-mercapto-1-methyl-
imidazoline (N2C4H6S) into 2-bromo-1-methyl-imidazole (N2C4H5Br) in
presence of copper(I) bromide†
Tarlok S. Lobana,*a Razia Sultanaa and Ray J. Butcherb
Received 13th July 2011, Accepted 30th August 2011
DOI: 10.1039/c1dt11327e
2-Mercapto-1-methyl-imidazoline (N2C4H6S) is converted
at room temperature into 2-bromo-1-methyl-imidazole
(N2C4H5Br) in presence of copper(I) bromide in acetonitrile-
chloroform mixture via extrusion of sulfur as sulfate and
oxidation of CuI into CuII. 2-Bromo-1-methyl-imidazole
was isolated as its self assembled tetranuclear CuII cluster,
1
1
[Cu4(g -N-(N2C4H5Br)4(l4-O)(l-Br)6] 1 {g -N-(N2C4H5Br) =
2-bromo-1-methyl-imidazole}.
Chart 1
Metal ion-mediated transformation of organic molecules is an
important chemical process as it provides facile synthetic routes for
the formation of many interesting molecules which are otherwise
difficult or even impossible to synthesize by conventional synthetic
procedures.1 For example, in organic chemistry bromination of an
aromatic amine occurs via preparation of its diazonium salt (eqn
(1)) and subsequent displacement of N2 X- with a bromo ion
+
(Sandmeyer reaction).2 Using this reaction, a variety of five- and
six-membered heterocyclic ring amines, such as, aminopyrazoles,
aminotriazoles, aminopyridine etc. have undergone diazotization
followed by halogenation (I–III, Chart 1).3 Surprisingly, there is no
bromination of heterocyclic rings with exocyclic thiones (IV–VI,
Chart 2).
Chart 2
halides with 2-mercapto-imidazolidine (N2C3H6S, VI, R = H) in
acetonitrile/acetonitrile-chloroform mixture formed dinuclear or
polynuclear complexes.6a–c But when copper(I) chloride/bromide
were allowed to react at room temperature in dimethyl sulfoxide
or dimethyl formamide, C–S bond rupture along with oxidation
of CuI to CuII occurred and formed a sulfate bridged polynu-
clear complex, [CuII{h -N,N-(N2C3H5)2S}(m-O,OSO2)(h -OH2)]n
{(N2C3H5)2S = 2,2¢-thio-di-2-imidazoline}.6e The N-substituted
analogous thio-ligand, namely, 2-mercapto-1-methyl-imidazoline
(N2C4H6S) with copper(I) chloride in acetonitrile or acetonitrile-
2
1
(1)
chloroform mixture yielded a dinuclear complex, [CuII2{h -
2
There is keen interest in the interaction of metal ions with
heterocyclic thioamides owing to the relevance of their metal
complexes in the biological systems.4–8 From this laboratory,
during the course of reactions of copper(I) halides with a
series of heterocyclic thioamides, it was observed that solvent
and reaction conditions played significant role in altering the
nature of the products. For example, reaction of copper(I)
N,N-(N2C4H5)2S}2(m-Cl)2Cl2] {(N2C4H5)2S = 1,1¢-dimethyl-2,2¢-
di-imidazolylsulfide} along with formation of free CuSO4·5H2O.6e
This reaction involved oxidation of CuI to CuII and in situ conver-
sion of 2-mercapto-1-methyl-imidazoline into 1,1¢-dimethyl-2,2¢-
di-imidazolylsulfide (Scheme 1).
The reaction of copper(I) bromide with 2-mercapto-1-methyl-
imidazoline (N2C4H6S, V, R = Me) in acetonitrile yielded a
trinuclear {Cu3(h -Br)3(m-SN2C4H6)3} complex,6d but in dimethyl
1
aDepartment of Chemistry, Guru Nanak Dev University, Amritsar, 143 005,
India. E-mail: tarlokslobana@yahoo.co.in; Fax: +91-183-2-258820
sulfoxide it formed sulfate chelated mononuclear CuII complex,
2
2
1
[CuII{h -N,N-(N2C4H5)2S}(h -O,OSO2)(h -OH2)].6e
bDepartment of Chemistry, Howard University, Washington DC, 20059, USA
Herein, in this communication we report reaction of copper(I)
bromide with 2-mercapto-1-methyl-imidazoline (N2C4H6S) in
acetonitrile-chloroform which has formed a CuII tetranuclear
† Electronic supplementary information (ESI) available: Synthesis for 1
and 2; additional crystallographic details and tables. CCDC reference
numbers 820399 (1) and 820400 (2).
11382 | Dalton Trans., 2011, 40, 11382–11384
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Scheme 1
1
cluster, [Cu4(h -N-(N2C4H5Br)4(m4-O)(m-Br)6] 1 (N2C4H5Br = 2-
bromo-1-methyl-imidazole) along with free CuSO4·5H2O 2.‡ Cop-
per has tendency to form such cores, for example, [Cu4OCl6(py)4].9
Copper(I) bromide in acetonitrile was added to the solution
of 2-mercapto-1-methyl-imidazoline in chloroform in 1 : 1 molar
ratio under atmospheric conditions (25–30 ◦C). The color of the
solution changed from colorless to brownish green in 2–3 days
and black crystals of composition C16H20Br10Cu4N8O were formed
in a week along with blue crystalline product of composition
H10CuSO9. An X-ray study of the black crystals has shown it to be a
1
1
CuII tetranuclear cluster, [Cu4(h -N-(N2C4H5Br)4(m4-O)(m-Br)6] 1
Fig.
2
Molecular structure of tetranuclear cluster [Cu4(h -N-
˚
(N2C4H5Br)4(m4-O)(m-Br)6] 1. Selected bond lengths (A) and an-
gles (◦): Cu1–O1 1.924(6), Cu1–N1A 1.962(7), Cu1–Br12 2.6436(14),
Cu1–Br13 2.4514(14), Cu1–Br14 2.5321(15) and O1–Cu1–N1A 178.1(3),
Br13–Cu1–Br14 142.58(6), Br13–Cu1–Br12 110.80(5), Br14–Cu1–Br12
104.83(5).
(N2C4H5Br = 2-bromo-1-methyl-imidazole) and blue crystals were
that of CuSO4·5H2O 2 (Scheme 2). The magnetic susceptibility
measurement of 1 showed the magnetic moment value of 1.972
BM per copper supporting the presence of CuII. The IR spectrum
showed that the diagnostic peak due to n(C S) in the free ligand at
930 cm-1 disappears in the spectrum of complex 1, which indicates
that there is extrusion of sulfur atom of the ligand. Further, the
electronic absorption spectrum of complex 1 in dmso showed a
band in 259 nm (e = 1.268 ¥ 104 L mol-1 cm-1) region due to
charge transfer (O→Cu) transition.10a It is weakly fluorescent at
lem = 329 nm corresponding to excitation wavelength of 270 nm
(Fig. 1).
˚
plex 1 {1.950(7)–1.965(8) A} are shorter than in complex
[CuII2{h -N,N-(N2C4H5)2S}2(m-Cl)2Cl2] {2.005(3), 2.026(3) A}.
2
6e
˚
The Cu–Br bond distances in complex 1 {2.4514(14)–2.6436(14)
˚
A} are longer than in complex {Cu6(m3-S-N2C3H6S)2(m-S-
6c
˚
N2C3H6S)4Br2(m-Br)4}n {2.4839(6)–2.5525(6) A}. Molecular
structure of complex 2 is given in supplementary. The ESI-mass
spectrum of complex 1 did not show molecular ion peak but it has
shown peaks which support various components of complex 1.
The important peak at m/z value 163.4 corresponds to 2-bromo-
1-methyl-imidazole (N2C4H5Br81). Another important peak at m/z
value 268.3 corresponds to Cu634O species. The peak at m/z
value 385.2 corresponds to (Cu63Br281N2C4H5Br79). Other peaks
at m/z values, 225.3 and 243.3 correspond to (Cu63Br281) and
(Cu65ON2C4H5Br81) respectively.
Scheme 2
The formation of 1 and 2 shows that there is oxidation of CuI
to CuII which in the presence of bromide ion as nucleophile is
believed to brominate 2-mercapto-1-methyl-imidazoline ring at
C2 carbon with the extrusion of sulfur as sulfate. Scheme 3 shows
plausible mechanism of formation of 2-bromo-1-methyl-imidazole
and its coordination to CuII in the tetranuclear complex 1. Under
the experimental conditions CuI oxidizes to CuII which in turn
oxidizes the thio moiety (A, B) into its disulfide, namely, 1,1¢-
dimethyl-2,2¢-di-imidazolyl disulphide (C). This disulfide is con-
verted to 2-bromo-1-methyl-imidazole (ring bromination) along
with formation of sulfate. Copper(II) in presence of bromide ions
coordinates to 2-bromo-1-methyl-imidazole forming tetranuclear
Fig. 1 Fluorescence spectrum of complex 1.
1
complex [Cu4(h -N-(N2C4H5Br)4(m4-O)(m-Br)6] 1. It is the first
1
Fig. 2 shows molecular structure of complex [Cu4II(h -N-
example of metal mediated in situ bromination of 2-mercapto-1-
methyl-imidazoline (N2C4H6S) into 2-bromo-1-methyl-imidazole
(N2C4H5Br). In literature this bromo-derivative has been prepared
by the metallation of 1-methyl-imidazole with n-butyl lithium at
-70 ◦C followed by treatment of tetrabromomethane.10b
(N2C4H5Br)4(m4-O)(m-Br)6] 1. Here, each copper is bonded to
three m-Br, one m4-O and one nitrogen atom of 2-bromo-
1-methyl-imidazole. The Br12, Br13 and Br14 atoms oc-
cupy basal plane, while O1 and N1A are the apical donor
atoms in the five-coordinated trigonal bipyramidal coordi-
nation around Cu(II). The Cu–N bond distances in com-
It is concluded that 2-mercapto-1-methyl-imidazoline has
been converted into 2-bromo-1-methyl-imidazole under ambient
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spectra [dmso, lmax, nm; e/L mol-1 cm-1]: 10-4M solution: 259 (1.269 ¥
104). Fluorescence spectrum: 10-5 M, lex = 270 nm lem = 329 nm. IR
absorptions: (n/cm-1) 3121–2960(s), n(C–H); 1535(sh), 1475(s), n(C–N) +
d(C–H); 754(s), d(N–CH3). Magnetic moment (BM): 1.972 per copper.
Mass spectrum (m/z): 161.4 (expected, 160, N2C4H5Br79), 163.4 (162,
N2C4H5Br81); 223.3 (223, Cu63Br79Br81), 225.3 (225, Cu63Br281), 227.3 (227,
Cu65Br281); 241.3 (241, Cu65ON2C4H5Br79), 243.3 (243, Cu65ON2C4H5Br81);
268.3 (268, Cu4630); 383.2 (383, Cu63Br279N2C4H5Br81), 385.2 (385,
Cu63Br281N2C4H5Br79), 387.2 (387, Cu63Br281N2C4H5Br81), 389.2 (389,
Cu63Br281N2C4H5Br81).
The single crystals of compounds 1 and 2 were mounted on glass fibers
and data were collected using Oxford Gemini diffractometer. The data
were processed with CrysAlisPro. The structures were solved by direct
methods using the program SHELXS-97 and refined by full-matrix least-
squares techniques based on F2 using SHELXL-97.12 Crystal data for
-1
˚
(1); C16H20Br10Cu4N8O, Mr = 1393.66 g mol , triclinic, a = 10.7306(7) A,
◦
˚
˚
b = 13.3805(5) A, c = 13.6898(5) A, a = 78.199(3), b = 75.841(4) , g =
◦
3
¯
˚
83.739(4) , V = 1861.98(16) A , T = 200(2)K, space group P1, rc = 2.486
Mg m-3, Z = 2, m(Mo-Ka) = 13.008 mm-1, 24314 reflections measured,
unique 11303 [R(int) = 0.0612]. The final R1 was 0.0669 for 6291 reflections
[I> 2.0s(I)] and wR2 was 0.1410 (24314).
1 B. K. Santra and G. K. Lahiri, J. Chem. Soc., Dalton Trans., 1998, 1613
and references cited therein.
2 (a) I. L. Finar, Organic Chemistry, ELBS, Longman, London, 1990, 1,
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S. C. Chan, Angew. Chem., Int. Ed., 2000, 39, 2468; (e) P. G. Jones and
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Boeyens, L. Denner and A. J. Markwell, J. Crystallogr. Spectrosc. Res.,
1986, 16, 763.
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Chim. Acta, 2009, 362, 5265 and references cited therein; (b) T. S.
Lobana, R. Sharma, G. Hundal and R. J. Butcher, Inorg. Chem.,
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A. Castineiras and P. Turner, Inorg. Chem., 2005, 44, 1914; (d) T. S.
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7 (a) J. J. M. Amoore, L. R. Hanton and M. D. Spicer, Dalton Trans.,
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Scheme 3
conditions with copper(I) bromide as a brominating agent. The
transformation of organic sulfur to inorganic sulfate is a possible
approach to a solution of the problem of pollution of organic
sulfur compounds.11a These reactions also mimic the metabolism
of organic sulfur in mammals.11b
Financial support (RS) from UGC(SAP) is gratefully acknowl-
edged.
10 (a) A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, London
1968; (b) C. Boga, E. D. Vecchio, L. Forlani and P. E. Todesco, J.
Organomet. Chem., 2000, 601, 233.
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Y. - G. Ya o , Inorg. Chem., 2005, 44, 3386; (b) T. S. Lobana, I. Kinoshita,
K. Kimura, T. Nishioka, D. Shiomi and K. Isobe, Eur. J. Inorg. Chem.,
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Notes and references
‡ To a solution of copper(I) bromide (0.025 g, 0.17 mmol) in acetonitrile
was added a solution of 2-mercapto-1-methyl-imidazoline (0.020 g,
0.17 mmol) in chloroform. After 2-3 days color of solution changed from
colorless to brownish green and black prismatic crystals of complex 1 were
formed along with blue crystals of CuSO4·5H2O 2. Yield of complex 1,
0.033 g, 54%, mp. 182–185 ◦C. Found: C, 14.05; H, 1.65; N, 8.22. Calc.
for C16H20Br10Cu4N8O: C, 13.78; H, 1.44; N, 8.04%. Electronic absorption
12 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
A64, 112–122.
11384 | Dalton Trans., 2011, 40, 11382–11384
This journal is
The Royal Society of Chemistry 2011
©