Ethyl 1-ethyl-7-fluoro-4-oxo-1,4-di­hydro­quinoline-3-carboxyl­ate: X-ray and DFT studies

The basic building unit in the structure of the title compound, C₁₄H₁₄FNO₃, is pairs of mol­ecules arranged in an anti­parallel fashion, enabling weak C—H...O inter­actions. Each mol­ecule is additionally involved in π–π inter­actions with neighbouring mol­ecules. The pairs of mol­ecules formed by the C—H...O hydrogen bonds and π–π inter­actions form ribbon-like chains running along the c axis. Theoretical calculations based on these pairs showed that, although the main inter­molecular inter­action is electrostatic, it is almost completely compensated by an exchange–repulsion contribution to the total energy. As a consequence, the dominating force is a dispersion inter­action. The F atoms form weak C—F...H—C inter­actions with the H atoms of the neighbouring ethyl groups, with H...F separations in the range 2.59–2.80 Å.

S1. Comment

Quinolones are known for a wide range of biological activities, such as antibacterial, anti-allergic, antiherpetic, anticarcinogenic etc., and are used in both human and veterinary medicine (Milata et al., 2000). As their antibacterial effect is enhanced with an F atom attached to the benzene ring, the majority of quinolone derivatives having medicinal applications have an F atom attached, typically in position 5, 6 or 8. The presence of a carbonyl group in position 4 and a carboxyl group in position 3 plays an important role in the interaction of a quinoline with DNA-gyrase 2, a key enzyme of bacterial cells in the process of DNA replication and transcription. Such a 4-oxo tautomeric form is best stabilized by an ethyl group attached to the N atom of the quinoline molecule. The skeleton of the title compound, (I), thus provides a good model for a family of potential drugs.

Compound (I) was synthesized within the framework of our continuing study (Langer et al., 2009, 2010) of the structure and properties of potential drugs based on fluoroquinolones, aimed at obtaining more insight into their structure–function relationships.

The numbering scheme and the overall conformation of (I) are shown in Fig. 1. The core of (I) is essentially planar (r.m.s. deviation 0.023 Å for the quinoline moiety plus the directly attached atoms F1, O1, C9 and C10), with atom C11 of the ethyl group displaced from this plane by 1.359 (3) Å. The C7—F1 bond distance (Table 1) agrees with the values previously found in structurally related compounds (Langer et al., 2009, 2010). A small rotation of the substituted carboxyl group along the C3—C9 bond is shown by the C2—C3—C9—O3 torsion angle [-174.7 (2)°].

The redistribution of electron density in an isolated molecule of (I) can be qualitatively described by a superposition of the resonance structures 1–4 (Fig. 3), obtained by natural bond orbital (NBO) analysis (Foster & Weinhold, 1980), from which structure 1 is the most probable. The obvious consequence of the presence of resonance structure 4 is shortening of the formally single C3—C9 bond compared with the expected value (1.487 Å; Allen et al., 1987). Consequently, the rigidity of the skeleton is increased, as free rotation around this bond is less favourable. The presence of resonance structures 2 and 3 is responsible for slight lengthening of the C4—O1 and C9═O2 bonds relative to an isolated C═O bond (Allen et al., 1987). From Table 1 it is evident that, apart from the C12—O3 bond, the calculated Wiberg bond orders are very close to the expected values. The deviation for this bond is due the nature of NBO analysis which, in general, assumes all bonds to be covalent, and hence a value significantly lower than expected points to the significant ionicity of the bond. Deviations from expected values can also be found for polar C—O bonds in other esters. For instance, our calculations for the ethyl esters of acetic and benzoic acids give bond orders of 0.89 and 0.87, respectively. Similarly, for the C7—F1 bond, the corresponding calculated bond order is significantly smaller than 1, due to the fact that the F atom is significantly ionic.

The basic building unit in the structure of (I) is pairs of molecules arranged in an antiparallel fashion, thus enabling weak C10—H10B···O2(-x, 1 - y, -z) interactions (Fig. 2). The BSSE (basis-set superposition error) corrected binding energy calculated for an isolated model dimer, with the geometry first optimized starting from the arrangement depicted in Fig. 2, is -5.24 kcal mol^-1 (1 kcal mol^-1 = 4.184 kJ mol^-1) per hydrogen bond. This value is very close to the energy of a hydrogen bond in a water dimer, for which the best theoretical estimate is -4.91 (7) kcal mol^-1 (Halkier et al., 1997); the experimentally obtained value cited in the same work is -5.4 (7) kcal mol^-1. Such a relatively large binding energy for a C—H···O hydrogen bond is mainly due to disperse interactions of two large parallel atomic systems (see below) and thus has a different origin than the hydrogen bond in the above-mentioned water dimer, the origin of which is largely electrostatic.

Each molecule of (I) is additionally involved in π–π interactions with neighbouring molecules (Fig. 4). The strength of the pair interactions was estimated using a simple model involving just three neighbouring molecules taken from the structure, A, Aⁱⁱ and Aⁱⁱⁱ [symmetry codes: (ii) -x, 1 - y, 1 - z; (iii) 1 - x, 1 - y, 1 - z]. Although the planes fitted to the molecules are almost parallel, the distance between the centres of gravity (Cg) of the heterorings (N1/C2–C4/C14/C15) of molecules A and Aⁱⁱ is significantly shorter than that for the pair A and Aⁱⁱⁱ: Cg(A)···Cg(Aⁱⁱ) = 3.456 (2) Å versus Cg(A)···Cg(Aⁱⁱⁱ) = 3.999 (2) Å. The difference in packing is also emphasized by the values of the mutual shift of the heterorings (slippage) calculated with respect to the positions of the centres of gravity: 2.003 Å for A and Aⁱⁱⁱ, compared with 1.041 Å for the A···Aⁱⁱ pair. Theoretical calculations based on these pairs showed that, although the main intermolecular interaction is electrostatic (the dipole moment of an isolated molecule calculated at the B3LYP/cc-pvTZ level is 4.5 D), it is almost completely compensated by the exchange-repulsion contribution to the total energy. As a consequence, the dominating force is a dispersion interaction. The sizes of the interaction energies between molecule A and its close neighbours Aⁱⁱ and Aⁱⁱⁱ, -18.8 and -22 kcal mol^-1, respectively, are then comparable with the values characteristic for medium-strong hydrogen bonds.

The pairs of molecules formed by the C—H···O hydrogen bonds and π–π interactions form ribbon-like chains running along the c axis (Fig. 4). The intermolecular interactions are completed by weak C—H···F contacts (<3 Å) with H12B(-x, 1 - y, 1 - z)···F1 = 2.69 Å, H11C(x, y, 1 + z)···F1 = 2.81 Å, H11B(x, 3/2 - y, 1/2 + z)···F1 = 2.84 Å H12A(1 - x, 1 - y, 1 - z)···F1 = 2.95Å and H11A(x, 3/2 - y, 1/2 + z)···F1 = 2.99 Å. These H···F separations are within the limits for this type of contact (Shimoni & Glusker, 1994).

Related literature

For related literature, see: Ahlrichs et al. (1989); Allen et al. (1987); Foster & Weinhold (1980); Frisch (1998); Glendening et al. (1993); Halkier et al. (1997); Langer et al. (2009, 2010); Milata et al. (2000); Shimoni & Glusker (1994); Treutler & Ahlrichs (1995).

S2. Experimental

To a flask containing ethyl 7-fluoro-1-ethyl-4-oxo-1,4-dihydroquinoline-3-carboxylate (1.00 g, 4.81 mmol) suspended in dimethylformamide (10 ml) was added K₂CO₃ (2.35 g, 19.5 mmol). The reaction mixture was heated for 10 min in an oil bath to 333 K and ethyl iodide (0.78 ml, 9.2 mmol) was added. After 3.5 h, the reaction mixture was poured into water (40 ml) and the resulting solution was extracted with chloroform (3 × 40 ml). The organic phases was collected, dried with sodium sulfate and evaporated to dryness, and the product obtained was purified using column chromatography (ethyl acetate/hexane 20:1 v/v) to give the title compound, (I), as a white powder (999 mg, 89%) [m.p. 388–393 K (uncorrected)].

S3. Refinement

Aromatic and secondary H atoms were refined isotropically, with U_iso(H) = 1.2U_eq(C), and their positions were constrained to an ideal geometry using an appropriate riding model (C—H = 0.95 Å for aromatic H or 0.99 Å for secondary H). For methyl groups, C—C—H angles (109.5°) were kept fixed, while the torsion angle was allowed to refine with the starting positions based on the circular Fourier synthesis averaged using the local three-fold axis, with U_iso(H) = 1.5U_eq(C), and a constrained distance C—H = 0.98 Å was applied.

Basic molecular calculations were carried out at the B3LYP/6-31+G** and B3LYP/cc-pvTZ levels of theory (GAUSSIAN98; Frisch et al., 1998). Dispersion calculations were carried out at the B97-d/cc-pvTZ level of theory using TURBOMOLE (Version 5.10; Ahlrichs et al., 1989) employing the RI DFT module (Treutler & Ahlrichs, 1995). Natural bond orbital (Foster & Weinhold, 1980) calculations were carried out using the NBO program (Version 3.1; Glendening et al. 1993) included in the GAUSSIAN98 package.

Computing details

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003) and SADABS (Sheldrick, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures

	Fig. 1. The structure of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. A pair of molecules of (I) interlinked by the C10—H10B···O2^i (C10^i—H10B^i···O2) hydrogen bond (dashed lines). [Symmetry code: (i): -x, 1 - y, -z.]
	Fig. 3. Possible resonance structures of the title compound.
	Fig. 4. The packing of the molecules of (I) in the crystal structure. The pairs of molecules formed by the C—H···O hydrogen bonds (long-dashed lines) and π–π interactions (short-dashed lines) form ribbon-like chains running along the c axis. [Symmetry codes: (i): -x, 1 - y, -z; (ii) -x, 1 - y, 1 - z; (iii) 1 - x, 1 - y, 1 - z; (iv) 1 + x, y, z.]

Ethyl 1-ethyl-7-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylate

Crystal data

C14H14FNO3	F(000) = 552
Mr = 263.26	Dx = 1.415 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 1848 reflections
a = 7.1426 (18) Å	θ = 2.7–24.1°
b = 21.156 (5) Å	µ = 0.11 mm^−1
c = 8.623 (2) Å	T = 153 K
β = 108.510 (5)°	Prism, colourless
V = 1235.6 (5) Å^3	0.22 × 0.16 × 0.07 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	2199 independent reflections
Radiation source: fine-focus sealed tube	1447 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.069
ϕ and ω scans	θmax = 25.2°, θmin = 2.7°
Absorption correction: multi-scan SADABS; Sheldrick, 2003	h = −8→8
Tmin = 0.976, Tmax = 0.992	k = −25→25
9173 measured reflections	l = −9→10

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.054	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.132	H-atom parameters constrained
S = 0.99	w = 1/[σ^2(Fo^2) + (0.0672P)^2 + 0.1543P] where P = (Fo^2 + 2Fc^2)/3
2199 reflections	(Δ/σ)max < 0.001
174 parameters	Δρmax = 0.19 e Å^−3
0 restraints	Δρmin = −0.23 e Å^−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
F1	0.1962 (2)	0.66892 (7)	0.88666 (17)	0.0444 (5)
O1	0.3665 (3)	0.40919 (8)	0.6077 (2)	0.0401 (5)
O2	0.1916 (3)	0.42710 (8)	0.0891 (2)	0.0411 (5)
O3	0.3026 (2)	0.36075 (8)	0.2999 (2)	0.0338 (5)
N1	0.1307 (3)	0.57313 (9)	0.3727 (2)	0.0256 (5)
C2	0.1561 (3)	0.52225 (11)	0.2897 (3)	0.0260 (6)
H2	0.1154	0.5255	0.1739	0.031*
C3	0.2354 (3)	0.46589 (11)	0.3568 (3)	0.0255 (6)
C4	0.2957 (3)	0.45779 (11)	0.5329 (3)	0.0274 (6)
C5	0.3120 (3)	0.51207 (12)	0.7916 (3)	0.0317 (6)
H5	0.3613	0.4737	0.8467	0.038*
C6	0.2894 (4)	0.56311 (12)	0.8822 (3)	0.0329 (6)
H6	0.3219	0.5607	0.9977	0.039*
C7	0.2180 (4)	0.61763 (11)	0.7987 (3)	0.0304 (6)
C8	0.1651 (4)	0.62360 (11)	0.6333 (3)	0.0292 (6)
H8	0.1138	0.6623	0.5809	0.035*
C9	0.2408 (3)	0.41725 (12)	0.2353 (3)	0.0295 (6)
C10	0.0434 (4)	0.63082 (11)	0.2815 (3)	0.0338 (7)
H10A	−0.0473	0.6504	0.3338	0.041*
H10B	−0.0348	0.6192	0.1682	0.041*
C11	0.2006 (4)	0.67842 (12)	0.2770 (3)	0.0439 (8)
H11A	0.2726	0.6921	0.3886	0.066*
H11B	0.1379	0.7151	0.2115	0.066*
H11C	0.2928	0.6588	0.2281	0.066*
C12	0.3026 (4)	0.31121 (12)	0.1843 (3)	0.0366 (7)
H12A	0.3960	0.3216	0.1243	0.044*
H12B	0.1691	0.3063	0.1041	0.044*
C13	0.3639 (4)	0.25194 (13)	0.2789 (4)	0.0512 (9)
H13A	0.4957	0.2575	0.3584	0.077*
H13B	0.3667	0.2173	0.2041	0.077*
H13C	0.2696	0.2418	0.3367	0.077*
C14	0.2657 (3)	0.51418 (11)	0.6222 (3)	0.0255 (6)
C15	0.1888 (3)	0.57086 (11)	0.5424 (3)	0.0255 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
F1	0.0607 (10)	0.0373 (9)	0.0364 (9)	−0.0009 (8)	0.0174 (8)	−0.0102 (7)
O1	0.0524 (12)	0.0299 (10)	0.0360 (11)	0.0134 (9)	0.0111 (9)	0.0061 (8)
O2	0.0579 (13)	0.0351 (11)	0.0315 (11)	0.0006 (9)	0.0160 (9)	−0.0025 (8)
O3	0.0412 (11)	0.0259 (10)	0.0353 (10)	0.0032 (8)	0.0137 (8)	−0.0046 (8)
N1	0.0294 (11)	0.0216 (11)	0.0243 (11)	−0.0004 (9)	0.0064 (9)	0.0015 (9)
C2	0.0274 (13)	0.0292 (14)	0.0225 (13)	−0.0051 (11)	0.0097 (10)	−0.0012 (11)
C3	0.0232 (13)	0.0250 (14)	0.0306 (14)	−0.0016 (10)	0.0117 (11)	−0.0004 (11)
C4	0.0221 (12)	0.0256 (14)	0.0334 (14)	0.0023 (11)	0.0073 (11)	0.0017 (11)
C5	0.0262 (14)	0.0331 (15)	0.0331 (14)	0.0032 (11)	0.0055 (11)	0.0049 (12)
C6	0.0337 (14)	0.0403 (16)	0.0225 (13)	−0.0027 (13)	0.0058 (11)	−0.0022 (12)
C7	0.0313 (14)	0.0286 (15)	0.0331 (15)	−0.0023 (12)	0.0127 (11)	−0.0085 (12)
C8	0.0323 (14)	0.0226 (14)	0.0317 (15)	−0.0028 (11)	0.0090 (11)	0.0009 (11)
C9	0.0270 (14)	0.0279 (15)	0.0334 (16)	−0.0023 (11)	0.0093 (11)	−0.0023 (12)
C10	0.0450 (16)	0.0248 (14)	0.0281 (14)	0.0047 (12)	0.0069 (12)	0.0016 (11)
C11	0.0661 (19)	0.0248 (15)	0.0379 (16)	−0.0075 (14)	0.0124 (14)	0.0049 (12)
C12	0.0415 (16)	0.0301 (15)	0.0418 (16)	−0.0007 (12)	0.0183 (13)	−0.0117 (12)
C13	0.0528 (18)	0.0337 (16)	0.061 (2)	0.0059 (15)	0.0092 (16)	−0.0111 (15)
C14	0.0203 (12)	0.0309 (14)	0.0245 (13)	−0.0001 (10)	0.0060 (10)	0.0010 (11)
C15	0.0215 (12)	0.0284 (14)	0.0259 (13)	−0.0029 (10)	0.0063 (10)	0.0002 (11)

Geometric parameters (Å, º)

F1—C7	1.361 (3)	C6—H6	0.9500
O1—C4	1.233 (3)	C7—C8	1.360 (3)
O2—C9	1.215 (3)	C8—C15	1.405 (3)
O3—C9	1.333 (3)	C8—H8	0.9500
O3—C12	1.446 (3)	C10—C11	1.518 (4)
N1—C2	1.336 (3)	C10—H10A	0.9900
N1—C15	1.389 (3)	C10—H10B	0.9900
N1—C10	1.478 (3)	C11—H11A	0.9800
C2—C3	1.367 (3)	C11—H11B	0.9800
C2—H2	0.9500	C11—H11C	0.9800
C3—C4	1.451 (3)	C12—C13	1.484 (4)
C3—C9	1.478 (3)	C12—H12A	0.9900
C4—C14	1.472 (3)	C12—H12B	0.9900
C5—C6	1.372 (3)	C13—H13A	0.9800
C5—C14	1.392 (3)	C13—H13B	0.9800
C5—H5	0.9500	C13—H13C	0.9800
C6—C7	1.369 (3)	C14—C15	1.405 (3)
C9—O3—C12	115.8 (2)	N1—C10—H10A	109.3
C2—N1—C15	119.35 (19)	C11—C10—H10A	109.3
C2—N1—C10	119.1 (2)	N1—C10—H10B	109.3
C15—N1—C10	121.5 (2)	C11—C10—H10B	109.3
N1—C2—C3	125.8 (2)	H10A—C10—H10B	107.9
N1—C2—H2	117.1	C10—C11—H11A	109.5
C3—C2—H2	117.1	C10—C11—H11B	109.5
C2—C3—C4	119.4 (2)	H11A—C11—H11B	109.5
C2—C3—C9	114.1 (2)	C10—C11—H11C	109.5
C4—C3—C9	126.4 (2)	H11A—C11—H11C	109.5
O1—C4—C14	120.4 (2)	H11B—C11—H11C	109.5
O1—C4—C3	125.6 (2)	O3—C12—C13	107.4 (2)
C14—C4—C3	114.1 (2)	O3—C12—H12A	110.2
C6—C5—C14	122.7 (2)	C13—C12—H12A	110.2
C6—C5—H5	118.6	O3—C12—H12B	110.2
C14—C5—H5	118.6	C13—C12—H12B	110.2
C7—C6—C5	117.1 (2)	H12A—C12—H12B	108.5
C7—C6—H6	121.5	C12—C13—H13A	109.5
C5—C6—H6	121.5	C12—C13—H13B	109.5
F1—C7—C8	117.9 (2)	H13A—C13—H13B	109.5
F1—C7—C6	117.9 (2)	C12—C13—H13C	109.5
C8—C7—C6	124.2 (2)	H13A—C13—H13C	109.5
C7—C8—C15	118.0 (2)	H13B—C13—H13C	109.5
C7—C8—H8	121.0	C5—C14—C15	117.9 (2)
C15—C8—H8	121.0	C5—C14—C4	119.8 (2)
O2—C9—O3	122.4 (2)	C15—C14—C4	122.3 (2)
O2—C9—C3	123.5 (2)	N1—C15—C8	120.8 (2)
O3—C9—C3	114.1 (2)	N1—C15—C14	119.0 (2)
N1—C10—C11	111.8 (2)	C8—C15—C14	120.1 (2)
C15—N1—C2—C3	0.4 (4)	C2—N1—C10—C11	−98.0 (3)
C10—N1—C2—C3	179.7 (2)	C15—N1—C10—C11	81.3 (3)
N1—C2—C3—C4	1.4 (4)	C9—O3—C12—C13	−176.6 (2)
N1—C2—C3—C9	178.5 (2)	C6—C5—C14—C15	−1.4 (4)
C2—C3—C4—O1	179.0 (2)	C6—C5—C14—C4	179.3 (2)
C9—C3—C4—O1	2.4 (4)	O1—C4—C14—C5	−1.9 (4)
C2—C3—C4—C14	−1.0 (3)	C3—C4—C14—C5	178.1 (2)
C9—C3—C4—C14	−177.6 (2)	O1—C4—C14—C15	178.9 (2)
C14—C5—C6—C7	0.0 (4)	C3—C4—C14—C15	−1.2 (3)
C5—C6—C7—F1	−179.4 (2)	C2—N1—C15—C8	179.7 (2)
C5—C6—C7—C8	1.4 (4)	C10—N1—C15—C8	0.4 (3)
F1—C7—C8—C15	179.6 (2)	C2—N1—C15—C14	−2.5 (3)
C6—C7—C8—C15	−1.3 (4)	C10—N1—C15—C14	178.2 (2)
C12—O3—C9—O2	−1.6 (3)	C7—C8—C15—N1	177.6 (2)
C12—O3—C9—C3	177.7 (2)	C7—C8—C15—C14	−0.2 (4)
C2—C3—C9—O2	4.6 (3)	C5—C14—C15—N1	−176.3 (2)
C4—C3—C9—O2	−178.6 (2)	C4—C14—C15—N1	2.9 (3)
C2—C3—C9—O3	−174.7 (2)	C5—C14—C15—C8	1.5 (3)
C4—C3—C9—O3	2.1 (3)	C4—C14—C15—C8	−179.2 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C10—H10B···O2^i	0.99	2.36	3.336 (3)	169

Symmetry code: (i) −x, −y+1, −z.