Microwave-Assisted Rapid and Selective Synthesis of cis- and trans-2,4,5-Triarylimidazolines from Aromatic Aldehydes

Abstract

Microwave irradiation of a mixture of aromatic aldehydes and hexamethyldisilazane in the presence of a solid catalyst such as alumina afforded methanediamines which were efficiently converted to either cis- or trans-2,4,5-triarylimidazolines depending on the base used. A one-pot selective synthesis of cis- and trans-imidazolines from aromatic aldehydes was achieved under microwave irradiation.

microwave - imidazoline - methanediamine - hexamethyldisilazane - solvent-free

References

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11

Basic, acidic, and neutral alumina showed similar results.

12

Irradiation in the presence of silica gel for 10 min gave 3a in 44% yield together with 2a and other unidentified products, whereas formation of 3a was not observed in the irradiation with alumina for 10 min.

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15

Irradiation of cis-3a with 1.0 equiv of t-BuOK/t-BuOH for 5 min gave trans-3a quantitatively.

16

Typical Procedure for Preparation of cis -2,4,5-Triphenylimidazoline ( cis -3a) from 1a. A mixture of benzaldehyde 1a (209 mg, 1.97 mmol), HMDS (620 µL, 480 mg, 2.95 mmol), and basic alumina (100 mg, Wako Pure Chemical Industries Ltd., column chromatography grade, 200 mesh, pH 9.0-11.0) was charged in a test tube equipped with a cooler. The mixture was irradiated under argon atmosphere for 5 min in a microwave oven (500 W, 2450 MHz). Then, DBU (100 µL, 105 mg, 0.69 mmol) was added and the mixture was irradiated for an additional 5 min. After cooling, alumina was filtered and washed with CH₂Cl₂. The obtained organic solution was concentrated under vacuum to leave a residue, which was purified by silica gel column chromatography (hexane/EtOAc 9:1) to give 164 mg (85%) of cis-3a.

17

Typical Procedure for Preparation of trans -2,4,5-Triphenylimidazoline ( trans -3a) from 1a. A mixture of 1a (209 mg, 1.97 mmol), HMDS (620 µL, 480 mg, 2.95 mmol) and, basic alumina (100 mg) was irradiated for 5 min, and then, t-BuOK (77 mg, 0.69 mmol) and DBU (100 µL, 105 mg 0.69 mmol) were added to the mixture which was further irradiated for an additional 5 min. Work-up and purification as described above gave 159 mg (81%) of trans-3a.