A Simple Synthesis of 2-Methyl-1,3-Dithiolium and Related Cations

Abstract

A simple, high-yielding synthesis of 2-methyl-1,3-dithiolium tetrafluoroborates is reported. The reaction of these salts with N,N-dimethylformamide and acetic anhydride gives rise to the previously unreported N,N-dimethyl 2-(1,3-dithiol-2-yliden)ethylideniminium tetrafluoroborates.

Key words

heterocycles - sulfur - 1,3-dithiolium cations - Meldrum’s acid

References and Notes

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• 8a

  Thione 1a is commercially available.

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• N,N-Diethyliminium analogues of 5 have been prepared by the reaction of 2-alkylsulfanyl-1,3-dithiolium salts with I₂ and Et₃N:
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7

The perchlorate of cation 4c has been isolated.⁶ Cation 4b has only been generated in aqueous solution as its perchlorate salt^3c or isolated in low yield as the corresponding triiodide.^6b

11

Isopropylidene 1,3-Dithiol-2-ylidenemalonates 3 - General Procedure
To a solution of the corresponding salt 2 (10 mmol) and Meldrum’s acid (1.44 g, 10 mmol) in EtOH (15 mL; 30 mL for 2c), DIPEA (1.9 mL, 11 mmol) was added. The mixture was refluxed under Ar for 3 h and then allowed to cool to r.t. The resulting solid was filtered off and purified by recrystallization from EtOH or column chromatography (silica gel, CH₂Cl₂) in the case of 3c.

12

Some of these isopropylidene malonates had already been prepared under different conditions than those described here, but their reported melting points^3c are lower than those measured by us. Therefore, full characterization data for compounds 3 follow:
Compound 3a: yield 69%; mp 244-245 °C (lit.^3c 235-237 °C). IR (Nujol): 1651 cm^-1. ¹H NMR (400 MHz, DMSO-d ₆): δ = 7.97 (s, 2 H), 1.66 (s, 6 H). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 182.0, 161.1, 128.4, 103.8, 93.4, 26.3. MS (EI): m/z (%) = 244 (60).
Compound 3b: yield 76%; mp 219-220 °C (lit.^3c 208-210 °C). IR (Nujol): 1665 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.30 (s, 6 H), 1.70 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 179.4, 162.1, 132.7, 104.2, 93.4, 26.9, 13.1. MS (EI): m/z (%) = 272 (70).
Compound 3c: yield 68%; mp 202-203 °C. IR (Nujol): 1665 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 7.62-7.60 (m, 2 H), 7.49-7.44 (m, 3 H), 7.43 (s, 1 H), 1.76 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 181.1, 161.8, 144.8, 130.4, 130.3, 129.5, 127.0, 119.6, 104.5, 94.5, 27.0. ESI-HRMS: m/z calcd for C₃₀H₂₄NaO₈S₄ [2 M + Na⁺]: 663.0246; found: 663.0222. HRMS: m/z calcd for C₁₅H₁₂NaO₄S₂ [M + Na⁺]: 343.0069; found: 343.0063.
Compound 3d: yield 56%; mp 194-195 °C (lit.^3c 187-189 °C). IR (Nujol): 1676 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 2.57 (s, 6 H), 1.73 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 179.4, 161.6, 135.2, 104.6, 26.9, 19.2. MS (EI): m/z (%) = 336 (95).

14

2-Methyl-1,3-Dithiolium Tetrafluoroborates 4 - General Procedure
To a solution of the corresponding compound 3 (2 mmol) in AcOH (10 mL; 20 mL for 3b and 3d), HBF₄ (54% in Et₂O, 4 mL) was added under Ar. The mixture was refluxed for 1 h and allowed to cool to r.t. Most of the AcOH was rotary evaporated and the resulting residue was slowly added to ice-cooled Et₂O (100 mL). The so formed solid was recovered by filtration, washed with Et₂O, dried, and optionally recrystallized from AcOH. In the case of 4d we have not been able to completely purify the desired salt.

15

Compound 4a: yield 82%; mp 109-110 °C. ¹H NMR (300 MHz, CF₃COOD): δ = 8.92 (s, 2 H), 3.43 (s, 3 H). ¹³C NMR (75 MHz, CF₃COOD): δ = 204.3, 145.1, 20.6. MS (MALDI, dithranol): m/z = 117 [C₄H₅S₂].
Compound 4b: yield 87%; mp 50-51 °C (dec.). ¹H NMR (300 MHz, CDCl₃): δ = 3.28 (s, 3 H), 2.69 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 193.9, 151.4, 19.5, 14.3. MS (MALDI, dithranol): m/z = 145 [C₆H₉S₂].
Compound 4c: yield 84%; mp 129-132 °C. ¹H NMR (300 MHz, CF₃COOD): δ = 8.73 (s, 1 H), 7.69-7.52 (m, 5 H), 3.40 (s, 3 H). ¹³C NMR (75 MHz, CF₃COOD): δ = 202.0, 135.9, 135.2, 132.6, 129.9, 20.6. MS (MALDI, dithranol): m/z = 193 [C₁₀H₉S₂].
Compound 4d: ¹H NMR (400 MHz, CDCl₃): δ = 3.32 (s, 3 H), 2.75 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 196.9, 151.8, 20.6, 19.9. MS (MALDI, dithranol): m/z = 209 [C₆H₉S₄].

20

N , N -Dimethyliminium Tetrafluoroborates 5 - General Procedure
To a solution of the corresponding salt 4 (0.86 mmol) in Ac₂O (10 mL), anhyd DMF (0.7 mL, 9 mmol) was added. The mixture was heated to 100 °C under Ar for 0.5 h, then allowed to cool to r.t., and finally poured into ice-cooled Et₂O (125 mL). The resulting solid was isolated by filtration, washed with Et₂O, dried, and recrystallized from AcOH if necessary.

21

Compound 5a: yield 59%; mp 161-162 °C. IR (Nujol): 1625 cm^-1. ¹H NMR (400 MHz, DMSO-d ₆): δ = 8.38 (d, 1 H, J = 11.8 Hz), 7.57 (s, 2 H), 6.91 (d, 1 H, J = 11.8 Hz), 3.41 (s, 3 H), 3.19 (s, 3 H). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 181.9, 157.8, 125.8, 99.1, 46.4. MS-FAB: m/z = 172 [C₇H₁₀NS₂].
Compound 5b: yield 65%; mp 193-194 °C. IR (Nujol): 1630 cm^-1. ¹H NMR (300 MHz, DMSO-d ₆): δ = 8.33 (d, 1 H, J = 11.8 Hz), 6.84 (d, 1 H, J = 11.8 Hz), 3.40 (s, 3 H), 3.18 (s, 3 H), 2.23 (s, 6 H). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 183.2, 162.6, 134.7, 104.1, 51.7, 44.3, 18.5, 18.1. MS (MALDI, dithranol): m/z = 200 [C₉H₁₄NS₂].
Compound 5c: yield 40%; mp 224 °C (dec.; darkening from 175 °C). IR (Nujol): 1626 cm^-1. ¹H NMR (400 MHz, DMSO-d ₆): δ = 8.49 (d, 0.5 H, J = 12.0 Hz), 8.46 (d, 0.5 H, J = 12.5 Hz), 7.89 (s, 1 H), 7.62-7.48 (m, 5 H), 7.02 (d, 0.5 H, J = 12.5 Hz), 6.98 (d, 0.5 H, J = 12.0 Hz), 3.45 (s, 3 H), 3.24 (s, 3 H). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 178.5, 158.3, 158.0, 139.3, 139.1, 129.9, 129.8, 129.6, 129.5, 126.5, 126.4, 119.9, 119.2, 100.5, 100.2, 46.7, 39.3. MS (MALDI, dithranol): m/z = 248 [C₁₃H₁₄NS₂].