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

: Non-fullerene acceptors have recently attracted much attention as components of organic solar cells. 1 H -indene-1,3(2 H )-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(3 H )-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 1 H -indene-1,3(2 H )-dione in a high yield. The structure of the newly synthesized compound was established by means of elemental analysis, high-resolution mass spectrometry


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 CH 2 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 CH 2 component is tert-butyl acetoacetate [9,[11][12][13][14][15].

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 1Hindene-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, 1 H, 13 C 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 Xray 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.

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 1Hindene-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, 1 H, 13 C 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 Xray 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.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, 1 H, 13 C 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.

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). 1 H and 13 C 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 ωscan technique), using graphite monochromatized Cu Kα-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 F 2 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: deposit@ccdc.cam.ac.uk).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.

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). 1 H and 13 C 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 CDCl 3 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 F 2 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: deposit@ccdc.cam.ac.uk).