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 title benzoxazine molecule, C18H18Br2N2O2, was prepared by a Mannich-type reaction of 4-bromo-phenol with ethane-1,2-di-amine and formaldehyde. The title compound crystallizes in the monoclinic space group C2/c with a centre of inversion located at the mid-point of the C-C bond of the central CH2CH2 spacer. The oxazinic ring adopts a half-chair conformation. The structure is compared to those of other functionalized benzoxazines synthesized in our laboratory. In the crystal, weak C-H⋯Br and C-H⋯O hydrogen bonds stack the mol-ecules along the b-axis direction.

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′-(ethane-1,2-di­yl)bis­(6-substituted-3,4-di­hydro-2H-1,3-benzoxazine) (Rivera et al., 2010 ▸, 2011 ▸, 2012a ▸,b ▸) 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.

Structural commentary

The asymmetric unit of the title compound (Fig. 1 ▸), C18H18Br2N2O2, contains one half of the organic mol­ecule, an inversion centre generates the other half of the mol­ecule (symmetry operation: 1 − x, 1 − y, 1 − z). The six-membered oxazine heterocyclic ring adopts a half-chair conformation, with puckering parameters Q = 0.512 (2) Å, θ =129.6 (2)°, φ = 283.6 (3)°. This ring is analysed with respect to the plane formed by O1/C3/C4/C5, with deviations of the C2 and N1 atoms from this plane of 0.300 (6) and −0.320 (4) Å, respectively.

Figure 1

The mol­ecular structure of the title compound, Displacement ellipsoids are drawn at the 50% probability level. Atoms labelled with the suffix A are generated using the symmetry operator (1 − x, 1 − y, 1 − z).

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—H5B⋯Br1 hydrogen bonds (Table 1 ▸) lead to the formation of inversion dimers with (12) ring motifs. These combine with the inversion symmetry of the mol­ecule to produce chains of mol­ecules along the c axis. Additional weak C2—H2B⋯O1 hydrogen bonds link these chains, stacking mol­ecules along the b-axis direction, Fig. 2 ▸.

Table 1

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C5—H5B⋯Br1i	0.99	3.04	3.951 (5)	154
C2—H2B⋯O1ii	0.99	2.64	3.506 (6)	146

Symmetry codes: (i) ; (ii) .

Figure 2

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

Database survey

A database search yielded four comparable structures, namely 3,3′-(ethane-1,2-di­yl)bis­(6-methyl-3,4-di­hydro-2H-1,3-benzoxazine) (AXAKAM; Rivera et al., 2011 ▸), 3,3′-ethyl­enebis(3,4-di­hydro-6-chloro-2H-1,3-benzoxazine), (NUQKAM; Rivera et al., 2010 ▸), 3,3′-(ethane-1,2-di­yl)bis­(6-meth­oxy-3,4-di­hydro-2H-1,3-benzoxazine) monohydrate (QEDDOU; Rivera et al., 2012b ▸), 3,3′-(ethane-1,2-di­yl)bis­(3,4-di­hydro-2H-1,3-benzoxazine) (SAGPUN; Rivera et al., 2012a ▸). The Cl-substituted compound (NUQKAM) and the title compound are isomorphous. However, AXAKAM and SAGOUN have different space groups and in QEDDOU a solvent water mol­ecule is included in the crystal packing.

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.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. All the H atoms were located in the difference electron-density map. C-bound H atoms were fixed geometrically (C—H = 0.95 or 0.99 Å) and refined using a riding-model approximation, with U iso(H) set to 1.2U eq of the parent atom.

Table 2

Experimental details

Crystal data
Chemical formula	C18H18Br2N2O2
M r	454.16
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	173
a, b, c (Å)	19.464 (2), 5.9444 (7), 17.2225 (19)
β (°)	121.557 (7)
V (Å3)	1698.0 (3)
Z	4
Radiation type	Mo Kα
μ (mm−1)	4.79
Crystal size (mm)	0.27 × 0.26 × 0.26
Data collection
Diffractometer	STOE IPDS II two-circle
Absorption correction	Multi-scan (X-AREA; Stoe & Cie, 2001 ▸)
T min, T max	0.905, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	3703, 1583, 1391
R int	0.078
(sin θ/λ)max (Å−1)	0.607
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.056, 0.143, 1.07
No. of reflections	1583
No. of parameters	110
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	1.40, −1.92

Computer programs: X-AREA (Stoe & Cie, 2001 ▸), SHELXS (Sheldrick, 2008 ▸), SHELXL2014/7 (Sheldrick, 2015 ▸) and XP in SHELXTL-Plus (Sheldrick, 2008 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989016016509/sj5510sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989016016509/sj5510Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989016016509/sj5510Isup3.cml

CCDC reference: 1510085

Additional supporting information: crystallographic information; 3D view; checkCIF report

C18H18Br2N2O2	F(000) = 904
Mr = 454.16	Dx = 1.777 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 19.464 (2) Å	Cell parameters from 3703 reflections
b = 5.9444 (7) Å	θ = 3.6–26.0°
c = 17.2225 (19) Å	µ = 4.79 mm−1
β = 121.557 (7)°	T = 173 K
V = 1698.0 (3) Å3	Block, colourless
Z = 4	0.27 × 0.26 × 0.26 mm

STOE IPDS II two-circle diffractometer	1391 reflections with I > 2σ(I)
Radiation source: Genix 3D IµS microfocus X-ray source	Rint = 0.078
ω scans	θmax = 25.6°, θmin = 3.6°
Absorption correction: multi-scan (X-Area; Stoe & Cie, 2001)	h = −20→23
Tmin = 0.905, Tmax = 1.000	k = −7→7
3703 measured reflections	l = −20→20
1583 independent reflections

Refinement on F2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F2 > 2σ(F2)] = 0.056	w = 1/[σ2(Fo2) + (0.099P)2] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.143	(Δ/σ)max < 0.001
S = 1.07	Δρmax = 1.40 e Å−3
1583 reflections	Δρmin = −1.92 e Å−3
110 parameters	Extinction correction: SHELXL-2014/7 (Sheldrick 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraints	Extinction coefficient: 0.0037 (8)

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.

	x	y	z	Uiso*/Ueq
Br1	0.60406 (3)	−0.02845 (9)	0.93353 (3)	0.0226 (3)
O1	0.67962 (18)	0.6369 (6)	0.7231 (2)	0.0176 (7)
N1	0.6072 (2)	0.4166 (7)	0.5842 (2)	0.0130 (8)
C1	0.5323 (2)	0.5473 (7)	0.5467 (3)	0.0141 (10)
H1A	0.5430	0.7067	0.5401	0.017*
H1B	0.5124	0.5406	0.5891	0.017*
C2	0.6783 (3)	0.5494 (8)	0.6434 (3)	0.0155 (10)
H2A	0.6811	0.6770	0.6082	0.019*
H2B	0.7268	0.4552	0.6639	0.019*
C3	0.6620 (3)	0.4788 (8)	0.7681 (3)	0.0141 (9)
C4	0.6259 (2)	0.2721 (8)	0.7288 (3)	0.0142 (9)
C5	0.6091 (2)	0.2158 (8)	0.6343 (3)	0.0140 (9)
H5A	0.6513	0.1123	0.6400	0.017*
H5B	0.5566	0.1372	0.5995	0.017*
C6	0.6083 (2)	0.1218 (8)	0.7781 (3)	0.0147 (9)
H6	0.5841	−0.0193	0.7526	0.018*
C7	0.6265 (3)	0.1804 (8)	0.8655 (3)	0.0164 (9)
C8	0.6619 (3)	0.3870 (9)	0.9044 (3)	0.0223 (10)
H8	0.6741	0.4248	0.9639	0.027*
C9	0.6787 (3)	0.5348 (8)	0.8552 (3)	0.0192 (11)
H9	0.7021	0.6768	0.8807	0.023*

	U11	U22	U33	U12	U13	U23
Br1	0.0261 (4)	0.0229 (4)	0.0255 (4)	0.00128 (17)	0.0181 (3)	0.00527 (19)
O1	0.0161 (15)	0.0160 (18)	0.0166 (15)	−0.0043 (12)	0.0058 (13)	−0.0018 (13)
N1	0.0078 (16)	0.0135 (19)	0.0148 (17)	0.0005 (13)	0.0039 (14)	0.0003 (15)
C1	0.010 (2)	0.013 (2)	0.014 (2)	0.0037 (16)	0.0029 (19)	−0.0009 (18)
C2	0.012 (2)	0.016 (2)	0.019 (2)	−0.0051 (16)	0.0081 (18)	−0.0034 (18)
C3	0.0114 (19)	0.014 (2)	0.014 (2)	0.0000 (15)	0.0047 (17)	0.0021 (17)
C4	0.0093 (18)	0.015 (2)	0.014 (2)	0.0023 (16)	0.0030 (15)	0.0016 (17)
C5	0.0105 (19)	0.012 (2)	0.016 (2)	0.0009 (15)	0.0039 (16)	−0.0021 (17)
C6	0.0098 (18)	0.015 (2)	0.018 (2)	0.0015 (15)	0.0062 (16)	0.0022 (18)
C7	0.020 (2)	0.014 (2)	0.020 (2)	0.0032 (16)	0.0137 (18)	0.0057 (18)
C8	0.028 (2)	0.023 (3)	0.017 (2)	−0.001 (2)	0.0126 (19)	−0.0036 (19)
C9	0.023 (2)	0.015 (2)	0.019 (2)	−0.0003 (17)	0.011 (2)	−0.0020 (18)

Br1—C7	1.909 (5)	C3—C4	1.402 (6)
O1—C3	1.372 (6)	C4—C6	1.393 (7)
O1—C2	1.455 (6)	C4—C5	1.519 (6)
N1—C2	1.450 (5)	C5—H5A	0.9900
N1—C5	1.462 (6)	C5—H5B	0.9900
N1—C1	1.470 (5)	C6—C7	1.397 (7)
C1—C1i	1.538 (8)	C6—H6	0.9500
C1—H1A	0.9900	C7—C8	1.396 (7)
C1—H1B	0.9900	C8—C9	1.373 (8)
C2—H2A	0.9900	C8—H8	0.9500
C2—H2B	0.9900	C9—H9	0.9500
C3—C9	1.398 (7)
C3—O1—C2	113.7 (4)	C6—C4—C5	121.9 (4)
C2—N1—C5	108.0 (3)	C3—C4—C5	118.9 (4)
C2—N1—C1	112.6 (4)	N1—C5—C4	112.2 (4)
C5—N1—C1	113.8 (3)	N1—C5—H5A	109.2
N1—C1—C1i	110.2 (4)	C4—C5—H5A	109.2
N1—C1—H1A	109.6	N1—C5—H5B	109.2
C1i—C1—H1A	109.6	C4—C5—H5B	109.2
N1—C1—H1B	109.6	H5A—C5—H5B	107.9
C1i—C1—H1B	109.6	C4—C6—C7	119.5 (4)
H1A—C1—H1B	108.1	C4—C6—H6	120.3
N1—C2—O1	113.4 (4)	C7—C6—H6	120.3
N1—C2—H2A	108.9	C8—C7—C6	121.3 (4)
O1—C2—H2A	108.9	C8—C7—Br1	119.4 (4)
N1—C2—H2B	108.9	C6—C7—Br1	119.2 (4)
O1—C2—H2B	108.9	C9—C8—C7	118.9 (4)
H2A—C2—H2B	107.7	C9—C8—H8	120.5
O1—C3—C9	117.2 (4)	C7—C8—H8	120.5
O1—C3—C4	122.5 (4)	C8—C9—C3	120.8 (5)
C9—C3—C4	120.3 (5)	C8—C9—H9	119.6
C6—C4—C3	119.2 (4)	C3—C9—H9	119.6
C2—N1—C1—C1i	151.1 (5)	C1—N1—C5—C4	−77.1 (4)
C5—N1—C1—C1i	−85.6 (6)	C6—C4—C5—N1	161.5 (4)
C5—N1—C2—O1	−64.9 (5)	C3—C4—C5—N1	−20.1 (5)
C1—N1—C2—O1	61.6 (5)	C3—C4—C6—C7	0.3 (6)
C3—O1—C2—N1	47.8 (5)	C5—C4—C6—C7	178.7 (4)
C2—O1—C3—C9	166.5 (4)	C4—C6—C7—C8	0.1 (7)
C2—O1—C3—C4	−15.8 (5)	C4—C6—C7—Br1	−178.8 (3)
O1—C3—C4—C6	−178.7 (4)	C6—C7—C8—C9	0.2 (7)
C9—C3—C4—C6	−1.1 (6)	Br1—C7—C8—C9	179.2 (4)
O1—C3—C4—C5	2.9 (6)	C7—C8—C9—C3	−1.0 (7)
C9—C3—C4—C5	−179.5 (4)	O1—C3—C9—C8	179.2 (4)
C2—N1—C5—C4	48.7 (5)	C4—C3—C9—C8	1.4 (7)

D—H···A	D—H	H···A	D···A	D—H···A
C5—H5B···Br1ii	0.99	3.04	3.951 (5)	154
C2—H2B···O1iii	0.99	2.64	3.506 (6)	146