1,2,3,4-Tetra­hydro­quinoxaline

The title compound, C₈H₁₀N₂, was synthesized from the hydrogenation reaction of quinoxaline. The crystal structure reveals a puckered piperazine ring fused to a planar aromatic ring. A series of N—H...N hydrogen bonds produces an infinite zigzag chain.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• Mean [sigma](C-C) = 0.001 Å
• R factor = 0.030
• wR factor = 0.087
• Data-to-parameter ratio = 9.6

checkCIF/PLATON results

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Alert level C
PLAT088_ALERT_3_C Poor Data / Parameter Ratio ....................       9.56

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   1 ALERT level C = Check and explain
   0 ALERT level G = General alerts; check

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   0 ALERT type 2 Indicator that the structure model may be wrong or deficient
   1 ALERT type 3 Indicator that the structure quality may be low
   0 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. Comment

Piperazines are an important class of compounds with antimalarial properties (Deprez-Poulain and Melnyk, 2005) and relevance to the problem of designer drugs (Maurer et al., 2004). We have recently been studying the coordination chemistry of diamines and diimines to copper(I) halides and pseudohalides (Graham et al., 2000; Pike et al., 2002; Maeyer et al., 2003; Wiles & Pike, 2006). In the course of this work, we have synthesized 1,2,3,4-tetrahydroquinoxaline (I). Several reduction strategies have been reported for the conversion of quinoxaline to I (Bohlmann, 1952; Hamer & Holliday, 1963; Bugle & Osteryoung, 1979; Nose & Kudo, 1984; Murata et al., 1987; Ranu et al., 1998; McKinney et al., 2005; Eary & Clausen, 2006). We prepared I by catalytic hydrogenation and are currently studying its network-forming coordination chemistry with copper(I) salts. Here we report the crystal structure of I (Figure 1).

Although 1,2,3,4-tetrahydroquinoxaline has been known for over 50 years, its structure has not yet been reported. Compound I crystallizes in the orthorhombic space group Pbca. The structure of I exhibits bond distances and angles that are unexceptional and are comparable to those of the relatively few previously reported tetrahydroquinoxaline structures (Ammon et al., 1979; Pniewska & Anulewicz, 1986; Epifani et al., 1987; Beddos et al., 1992; Brown et al., 1995; Nair et al., 2004).

The non-planar piperazine ring is fused to the planar aromatic ring with fairly minor deviations from the C3–C8 best plane: N1 = 0.1021 (14) Å, N2 = -0.1112 (14) Å, C1 = -0.4066 (17) Å, and C2 = 0.2353 (18) Å. A series of hydrogen bonds, N1···N2 = 3.0899 (12) and N2···N1 = 3.1740 (12), creates a zigzag chain between adjacent molecules that are nearly perpendicular (interplanar angle = 75.58 (2)°). The hydrogen-bonded chain propagates in a direction parallel to the b-axis (Figure 2).

Related literature

For related literature, see: Ammon et al. (1979); Beddoes et al. (1992); Bohlmann (1952); Brown et al. (1995); Bugle & Osteryoung (1979); Deprez-Poulain & Melnyk (2005); Eary & Clausen (2006); Epifani et al. (1987); Farrugia (1997); Graham et al. (2000); Hamer & Holliday (1963); Maeyer et al. (2003); Maurer et al. (2004); McKinney et al. (2005); Murata et al. (1987); Nair et al. (2004); Nose & Kudo (1984); Pike et al. (2002); Pniewska & Anulewicz (1986); Ranu et al. (1998); Wiles & Pike (2006).

S2. Experimental

Freshly sublimed quinoxaline (1.24 g, 9.51 mmol) and 5% rhodium on alumina catalyst (198 mg) were dissolved in 100% EtOH (20 ml). The mixture was placed in a glass Parr hydrogenation vessel and was pressurized to 3.4 atm with hydrogen gas. The mixture was shaken at room temp. for 18 h and then filtered and evaporated. The crude orange product was recrystallized by dissolving it in diethyl ether and then cooling, providing 792 mg of 1 as slightly orange crystals (62.0% yield); mp 88–90° C.

S3. Refinement

All hydrogen atoms were located in the difference map and refined isotropically.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT-Plus (Bruker, 2004); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997) and XSHELL (Bruker, 2004); molecular graphics: ORTEP-3 (Farrugia, 1997); Mercury. (Version 4.2.1; Macrae et al., 2006); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. ORTEP picture (Farrugia, 1997) of (1) Displacement ellipsoids have been drawn at the 50% probability level.
	Fig. 2. Ball and stick packing diagram of (1) showing hydrogen-bonding chains.

1,2,3,4-Tetrahydroquinoxaline

Crystal data

C8H10N2	Z = 8
Mr = 134.18	F(000) = 576
Orthorhombic, Pbca	Dx = 1.254 Mg m^−3
Hall symbol: -P 2ac 2ab	Cu Kα radiation, λ = 1.54178 Å
a = 9.8609 (2) Å	µ = 0.60 mm^−1
b = 8.4986 (2) Å	T = 100 K
c = 16.9639 (4) Å	Block, colorless
V = 1421.64 (6) Å^3	0.34 × 0.22 × 0.22 mm

Data collection

Bruker SMART APEXII CCD diffractometer	1262 independent reflections
Radiation source: fine-focus sealed tube	1209 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.028
ω and ψ scans	θmax = 67.0°, θmin = 5.2°
Absorption correction: multi-scan (SADABS; Sheldrick, 2004)	h = −11→11
Tmin = 0.821, Tmax = 0.879	k = −9→10
14760 measured reflections	l = −20→18

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.030	All H-atom parameters refined
wR(F^2) = 0.087	w = 1/[σ^2(Fo^2) + (0.0523P)^2 + 0.2875P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max < 0.001
1262 reflections	Δρmax = 0.22 e Å^−3
132 parameters	Δρmin = −0.12 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0017 (3)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N1	0.54142 (8)	0.11160 (9)	0.25782 (4)	0.0213 (2)
N2	0.70309 (9)	0.38487 (10)	0.25085 (4)	0.0224 (2)
C1	0.55804 (10)	0.19734 (12)	0.18369 (5)	0.0228 (3)
C2	0.69619 (10)	0.27641 (12)	0.18412 (5)	0.0233 (3)
C3	0.65474 (9)	0.32959 (11)	0.32396 (5)	0.0205 (3)
C4	0.68628 (10)	0.40793 (12)	0.39369 (6)	0.0273 (3)
C5	0.63124 (11)	0.36107 (12)	0.46517 (6)	0.0306 (3)
C6	0.54215 (10)	0.23452 (12)	0.46790 (6)	0.0276 (3)
C7	0.51080 (9)	0.15467 (11)	0.39898 (5)	0.0222 (3)
C8	0.56618 (9)	0.19922 (10)	0.32676 (5)	0.0194 (2)
H1A	0.4851 (12)	0.2794 (13)	0.1754 (6)	0.024 (3)*
H1B	0.5532 (11)	0.1203 (14)	0.1400 (7)	0.029 (3)*
H1N	0.4626 (14)	0.0554 (15)	0.2602 (7)	0.035 (3)*
H2N	0.7827 (14)	0.4367 (16)	0.2546 (7)	0.035 (3)*
H2A	0.7695 (12)	0.1924 (14)	0.1874 (6)	0.028 (3)*
H2B	0.7108 (11)	0.3363 (14)	0.1342 (6)	0.028 (3)*
H4	0.7473 (13)	0.4979 (15)	0.3899 (6)	0.033 (3)*
H5	0.6535 (12)	0.4214 (15)	0.5133 (7)	0.040 (3)*
H6	0.5002 (13)	0.2045 (14)	0.5183 (7)	0.034 (3)*
H7	0.4479 (12)	0.0639 (14)	0.4000 (6)	0.028 (3)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0211 (5)	0.0213 (4)	0.0215 (4)	−0.0022 (3)	0.0009 (3)	0.0002 (3)
N2	0.0200 (5)	0.0237 (5)	0.0234 (4)	−0.0030 (3)	0.0007 (3)	0.0037 (3)
C1	0.0250 (5)	0.0237 (5)	0.0198 (5)	0.0009 (4)	0.0006 (4)	−0.0002 (4)
C2	0.0231 (5)	0.0238 (5)	0.0231 (5)	0.0026 (4)	0.0035 (4)	0.0031 (4)
C3	0.0169 (5)	0.0213 (5)	0.0233 (5)	0.0016 (4)	−0.0008 (3)	0.0041 (3)
C4	0.0266 (5)	0.0272 (5)	0.0280 (5)	−0.0064 (4)	−0.0030 (4)	0.0013 (4)
C5	0.0356 (6)	0.0345 (6)	0.0217 (5)	−0.0056 (5)	−0.0030 (4)	−0.0021 (4)
C6	0.0279 (5)	0.0331 (6)	0.0216 (5)	−0.0012 (4)	0.0011 (4)	0.0040 (4)
C7	0.0195 (5)	0.0225 (5)	0.0247 (5)	−0.0006 (4)	−0.0002 (4)	0.0037 (4)
C8	0.0162 (5)	0.0192 (5)	0.0227 (5)	0.0029 (3)	−0.0018 (3)	0.0014 (3)

Geometric parameters (Å, º)

N1—C8	1.4078 (11)	C3—C4	1.3925 (13)
N1—C1	1.4626 (12)	C3—C8	1.4115 (14)
N1—H1N	0.913 (14)	C4—C5	1.3869 (14)
N2—C3	1.4094 (11)	C4—H4	0.975 (13)
N2—C2	1.4613 (12)	C5—C6	1.3895 (14)
N2—H2N	0.903 (15)	C5—H5	0.989 (13)
C1—C2	1.5191 (14)	C6—C7	1.3867 (13)
C1—H1A	1.011 (11)	C6—H6	0.984 (12)
C1—H1B	0.990 (12)	C7—C8	1.3938 (12)
C2—H2A	1.017 (12)	C7—H7	0.990 (12)
C2—H2B	0.998 (11)
C8—N1—C1	115.56 (7)	C4—C3—C8	119.00 (8)
C8—N1—H1N	112.8 (7)	C4—C3—N2	120.83 (8)
C1—N1—H1N	113.2 (8)	C8—C3—N2	120.04 (8)
C3—N2—C2	117.10 (8)	C5—C4—C3	121.20 (9)
C3—N2—H2N	113.3 (7)	C5—C4—H4	121.7 (6)
C2—N2—H2N	113.7 (8)	C3—C4—H4	117.1 (6)
N1—C1—C2	108.46 (7)	C4—C5—C6	119.92 (9)
N1—C1—H1A	112.5 (6)	C4—C5—H5	119.1 (7)
C2—C1—H1A	109.5 (6)	C6—C5—H5	120.9 (7)
N1—C1—H1B	108.0 (6)	C7—C6—C5	119.45 (9)
C2—C1—H1B	109.9 (6)	C7—C6—H6	120.8 (7)
H1A—C1—H1B	108.5 (9)	C5—C6—H6	119.7 (7)
N2—C2—C1	108.94 (8)	C6—C7—C8	121.39 (9)
N2—C2—H2A	111.5 (6)	C6—C7—H7	120.5 (6)
C1—C2—H2A	109.1 (7)	C8—C7—H7	118.2 (6)
N2—C2—H2B	109.2 (6)	C7—C8—N1	121.24 (8)
C1—C2—H2B	110.5 (6)	C7—C8—C3	119.03 (8)
H2A—C2—H2B	107.5 (9)	N1—C8—C3	119.64 (8)
C8—N1—C1—C2	51.72 (11)	C5—C6—C7—C8	−0.38 (15)
C3—N2—C2—C1	45.09 (11)	C6—C7—C8—N1	175.67 (9)
N1—C1—C2—N2	−59.95 (10)	C6—C7—C8—C3	−0.75 (14)
C2—N2—C3—C4	164.45 (9)	C1—N1—C8—C7	157.24 (8)
C2—N2—C3—C8	−19.61 (12)	C1—N1—C8—C3	−26.36 (12)
C8—C3—C4—C5	−0.67 (15)	C4—C3—C8—C7	1.26 (13)
N2—C3—C4—C5	175.30 (9)	N2—C3—C8—C7	−174.74 (8)
C3—C4—C5—C6	−0.47 (16)	C4—C3—C8—N1	−175.22 (8)
C4—C5—C6—C7	0.99 (16)	N2—C3—C8—N1	8.78 (13)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2N···N1^i	0.903 (15)	2.284 (15)	3.1740 (12)	168.4 (11)
N1—H1N···N2^ii	0.913 (14)	2.192 (14)	3.0900 (12)	167.6 (11)

Symmetry codes: (i) −x+3/2, y+1/2, z; (ii) −x+1, y−1/2, −z+1/2.