Dimethyl 3,7-diamino-4,8-bis((2-methoxy-2-oxoethyl)thio)benzo[1,2-b:4,5-b’]dithiophene-2,6-dicarboxylate

Abstract

The convenient and multigram synthesis of a linear benzo[b]dithiophene (BDT) structure is proposed (dimethyl 3,7-diamino-4,8-bis((2-methoxy-2-oxoethyl)thio)benzo[1,2-b:4,5-b’]dithiophene-2,6-dicarboxylate). The starting materials are commercially available 2,3,5,6-tetrachloroterephthalonitrile and methyl 2-mercaptoacetate, which undergo aromatic substitution, followed by a cyclization reaction in basic conditions to form suitable BDT for potential optoelectronic applications.

1. Introduction

Due to the ever increasing demand for renewable energy, new organic semiconducting materials are required for optoelectronic applications, such as organic photovoltaics cells (OPCs), organic field-effect transistors (OFETs) [1], etc. One of the most promising building blocks for such a material is linear benzo[b]dithiophene [2].

2. Results and Discussion

The target BDT molecule is a suitable precursor for the further molecular design of organic semiconducting materials, as it features readily convertible functional groups. The synthesis of BDT is a continuation of previous work of our research group [3], and it pertains to studying the reaction of 2,3,5,6-tetrachloroterephthalonitrile with methyl 2-mercaptoacetate in the presence of potassium carbonate in ethanol (Scheme 1). The reaction mechanism was previously studied on benzo[b]thiophenes [4]. Similarly, in this case, the first step of the mechanism could be aromatic substitution via the S_NAr or S_RN1 of methyl 2-mercaptoacetate for chlorine from 2,3,5,6-tetrachloroterephthalonitrile, followed by cyclization via the Ad_N of carbon nucleophile on the nitrile group in basic conditions.

3. Materials and Methods

3.1. General Information

The ¹H and ¹³C NMR spectra were acquired on a Varian Inova-300 spectrometer (Bratislava, Slovakia) with a frequency of 299.96 MHz for the ¹H nucleus and 75.43 MHz for the ¹³C nucleus. Chemical shifts were recorded in ppm relative to the solvent resonance as the reference. Data were reported as follows: chemical shift, multiplicity (s—singlet; d—doublet; t—triplet; q—quaternary; m—multiplet; br—broad) and coupling constants J (Hz). The NMR spectra were processed using MestReNova 14.2.2 software (Bratislava, Slovakia).

The IR spectrum was recorded on a Perkin Elmer Spectrum Two apparatus (Bratislava, Slovakia). Wavenumbers were recorded in cm⁻¹.

The melting point was measured on a Boetius apparatus (Nagema, Radebeul, Germany) using a high-precision thermometer TD 121 from VWR. M.P. was left uncorrected.

An elemental analysis was carried out on a FlashEA 1112 (Thermo Scientific, Waltham, MA, USA) from the Thermo electron corporation.

The single-crystal X-ray analysis was performed on an Oxford Diffraction Gemini R four-circle diffractometer (Oxford Diffraction Ltd., Abingdon, UK) with CrysAlis [5], using Cu-Ka radiation for BDT at 100(1) K. Data reduction was performed with CrysAlis RED [5]. The structure was solved with the charge-flipping algorithm superflip [6] using OLEX2 [7]. A refinement was carried out on F², and scattering factors incorporated in SHELXL-2013 [8] were used. All nonhydrogen atoms were refined with anisotropic thermal parameters. Crystal data, data collection procedures, structure determinations and refinements were summarized in

Table 1. Crystal data and structure refinement for BDT.

Compound	BDT
Empirical formula (g/mol)	C10H10NO4S2C14
Temperature (K)	100(1)
Wavelength (Å)	1.54184
Crystal system	triclinic
Space group	P-1
Unit cell dimensions	a = 6.3851(3) Å, α = 73.558(3)° b = 9.5602(3) Å, β = 79.571(3)° c = 10.0541(4) Å, γ = 81.722(3)°
Formula weight	272.31
Volume (Å3)	576.11
Z/calculated density (Mg/m3)	2/1.570
Absorption coefficient (mm−1)	4.247
F(000)	282
Crystal size (mm)	0.267 × 0.038 × 0.026
Theta range for data collection	4.637 to 73.894°
Index ranges	−7 ≤ h ≤ 7 −11 ≤ k ≤ 11 −12 ≤ l ≤ 11
Reflections collected	8848
Independent reflections	2270 (Rint = 0.0310, Rsigma = 0.0231)
Completeness to 2Θ = 25.000°	99.95 %
Refinement method	Full-matrix least-squares on F2
Data/restraints/parameters	2270/0/194
Goodness-of-fit on F2	1.024
Final R indices (I > 2σ(I))	R1 = 0.0325 wR2 = 0.0876
R indices (all data)	R1 = 0.0375 wR2 = 0.0917
Extinction coefficient	n/a
Largest diff. peak and hole (e.Å-3)	0.31 and −0.24

and Figure 1. The positions of the H atoms were geometrically optimized and constrained to ride on their parent atoms, with a bond length of C–H = 0.96 Å (CH₃); the N–H hydrogen was obtained from Fourier synthesis. Hydrogen‘s temperature factors were Uiso(H) = 1.2 Ueq(C,N). The DIAMOND program package was used for the molecular structure drawing [9]. Cambridge Crystallographic Data Centre (CCDC) 2203334 contains the supplementary crystallographic data for this article. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on: 24 August 2022) or from the Cambridge Crystallographic Data Centre, 12 Uni-on Road, CambridgeCB2 1EZ, UK; fax: +44-1223-336-033; or e-mail: [email protected].

3.2. Dimethyl 3,7-diamino-4,8-bis((2-methoxy-2-oxoethyl)thio)benzo[1,2-b:4,5-b’]dithiophene-2,6-dicarboxylate

A 250 mL three-necked round-bottom flask was equipped with a thermometer, dropping funnel and condenser. The flask was charged with 2,3,5,6-tetrachloroterephthalonitrile (2.66 g, 10 mmol, 1 eq.) and potassium carbonate (6.92 g, 50 mmol, 5 eq.) in 80 mL of ethanol. The dropping funnel was filled with methyl 2-mercaptoacetate (5.31 g, 50 mmol, 5 eq.) in 80 mL of ethanol. A solution of methyl 2-mercaptoacetate was added dropwise into the stirring reaction mixture at room temperature. The colorless reaction mixture turned yellow, and the orange precipitate was formed. After the addition of methyl 2-mercaptoacetate was completed, the reaction mixture was heated to reflux for 5 h. The reaction was monitored with TLC (ethyl acetate:isohexane 1:1, Rf for BDT was 0.4). After cooling, the formed precipitate was filtered off and washed with cold water and ethanol, resulting in 5.16 g (95%, 9.5 mmol) of crude product. The crude product was crystallized from dimethyl carbonate to form red needles (m.p. 211 °C).

¹H NMR (DMSO, 300 MHz): δ = 3.30 ppm (s, 6 H), 3.34 ppm (s, 6 H), 3.69 ppm (s, 4 H), 7.29 ppm (br, 4 H) (See Supplementary Materials).

¹³C NMR (DMSO, 75 MHz): δ = 168.68, 164.49, 149.62, 146.21, 130.37, 124.31, 98.5, 52.22, 51.63, 37.26.

Anal. Calcd. (%) for C₂₀H₂₀N₂O₈S₄: C 44.11; H 3.70; N 5.14; O 23.50; S 23.55. Found: C 44.02; H 3.79; N 5.22; O 23.43; S 23.48.

IR (cm⁻¹): 3448.7; 3335.7; 2948.4; 1734.9; 1672.6; 1592.0; 1522.7.

X-ray analysis: The crystal structure of BDT was deposited at the Cambridge Crystallographic Data Centre with the deposition number CCDC 2203334.

4. Conclusions

In conclusion, the convenient and multigram synthesis of a linear benzo[b]dithiophene heterocycle is described with corresponding spectral, and crystallographic data for this structure. The title compound could be utilized as a building block for organic semiconducting materials, as it features readily convertible functional groups suitable for further molecular design.