Microwave-Assisted Synthesis of N-Heterocyclic Carbene Precursors

Abstract

A very simple and efficient procedure is reported for the synthesis of 1,3-diarylimidazolinium chlorides by cyclization of N,N′-diarylethylenediamines dihydrochlorides with triethyl orthoformate under microwave irradiation.

Key words

amines - cyclizations - heterocycles - imidazolinium salts - ligands

References and Notes

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16

General Procedure for the Microwave-Assisted Synthesis of 1,3-Diarylimidazolinium Chlorides. A 10-mL glass vial equipped with a stirring bar was charged with a N,N′-diarylethylenediamine dihydrochloride (1 mmol) and triethyl orthoformate (1-2 mL). The vial was capped and irradiated 5 min at 145 °C (monitored by IR sensor) in a CEM Discover instrument with a 50-W microwave power. No ramp and no simultaneous cooling were applied. After rapid air cooling by the unit, the reaction mixture was diluted with Et₂O (3 mL) and filtered under vacuum. The precipitate was rinsed with a few milliliters of Et₂O and dried under vacuum to afford pure 1,3-diaryl-imidazolinium chloride.

17

¹H NMR (400 MHz, DMSO-d ₆): δ = 2.35 (s, 6 H, para-CH₃), 4.55 ppm (s, 4 H, CH₂), 7.35 (d, J = 4.0 Hz, 2 H, CH _ortho ), 7.60 (d, J = 4.0 Hz, 2 H, CH _meta ), 10.13 (s, 1 H, im-CH²). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 20.5 (para-CH₃), 48.3 (CH₂), 118.4 (CH _ortho ), 130.0 (CH _meta ), 133.8 (C _para ), 136.4 (C _ipso ), 151.1 (im-CH₂).

18

Maj, A.; Delaude, L.; Demonceau, A.; Noels, A. F. manuscript in preparation.

20

Scaled-up Preparation of 1,3-Dimesitylimidazolinium Chloride.
A 20-mL glass vial equipped with a stirring bar was charged with N,N′-dimesitylethylenediamine dihydrochloride (7.39 g, 20 mmol) and triethyl orthoformate (15 mL). The vial was capped and irradiated 5 min at 145 °C (monitored by IR sensor) in a Biotage Emrys Optimizer instrument with a 75-W microwave power. No ramp was applied. After rapid cooling in an ice bath, the reaction mixture was diluted with Et₂O (20 mL) and filtered under vacuum. The precipitate was rinsed with Et₂O (2 × 10 mL) and dried under vacuum to afford pure 1,3-dimesitylimidazolinium chloride (6.57 g, 97% yield).