2-(Phenyl­amino­methyl­idene)­cyclo­hexane-1,3-dione

In the title compound, C₁₃H₁₃NO₂, there is polarization of π-electron density from the amine N atom to the acceptor carbonyl groups: as a result, the mol­ecule exists predominantly in an azomethino-1,3-diketone tautomeric form. There is crystallographic evidence that the phenyl ring, although roughly coplanar with the rest of the mol­ecule, is deconjugated with the adjacent π system of the mol­ecule. The cyclo­hexane ring adopts an unsymmetrical half-chair conformation and converts between two inversion-related conformers. The mol­ecule is stabilized by an intramolecular hydrogen bond, while the intermolecular packing is dominated by a number of short C—H...O contacts.

S1. Comment

Recently, as a part of our ongoing study on the structure–activity relationships of the biologically active compounds, we focused our attention to the title compound, (I), following reports that such derivatives exhibit photobleaching activity as assayed in tobacco (Nicotiana tabacum) cultured cells (Wang et al., 1997). It was shown that the chlorophyll and carotenoid contents of the cells treated with the cyclic dione disapeared within 24 h under light but not dark condition. However, the rapid onset of photobleaching activity did not result from inhibition of protoporphyrinogen oxidase, indicating that some other mechanism of action might operate, for example, electron transport between the drug molecule and the photosynthetic pigments. Thus, to shed some light on the molecular mechanism of action, we initiated a study of the π-electronic structure of the dione in both ground and photo-activated states by a combination of theoretical and experimental techniques. In this communication, we report on the crystal structure of (I).

The molecular structure along with the atom-numbering scheme is shown in Fig. 1. As shown in the figure, atoms C5, C6 and O1 are disordered between two positions, denoted by unprimed (major site) and primed (minor site) labels, respectively. The disorder originates from two conformations of the cyclohexane ring, which are related by an inversion of the ring. A calculation of the least-squares planes has shown that the ring is puckered in such a manner that atoms C1, C2, C3 and C4 are coplanar to within experimental error [r.m.s.deviation = 0.002 (2) Å], while atoms C5 and C6 are unequally displaced from this plane on opposite sides, with out-of-plane displacements of −0.531 (7) and 0.227 (12) Å, respectively. The corresponding displacements of atoms C5' and C6' in the inverted conformation are 0.518 (9) and −0.264 (15) Å, respectively. Thus, the conformation of the two puckered rings can be described according to Duax et al. (1976) as intermediate between half-chair, with a local pseudo twofold axis along the mid-points of the C2—C3 and C5—C6 (or C5'—C6') bonds, and sofa, with a local pseudo mirror along the C2···C5 (or C2···C5') direction. The puckering parameters according to Cremer & Pople (1975) are Q = 0.499 (6) Å, θ = 50.8 (3)° and ϕ = −104.0 (7)° (calculated for the sequence C1/C2/C3/C4/C5/C6); the corresponding parameters for the inverted ring are Q = 0.525 (8) Å, θ = 128.7 (2)° and ϕ = 79.3 (7)° (sequence C1/C2/C3/C4/C5'/C6').

As noted above, the main purpose of this work was to establish the (π) electron ditribution within the π-electronic portion of the molecule. Firstly, as shown in Table 1, the the C7—N1—C8 valence angle is even larger than 120°, i.e. the amine N atom is sp²-hybridized, with the lone-pair electrons available for π-bonding. Secondly, as estimated from the bond-order–bond-length curves proposed by Burke-Laing & Laing (1976), the bond order of the N1—C7 single bond is even higher (ca 1.7) than that of the C2—C7 double bond (ca 1.5). Furthermore, the C1—C2 and C2—C3 bond distances are significantly shorter than the normal value of 1.487 Å reported for the C(sp²)-C(sp²) single bond (Shmueli et al., 1973). These findings, coupled with the lengthening of the two carbonyl bonds with respect to the range (1.20–1.22 Å) normally accepted for a C=O double bond, imply that there is an extensive π-electron delocalization from atom N1 to the carbonyl groups, leading to the development of negative charges on atoms O1 and O3. A similar pattern of bond lengths and angles has also been observed in other compounds containing the aminomethylene-1,3- diketone substructure (e.g. DeGarcia-Martin et al., 1987), as revealed by a search of the Cambridge Structural Database (Allen et al., 1983). Thus, the electronic structure of the title and related compounds can better be described by the azomethino-1,3-diketone rather than the aminomethylene tautomerism. That the lone-pair electrons on atom N1 are delocalized through conjugation with the methylene-1,3-diketone moiety rather than the adjacent phenyl ring is also seen in the N1—C8 bond distance [1.421 (2) Å], which is not significantly different from the value [1.425 (3) Å] reported for a pure N(sp²)-C(sp²) single bond (Adler at al., 1976). Even though the phenyl ring is deconjugated with the azomethino-1,3-diketone moiety, both fragments are roughly coplanar to one another; the C9—C8—N1—C7 torsion angle is −16.2 (4)°.

The charge distribution in the molecule can also be guessed from the formation of an intramolecular hydrogen bond between the N1—H1 donor and O2 acceptor (Table 2). Besides this hydrogen bond, there are several short intermolecular C—H···O contacts, which, on the basisi of their H···O distances, can be regarded as weak hydrogen-bonding interactions (Table 2; Taylor & Kennard, 1982).

S2. Experimental

The title compound, (I), was synthesized by a three-component reaction of equimolar amounts of aniline, triethyl orthoformate and cyclohexane- 1,3-dione in ethanol under reflux according to Wolfbeis & Ziegler (1976), as described in detail by Marko et al. (2004). The product (m.p. 434 K) was recrystallized from ethanol to give single crystals suitable for X-ray analysis.

S3. Refinement

The disorder of the cyclohexane ring was modelled by resolving the positions of atoms C5, C6 and O1 into two components (C5/C5', C6/C6', and O1/O1') and using a total of 35 restraints on corresponding bond distances and anisotropic displacement parameters [a combination of DFIX and SIMU options in SHELXL97 (Sheldrick, 1997)]. The refined occupancies for the unprimed (major) and primed (minor) sites were 58.2 (6) and 41.8 (6)%, respectively. H atoms were refined with fixed geometry, riding on their carrier atoms, with U_iso(H) set to 1.2U_eq of the parent atom.

Computing details

Data collection: XSCANS (Siemens, 1991); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLUTON (Spek, 1992); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. Displacement ellipsoid plot of (I), with the labelling scheme for the non-H atoms, which are drawn as 35% probability ellipsoids.

2-(phenylaminomethylidene)cyclohexane-1,3-dione

Crystal data

C13H13NO2	F(000) = 228
Mr = 215.24	Dx = 1.298 Mg m^−3
Triclinic, P1	Melting point: 434 K
a = 5.678 (2) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.424 (3) Å	Cell parameters from 20 reflections
c = 12.404 (5) Å	θ = 7–20°
α = 100.17 (3)°	µ = 0.09 mm^−1
β = 93.85 (3)°	T = 293 K
γ = 107.96 (4)°	Prism, colourless
V = 550.8 (4) Å^3	0.30 × 0.20 × 0.15 mm
Z = 2

Data collection

Siemens P4 diffractometer	Rint = 0.037
Radiation source: fine-focus sealed tube	θmax = 30.0°, θmin = 1.7°
Graphite monochromator	h = −1→7
ω/2θ scans	k = −11→11
4085 measured reflections	l = −17→17
3188 independent reflections	3 standard reflections every 97 reflections
1818 reflections with I > 2σ(I)	intensity decay: 2%

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)] = 0.065	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.199	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.0636P)^2 + 0.2041P] where P = (Fo^2 + 2Fc^2)/3
3188 reflections	(Δ/σ)max = 0.001
178 parameters	Δρmax = 0.21 e Å^−3
35 restraints	Δρmin = −0.17 e Å^−3

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
C1	0.4552 (4)	0.6907 (4)	0.56698 (19)	0.0723 (8)
C2	0.4521 (4)	0.6600 (3)	0.67881 (16)	0.0500 (5)
C3	0.6640 (4)	0.7432 (3)	0.76235 (17)	0.0529 (5)
C4	0.8945 (4)	0.8666 (3)	0.73295 (19)	0.0659 (7)
H4A	0.8927	0.9820	0.7577	0.079*	0.582 (6)
H4B	1.0412	0.8566	0.7722	0.079*	0.582 (6)
H4C	1.0171	0.8094	0.7185	0.079*	0.418 (6)
H4D	0.9666	0.9613	0.7953	0.079*	0.418 (6)
C5	0.9149 (7)	0.8370 (7)	0.6109 (3)	0.0702 (14)	0.582 (6)
H5A	0.9426	0.7288	0.5874	0.084*	0.582 (6)
H5B	1.0558	0.9267	0.5961	0.084*	0.582 (6)
C6	0.6762 (11)	0.8356 (12)	0.5469 (6)	0.076 (3)	0.582 (6)
H6A	0.6917	0.8204	0.4686	0.091*	0.582 (6)
H6B	0.6486	0.9438	0.5704	0.091*	0.582 (6)
C5'	0.8372 (12)	0.9349 (7)	0.6320 (4)	0.0721 (18)	0.418 (6)
H5'1	0.7313	1.0045	0.6491	0.086*	0.418 (6)
H5'2	0.9913	1.0061	0.6124	0.086*	0.418 (6)
C6'	0.7070 (16)	0.7887 (16)	0.5356 (6)	0.074 (3)	0.418 (6)
H6'1	0.8070	0.7143	0.5208	0.089*	0.418 (6)
H6'2	0.6824	0.8321	0.4698	0.089*	0.418 (6)
C7	0.2363 (4)	0.5429 (3)	0.70021 (16)	0.0501 (5)
H7	0.1048	0.4931	0.6432	0.060*
C8	−0.0070 (4)	0.3819 (2)	0.82497 (15)	0.0473 (5)
C9	−0.2384 (4)	0.3293 (3)	0.76301 (18)	0.0583 (6)
H9	−0.2597	0.3667	0.6980	0.070*
C10	−0.4390 (4)	0.2194 (3)	0.7994 (2)	0.0696 (7)
H10	−0.5958	0.1827	0.7579	0.084*
C11	−0.4101 (5)	0.1638 (3)	0.8957 (2)	0.0701 (7)
H11	−0.5463	0.0914	0.9198	0.084*
C12	−0.1784 (5)	0.2165 (3)	0.95562 (19)	0.0661 (7)
H12	−0.1575	0.1780	1.0203	0.079*
C13	0.0245 (4)	0.3256 (3)	0.92158 (17)	0.0568 (6)
H13	0.1811	0.3611	0.9631	0.068*
N1	0.2070 (3)	0.4985 (2)	0.79583 (13)	0.0528 (5)
H1	0.3320	0.5454	0.8469	0.063*
O1	0.2915 (10)	0.5952 (8)	0.4895 (5)	0.0732 (17)	0.582 (6)
O1'	0.2559 (14)	0.6629 (13)	0.5050 (8)	0.084 (3)	0.418 (6)
O2	0.6626 (3)	0.7175 (2)	0.85724 (13)	0.0793 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0476 (12)	0.103 (2)	0.0540 (13)	−0.0041 (12)	0.0032 (10)	0.0375 (13)
C2	0.0424 (10)	0.0585 (12)	0.0431 (10)	0.0030 (9)	0.0057 (8)	0.0193 (9)
C3	0.0490 (11)	0.0537 (12)	0.0494 (11)	0.0050 (9)	0.0038 (9)	0.0157 (9)
C4	0.0491 (12)	0.0681 (15)	0.0622 (13)	−0.0064 (11)	0.0021 (10)	0.0140 (11)
C5	0.054 (2)	0.072 (3)	0.069 (2)	−0.007 (2)	0.0148 (18)	0.022 (2)
C6	0.060 (3)	0.100 (5)	0.064 (3)	0.004 (3)	0.014 (2)	0.043 (3)
C5'	0.067 (3)	0.059 (3)	0.071 (3)	−0.013 (3)	0.020 (3)	0.020 (3)
C6'	0.058 (3)	0.096 (6)	0.060 (3)	−0.001 (4)	0.018 (3)	0.036 (3)
C7	0.0432 (10)	0.0597 (12)	0.0420 (10)	0.0052 (9)	0.0044 (8)	0.0181 (9)
C8	0.0461 (10)	0.0485 (11)	0.0434 (10)	0.0054 (8)	0.0106 (8)	0.0163 (8)
C9	0.0505 (12)	0.0721 (14)	0.0523 (11)	0.0098 (10)	0.0086 (9)	0.0306 (11)
C10	0.0459 (12)	0.0837 (17)	0.0737 (15)	0.0039 (11)	0.0071 (11)	0.0336 (13)
C11	0.0621 (14)	0.0747 (16)	0.0695 (15)	0.0028 (12)	0.0219 (12)	0.0350 (13)
C12	0.0761 (16)	0.0682 (14)	0.0515 (12)	0.0093 (12)	0.0136 (11)	0.0296 (11)
C13	0.0577 (12)	0.0626 (13)	0.0441 (10)	0.0063 (10)	0.0038 (9)	0.0202 (9)
N1	0.0447 (9)	0.0604 (11)	0.0440 (9)	−0.0004 (8)	0.0020 (7)	0.0198 (8)
O1	0.0527 (19)	0.106 (4)	0.048 (2)	0.002 (2)	0.0017 (15)	0.029 (3)
O1'	0.061 (3)	0.120 (6)	0.055 (3)	−0.003 (3)	−0.006 (3)	0.043 (4)
O2	0.0673 (11)	0.0951 (13)	0.0519 (9)	−0.0111 (9)	−0.0091 (7)	0.0287 (9)

Geometric parameters (Å, º)

C1—O1	1.257 (5)	C5'—C6'	1.515 (7)
C1—O1'	1.258 (7)	C5'—H5'1	0.9700
C1—C2	1.455 (3)	C5'—H5'2	0.9700
C1—C6	1.527 (6)	C6'—H6'1	0.9700
C1—C6'	1.529 (8)	C6'—H6'2	0.9700
C2—C7	1.391 (3)	C7—N1	1.311 (2)
C2—C3	1.438 (3)	C7—H7	0.9300
C3—O2	1.234 (2)	C8—C9	1.378 (3)
C3—C4	1.512 (3)	C8—C13	1.385 (3)
C4—C5	1.508 (4)	C8—N1	1.421 (2)
C4—C5'	1.523 (5)	C9—C10	1.387 (3)
C4—H4A	0.9700	C9—H9	0.9300
C4—H4B	0.9700	C10—C11	1.376 (3)
C4—H4C	0.9700	C10—H10	0.9300
C4—H4D	0.9700	C11—C12	1.368 (3)
C5—C6	1.520 (6)	C11—H11	0.9300
C5—H5A	0.9700	C12—C13	1.378 (3)
C5—H5B	0.9700	C12—H12	0.9300
C6—H6A	0.9700	C13—H13	0.9300
C6—H6B	0.9700	N1—H1	0.8600
O1—C1—C2	121.2 (4)	C6'—C5'—C4	110.2 (6)
O1'—C1—C2	121.3 (5)	C6'—C5'—H5'1	109.6
O1—C1—C6	121.8 (4)	C4—C5'—H5'1	109.6
O1'—C1—C6	114.5 (6)	C6'—C5'—H5'2	109.6
C2—C1—C6	116.9 (3)	C4—C5'—H5'2	109.6
O1'—C1—C6'	120.6 (6)	H5'1—C5'—H5'2	108.1
C2—C1—C6'	117.1 (4)	C5'—C6'—C1	107.4 (6)
C7—C2—C3	121.58 (17)	C5'—C6'—H6'1	110.2
C7—C2—C1	116.70 (18)	C1—C6'—H6'1	110.2
C3—C2—C1	121.69 (18)	C5'—C6'—H6'2	110.2
O2—C3—C2	121.99 (19)	C1—C6'—H6'2	110.2
O2—C3—C4	119.13 (19)	H6'1—C6'—H6'2	108.5
C2—C3—C4	118.88 (18)	N1—C7—C2	124.01 (18)
C5—C4—C3	113.4 (2)	N1—C7—H7	118.0
C3—C4—C5'	112.2 (3)	C2—C7—H7	118.0
C5—C4—H4A	108.9	C9—C8—C13	120.53 (18)
C3—C4—H4A	108.9	C9—C8—N1	122.59 (17)
C5—C4—H4B	108.9	C13—C8—N1	116.86 (18)
C3—C4—H4B	108.9	C8—C9—C10	118.8 (2)
H4A—C4—H4B	107.7	C8—C9—H9	120.6
C3—C4—H4C	109.2	C10—C9—H9	120.6
C5'—C4—H4C	109.2	C11—C10—C9	121.1 (2)
C3—C4—H4D	109.2	C11—C10—H10	119.4
C5'—C4—H4D	109.2	C9—C10—H10	119.4
H4C—C4—H4D	107.9	C12—C11—C10	119.2 (2)
C4—C5—C6	109.8 (5)	C12—C11—H11	120.4
C4—C5—H5A	109.7	C10—C11—H11	120.4
C6—C5—H5A	109.7	C11—C12—C13	121.0 (2)
C4—C5—H5B	109.7	C11—C12—H12	119.5
C6—C5—H5B	109.7	C13—C12—H12	119.5
H5A—C5—H5B	108.2	C12—C13—C8	119.4 (2)
C5—C6—C1	109.8 (5)	C12—C13—H13	120.3
C5—C6—H6A	109.7	C8—C13—H13	120.3
C1—C6—H6A	109.7	C7—N1—C8	127.64 (17)
C5—C6—H6B	109.7	C7—N1—H1	116.2
C1—C6—H6B	109.7	C8—N1—H1	116.2
H6A—C6—H6B	108.2
O1—C1—C2—C7	−12.2 (5)	O1'—C1—C6—C5	−169.9 (8)
O1'—C1—C2—C7	23.8 (7)	C2—C1—C6—C5	39.6 (9)
C6—C1—C2—C7	172.1 (5)	C4—C5'—C6'—C1	64.4 (10)
C6'—C1—C2—C7	−167.5 (7)	O1'—C1—C6'—C5'	126.3 (9)
O1—C1—C2—C3	166.2 (4)	C2—C1—C6'—C5'	−42.6 (11)
O1'—C1—C2—C3	−157.8 (6)	C3—C2—C7—N1	−0.1 (4)
C6—C1—C2—C3	−9.5 (6)	C1—C2—C7—N1	178.3 (2)
C6'—C1—C2—C3	10.9 (7)	C13—C8—C9—C10	0.2 (4)
C7—C2—C3—O2	−1.6 (4)	N1—C8—C9—C10	−178.0 (2)
C1—C2—C3—O2	−179.9 (3)	C8—C9—C10—C11	0.3 (4)
C7—C2—C3—C4	178.8 (2)	C9—C10—C11—C12	−0.8 (4)
C1—C2—C3—C4	0.5 (4)	C10—C11—C12—C13	0.8 (4)
O2—C3—C4—C5	157.5 (3)	C11—C12—C13—C8	−0.3 (4)
C2—C3—C4—C5	−22.9 (4)	C9—C8—C13—C12	−0.2 (4)
O2—C3—C4—C5'	−158.3 (4)	N1—C8—C13—C12	178.1 (2)
C2—C3—C4—C5'	21.3 (4)	C2—C7—N1—C8	179.2 (2)
C3—C4—C5—C6	53.4 (6)	C9—C8—N1—C7	−16.2 (4)
C4—C5—C6—C1	−61.3 (8)	C13—C8—N1—C7	165.6 (2)
O1—C1—C6—C5	−136.0 (6)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O2	0.86	1.97	2.641 (3)	134
C7—H7···O1	0.93	2.41	2.749 (8)	102
C7—H7···O1′	0.93	2.46	2.780 (10)	100
C5—H5A···O1′^i	0.97	2.27	3.006 (11)	132
C6′—H6′1···O1^ii	0.97	2.47	3.196 (16)	131
C7—H7···O1^iii	0.93	2.52	3.436 (6)	167
C7—H7···O1′^iii	0.93	2.51	3.434 (8)	175
C9—H9···O1^iii	0.93	2.41	3.317 (8)	166
C9—H9···O1′^iii	0.93	2.49	3.332 (11)	151

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