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organic papers
Acta Crystallographica Section E

Structure Reports Online
ISSN 1600-5368

2-(4-Bromophenyl)oxazolo[3,2-a]pyrimidinium perchlorate

Victor B. Rybakov,* Vadim L. Alifanov and Eugene V. Babaev
Department of Chemistry, Moscow State University, 119992 Moscow, Russian Federation Correspondence e-mail: rybakov20021@yandex.ru

The title compound, C12H8BrN2O+ÑClO4Ð, was synthesized and characterized by 1H NMR and single-crystal X-ray diffraction. The essentially planar p-bromophenyl fragment and plane of the heterocyclic system form a dihedral angle of À 1.79 (7) . Short intermolecular contacts (ca 3.0 A) involving O atoms of a perchlorate anion may indicate the positive charge concentration on the bridgehead N atom of the cation.

Received 15 August 2006 Accepted 13 September 2006

Comment
Key indicators Single-crystal X-ray study T = 293 K À Mean (C-C) = 0.003 A R factor = 0.041 wR factor = 0.111 Data-to-parameter ratio = 14.3 For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e.

In a previous communication (Rybakov et al., 2006), we reported the synthesis and crystal structure of 1-(4-chlorobenzoylmethyl)pyrimidin-2(1H)-one. Here we report the synthesis of the p-bromphenyl analog (1) and the cyclization reaction to form the title aromatic oxazolo[3,2-a]pyrimidinium cation, (2). This class of oxazolo[3,2-a]pyrimidinium cationic bicyclic system can be prepared by two synthetic routes, either from oxazole or from pyrimidine. The first route is by condensation of a 4,5-disubstituted 2-aminooxazole with acetylacetone (Chuiguk & Leshenko, 1974), leading to a 5,7dimethyloxazolo[3,2-a]pyrimidinium salt. Alternatively, 7-diaryloxazolo[3,2-a]pyrimidinium salts have been obtained from N-phenacyl-4-arylpyrimidinethiones by two different methods (Reimer et al., 1993; Liebscher & Hassoun, 1988). One method involves the cyclization of N-phenacyl-4-aryl-pyrimid-2-ones by a somewhat unsafe combination of a dehydratating agent (acetic anhydride) with perchloric acid (Liebscher & Hassoun, 1988). Applying this method to phenacylpyrimidone (1), we obtained only the perchlorate of the starting material [(1)ÑHClO4]. Nevertheless, fuming sulfuric acid (20 mass% of SO3) turned out to be the reagent of choice, and its use resulted in formation of the desired salt (2) from (1) in excellent yield. Compound (2) was characterized by 1HNMR. The singlet of the CH2 group [initially observed in parent (1)] disappeared in (2), and a new down-field singlet of the formed oxazole ring appeared at 9.31 p.p.m. Signals of aromatic protons appeared as a characteristic multiplet (AA0 BB0 system), so ortho-protons are equivalent (at least in solution).

# 2006 International Union of Crystallography All rights reserved

The bond lengths in the oxazolopyrimidinium ring system of (2) (Fig. 1) confirm its aromatic nature. The dihedral angle between the essentially planar nine-membered bicyclic system
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organic papers
and the benzene ring is 1.79 (7) , indicating the presence of conjugation between these aromatic fragments. Similar values for this dihedral angle [3.77 (7) and 5.33 (15) ] were found in previously reported oxazolopyridinium salts (Albov et al., 2004a,b). It is arguable that the positive charge of the cation is concentrated on atom N1, as fairly short intermolecular contacts exist between the cation and the O atoms of the perchlorate anion (having a low-density negative charge) [O13ÑÑÑN1 = 2.998 (4), O14ÑÑÑC2 = 3.157 (4), O13ÑÑÑC5 = À 2.973 (4), O12ÑÑÑC9 = 2.996 (4) and O14ÑÑÑC9 = 2.991 (4) A]. There is a weak intramolecular hydrogen bond [H11ÑÑÑO4 = À À 2.47 A, C11ÑÑÑO4 = 2.798 (3) A and C11 H11ÑÑÑO4 = 101 ]. This same weak interaction is observed in the structures of related oxazolopyridine salts (Albov et al., 2004a,b; Babaev et al., 1997; Babaev, Bush et al., 1999; Babaev, Rybakov et al., 1999). From a search of the Cambridge Structural Database (Version 5.27; Allen, 2002), we found NÑÑÑH hydrogen bonding for two analogs of the salt (2), viz. 6-nitro-2-phenylimidazo[1,2-a]pyrimidine (Aslanov et al., 1983) and 2-phenylimidazo[1,2-a]pyrimidine (Tafeenko et al., 1996), with NÑÑÑH À distances of 2.53 and 2.50 A, respectively.

Figure 1
The molecular structure of the title compound, showing the atom numbering scheme. Displacement ellipsoids are drawwn at the 50% probability level with H atoms shown as spheres of arbitrary radius.

Table 1
N1--C5 N1--C9 N1--C2 C2--C3 C3--O4 C3--C10

À Selected geometric parameters (A, ).

Experimental
Dried N phenacylpyrimid 2 one, (1) (0.02 mol), was carefully added to a stirred mixture of 17 ml fuming sulfuric acid (60 mass% of SO3) and 25 ml of concentrated sulfuric acid. The temperature was kept in the range 273 276 K. The mixture was stirred below 273 K until compound (1) had completely dissolved and was kept for 4 h at room temperature. After this period, the reaction mixture was carefully poured into 500 g of crushed ice, and 10 ml of concentrated perchloric acid was added. The product was isolated by suction and washed with ethanol and diethyl ether, and recrystallized from CH2Cl2 CF3COOH (20:1) (yield 95%, m.p. 552 554 K). Crystal data
C12H8BrN2O+ÑClO4 Mr = 375.56 Monoclinic, P21 =n À a = 6.291 (2) A À b = 12.052 (2) A À c = 17.688 (4) A  = 98.19 (2) À V = 1327.4 (6) A3 Z=4 Dx = 1.879 Mg m 3 Cu K radiation = 6.33 mm 1 T = 293 (2) K Prism, colorless 0.15 Ò 0.15 Ò 0.15 mm

1.352 1.359 1.384 1.342 1.383 1.447

(3) (3) (3) (3) (3) (3)

O4--C5 C5--N6 N6--C7 C7--C8 C8--C9 N6--C5--O4 N6--C5--N1 O4--C5--N1 C5--N6--C7 N6--C7--C8 C9--C8--C7 N1--C9--C8

1.322 1.317 1.330 1.392 1.360 124.2 125.9 109.9 114.1 124.0 119.2 117.0

(3) (3) (3) (4) (4)

C5--N1--C9 C5--N1--C2 C9--N1--C2 C3--C2--N1 C2--C3--O4 C2--C3--C10 O4--C3--C10 C5--O4--C3

119.7 (2) 107.63 (19) 132.6 (2) 106.4 (2) 109.3 (2) 133.6 (2) 117.12 (19) 106.82 (17)

(2) (2) (2) (2) (2) (2) (2)

À All H atoms were refined using a riging model, with C H 0.93 A and Uiso(H) 1.2Ueq(C). Data collection: CAD 4 EXPRESS (Enraf Nonius, 1994); cell refinement: CAD 4 EXPRESS; data reduction: XCAD4 (Harms & Wokadlo, (1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP 3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX publication routines (Farrugia, 1999).

Data collection
Enraf Nonius CAD-4 diffractometer ! scans Absorption correction: none 2811 measured reflections 2714 independent reflections 2375 reflections with I > 2 (I) Rint = 0.084 max = 74.9 1 standard reflection frequency: 60 min intensity decay: 3%

The authors are indebted to the Russian Foundation for Basic Research for covering the licence fee for use of the Cambridge Structural Database.

References
Albov, D. V., Rybakov, V. B., Babaev, E. V. & Aslanov, L. A. (2004a). Acta Cryst. E60, o1096 o1097. Albov, D. V., Rybakov, V. B., Babaev, E. V. & Aslanov, L. A. (2004b). Acta Cryst. E60, o1301 o1302. Allen, F. H. (2002). Acta Cryst. B58, 380 388. Aslanov, L. A., Tafeenko, V. A., Paseshnichenko, K. A., Bundel, Yu. G., Gromov, S. P. & Gerasimov, B. G. (1983). Zh. Struct. Khim. (Russ.) (J. Struct. Chem.), 24, 115 122. Babaev, E. V., Bozhenko, S. V., Maiboroda, D. A., Rybakov, V. B. & Zhukov, S. G. (1997). Bull. Soc. Chim. Belg. 106, 631 638.

Refinement
Refinement on F 2 R[F 2 > 2 (F 2)] = 0.041 wR(F 2) = 0.111 S = 0.92 2714 reflections 190 parameters H-atom parameters constrained w = 1/[ 2(Fo2) + (0.0909P)2 + 0.38P] where P = (Fo2 + 2Fc2)/3 (Ñ/ )max = 0.001 À Ñmax = 0.48 e A 3 À Ñmin = 0.47 e A 3

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Babaev, E. V., Bush, A. A., Orlova, I. A. & Rybakov, V. B. (1999). Tetrahedron Lett. 40, 7553 7556. Babaev, E. V., Rybakov, V. B., Zhukov, S. G. & Orlova, I. A. (1999). Chem. Heterocycl. Compd. 35, 479 485. Chuiguk, V. A. & Leshenko, E. A. (1974). Ukr. Chim. Zh. 40, 633 635; Chem. Abstr. (1974), 81, 105438. Enraf Nonius (1994). CAD-4 EXPRESS. Version 5.0. Enraf Nonius, Delft, The Netherlands. Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837 838. Harms, K. & Wokadlo, S. (1995). XCAD4. University of Marburg, Germany. Liebscher, J. & Hassoun, A. (1988). Synthesis, pp. 816 820. Reimer, B., Patzel, M., Hassoun, A., Liebscher, J., Friedrichsen, W. & Jones, P. G. (1993). Tetrahedron, 49, 3767 3780. Rybakov, V. B., Tsisevich, A. A. V., Nikitin, K. L., Alifanov, V. & Babaev, E. V. (2006). Acta Cryst. E62, o2546 o2547. Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of ? Gottingen, Germany. Tafeenko, V. A., Paseshnichenko, K. A. & Shenk, H. (1996). Z. Kristallogr. 211, 457 463.

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