Ionic liquids as an alternative reaction medium for HMDST based synthesis of thioaldehydes

References

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• Typical procedure. A mixture of 0.5 mL of ionic liquid (maintained under high vacuum prior to use), benzaldehyde (30 mg, 0.28 mmol) and 2,3-dimethyl-1,3-butadiene (46 mg, 0.56 mmol) was treated under inert atmosphere at room temperature with HMDST (100 mg, 0.56 mmol) and CoCl2·6H2O (14 mg, 0.056 mmol) (or TfOTMS, 12 mg, 0.056 mmol). The progress of the reaction was monitored by TLC (petroleum ether/diethyl ether 10:1). After completion of the reaction (2–3 h), the mixture was diluted with diethyl ether. The organic phase was then washed with NH4Cl (3×1 mL) and extracted with diethyl ether (3×2 mL). The combined organic phases were dried over Na2SO4 and the solvent was evaporated under vacuum. TLC purification (petroleum ether/diethyl ether 50:1) afforded 44 mg (78%) of 4,5-dimethyl-2-phenyl-3,6-dihydro-2H-thiopyran 2a.5c 4,5-dimethyl-2- (4- (trifluoromethyl)-phenyl)-3,6-dihydro-2H-thiopyran 2b. 1H NMR (200 MHz, CDCl3), δ (ppm): 1.73 (3H, s), 1.77 (3H, s), 2.45–2.55 (2H, m), 2.91 (1H, bd, J = 16.8 Hz), 3.42 (1H, bd, J = 16.8 Hz), 4.02 (1H, dd, J = 8.4 Hz, 4.5 Hz), 7.45 (2H, ap d, J = 8.3 Hz), 7.61 (2H, ap d, J = 8.3 Hz). MS, m/z (Irel, %): 272 (26) [M+], 239 (16), 190 (59), 82 (100). 4,5-dimethyl-2- (2- (trifluoromethyl)phenyl)-3,6-dihydro-2H-thiopyran 2c. 1H NMR (200 MHz, CDCl3), δ (ppm): 1.72 (3H, s), 1.74 (3H, s), 2.33–2.51 (2H, m), 2.93 (1H, bd, J = 17.0 Hz), 3.44 (1H, d, J = 17.0 Hz), 4.16 (1H, dd, J = 10.0 Hz, 4.5 Hz), 7.30–7.38 (1H, m), 7.50–7.56 (1H, m), 7.79–7.83 (1H, m), 7.95–8.01 (1H, m). MS, m/z (Irel, %): 272 (24) [M+], 239 (12), 190 (62), 82 (100).
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