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Synlett 2003(14): 2167-2170  
DOI: 10.1055/s-2003-42100
LETTER
© Georg Thieme Verlag Stuttgart · New York

A Novel and Convenient Approach to 2-Unsubstituted Imidazolium and Thiazolium Salts

Georges Morel*
Synthèse et Electrosynthèse Organiques, UMR 6510, Université de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
Fax: +33(223)236738; e-Mail: Georges.Morel@univ-rennes1.fr;
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Publication History

Received 19 June 2003
Publication Date:
29 October 2003 (online)

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Abstract

The selective reductive cleavage of highly functionalized 5-(tert-butylamino)-2-(phenylthio)thiazolium chlorides and mesoionic 2-(phenylthio)thiazolium-5-thiolates has been effected efficiently at room temperature using an excess of PhSH in the presence of Et3N to give the corresponding 2-unsubstituted imidazolium and thiazolium-5-thiolates. The NaBH4 reduction of 2-(methylthio)imidazolium chlorides and the H2O2 treatment of a thiazoline-2-thione are also reported.

Key words

thiazoles - imidazoles - mesoionic compounds - reduction

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Experimental Procedure: NaBH4 (0.25 g, 6.6 mmol) was added portionwise over a period of 3 min to a solution of 1-methyl imidazolium chloride 1 (0.68 g, 2 mmol) in 95% EtOH (15 mL). After 45 min at r.t., the mixture was poured into 60 mL of water and washed with Et2O. The aq phase was saturated with NaCl and extracted with CH2Cl2 (2 × 10 mL). The combined CH2Cl2 layers were dried over Na2SO4 and concentrated to dryness. Trituration of the residue with CH2Cl2-Et2O gave a crystalline material (5, 0.28 g, 48%); mp 222 °C. 1H NMR (300 MHz, CDCl3): δ = 0.97 (d, J = 6.2 Hz, 6 H), 1.66 (d, J = 6.6 Hz, 6 H), 2.99 (m, 1 H), 3.82 (s, 3 H), 4.58 (d, J = 4 Hz, NH), 5.02 (m, 1 H), 7.50 (m, 5 H), 10.34 (s, 1 H). 1 3C NMR (75.5 MHz, CDCl3): δ = 23.0, 23.1, 34.8 (3 × qd), 47.8, 48.6 (2 dm), 121.5 (m), 125.9, 129.3, 130.1, 130.5 (arom C), 131.7 (dm, 1 J = 215 Hz), 134.9 (m). MS (FAB): m/z [M]+ calcd for C16H24N3: 258.1970; found: 258.1955. Anal. Calcd for C16H24ClN3: C, 65.40; H, 8.23; N, 14.30. Found C, 65.60; H, 8.28; N, 14.17.

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Procedure for Preparation of Thiazolium Hexafluoro-phosphates 7 and 8: To a solution of 6 (0.5 g, 2 mmol) in acetone (12 mL) were added HPF6 (3 mmol, from 60% solution in water) and H2O2 (10 mmol, from 35% solution in water). The reaction medium was stirred at 0 °C for 30 min then at r.t. for 45 min and concentrated in vacuo. The residual syrup was triturated with Et2O to afford a mixture of salts 7 and 8 as insoluble viscous oil.
NMR data for salt 7 (in admixture with 8): 1H NMR (200 MHz, CDCl3): δ =2.47 (s, 3 H), 3.95 (s, 3 H), 7.50 (m, 5 H), 9.65 (s, 1 H). 13C NMR (75.5 MHz, CDCl3): δ = 20.3 (q,
1 J = 143 Hz), 42.6 (qd, 1 J = 146 Hz, 3 J = 2 Hz), 125.9 (m), 130.0, 130.7, 132.0 138.0 (arom C), 147.5 (m), 158.0 (dq, 1 J = 216 Hz, 3 J = 4 Hz).
A similar treatment for 6 h gave a viscous oil insoluble in CH2Cl2. 1H NMR spectroscopy analysis confirmed that the sulfoxide 8 was greatly preponderant ( 95%) in this final product; (0.54 g, 70% yield). 1H NMR (500 MHz, D2O): δ = 2.93 (s, 3 H), 3.94 (s, 3 H), 7.60 (m, 5 H), 10.26 (s, 1 H). 1 3C NMR (125.8 MHz, D2O): δ = 41.4 (q, 1 J = 142 Hz), 41.7 (qd, 1 J = 147 Hz, 3 J = 2.3 Hz), 123.7, 129.7, 129.9, 132.4 (arom C), 144.9, 148.5 (2 × m), 162.3 (dq, 1 J = 220 Hz, 3 J = 4.7 Hz). MS (EI): m/z [M - HPF6]+ · calcd for C11H11NOS2: 237.02821; found: 237.02835.

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Due to its extremely unpleasant odour, manipulations of PhSH were carried out in a well-ventilated fume hood. Typical Experimental Procedure for the Synthesis of Representative Examples 13a and 14a and Their Spectroscopic Data: PhSH (0.33 g, 3 mmol) and Et3N (0.3 g, 3 mmol) were added to a solution of mesoionic heterocycle 3a (0.32 g, 1 mmol) in anhyd THF (5 mL). A yellowish material precipitated slowly. The mixture was maintained at r.t. for 1 d. Thiazole 13a was collected by filtration, washed with Et2O, water and recrystallized from MeOH (0.17 g, 0.8 mmol); mp 228-230 °C. 1H NMR (200 MHz, CDCl3): δ = 3.74 (s, 3 H), 7.49 (m, 5 H), 8.15 (s, 1 H). MS (EI): m/z [M]+ · calcd for C10H9NS2: 207.01764; found: 207.01791. Anal. Calcd C, 57.97; H, 4.35; N, 6.76; S, 30.92. Found C, 57.83; H, 4.35; N, 6.58; S, 31.31. The 13C NMR spectrum could not be recorded because of the low solubility of 13a in CDCl3 and other usual solvents.
A suspension of 13a in CHCl3 was treated with 2 equiv of CH3I at r.t. for 1 h. The resulting solution was concentrated in vacuo to give a greyish solid that was recrystallized from CH2Cl2-Et2O (14a, 94% yield); mp 152-154 °C. 1H NMR (300 MHz, CDCl3): δ = 2.53 (s, 3 H), 4.18 (s, 3 H), 7.60 (s, 5 H), 11.07 (s, 1 H). 13C NMR (75.5 MHz, CDCl3): δ = 20.3 (q, 1 J = 143 Hz), 43.1 (qd, 1 J = 146 Hz, 3 J = 2.3 Hz), 125.6 (m), 129.5, 130.6, 131.4, 137.2 (arom C), 147.0 (m), 160.0 (dq, 1 J = 219 Hz, 3 J = 4.9 Hz). MS (EI): m/z [M - CH3I]+ · calcd for C10H9NS2: 207.01764; found: 207.01791. Anal. Calcd for C11H12INS2: C, 37.82; H, 3.44; N, 4.01; S, 18.34. Found C, 37.84; H, 3.46; N, 4.05; S, 18.67.

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Compound 16; amorphous semi-solid. 1H NMR (200 MHz, CDCl3): δ = 0.97 (m, 2 H), 1.42 (m, 2 H), 1.98 (s, 9 H), 2.36 (s, 3 H), 3.81 (t, J = 7 Hz, 2 H), 7.22 (d, J = 8 Hz, 2 H), 7.38 (d, J = 8 Hz, 2 H), 7.76 (s, 1 H). MS (FAB): m/z [M + H]+ calcd for C34H47N4S2: 575.32422; found: 575.32410.