1-(Dicyanomethylene)-3-hydroxy-1H-indene-2-carboxylic Acid

Abstract

Bulk heterojunction solar cells are among the most promising organic solar cells (OSCs). One of the two important parts of OSCs are acceptors, and the development of the design and synthesis of non-fullerene acceptors involves an electron-deficient heterocyclic central core and anchor acceptor malonitrile derivatives of 3-methylene-2,3-dihydro-1H-inden-1-ones. In this communication, an intermediate for the synthesis of this compound, 1-(dicyanomethylene)-3-hydroxy-1H-indene-2-carboxylic acid, was prepared by the Perkin reaction of 2-(3-oxoisobenzofuran-1(3H)-ylidene)malononitrile with tert-butyl acetoacetate in the presence of acetic anhydride and triethylamine. The structure of the newly synthesized compound was established by means of elemental analysis, high-resolution mass spectrometry, ¹H NMR, ¹³C NMR and IR spectroscopy, and mass spectrometry.

1. Introduction

The most promising area of commercialization for converting solar radiation into electricity is organic solar cells (OSCs), i.e., solar cells and modules in which the photovoltaic active layer consists of organic semiconductors [1,2]. Bulk heterojunction OSCs containing donor and acceptor elements currently occupy a leading position among such devices with high potential for large-scale production; based on them, a power-conversion efficiency of more than 19% has been achieved [3,4]. Due to the significant disadvantages of previously used fullerene acceptors, such as the high cost of synthetic material and low light-harvesting ability, in the last 5 years, the chemistry of non-fullerene acceptors (NFAs) has begun to rapidly develop, which has revolutionized the field of OSCs. It turned out that with their help, it is possible to achieve much higher values of photovoltaic efficiency [5,6]. The most promising NFAs contain 1-(dicyanomethylene)-3-oxo-2,3-dihydro-1H-indenes 1 as an anchor residue with an activated methylene group [7,8,9]. Typically, this protocol involves the Perkin reaction of phthalic anhydrides followed by the Knoevenagel reaction of 1H-indene-1,3(2)-dione 2 and its derivatives with malononitrile in the presence of a catalyst [10,11,12]. However, the yields of 1-(dicyanomethylene)-3-oxo-2,3-dihydro-1H-indenes 1 in this reaction are often low due to the side reaction of bindone 3 formation [13,14,15]. This is because the formation of bindons 3 from 1H-indene-1,3(2)-diones 2 occurs under the same conditions when treated with sodium acetate in ethanol [16,17]. To avoid the possibility of this side reaction, one could first introduce a dicyanomethylene group into the phthalic anhydride, and then carry out the Perkin reaction to form the target compound 1 (Scheme 1).

The synthesis of 2-(3-oxoisobenzofuran-1(3H)-ylidene) malononitrile 4 from phthalic anhydride and malononitrile is described in the literature [18]. Herein, we report the reaction of 2-(3-oxoisobenzofuran-1(3H)-ylidene) malononitrile with tert-butyl acetoacetate and synthesis of 1-(dicyanomethylene)-3-hydroxy-1H-indene-2-carboxylic acid 5 as a precursor for the preparation of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile 1 (R = H).

2. Results and Discussion

It was found that the reaction of 2-(3-oxoisobenzofuran-1(3H)-ylidene) malononitrile 4 with tert-butylacetoacetate in the presence of triethylamine and acetic anhydride for 72 h at room temperature, followed by treatment of the resulting precipitate with an HCl solution at 70 °C within 3 h, led to compound 5 (Scheme 2).

The structure of 1-(dicyanomethylene)-3-hydroxy-1H-indene-2-carboxylic acid 5 as monohydrate was established by means of elemental analysis, high-resolution mass spectrometry, ¹H NMR, ¹³C NMR, IR spectroscopy, and mass spectrometry. A two-dimensional HSQC NMR spectrum was recorded in order to correlate the signals in the ¹H and ¹³C spectra. It turned out that the structure of compound 5 has four tertiary carbon atoms of the aromatic ring, which correspond to signals in the ¹H NMR spectrum. Using the ¹³C DEPT NMR spectrum of compound 5, the remaining nine carbon atoms were found to be quaternary. The enol structure of acid 5 is confirmed by the presence of a broadened signal in the low-field region of the ¹H NMR spectrum at δ = 10.30 ppm, corresponding to the proton of the carboxyl group, and the signal of the hydroxyl group obviously coincides with the signal of water in DMSO-d₆, which is confirmed by an increase in the integral of water at δ = 3.36 ppm when compound 5 is dissolved in DMSO-d₆ by approximately three protons (see Figures S1 and S2 in Supplementary Materials).

It should be noted that proving the structure of 1,3-dioxo-2,3-dihydro-1H-indene-2-carboxylic acid 6, which is similar in structure to compound 5, was also very difficult and required the use of quantum-chemical calculations and solid-state ¹³C NMR and IR spectroscopy (Scheme 3). It has been established that this compound exists as 3-hydroxy-1-oxo-1H-indene-2-carboxylic acid [18,19].

In conclusion, it was shown that the reaction of 2-(3-oxoisobenzofuran-1(3H)-ylidene) malononitrile with tert-butyl acetoacetate led to 1-(dicyanomethylene)-3-hydroxy-1H-indene-2-carboxylic acid 5, which is intermediate in the synthesis of 1-(dicyanomethylene)-3-oxo-2,3-dihydro-1H-indene 1.

3. Materials and Methods

2-(3-Oxoisobenzofuran-1(3H)-ylidene) malononitrile 4 was prepared according to the published method [20]. The solvents and reagents were purchased from commercial sources and used as received. Elemental analysis was performed on a 2400 Elemental Analyzer (Perkin Elmer Inc., Waltham, MA, USA). Melting point was determined on a Kofler hot-stage apparatus and was uncorrected. ¹H and ¹³C NMR spectra were taken with a QOne Quantum-I-plus AS600 (Q.One Instruments Ltd., Wuhan, Hubei Province, China), at frequencies of 600 and 150 MHz, in DMSO-d₆ solutions, with TMS as the standard. J values are presented in Hz. The MS spectrum (EI, 70 eV) was obtained with a Finnigan MAT INCOS 50 instrument (Hazlet, NJ, USA). The IR spectrum was measured with a Bruker “Alpha-T” instrument in the KBr pellet. High-resolution MS spectra were measured on a Bruker micrOTOF II instrument (Bruker Daltonik Gmbh, Bremen, Germany) using electrospray ionization (ESI).

Synthesis of 1-(dicyanomethylene)-3-hydroxy-1H-indene-2-carboxylic acid 5 (Supplementary Materials).

A mixture of 2-(3-oxoisobenzofuran-1(3H)-ylidene)malononitrile 4 (300 mg, 1.53 mmol), tert-butyl acetoacetate (302 mg, 314 µL, 1.91 mmol) in triethylamine (1.35 mL), and acetic anhydride (2.04 mL) was stirred at room temperature for 72 h. Then, a 2 N solution of HCl (10 mL) was added and the reaction mixture was stirred at 70 °C for 2 h. After cooling to room temperature, the solid was filtered off and washed with water. Yield 302 mg (83%, 1.269 mmol), orange solid, mp > 300 °C. Anal. Calcd for C₁₃H₆N₂O₃*H₂O (256): C, 60.94; H, 3.15; N, 10.93. Found: C, 61.03; H, 3.34; N, 11.17. ¹H NMR (600 MHz, DMSO-d₆, δ, ppm): 10.29 (br s, 1H, OH), 8.04 (d, J = 7.6, 1H, CH), 7.60–7.58 (m, 1H, CH), 7.50–7.54 (m, 2H, CH). ¹³C NMR (150 MHz, DMSO-d₆, δ, ppm): 185.5 (=C-OH), 165.4 (C=C(CN)₂), 160.5 (CO₂H), 160.2, 138.1, 137.4, 132.6 (CH), 131.9 (CH), 122.6 (CH), 122.1 (CH), 118.5 (CN), 102.1 (CCO₂H), 79.1 (=C(CN)₂). HRMS-ESI (m/z): [M+H]⁺ calcd for (C₁₃H₆N₂O₃) 239.0451, found 239.0455. MS (EI, 70eV), m/z (I, %): 238 (95, [M]), 220 (18, [M-H₂O]), 195 (12), 139 (25), 127 (15), 112 (8), 100 (10), 88 (11), 86 (11), 74 (9), 63 (12), 50 (15), 44 (100, CO₂), 39 (16). IR, ν, cm⁻¹: 3429 (OH), 3122 and 2979 (CH), 2221 (C≡N), 1677 (C=O), 1618 (C=N), 1590, 1550, 1483, 1463, 1379, 1315, 1273, 1226, 1178, 1082, 941, 853. R_f = 0.10 (petroleum ether/MeOH—10:1).