tert-Butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate

Abstract

Non-fullerene acceptors have recently attracted much attention as components of organic solar cells. 1H-indene-1,3(2H)-dione is a key compound for the synthesis of the end-capping component of non-fullerene acceptors. In this communication, an intermediate for the synthesis of this compound, tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate, was prepared by the reaction between phthalic anhydride and tert-butyl acetoacetate. Further treatment with sodium methoxide in methanol led to the formation of 1H-indene-1,3(2H)-dione in a high yield. The structure of the newly synthesized compound was established by means of elemental analysis, high-resolution mass spectrometry, ¹H, ¹³C NMR, IR spectroscopy, mass spectrometry and X-ray analysis.

1. Introduction

Organic solar cells (OSCs) have made significant progress over the past decades due to the urgent need to replace exhaustible energy sources with renewable and clean solar energy. Among them, bulk heterojunction solar cells are of particular interest because of their remarkable flexibility, semi-transparency, and high potential for large-scale production and achieved power-conversion efficiency of more than 17% [1,2]. One of the key parts of organic solar cells are acceptors, including the rapidly developing non-fullerene ones (NFAs) [3,4,5]. The most frequently studied NFA structure is the A-D-A, where 1H-indene-1,3(2H)-dione derivatives play the role of end-capping acceptors with activated methylene group [6,7,8]. However, despite its simplicity, the synthesis of 1H-indene-1,3(2H)-dione acceptors often proceeds with low yields (20–30%) [9,10,11,12], and the procedure for preparing these compounds needs to be improved. Typically, this protocol involves the reaction of phthalic anhydride and its derivatives with a compound containing an activated CH₂ group in acetic anhydride and triethylamine. The mechanism of this multi-step transformation has not been studied in detail, but it seems to involve Knoevenagel condensation, elimination of the acetyl and ester groups to form isobenzofuranone, and rearrangement to indanedione (Scheme 1). The most commonly used CH₂ component is tert-butyl acetoacetate [9,11,12,13,14,15].

Herein, we report the synthesis of tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate 1 and its conversion to 1H-indene-1,3(2H)-dione.

2. Results and Discussion

tert-Butyl-(E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate 1 was obtained using a simple and convenient procedure, which included the reaction of phthalic anhydride and tert-butyl acetoacetate in acetic anhydride in the presence of triethylamine at room temperature for 20 h (Scheme 2). The reaction proceeded with high yield (83%) and regioselectivity. It is interesting to note that in the reaction between phthalic anhydride and silylenol ether, obtained from tert-butyl acetoacetate, product 1 was not isolated [16]. To confirm the possibility of using product 1 as an intermediate in the synthesis of 1H-indene-1,3(2H)-dione, it was shown that the reaction of ester 1 with sodium methoxide in methanol at room temperature for 20 h, followed by treatment with hydrochloric acid, produced 1H-indene-1,3(2H)-dione in a high yield (80%).

The structure of tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate 1 was confirmed by means of elemental analysis, high-resolution mass spectrometry, ¹H, ¹³C NMR, IR spectroscopy as well as mass spectrometry. The configuration of the double bond of ether 1 was established on the basis of NOESY experiments (Figure 1). The singlet corresponding to the hydrogen atoms of tert-butyl (1.18 ppm) correlates with aromatic protons (8.26 ppm), while the methyl singlet (3.14 ppm) does not correlate with aromatic protons of the system, which proves the existence of a single isomer 1 of the proposed compound. The structure of compound 1 was also unambiguously confirmed using an X-ray diffraction analysis (Figure 2). X-ray diffraction analysis in combination with NOESY NMR spectra showed the presence of only one E-regiomer in the structure, in which the bulky tert-butyloxycarbonyl group is closer to the aromatic ring.

In conclusion, it was shown that the reaction of phthalic anhydride with tert-butyl acetoacetate is regioselective and led to tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate 1, which is an intermediate in the synthesis of 1H-indene-1,3(2H)-dione from phthalic anhydride.

3. Materials and Methods

The solvents and reagents were purchased from commercial sources and used as received. Elemental analysis was performed on a 2400 Elemental Analyzer (Perkin ElmerInc., Waltham, MA, USA). ¹H and ¹³C NMR spectra were taken with a Bruker AM-300 machine (Bruker AXS Handheld Inc., Kennewick, WA, USA) (at frequencies of 300 and 75 MHz) in CDCl₃ solution, with TMS as the standard. J values are given in Hz. MS spectrum (EI, 70 eV) was obtained with a Finnigan MAT INCOS 50 instrument (Hazlet, NJ, USA). High-resolution MS spectrum was measured on a Bruker micrOTOF II instrument (Bruker Daltonik Gmbh, Bremen, Germany) using electrospray ionization (ESI). IR spectrum was measured with a Bruker “Alpha-T” instrument in KBr pellet.

X-ray diffraction data were collected at 100 K on a four-circle Rigaku Synergy S diffractometer equipped with a HyPix600HE area-detector (kappa geometry, shutterless ω-scan technique), using graphite monochromatized Cu K_α-radiation. The intensity data were integrated and corrected for absorption and decay using the CrysAlisPro program [17]. The structure was solved with direct methods using SHELXT [18] and refined on F² using SHELXL-2018 [19] in the OLEX2 program [20]. All non-hydrogen atoms were refined with individual anisotropic displacement parameters. All hydrogen atoms were placed in ideally calculated positions and refined as riding atoms with relative isotropic displacement parameters. The Mercury program suite [21] was used for molecular graphics. Cambridge Crystallographic Data Centre contains the supplementary crystallographic data for this paper No. CCDC 2247414. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 7 March 2023) (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; e-mail: [email protected]).

Synthesis of tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate 1 (Supplementary Materials).

Triethylamine (0.7 mL, 4.7 mmol) was added dropwise with vigorous stirring to a solution of phthalic anhydride (200 mg, 1.35 mmol) and tert-butyl acetoacetate (490 mg, 2 mmol) in acetic anhydride (1.3 mL, 13.7 mmol). The mixture was stirred overnight at room temperature, poured into cold water (5 mL) and the crude product was collected using filtration and washed with water. Yield 324 mg (83%), white solid, mp 150–152 °C. R_f = 0.8 (CH₂Cl₂). ¹H NMR (300 MHz, CDCl₃): 8.26 (d, J = 6.3, 1H), 8.02 (d, J = 5.1, 1H), 7.81 (td, J = 5.7, J = 0.9, 1H), 7.74 (td, J = 5.4, J = 0.6, 1H), 2.64 (s, 3H), 1.62 (s, 9H). ¹³C NMR (75 MHz, CDCl₃): 195.6, 164.4, 163.4, 152.0, 136.7, 135.3, 132.7, 126.0, 125.9, 125.4, 117.8, 83.7, 31.8, 28.0 (3 CH₃). MS (EI, 70 eV), m/z (I, %): 288 (7), 232 (8), 217 (20), 190 (15), 173 (100), 57 (33), 43 (36). HRMS-ESI (m/z): calcd. for (C₁₆H₁₇O₅) 289.1071, found m/z 289.1074. IR, ν, cm⁻¹: 3433, 3092, 2986, 1814, 1726, 1640, 1472, 1372, 1254, 1160, 1056, 997, 837, 691. Anal. calcd. For C₁₆H₁₆O₅ (288.2952): C, 66.66; H, 5.59. Found: C, 66.84; H, 5.72%.

Synthesis of 1H-indene-1,3(2H)-dione from tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate 1 (Supplementary Materials).

A 5.4 M solution of sodium methoxide in MeOH (0.04 mL, 1.1 mmol) was added to a solution of tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate 1 (200 mg, 0.7 mmol) in dry methanol (7 mL), and the mixture was stirred overnight at room temperature. The solvent was evaporated under reduced pressure and the residue was treated with a mixture of hydrochloric acid (3 mL) and ice-water (4 mL). The resulting mixture was refluxed for 1 h, cooled to room temperature, diluted with water (3 mL) and extracted with CH₂Cl₂ (3 × 10 mL). The combined organic extracts were dried over MgSO₄ and evaporated under reduced pressure. The residue was purified using column chromatography on silica gel (Silica gel Merck 60, eluent dichloromethane). Yield 82 mg (80%), pale yellow solid, mp 128–131 °C (lit. mp 130–132 °C [22]). R_f = 0.5 (dichloromethane). The data on the ¹H and ¹³C NMR spectra correspond to data in the literature [23].

Crystallographic data of the compound 1 are given in

Table 1. Crystal data and structure refinement for compound 1.

Empirical Formula	C16H16O5
Formula weight	288.29
Temperature	99.9(2) K
Wavelength	1.54184 Å
Crystal system	Monoclinic
Space group	P 21/c
Unit cell dimensions	a = 6.68587(5) Å a = 90° b = 10.54420(10) Å b = 94.2747(7)° c = 20.48957(18) Å g = 90°
Volume	1440.44(2) Å3
Z	4
Density (calculated)	1.329 g/cm3
Absorption coefficient	0.824 mm−1
F(000)	608
Crystal size	0.4 × 0.16 × 0.03 mm3
Theta range for data collection	4.328 to 77.931°.
Index ranges	−7 <= h <= 8, −13 <= k <= 13, −25 <= l <= 25
Reflections collected	16,388
Independent reflections	3062 [R(int) = 0.0234]
Observed reflections	2878
Completeness to theta = 67.684°	100.0%
Absorption correction	Gaussian
Max. and min. transmission	1.000 and 0.434
Refinement method	Full-matrix least-squares on F2
Data/restraints/parameters	3062/0/194
Goodness-of-fit on F2	1.055
Final R indices [I > 2sigma(I)]	R1 = 0.0338, wR2 = 0.0874
R indices (all data)	R1 = 0.0356, wR2 = 0.0890
Largest diff. peak and hole	0.194 and −0.230 e.Å−3

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