(RS)-6-Ethyl 2-carboxy-1,2,3,4-tetra­hydro­azulene-6-carboxylate

The title compound, C₁₄H₁₆O₄, was obtained during the synthesis of 2,6-disubstituted azulene derivatives. In the partially reduced azulene skeleton, the absence of a H atom at the ester substitutent position of the seven-membered ring, as well as lengthened double bonds, indicate a conjugative stabilized system with two overlaid tautomers.

S1. Comment

The synthesis of the azulene skeleton has received a good deal of attention over the past few decades (Nefedov, 1973). Luhowy & Keehn (1977) synthesized azulenophane in order to study the charge-transfer interaction, and later Zindel et al. (1996) reported the synthesis of 2,6-substituted push–pull azulene. Recently, Mori et al. (2003) synthesized the 1,3-diazaazulene skeleton and described the mesomorphic properties of its various derivatives. Our primary interest was in synthesizing the perhydroazulene skeleton and in studying the various new materials based on this system. During our work on synthesizing the core skeleton, we employed various synthetic strategies to obtain 2,6-disubstituted perhydroazulene derivatives. We employed a slightly modified synthetic strategy as reported by Zindel et al. (1996) to obtain the title compound, (I), in order to investigate its structural and stereochemical features for input into further studies.

We prepared compound (I) by adding formic acid to its diester and stirring the mixture at room temperature for 15 h. Compound (I), obtained first as a light-coloured solid, was recrystallized from hexane to obtain colourless irregular crystals. The structure analysis revealed an azulene framework which was partially hydrogenated at the pentacyclic subunit. The geometric parameters of the seven-membered ring further revealed the presence of disorder between two tautomers. Their structures, shown as two overlaid molecules in Fig. 1, were refined with a disorder model involving atoms C5, C6, C8 and C9, and C5A, C6A, C8A and C9A [with 53(s.u.?):47(s.u.?)% refined occupancy, minor sites linked with open bonds in Fig. 1].

We did not find an H atom at C7 in the difference Fourier map, but did observe two H atoms at each of C5A and C9, as well as only one H atom at C5 and at C9A. Due to the disorder of the seven-membered ring, these H atoms were included in the refinement in calculated positions which were re-idealized after every refinement run.

The disorder of the two tautomers is evident in an analysis of selected geometric parameters, listed in Table 1. The average value of the distances C5—C6, C5A—C6A, C8—C9 and C8A—C9A [1.392 (5) Å] is longer by about 0.07 Å than the average for chemically equivalent olefinic bonds (Allen et al., 1995). For comparison, the C4═C10 double bond has a length of 1.337 (2) Å, which is in the expected range. Furthermore, the average value of 1.412 (4) Å for the bonds C6—C7, C6A—C7, C7—C8 and C7—C8A is about 0.05 Å shorter than comparable single-bond lengths tabulated by Allen et al. (1995). In addition, a very `flat' conformation at the top atom, C7, was detected (selected torsion angles are given in Table 1), thus indicating conjugative interaction of the π system with the ester substituent.

In the extended structure of (I), stabilizing hydrogen bonds were found [O4—H4O···O3ⁱ 1.74 (3) Å, O4···O3ⁱ 2.635 (3) Å, O4—H4O···O3ⁱ 176.9 (2)°; symmetry code: (i) −x,-y + 1,-z − 1], mediating aggregation of the azulene derivatives into dimeric pairs across a crystallographic centre of symmetry.

S2. Experimental

A solution of the diester of (I) (1.0 mmol) in formic acid (50 ml) was stirred at room temperature for 15 h. The solvent was removed under reduced pressure and the crude product, originally obtained as a slightly coloured solid, was recrystallized from hexane, to give a 95% yield of (I) as colourless irregular crystals.

S3. Refinement

Each of the atoms C5, C6, C8 and C9 was refined at two positions with 53(s.u.?):47(s.u.?)% refined occupancy. The displacement parameters of their disordered positions were only refined isotropically because some of the disorder sites are rather close together. Their positions were not initially located as separate sites in a difference map, but were rather generated by splitting the atomic positions with large anisotropic displacement parameters, as per a suggestion by SHELXTL (Bruker, 2001). The H atom of the carboxylic acid group was located in a difference Fourier map and refined isotropically. All other H atoms were found in a difference map but then placed in calculated positions and included in the refinement with U_iso(H) = 1.2U_eq(C) [1.5U_eq(C_methyl)], and their positions were re-idealized before each refinement cycle. The C—H bond distances of 0.95–0.99 Å were established according to criteria described in the SHELXTL manual (Bruker, 2001), with an increase of 0.01 or 0.02 Å for low-temperature refinement.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SMART; data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXTL (Bruker, 2001); program(s) used to refine structure: SHELXTL; molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXTL.

Figures

Fig. 1. The molecular structure of (I), showing the atom-labelling scheme. The unshaded bonds indicate the minor sites of two overlaid tautomeric molecules. Displacement ellipsoids are drawn at the ??% probability level [Please complete] and H atoms are shown as small spheres of arbitrary radii.

(RS)-1,2,3,4-tetrahydroazulene-2,6-dicarboxylic acid 6-ethyl ester

Crystal data

C14H16O4	F(000) = 528
Mr = 248.27	Dx = 1.375 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 4.1002 (4) Å	Cell parameters from 4258 reflections
b = 36.118 (4) Å	θ = 2.3–28.3°
c = 8.3164 (8) Å	µ = 0.10 mm^−1
β = 103.157 (2)°	T = 100 K
V = 1199.2 (2) Å^3	Irregular, colourless
Z = 4	0.18 × 0.15 × 0.14 mm

Data collection

Bruker SMART APEX CCD area-detector diffractometer	2953 independent reflections
Radiation source: fine-focus sealed tube	2710 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
ϕ and ω scans	θmax = 28.3°, θmin = 2.3°
Absorption correction: multi-scan (SADABS; Sheldrick, 2003)	h = −5→5
Tmin = 0.982, Tmax = 0.986	k = −48→48
11969 measured reflections	l = −11→11

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)] = 0.061	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.136	H atoms treated by a mixture of independent and constrained refinement
S = 1.17	w = 1/[σ^2(Fo^2) + (0.0468P)^2 + 0.8094P] where P = (Fo^2 + 2Fc^2)/3
2953 reflections	(Δ/σ)max = 0.001
165 parameters	Δρmax = 0.37 e Å^−3
0 restraints	Δρmin = −0.22 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. The highest peak is located 0.73 Å from atom C2 and the deepest hole 0.28 Å from atom H9A.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1	0.4078 (4)	0.66224 (4)	0.55926 (16)	0.0350 (4)
O2	0.1767 (3)	0.70239 (3)	0.36272 (14)	0.0231 (3)
O3	0.0919 (3)	0.54329 (3)	−0.45087 (15)	0.0240 (3)
O4	−0.1732 (4)	0.50371 (3)	−0.31913 (16)	0.0256 (3)
C1	0.0660 (4)	0.60205 (4)	−0.22418 (19)	0.0186 (3)
H1A	0.1991	0.6025	−0.3097	0.022*
H1B	−0.1249	0.6194	−0.2564	0.022*
C2	−0.0563 (4)	0.56264 (4)	−0.20010 (19)	0.0169 (3)
H2	−0.2947	0.5642	−0.1906	0.020*
C3	0.1595 (4)	0.54850 (4)	−0.0321 (2)	0.0197 (3)
H3A	0.0168	0.5375	0.0367	0.024*
H3B	0.3250	0.5299	−0.0496	0.024*
C4	0.3306 (4)	0.58299 (4)	0.04658 (19)	0.0173 (3)
C5	0.5131 (14)	0.58290 (11)	0.2175 (5)	0.0182 (11)*	0.531 (11)
H5	0.6009	0.5600	0.2641	0.022*	0.531 (11)
C6	0.5676 (14)	0.61343 (10)	0.3160 (4)	0.0187 (9)*	0.531 (11)
H6	0.7242	0.6110	0.4190	0.022*	0.531 (11)
C5A	0.5828 (16)	0.58411 (13)	0.2095 (6)	0.0189 (13)*	0.469 (11)
H5A1	0.6107	0.5590	0.2586	0.023*	0.469 (11)
H5A2	0.8025	0.5924	0.1921	0.023*	0.469 (11)
C6A	0.4663 (15)	0.61007 (12)	0.3233 (5)	0.0191 (10)*	0.469 (11)
H6A	0.4264	0.6013	0.4248	0.023*	0.469 (11)
C7	0.4146 (5)	0.64811 (5)	0.2800 (2)	0.0239 (4)
C8	0.4636 (14)	0.66424 (10)	0.1358 (5)	0.0174 (9)*	0.531 (11)
H8	0.4971	0.6903	0.1406	0.021*	0.531 (11)
C9	0.4708 (15)	0.64762 (11)	−0.0163 (5)	0.0193 (10)*	0.531 (11)
H9A	0.7077	0.6429	−0.0184	0.023*	0.531 (11)
H9B	0.3815	0.6656	−0.1051	0.023*	0.531 (11)
C8A	0.3647 (16)	0.66551 (12)	0.1212 (5)	0.0178 (10)*	0.469 (11)
H8A	0.3138	0.6912	0.1167	0.021*	0.469 (11)
C9A	0.3820 (15)	0.64958 (12)	−0.0189 (5)	0.0170 (11)*	0.469 (11)
H9C	0.4652	0.6635	−0.0979	0.020*	0.469 (11)
C10	0.2797 (4)	0.61184 (4)	−0.05693 (19)	0.0181 (3)
C11	0.3359 (4)	0.67066 (5)	0.4162 (2)	0.0189 (3)
C12	0.0872 (5)	0.72496 (5)	0.4898 (2)	0.0239 (4)
H12A	0.2914	0.7333	0.5696	0.029*
H12B	−0.0518	0.7105	0.5502	0.029*
C13	−0.1056 (6)	0.75768 (5)	0.4068 (3)	0.0330 (4)
H13A	0.0362	0.7721	0.3498	0.049*
H13B	−0.1742	0.7732	0.4899	0.049*
H13C	−0.3047	0.7491	0.3266	0.049*
C14	−0.0382 (4)	0.53615 (4)	−0.33691 (19)	0.0168 (3)
H4O	−0.148 (6)	0.4884 (7)	−0.400 (3)	0.044 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0581 (10)	0.0273 (7)	0.0167 (6)	0.0088 (6)	0.0022 (6)	−0.0013 (5)
O2	0.0332 (7)	0.0204 (6)	0.0147 (6)	0.0036 (5)	0.0034 (5)	−0.0034 (4)
O3	0.0376 (7)	0.0178 (6)	0.0192 (6)	−0.0039 (5)	0.0120 (5)	−0.0040 (5)
O4	0.0411 (8)	0.0176 (6)	0.0218 (6)	−0.0073 (5)	0.0147 (5)	−0.0065 (5)
C1	0.0267 (8)	0.0138 (7)	0.0140 (7)	0.0021 (6)	0.0021 (6)	−0.0008 (6)
C2	0.0201 (7)	0.0155 (7)	0.0151 (7)	0.0012 (6)	0.0039 (6)	−0.0020 (6)
C3	0.0270 (8)	0.0166 (7)	0.0150 (7)	−0.0003 (6)	0.0038 (6)	0.0007 (6)
C4	0.0201 (7)	0.0165 (7)	0.0155 (7)	0.0026 (6)	0.0042 (6)	−0.0013 (6)
C7	0.0280 (9)	0.0232 (8)	0.0204 (8)	0.0010 (7)	0.0055 (7)	−0.0054 (6)
C10	0.0225 (8)	0.0166 (7)	0.0152 (7)	0.0011 (6)	0.0045 (6)	−0.0027 (6)
C11	0.0196 (8)	0.0196 (8)	0.0169 (7)	−0.0040 (6)	0.0030 (6)	−0.0019 (6)
C12	0.0296 (9)	0.0238 (8)	0.0186 (8)	0.0009 (7)	0.0062 (7)	−0.0071 (6)
C13	0.0396 (11)	0.0259 (9)	0.0337 (10)	0.0072 (8)	0.0086 (8)	−0.0043 (8)
C14	0.0180 (7)	0.0161 (7)	0.0148 (7)	0.0019 (6)	0.0005 (6)	−0.0008 (6)

Geometric parameters (Å, º)

O1—C11	1.198 (2)	C5A—C6A	1.486 (7)
O2—C11	1.344 (2)	C5A—H5A1	0.9900
O2—C12	1.447 (2)	C5A—H5A2	0.9900
O3—C14	1.216 (2)	C6A—C7	1.424 (4)
O4—C14	1.318 (2)	C6A—H6A	0.9500
O4—H4O	0.89 (3)	C7—C8	1.388 (4)
C1—C10	1.508 (2)	C7—C8A	1.435 (5)
C1—C2	1.537 (2)	C7—C11	1.488 (2)
C1—H1A	0.9900	C8—C9	1.406 (5)
C1—H1B	0.9900	C8—H8	0.9500
C2—C14	1.501 (2)	C9—C10	1.509 (4)
C2—C3	1.560 (2)	C9—H9A	0.9900
C2—H2	1.0000	C9—H9B	0.9900
C3—C4	1.504 (2)	C8A—C9A	1.315 (6)
C3—H3A	0.9900	C8A—H8A	0.9500
C3—H3B	0.9900	C9A—C10	1.440 (4)
C4—C10	1.337 (2)	C9A—H9C	0.9500
C4—C5	1.448 (4)	C12—C13	1.500 (3)
C4—C5A	1.506 (5)	C12—H12A	0.9900
C5—C6	1.362 (5)	C12—H12B	0.9900
C5—H5	0.9500	C13—H13A	0.9800
C6—C7	1.403 (4)	C13—H13B	0.9800
C6—H6	0.9500	C13—H13C	0.9800
C11—O2—C12	115.08 (13)	C8—C7—C11	121.8 (2)
C14—O4—H4O	110.1 (17)	C6—C7—C11	119.4 (2)
C10—C1—C2	103.61 (13)	C6A—C7—C11	112.4 (2)
C10—C1—H1A	111.0	C8A—C7—C11	116.8 (2)
C2—C1—H1A	111.0	C7—C8—C9	129.4 (3)
C10—C1—H1B	111.0	C7—C8—H8	115.3
C2—C1—H1B	111.0	C9—C8—H8	115.3
H1A—C1—H1B	109.0	C8—C9—C10	116.8 (3)
C14—C2—C1	114.68 (13)	C8—C9—H9A	108.1
C14—C2—C3	110.75 (13)	C10—C9—H9A	108.1
C1—C2—C3	106.53 (13)	C8—C9—H9B	108.1
C14—C2—H2	108.2	C10—C9—H9B	108.1
C1—C2—H2	108.2	H9A—C9—H9B	107.3
C3—C2—H2	108.2	C9A—C8A—C7	126.8 (4)
C4—C3—C2	103.41 (13)	C9A—C8A—H8A	116.6
C4—C3—H3A	111.1	C7—C8A—H8A	116.6
C2—C3—H3A	111.1	C8A—C9A—C10	122.5 (4)
C4—C3—H3B	111.1	C8A—C9A—H9C	118.8
C2—C3—H3B	111.1	C10—C9A—H9C	118.8
H3A—C3—H3B	109.0	C4—C10—C9A	126.9 (2)
C10—C4—C5	127.1 (2)	C4—C10—C1	112.57 (14)
C10—C4—C3	112.09 (14)	C9A—C10—C1	120.3 (2)
C5—C4—C3	120.7 (2)	C4—C10—C9	121.7 (2)
C10—C4—C5A	122.2 (2)	C1—C10—C9	124.81 (19)
C3—C4—C5A	124.8 (2)	O1—C11—O2	122.42 (15)
C6—C5—C4	124.5 (3)	O1—C11—C7	124.73 (16)
C6—C5—H5	117.7	O2—C11—C7	112.84 (14)
C4—C5—H5	117.7	O2—C12—C13	107.62 (14)
C5—C6—C7	126.2 (4)	O2—C12—H12A	110.2
C5—C6—H6	116.9	C13—C12—H12A	110.2
C7—C6—H6	116.9	O2—C12—H12B	110.2
C6A—C5A—C4	109.6 (4)	C13—C12—H12B	110.2
C6A—C5A—H5A1	109.8	H12A—C12—H12B	108.5
C4—C5A—H5A1	109.8	C12—C13—H13A	109.5
C6A—C5A—H5A2	109.8	C12—C13—H13B	109.5
C4—C5A—H5A2	109.8	H13A—C13—H13B	109.5
H5A1—C5A—H5A2	108.2	C12—C13—H13C	109.5
C7—C6A—C5A	119.9 (4)	H13A—C13—H13C	109.5
C7—C6A—H6A	120.1	H13B—C13—H13C	109.5
C5A—C6A—H6A	120.1	O3—C14—O4	123.06 (14)
C8—C7—C6	114.4 (3)	O3—C14—C2	124.16 (15)
C8—C7—C6A	125.5 (3)	O4—C14—C2	112.77 (14)
C6A—C7—C8A	129.6 (3)
C10—C1—C2—C14	−135.68 (14)	C3—C4—C10—C1	−0.1 (2)
C10—C1—C2—C3	−12.79 (16)	C5A—C4—C10—C1	169.2 (3)
C14—C2—C3—C4	138.06 (14)	C5—C4—C10—C9	12.7 (5)
C1—C2—C3—C4	12.74 (17)	C3—C4—C10—C9	−169.8 (3)
C2—C3—C4—C10	−8.04 (18)	C8A—C9A—C10—C4	−39.6 (6)
C2—C3—C4—C5	169.7 (3)	C8A—C9A—C10—C1	134.6 (4)
C2—C3—C4—C5A	−177.0 (3)	C2—C1—C10—C4	8.39 (19)
C10—C4—C5—C6	22.9 (8)	C2—C1—C10—C9A	−166.6 (3)
C3—C4—C5—C6	−154.5 (5)	C2—C1—C10—C9	177.7 (3)
C4—C5—C6—C7	11.2 (10)	C8—C9—C10—C4	−53.5 (5)
C10—C4—C5A—C6A	69.0 (6)	C8—C9—C10—C1	138.1 (3)
C3—C4—C5A—C6A	−123.1 (4)	C12—O2—C11—O1	−2.2 (2)
C4—C5A—C6A—C7	−60.6 (7)	C12—O2—C11—C7	178.59 (14)
C5—C6—C7—C8	−58.1 (8)	C8—C7—C11—O1	−151.3 (3)
C5—C6—C7—C11	145.1 (5)	C6—C7—C11—O1	3.7 (4)
C5A—C6A—C7—C8A	19.8 (10)	C6A—C7—C11—O1	22.5 (4)
C5A—C6A—C7—C11	−173.5 (5)	C8A—C7—C11—O1	−169.0 (3)
C6—C7—C8—C9	38.4 (7)	C8—C7—C11—O2	27.8 (3)
C11—C7—C8—C9	−165.5 (4)	C6—C7—C11—O2	−177.2 (3)
C7—C8—C9—C10	25.7 (6)	C6A—C7—C11—O2	−158.3 (3)
C6A—C7—C8A—C9A	−5.6 (9)	C8A—C7—C11—O2	10.2 (3)
C11—C7—C8A—C9A	−171.8 (4)	C11—O2—C12—C13	−176.49 (15)
C7—C8A—C9A—C10	34.0 (8)	C1—C2—C14—O3	7.1 (2)
C3—C4—C10—C9A	174.4 (3)	C3—C2—C14—O3	−113.49 (18)
C5A—C4—C10—C9A	−16.3 (5)	C1—C2—C14—O4	−174.21 (14)
C5—C4—C10—C1	−177.7 (3)	C3—C2—C14—O4	65.21 (18)