Benzo[d][1,3]oxathiole-2-thione

Abstract

Although known for 120 years, the title compound has not been adequately characterised before. In this paper, it is fully characterised by ¹H and ¹³C NMR and IR spectroscopy and its X-ray structure has been determined for the first time.

1. Introduction

The fundamental heterocyclic compound benzo[d][1,3]oxathiole-2-thione 1 was first described in 1904 [1] when it was obtained as a side-product in the steam distillation of the hydroxyphenyl xanthate 2, which itself was obtained by diazotisation of o-aminophenol and a reaction with potassium ethyl xanthate (Scheme 1). A more convenient synthesis was reported in 1953 [2], involving the treatment of o-mercaptophenol 3 with thiophosgene in aqueous alkali. In these early studies, the only characterisation was by melting point and elemental analysis. More recently, a third route to 1 involving reaction of 3 with Me₂N⁺=CCl₂ Cl⁻ (“Viehe’s salt”) in pyridine followed by treatment with NaSH was described, and in this case ¹³C (but not ¹H) NMR data were given for the product [3]. There have only been a few further mentions of compound 1 in the literature, including its reaction with benzyne to give the isomeric benzo[d][1,3]-dithiol-2-one by an interesting cycloaddition–cycloreversion mechanism [4], and its reduction with diisobutylaluminium hydride to regenerate 3 [5]. In this paper, we present the ¹H NMR and IR spectra for 1, a full assignment of the NMR data, and determination of its structure by X-ray diffraction.

2. Results

Compound 1 was readily prepared from 3 by reaction with thiophosgene in aqueous sodium hydroxide, as reported [2], and was obtained as orange crystals after column chromatography.

The ¹H NMR spectrum, which has not been documented before, turned out to be exceptionally complex for a simple ortho-disubstituted benzene derivative (Figure 1). This was, however, successfully simulated using the chemical shift values in

Table 1. ¹H and ¹³C chemical shifts (ppm) for 1.

Position	δC	δH
2	201.7	–
3a	126.6	–
4	121.2	7.412
5	126.0	7.337
6	128.0	7.403
7	112.2	7.400
7a	155.1	–

and the coupling constants in

Table 2. ¹H–¹H coupling constants (Hz) in 1.

Compound	3JH4–H5	4JH4–H6	5JH4–H7	3JH5–H6	4JH5–H7	3JH6–H7
1	8.0	1.8	0.3	7.4	1.7	8.5

to give agreement of all 30 signals to within 0.001 ppm (Figure 1). The most obvious discrepancy is that the signals appearing as doublets at 7.345/7.343 and 7.326/7.325 in the simulation are incompletely resolved in the experimental spectrum, but shoulders are visible on these signals. The precise match achieved for even the smallest peaks to the right of the spectrum gives confidence in the derived parameters. Once the data have been extracted, the reason for the complexity is clear: all four protons are coupling to each other and have similar chemical shifts, with two of them very close indeed, leading to significant second-order effects.

While the simulation yielded a viable set of chemical shifts and coupling constants, unambiguous assignment of these to the protons in the structure required consideration of the ¹³C NMR data and particularly HSQC and HMBC studies. The observed chemical shifts (see Supplementary Materials) are consistent with those reported earlier [3], although the assignment for the quaternary ring-junction carbons C-3a and C-7a is opposite to that made in the earlier publication (“153.3 C-3a, 126.8 C-7a”). We believe this may be due to erroneous numbering there since the signal at 155.1 is clearly the carbon directly joined to oxygen (C-7a). This value is also consistent with that of 153.9 for C-7a, which we recently reported for the corresponding cyclic thiosulfite benzo[d][1,2,3]oxadithiole 2-oxide [6]. The HSQC spectrum allowed unambiguous association of the CH carbon signals to the relevant protons (Table 1) but consideration of the HMBC results was key to arriving at a final assignment, as summarised in Figure 2. In particular, the carbon signals at 155.1 and 121.2 were linked on HMBC to the proton signal at 7.403, the carbon signal at 112.2 was linked to the proton signal at 7.337, the carbon signal at 128.0 was linked to the proton signal at 7.412, and the carbon signal at 126.0 was linked to the proton signal at 7.400. Taken together, this pattern is fully consistent with the assignment of Figure 2, with the HMBC spectrum consistently showing correlations for ³J_C–H but not ²J_C–H.

Since compound 1 formed suitable crystals after chromatography, we were able to determine its structure by X-ray diffraction (Figure 3). The molecular structure is completely planar, and the heterocyclic ring features the expected long C–S bonds to accommodate the large sulfur atom with the interior angle at sulfur just over 90° (

Table 3. Heterocyclic ring dimensions for 1 (Å, °).

Bond Length		Angle
S(3)–C(2)	1.7399(16)	C(3a)–S(3)–C(2)	91.42(7)
C(2)–S(1)	1.6293(16)	S(3)–C(2)–O(1)	111.77(11)
C(2)–O(1)	1.3625(18)	S(3)–C(2)–S(1)	125.58(10)
O(1)–C(7a)	1.3891(17)	S(1)–C(2)–O(1)	122.65(12)
C(7a)–C(3a)	1.393(2)	C(2)–O(1)–C(7a)	112.89(11)
C(3a)–S(3)	1.7422(15)	O(1)–C(7a)–C(3a)	114.29(13)
		C(7a)–C(3a)–S(3)	109.61(11)

).

Weak non-classical C-H···O hydrogen bonds between C(4) and O(1) (O···H 2.545(1) Å, C···O 3.3830(19) Å) link adjacent molecules along the (1 0 0) axis (Figure 4). These chains form sheets in the [0 1 0] plane via C-H···S contacts (H···S 2.9238(4) Å, C···S 3.7561(16) Å). This results in adjacent molecules forming head-to-tail stacks along the (1 0 0) axis (Figure 4), although at distances too long for π-π stacking to play a significant role.

As far as we are aware, this is the first 1,3-benzoxathiole-2-thione to be crystallographically characterised and a search of the Cambridge Structural Database (CSD) showed that only a few comparable structures have been determined before (Figure 5). There are only two previous 1,3-oxathiole-2-thione structures, 4 and 5 [7], while for the saturated 1,3-oxathiolane-2-thione ring system, there are only four structures: the tetramethyl compound 6 [8], the limonene-derived bicyclic structure 7 [9], the amino compound 8, which is of interest for ring-opening polymerisation [10], and the chiral phthalimido compound 9 [11]. As might be expected, the heterocyclic rings of 4 and 5 are essentially planar like 1 but removal of the double bond leads to a significant degree of twisting in structures 6–9.

In summary, we have been able to fully assign the ¹H and ¹³C NMR spectra for compound 1 for the first time. The X-ray structure of 1 features a completely planar molecule with an internal angle at the ring sulfur atom of just over 90°.

3. Experimental

3.1. General Experimental Details

Melting points were recorded using a Reichert hot-stage microscope (Reichert, Vienna, Austria) and are uncorrected. NMR spectra were obtained using a Bruker AVII-400 instrument (Bruker, Billerica, MA, USA). Spectra were run with internal Me₄Si as the reference and chemical shifts are reported in ppm to the high frequency of the reference. NMR spectra were processed, and simulations produced using iNMR reader, version 6.3.3 (Mestrelab Research, Santiago de Compostela, Spain). The IR spectrum was run on a Shimadzu IRAffinity instrument using the ATR technique.

3.2. Synthesis of Benzo[d][1,3]oxathiole-2-thione 1

2-Mercaptophenol (0.50 g, 4.0 mmol) was dissolved in water (6 mL) containing sodium hydroxide (0.4 g, 10 mmol) and stirred while thiophosgene (0.38 mL, 0.57 g, 5.0 mmol) was added. After stirring at RT for 18 h, the mixture was extracted with CH₂Cl₂ (2 × 5 mL) and the extracts were dried and evaporated. Column chromatography of the residue (SiO₂, Et₂O/hexane 1:1) gave the product (0.10 g, 15%) as orange-red crystals, mp 93–94 °C (lit. [2] 97–98 °C). IR (ATR): ν_max /cm^–1 1450, 1314, 1175 (S=O), 1153, 1084, 1022, 1009, 741, 656. ¹H NMR and ¹³C NMR (CDCl₃): see Table 1 and Table 2 and Figure 2.

3.3. X-ray Structure Determination of 1

X-ray diffraction data were collected at 100 K using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71073 Å)]. Data were collected (using a calculated strategy) and processed (including a multi-scan absorption correction) using CrysAlisPro [12]. The structure was solved by dual-space methods (SHELXT) [13] and refined by full-matrix least-squares against F² (SHELXL-2019/3) [14]. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. All calculations were performed using the Olex2 interface [15].

Crystal data for C₇H₄OS₂, M = 168.22 g mol^–1, yellow prism, crystal dimensions 0.12 × 0.09 × 0.03 mm, triclinic, space group P–1 (No. 2), a = 6.8478(3), b = 7.4095(4), c = 7.9830(4) Å, α = 64.683(5), β = 83.960(4), γ = 70.423(4) °, V = 344.63(3) ų, Z = 2, D_calc = 1.621 g cm^–3, T = 100 K, goodness of fit on F² 1.064, 7573 reflections measured, 1615 unique (R_int = 0.0426), which were used in all calculations. The final R₁ [I > 2σ(I)] was 0.0287 and wR₂ (all data) was 0.0805. Data have been deposited at the Cambridge Crystallographic Data Centre as CCDC 2382164. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/structures (accessed on 6 September 2024).