4-Bromo-2H-1,3-oxazine-2,6(3H)-dione

The title compound, C4H2BrNO3, is one of a series of three substituted oxauracils prepared as precursors in the preparation of 1-aza-1,3-butadienes. Although each structure has identical potential for N—H⋯O intermolecular hydrogen bonds, each forms a distinctive intermolecular network. In the title compound, there are two independent molecules in the asymmetric unit, with a non-crystallographic twofold screw-like relationship between them. The two indpendent molecules are linked by an intermolecular N—H⋯O hydrogen bond. In the crystal structure, this hydrogen-bonded pair is linked to translationally related molecules through further intermolecular N—H⋯O hydrogen bonds, forming one-dimensional chains along [100]. The crystal structure also has short Br⋯O=C intermolecular contacts with distances of 2.843 (4) and 2.852 (4) Å.

The title compound, C 4 H 2 BrNO 3 , is one of a series of three substituted oxauracils prepared as precursors in the preparation of 1-aza-1,3-butadienes. Although each structure has identical potential for N-HÁ Á ÁO intermolecular hydrogen bonds, each forms a distinctive intermolecular network. In the title compound, there are two independent molecules in the asymmetric unit, with a non-crystallographic twofold screwlike relationship between them. The two indpendent molecules are linked by an intermolecular N-HÁ Á ÁO hydrogen bond. In the crystal structure, this hydrogen-bonded pair is linked to translationally related molecules through further intermolecular N-HÁ Á ÁO hydrogen bonds, forming onedimensional chains along [100]. The crystal structure also has short BrÁ Á ÁO C intermolecular contacts with distances of 2.843 (4) and 2.852 (4) Å .
In the title compound there are two crystallographically independent molecules in the asymmetric unit ( Fig. 1). These two molecules are arranged in a planar, pseudo-2-fold screw relationship, as shown in Figure 2. There is a hydrogen bond between the two molecules, N3···O2A, and between the second molecule with a translation related molecule one, N3A···O2C. These two hydrogen bonds are not related by crystallographic symmetry.
There are short, non-bonded contacts between the bromines and the O6 oxygen of the translation related molecules (

Experimental
Bromomaleic anhydride (3-bromofuran-2,5-dione, 2.0 ml, 22 mmol) was disolved in 10 ml dichloromethane and and trimethylsilyl azide (3.1 ml, 23 mmol) were added dropwise maintaining the reaction temperature below 278K. The solution was stirred under nitrogen for 4 h and then at room temperature for 20 h. To the suspension was added absolute ethanol (6 ml). The resulting mixture was stirred at room temperature for an additional 2 hrs. The white precipitate was filtered, washed with dichlormethane, and then dried in vacuo to give the final compound as a white solid (0.85 g, 21%).

Refinement
Hydrogen positions were calculated and refined using a riding model using the following C-H distances: methylene 0.93 Å, and N-H 0.86 Å. The U iso values for the H atoms were set at 20% above that of the bonded C or N atom.
supplementary materials sup-2 Figures   Fig. 1. The molecular structure of the title compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms. The dashed line indicates a hydrogen bond. Fig. 2. The two indetendent molecules in the asymmetric unit plus a pair related by translation along the a axis (O2A is identical to O2AA by translation, as are N3 and N3B). The psuedo-2-fold screw runs approximately through O2 and O2A. 4-Bromo-2H-1,3-oxazine-2,6(3H)-dione Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Successful refinement of the structure in space group P22~1~2~1~ confirms the assignment of this symmetry, which was not the intial choice based on the systematic absences. The pseudo-2-fold screw between the two molecules in the asymmetric unit likely results in the near extinction of the h = 2n + 1 reflections in the h00 line. Only two reflections, -7 0 0 and -9 0 0, have an observed structure factor with a sigma greater than 1, approximately 2. The agreement of the observed and calculated structure factors for these two reflections is good. Although these reflections are, indeed, quite weak the observed structure factors are 2 to 10 times the those of the supplementary materials sup-4 unobserved k = 2n + 1 and l = 2n + 1 reflections on the 0k0 and 00l lines. These screw-required absent reflections have an intensity of less than one sigma.
Note: Checkcif offers conflicting instructions on the choice of the space group. Oiginally solved as P2\~1\~2\~1\~2 checkcif gave PLAT158: Unless for special reasons related to the structure/content, a unitcell and structure is best reported with reference to the Niggli Reduced Cell. Thus I redid the structure as P22\~1\~2\~1\~ and checkcif gave PLAT128 The reported monoclinic space-group is in a non-standard setting. Transformation to the conventional setting is indicated unless there is a good (scientific) reason not to do so.