Crystal structure of 1,2-bis(6-bromo-3,4-dihydro-2H-benz[e][1,3]oxazin-3-yl)ethane: a bromine-containing bis-benzoxazine

The solid-state structure of a 4-bromobenzoxazine has been determined. The whole molecule of the title compound is generated by inversion symmetry. This is a potential benzoxazine monomer for the preparation of phenolic materials.


Chemical context
In a continuation of our work on the synthesis and characterization of bis-1,3-benzoxazines, we have studied some of the chemical properties and determined the crystal structure of the title compound. Benzoxazines form an important class of benzo-fused heterocycles with a wide spectrum of biological activities. They are also emerging as desirable phenolic resin precursors because benzoxazines can undergo ring opening without emitting volatile materials during the curing process. This leads to a final cured product with excellent properties (Pilato, 2010). Normally, the incorporation of bromine can increase the flame-retarding properties and reduce the flammability of polymers (Li, et al., 2010). Recently, we have investigated the crystal structures of analogous bifunctional benzoxazines namely 3,3 0 -(ethane-1,2-diyl)bis(6-substituted-3,4-dihydro-2H-1,3-benzoxazine) (Rivera et al., 2010(Rivera et al., , 2011(Rivera et al., , 2012a that were prepared to investigate whether replacement of the substituents at the para position of the phenyl ring affects the electrophilic anomeric effect in the N-C-O sequence of the adjacent oxazine ring. In addition, as benzoxazine contains a tertiary nitrogen atom, the lone-pair electrons may play an important role in the interaction with guest molecules but there are no reports on the inclusion properties of polybenzoxazines (Chirachanchai et al., 2011). An X-ray structural study may therefore provide a better understanding of the ability of benzoxazines to act as novel host-guest compounds. In our opinion, the title compound also has potential applications in the production of new bromine-containing phenolic resins.
The C-C bond distances and angles of the aromatic rings were found to be normal. The C3-O1 bond length [1.372 (6) Å ] is comparable with other previously reported C-O bond lengths for related structures [1.370 (10) and 1.388 (9) Å (Rivera et al., 2012a) and 1.376 (1) Å (Rivera et al., 2011)], but is found to be shorter in the p-chloro derivative where C-O = 1.421 (2) Å (Rivera et al., 2010). Interestingly, the C2-N1 and C2-O1 distances, 1.450 (5) and 1.456 (6) Å , respectively, are significantly different from the corresponding distances in the p-chloro derivative [1.369 (2) and 1.529 (2) Å , respectively; Rivera et al., 2010]. Indeed, the values observed here are closer to those found in the analogous compound with no p-substituent on the aromatic ring (1.424 and 1.463 Å , respectively; Rivera et al., 2012a). This may indicate that the presence of the electron-withdrawing bromine atom does not significantly affect the adjacent oxazinic ring.

Supramolecular features
In the crystal, weak C5-H5BÁ Á ÁBr1 hydrogen bonds (Table 1) lead to the formation of inversion dimers with R 2 2 (12) ring motifs. These combine with the inversion symmetry of the molecule to produce chains of molecules along the c axis. Additional weak C2-H2BÁ Á ÁO1 hydrogen bonds link these chains, stacking molecules along the b-axis direction, Fig. 2.

Synthesis and crystallization
An aqueous solution of formaldehyde (1.5 mL, 20 mmol) was added dropwise to a mixture of ethane-1,2-diamine (0.34 ml, 5 mmol) and 4-bromophenol (1.73 g, 10 mmol) dissolved in dioxane (10 ml). The reaction mixture was stirred for 4 h at room temperature. Single crystals were obtained from this solution by slow evaporation of the solvent.

Figure 2
Packing diagram for the title compound, viewed along the b axis, with hydrogen bonds drawn as dashed lines.

1,2-Bis(6-bromo-3,4-dihydro-2H-benz[e][1,3]oxazin-3-yl)ethane
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.