A Convenient Synthesis of Oxazolines and Imidazolines from Aromatic Aldehydes with Pyridinium Hydrobromide Perbromide in Water

Abstract

Various 2-oxazolines were prepared from aromatic aldehydes and 2-aminoethanol with pyridinium hydrobromide per­bromide in water at room temperature. 2-Imidazolines were also obtained in good yields from aromatic aldehydes and ethylenediamine under the same reaction conditions.

oxazoline - imidazoline - pyridinium hydrobromide perbromide - aldehyde - 2-aminoethanol - ethylenediamine

References and Notes

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10

The reaction of ethylenediamine and aromatic dialdehydes such as terephthalaldehyde, isophthalaldehyde, and phthalaldehyde with PHPB took place to give no respective diimidazilines under the same reaction conditions.

11

It was assumed that steric hindrance between the formyl group and the methyl group exerted influence on the decrease in the yield of the corresponding oxazoline.

12

The products were complicated. The steric hindrance of two formyl groups in phthalaldehyde was supposed to inhibit generation of the respective dioxazoline.

13

The yields of nonanal (ca. 50%) and hydroxyimine (ca. 20%) were determined by ¹H NMR analysis of crude products.

14

Typical Procedure for Preparation of Imidazoline from Aldehyde.
To a solution of 4-bromobenzaldehyde (1, 46 mg, 0.25 mmol) in H₂O (6 mL) were added PHPB (160 mg, 0.5 mmol). Etylenediamine [90 mg (100 µL), 1.5 mmol] was added. After stirring for 15 h at r.t., the reaction mixture was treated with 0.5 M aq Na₂S₂O₃ and extracted with EtOAc. The organic layer was washed with 0.5 M aq Na₂S₂O₃ and successively washed with sat. aq NaCl and dried over MgSO₄. After removal of the solvent in vacuo, the residue was purified by column chromatography on silica gel (Wakogel C-200) with CHCl₃. Imidazoline 1a (50 mg, 0.22 mmol) was obtained in 88% yield.

15

Typical Procedure for Preparation of Oxazoline from Aldehyde.
To a solution of 4-bromobenzaldehyde (1, 46 mg, 0.25 mmol) in H₂O (6 mL) were added PHPB (240 mg, 0.75 mmol). 2-Aminoethanol [92 mg (90 µL), 1.5 mmol] was added. After stirring for 23 h at r.t., the reaction mixture was treated with 0.5 M aq Na₂S₂O₃ and extracted with EtOAc. The organic layer was washed with 0.5 M aq Na₂S₂O₃ and successively washed with sat. aq NaCl and dried over MgSO₄. After removal of the solvent in vacuo, the residue was purified by column chromatography on silica gel (Wakogel C-200) with CHCl₃. Oxazoline 1c (50 mg, 0.22 mmol) was obtained in 89% yield.