rac-(4aR,8aR)-2,3-Diphenyl-4a,5,6,7,8,8a-hexa­hydro­quinoxaline

Fang Chen; Heng-Yun Ye

doi:10.1107/S1600536808016103

. 2008 Jun 7;64(Pt 7):o1191. doi: 10.1107/S1600536808016103

rac-(4aR,8aR)-2,3-Diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline

Fang Chen ^a, Heng-Yun Ye ^a,^*

PMCID: PMC2961730 PMID: 21202833

Abstract

The structure of the title racemic compound, C₂₀H₂₀N₂, shows close similarity to that of the enantiomerically pure (4aR,8aR)-2,3-diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline [Wang & Ye (2008 ▶). Acta Cryst. E64, o359–o359]. The similarity applies to the unit-cell parameters as well as to the packing of the constituent molecules. Similar packing is conditioned by a lack of directed intermolecular interactions such as hydrogen bonds in either structure.

Related literature

For examples of the synthesis of non-centrosymmetric solid materials by the reaction of chiral organic ligands and inorganic salts, see: Qu et al. (2004 ▶). For geometric parameters of C=N bonds, see: Figuet et al. (2001 ▶); Kennedy & Reglinski (2001 ▶). For our previous work regarding the enantiomerically pure (4aR,8aR)-2,3-diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline, see: Wang & Ye (2008 ▶).

Experimental

Crystal data

C₂₀H₂₀N₂
M _r = 288.38
Orthorhombic,
a = 15.278 (3) Å
b = 18.388 (4) Å
c = 5.6638 (11) Å
V = 1591.2 (5) Å³
Z = 4
Mo Kα radiation
μ = 0.07 mm⁻¹
T = 293 (2) K
0.25 × 0.15 × 0.10 mm

Data collection

Rigaku SCXmini diffractometer
Absorption correction: multi-scan (CrystalClear; Rigaku, 2005 ▶) T _min = 0.831, T _max = 1.000 (expected range = 0.825–0.993)
16117 measured reflections
2004 independent reflections
1558 reflections with I > 2σ(I)
R _int = 0.071

Refinement

R[F ² > 2σ(F ²)] = 0.047
wR(F ²) = 0.100
S = 1.10
2004 reflections
199 parameters
1 restraint
H-atom parameters constrained
Δρ_max = 0.13 e Å⁻³
Δρ_min = −0.17 e Å⁻³

Data collection: CrystalClear (Rigaku, 2005 ▶); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: SHELXTL (Sheldrick, 2008 ▶); software used to prepare material for publication: SHELXTL.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536808016103/fb2093sup1.cif

e-64-o1191-sup1.cif^{(17.7KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808016103/fb2093Isup2.hkl

e-64-o1191-Isup2.hkl^{(98.6KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

This work was supported by a Start-up Grant awarded to HYY by Southeast University.

supplementary crystallographic information

Comment

Presence of chiral centres in organic ligands is very important for design and synthesis of noncentrosymmetric or chiral coordination polymers which exhibit desirable physical properties such as a ferroelectric behaviour (Qu et al., 2004). We have recently reported the crystal structure of the enantiomerically pure ligand (4aR,8aR)-2,3-diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline (Wang & Ye, 2008). As a part of our ongoing investigations in this field we have determined the crystal structure of the title compound, rac-(4aR,8aR)-2,3- diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline.

The title compound can be regarded as a derivative of hexahydroquinoxaline by substitution of two H atoms in each of the positions 2 and 3 by the phenyl rings. The heterocyclic ring of the quinoxaline system has a twist-boat configuration, while the cyclohexane ring has a chair configuration. The torsion angle N2—C1—C6—N1 is -58.3 (3)°. The C═N double bonds (C7═N1, 1.272 (2) Å; C14═N2, 1.279 (2) Å) are in the range of 1.27–1.38 Å that have been found in other Schiff base complexes (Figuet et al., 2001; Kennedy & Reglinski, 2001; Wang & Ye, 2008). C7, C14 show typical sp² geometry environment. Comparing the bond angles around sp² N atoms (N1, N2) with those around the sp² C atoms (C7, C14), the latter are somewhat more close to 120°. N1C8C7C14 and N2C15C14C7 are almost coplanar with the mean deviations equal to 0.0119 and 0.0052 Å, respectively. The angle (29.76 (14)°) between the planes of N1C7C8C14 and N2C7C14C15 is very close to that (29.65 (14)°) in the enantiomerically pure compound (4aR,8aR)-2,3-diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline (Wang & Ye, 2008). The angle (64.3 (1)°) between both phenyl rings in the title structure equals within the precision of the experiments to that (64.3 (1)°) of the enantiomerically pure compound (4aR,8aR)-2,3-diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline.

The title racemic compound crystallizes in the space group of Pna2₁. Figs. 2 and 3 contain the respective views of the unit cells of the title compound and its enantiomerically pure counterpart (Wang & Ye, 2008)). [The enantiomerically pure structure in Fig. 3 has been obtained by the following transformations of the published data (Wang & Ye, 2008): (a,b,c)₂ = (a,b,c)₁(0 1 0/0 0 1/1 0 0) followed by the shift of the origin by -1/4 1/2 -1/4 with the corresponding change in the translational parts of the symmetry operators. (0 1 0/0 0 1/1 0 0) is the transformation matrix where each triplet of the numbers corresponds to its row.]

In spite of the fact that a half of the molecules in the title structure are the opposite enantiomers (Fig. 2) in contrast to the structure composed of the enantiomers of one kind in Wang & Ye (2008) (Fig. 3) both structures look alike when wieved along the shortest unit-cell axis. It can not be excluded that both enantiomers form solid solutions in some composition interval. The experiments that would confirm the hypothesis about the formation of the solid solutions are going to be carried out in near future. The melting point of the enantiomerically pure structure (Wang & Ye, 2008) is 194–198°C.

(Note: The setting P2₁nb is directly related to that of the reported structure of the enantiomerically pure compound (Wang & Ye, 2008). In the setting P2₁nb the unit cell axes are ordered according to their length from the minimal to the maximal.)

Experimental

rac-(1R,2R/1S,2S)-diaminocyclohexane was obtained from Adrich. The title compound was prepared by an analogous procedure to that regarding the enantiomerically pure (4aR,8aR)-2,3-diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline (Wang & Ye, 2008) using rac-1,2-diaminocyclohexane instead of (-)-(1R,2R)-diaminocyclohexane. Yellow block-like crystals, suitable for X-ray analysis, were obtained by slow evaporation of the ethanol solution of the crude product.