Iron-Catalyzed Direct Transformation of Benzylic Amines into Carbonyl Compounds in Water

Maki Minakawa; Takashi Sasaki

doi:10.1055/s-0037-1611881

Synlett, Table of Contents

Synlett 2019; 30(13): 1597-1601
DOI: 10.1055/s-0037-1611881

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Iron-Catalyzed Direct Transformation of Benzylic Amines into Carbonyl Compounds in Water

Maki Minakawa∗

Department of Applied Chemistry, Chemical Engineering and Biochemical Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa, Yamagata 992-8510, Japan Email: minakawa@yz.yamagata-u.ac.jp

,

Takashi Sasaki

› Author Affiliations

Abstract

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Abstract

Fe-catalyzed direct transformation of benzylic amines into carbonyl compounds was performed in H₂O. The reaction of benzylic amines with formaldehyde in the presence of FeCl₃·6H₂O in H₂O afforded the corresponding carbonyl compounds (80 °C to reflux conditions; 14 examples, up to 94% yield). O¹⁸-labeling experiments indicated that the O atom in the generated carbonyl is derived from H₂O.

Key words

direct transformation - benzylic amines - carbonyl compounds - iron catalysis - formaldehyde

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References

References and Notes

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5 The reaction with an excess of amine to give an aminal seems to predominate over the conventional reaction (Scheme 1). On the other hand, the reaction with an excess of formaldehyde prevented the formation of an aminal in the presence of FeCl₃·6H₂O.
6 0.2 M aq HCl. The concentration was calculated for 1.0 mol equiv of HCl formed from FeCl₃·6H₂O (20 mol% based on amine).
7 0.6 M aq HCl. The concentration was calculated for 3.0 mol equiv of HCl formed from FeCl₃·6H₂O (20 mol% based on amine).
8 Benzophenone (3a); Typical Procedure (Table [2], Entry 1) A vial was charged with (diphenylmethyl)amine (1a; 91.6 mg, 0.5 mmol), FeCl₃·6H₂O (27.0 mg, 20 mol%), and paraformaldehyde (2; 19.2 mg, 0.6 mmol) under air. H₂O (0.5 mL) was added and the mixture was stirred at 100 °C for 18 h. The aqueous phase was then extracted with EtOAc (×3) and dried (Na₂SO₄). The extract was filtered through paper and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–hexane (1:10)] to give a white solid; yield: 85.2 mg (94%); mp 47.5–49.0 °C. ¹H NMR (500 MHz, CDCl₃): δ = 7.49 (t, J = 6.2 Hz, 4 H), 7.58–7.61 (m, 2 H), 7.81 (d, J = 6.2 Hz, 4 H). ¹³C NMR (125 MHz, CDCl₃): δ = 128.4, 130.2, 132.6, 137.7, 196.9.
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11 Although the reactions of (pyridin-4-ylmethyl)amine, (pyrazin-2-ylmethyl)amine, and furfurylamine with formaldehyde were also performed under similar reaction conditions, the corresponding carbonyl compounds were not obtained. In addition, 4-nitrobenzylamine with formaldehyde gave 4-nitrobenzaldehyde in 7% yield under similar reaction conditions.
12 Without FeCl₃·6H₂O, N-(phenylmethylene)methanamine (6) was detected in quantitative yield after the reaction.
13 The yields were determined by ¹H NMR analysis of the crude mixtures (based on an internal standard). After the reaction, benzaldehyde (3f) was also detected in 35% yield.
14 The products ratio (O¹⁸/O¹⁶) was determined by GC–MS analysis.

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