Methyl 3-(Propan-2-ylidene)-3a,9a-dihydro-3H-cyclopenta[a]azulene-9-carboxylate

Abstract

Methyl 3-(propan-2-ylidene)-3a,9a-dihydro-3H-cyclopenta[a]azulene-9-carboxylate (2) was synthesized in moderate yield via an [8 + 2] cycloaddition of methyl 2-oxo-2H-cyclohepta[b]furan-3-carboxylate (1) with 6,6-dimethylfulvene. The resulting compound was characterized by ¹H and ¹³C NMR spectroscopy, high-resolution mass spectrometry, and single-crystal X-ray diffraction analysis.

1. Introduction

Nonbenzenoid π-electron systems are important molecular platforms for examining how aromaticity, bond localization, charge polarization, and orbital interactions govern structure and optical properties. Among them, azulene is a representative nonalternant aromatic hydrocarbon whose fused five- and seven-membered-ring framework gives rise to electronic and spectroscopic properties that differ markedly from those of benzenoid hydrocarbons [1,2]. However, azulene-related molecules cannot always be described simply as fully delocalized 10π aromatic systems. In some cases, their structures are better interpreted in terms of localized fulvenoid- or heptafulvene-type electronic structures. Because aromaticity is a multidimensional concept, structural, magnetic, spectroscopic, and computational criteria must be considered together rather than relying on a single descriptor [3,4].

Cycloaddition reactions of 2H-cyclohepta[b]furan-2-one derivatives provide useful access to structurally unusual azulenes and their analogues [5,6]. In particular, the reactions with 6,6-dimethylfulvene are known to show condition-dependent selectivity: lower-boiling solvents can favor [4 + 2] cycloadducts, whereas refluxing xylene promotes formation of the corresponding [8 + 2] cycloadduct [7]. This divergence suggests competition between kinetically and thermodynamically controlled pathways, but the relationship between the cycloaddition mode, bond localization, and optical behavior of the resulting products has not been fully clarified. In addition, compact nonbenzenoid frameworks may permit through-space orbital interactions between spatially adjacent but formally nonconjugated π units, a phenomenon related to homoconjugation.

In this study, we synthesized methyl 3-(propan-2-ylidene)-3a,9a-dihydro-3H-cyclopenta[a]azulene-9-carboxylate (2) through the [8 + 2] cycloaddition of methyl 2-oxo-2H-cyclohepta[b]furan-3-carboxylate (1) with 6,6-dimethylfulvene under refluxing conditions in xylene. The structure of 2 was elucidated by single-crystal X-ray diffraction analysis, multidimensional NMR spectroscopy, chemical-shift calculations, UV/Vis spectroscopy, and TD-DFT calculations. These combined analyses enabled us to define the bond-localized character of 2, evaluate the contribution of fulvenoid forms, and clarify the origin of its characteristic absorption behavior. Thus, this study demonstrates that even a known cycloadduct can serve as a useful model compound for understanding how localized nonbenzenoid π-electron frameworks and through-space orbital interactions influence molecular structure and optical properties.

2. Results and Discussion

Methyl 3-(propan-2-ylidene)-3a,9a-dihydro-3H-cyclopenta[a]azulene-9-carboxylate (2) was obtained as a reddish-brown solid in 40% yield through the [8 + 2] cycloaddition of methyl 2-oxo-2H-cyclohepta[b]furan-3-carboxylate (1) with 6,6-dimethylfulvene under refluxing conditions in xylene, followed by chromatographic purification (Scheme 1). The use of refluxing xylene is particularly important for this transformation, because related reactions of 1 with 6,6-dimethylfulvene have been reported to give the [8 + 2] cycloadduct selectively in xylene, whereas lower-boiling solvents such as ethanol or benzene favor formation of the corresponding [4 + 2] adducts. Thus, the present result is consistent with the thermodynamically controlled formation of the [8 + 2] cycloadduct under high-temperature conditions.

Single crystals of 2 suitable for single-crystal X-ray diffraction analysis were obtained by slow evaporation from ethyl acetate. The molecular structure of 2 is shown in Figure 1. In the seven-membered ring, alternating C-C bond lengths of approximately 1.35 and 1.43–1.45 Å were observed, indicating pronounced bond localization and substantial olefinic character. In addition, one of the C-C bonds adjacent to the seven-membered ring was approximately 1.37 Å, which is also consistent with significant double-bond character. This structural feature is markedly different from the bond-length equalization expected for an azulene-like aromatic 10π-electron system [8,9]. Instead, the observed bond alternation supports a localized heptafulvene-type electronic structure. This interpretation is also consistent with previous crystallographic and computational studies of azulene-related systems, in which enhanced bond alternation in the seven-membered ring was correlated with increased heptafulvene character.

Furthermore, in the crystal structure of 2, two molecules adopt a slipped packing arrangement, in which the five-membered ring of one molecule overlaps with the seven-membered ring of the neighboring molecule. The interplanar distance between these rings is approximately 3.45 Å, which falls within the range generally associated with close contacts between π-conjugated molecular surfaces. Because 2 is better described as a bond-localized heptafulvene-like framework rather than as a fully delocalized azulene-type aromatic system, this interaction should be regarded as a π-stacking-type contact between conjugated, partially localized π-systems. The slipped arrangement is also reasonable in light of the general tendency of π-stacked molecules to avoid perfect face-to-face overlap and instead adopt offset geometries that reduce repulsive interactions while preserving favorable dispersion and electrostatic contacts [10,11].

Figure 1. (a) Bond lengths and (b) packing structure of 2 (CCDC 1838803) [12].

Figure 1. (a) Bond lengths and (b) packing structure of 2 (CCDC 1838803) [12].

The NMR spectroscopic assignments of 2 were established on the basis of COSY, HMQC, and HMBC experiments (see Supplementary Materials). The experimentally observed ¹H and ¹³C NMR chemical shifts, together with those calculated at the B3LYP/6-311G* level, are summarized in

Table 1. Assignments of the experimentally observed ¹H and ¹³C NMR signals of 2, together with the chemical shifts calculated at the B3LYP/6-311G* level.

	1H NMR		13C NMR
Position	Experimental [ppm]	Calculated [ppm]	Experimental [ppm]	Calculated [ppm]
1	5.93	6.07	130.48	145.73
2	6.42	6.42	131.21	136.44
3	−	−	141.14	151.83
3a	4.08	3.95	49.71	56.48
3b	−	−	153.59	173.07
4	6.09–6.04	6.07	126.75 or 133.18	133.07
5	6.01	6.05	132.89	136.67
6	5.89	5.94	135.74	137.10
7	6.09–6.04	6.15	126.75 or 133.18	138.41
8	7.50	8.10	131.25	140.49
8a	−	−	160.36	163.10
9	−	−	121.61	123.71
9a	3.92	3.95	54.02	60.63
methylidene	−	−	123.50	131.29

. The calculated values showed an overall satisfactory correlation with the experimental data, particularly for the diagnostic proton resonances, thereby supporting the reliability of the signal assignments. Although relatively large deviations were observed for several quaternary carbons, such deviations are not unusual in unscaled DFT-based ¹³C NMR chemical-shift calculations and do not affect the structural assignment, which is independently supported by two-dimensional NMR correlations and single-crystal X-ray diffraction analysis [13,14].

Notably, the protons assigned to the seven-membered ring of 2 resonated in the upfield olefinic region, in marked contrast to the seven-membered-ring protons of typical azulene derivatives, which generally appear in the aromatic region. This pronounced upfield shift indicates a substantial attenuation of azulene-type diatropic ring-current effects and is consistent with localized olefinic character in the seven-membered ring [15]. Since proton chemical shifts alone should not be regarded as definitive aromaticity criteria, this conclusion is drawn from the combined evidence provided by NMR spectroscopy, calculated chemical shifts, and X-ray crystallographic bond-length analysis [16]. Overall, these results strongly support the interpretation that 2 is better described as a bond-localized heptafulvene-like structure rather than as an aromatic azulene framework.

To investigate the optical properties of 2, its UV/Vis absorption spectrum was measured in dichloromethane (CH₂Cl₂). The spectrum is shown in Figure 2, and the experimentally observed λ_max and log ε values are summarized in

Table 2. The absorption maxima [nm] and their coefficients (log ε) of 2 in CH₂Cl₂, and electronic transitions of 2 derived from the computed values based on the TD-DFT calculations at the B3LYP/6-311G* level.

Experimental		Computed Values
λmax [nm] (log ε)	Excitation Energy [eV]	λmax (Oscillator Strength)	Excitation Energy [eV]	Composition of Band (Contribution)
577 sh (2.20)	2.15	472 (0.0170)	2.62	H → L (0.9694)
530 sh (2.58)	2.34
499 sh (2.73)	2.49
455 sh (2.90)	2.73
377 (4.16)	3.29	391 (0.0027)	3.17	H − 1 → L (0.9789)
		362 (0.0299)	3.42	H − 1 → L + 1 (0.7393) H → L + 1 (0.2355)

, together with the electronic transitions estimated by TD-DFT calculations at the B3LYP/6-311G* level.

The absorption spectrum of 2 in CH₂Cl₂ exhibited an intense band at λ_max = 377 nm, accompanied by several lower-energy shoulder bands in the 440–680 nm region, which are attributable to vibronic fine structure and weak low-energy electronic transitions (Figure 2). To obtain further insight into the origin of these absorption bands, TD-DFT calculations were performed at the B3LYP/6-311G* level. The frontier Kohn–Sham orbitals of 2 are shown in Figure 3. The HOMO of 2 is predominantly localized on the heptafulvene moiety, whereas the LUMO is mainly distributed over the cycloheptatriene moiety. The calculated results indicate that the broad absorption extending to approximately 680 nm in the long-wavelength region is primarily associated with the HOMO-to-LUMO transition. This transition is therefore assigned to a low-energy π–π* excitation involving the heptafulvene-based HOMO and the cycloheptatriene-based LUMO.

In contrast, the intense absorption band at λ_max = 377 nm is composed of several higher-energy transitions, mainly H − 1 → L, H − 1 → L + 1, and H → L + 1. These transitions can be ascribed to π–π* excitations within the heptafulvene framework, together with electronic communication between spatially adjacent orbitals located on the heptafulvene and dihydropentafulvene moieties. Such through-space orbital interactions are consistent with the concept of homoconjugation, in which nonconjugated π systems interact through spatial orbital overlap rather than through a continuous covalent π-conjugation pathway [17,18,19]. The present orbital analysis therefore suggests that the characteristic absorption profile of 2 reflects not only localized π–π* excitation of the heptafulvene unit, but also homoconjugative interaction between the proximate unsaturated fragments.

3. Materials and Methods

General: Melting points were determined with a Yanagimoto MPS3 micromelting (Tokyo, Japan) apparatus. UV/Vis spectra were measured with a JASCO V-670 spectrophotometer (Tokyo, Japan). ¹H and ¹³C NMR spectra were recorded with a JEOL ECZ400 (Tokyo, Japan) at 400 MHz (¹H NMR) and 100 MHz (¹³C NMR), respectively. The HRMS data was obtained with a Bruker Daltonics autoflex maX (Billerica, Massachusetts, USA) (MALDI-TOF). The NMR chemical shifts and time-dependence density functional (TD-DFT) calculations were performed with Spartan’10 version 1.1.0, Wave function, Irvine, CA, USA.

Methyl 3-(propan-2-ylidene)-3a,9a-dihydro-3H-cyclopenta[a]azulene-9-carboxylate: 6,6-Dimethylfulvene (1.59 g, 15.0 mmol) was added to a solution of methyl 2H-cyclohepta[b]furan-3-carboxylate (1) (1.02 g, 5.00 mmol) in xylene (10 mL), and the resulting mixture was refluxed for 12 h using an oil bath. After cooling to room temperature, the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using CHCl₃/AcOEt (4:1) as the eluent to give methyl 3-(propan-2-ylidene)-3a,9a-dihydro-3H-cyclopenta[a]azulene-9-carboxylate (2) (533 mg, 40%) as reddish-brown crystals. M.p. 100–101 °C; ¹H NMR (400 MHz, CDCl₃): δ_H = 7.50 (d, J = 12.0 Hz, 1H), 6.42 (dd, J = 5.7, 2.2 Hz, 1H), 6.09–5.99 (m, 3H), 5.94–5.88 (m, 2H), 4.08 (d, J = 7.4 Hz, 1H), 3.92 (d, J = 7.4 Hz, 1H), 3.73 (s, 3H), 1.89 (s, 3H), 1.81 (s, 3H) ppm; ¹³C NMR (100 MHz, CDCl₃): δ_C = 166.59, 160.36, 153.59, 141.14, 135.74, 133.18, 132.89, 131.25, 131.21, 130.48, 126.75, 123.50, 121.61, 54.02, 51.02, 49.71, 23.01, 20.99 ppm; HRMS (MALDI−TOF, positive): calcd for [C₁₈H₁₈O₂]⁺ 266.1301, found: 266.1281.

4. Conclusions

Methyl 3-(propan-2-ylidene)-3a,9a-dihydro-3H-cyclopenta[a]azulene-9-carboxylate (2) was synthesized in 40% yield by the [8 + 2] cycloaddition of methyl 2-oxo-2H-cyclohepta[b]furan-3-carboxylate (1) with 6,6-dimethylfulvene under reflux in xylene.

The structure and electronic properties of 2 were elucidated by single-crystal X-ray diffraction analysis, multidimensional NMR spectroscopy, chemical-shift calculations, UV/Vis spectroscopy, and TD-DFT calculations. X-ray crystallographic analysis revealed pronounced bond alternation in the seven-membered ring, indicating substantial bond localization and olefinic character rather than the bond-length equalization expected for a fully delocalized azulene-type 10π-electron system. This conclusion was further supported by the upfield olefinic resonances of the seven-membered-ring protons in the ¹H NMR spectrum, which indicate a significant attenuation of azulene-like diatropic ring-current effects.

The UV/Vis absorption spectrum of 2 in CH₂Cl₂ exhibited an intense band at 377 nm, together with weak shoulder bands extending into the visible region. TD-DFT calculations indicated that the low-energy absorption is mainly associated with the HOMO to LUMO transition, whereas the intense higher-energy band arises from multiple π–π* transitions involving the heptafulvene framework. Frontier orbital analysis further suggested that through-space interaction between the spatially adjacent heptafulvene and dihydropentafulvene moieties contributes to the characteristic absorption profile of 2. Thus, this study demonstrates that the optical properties of 2 are governed not only by localized π-electron excitation, but also by homoconjugative orbital communication within its compact nonbenzenoid framework.