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Synlett 2008(9): 1289-1292  
DOI: 10.1055/s-2008-1072748
LETTER
© Georg Thieme Verlag Stuttgart · New York

Hydrogen-Transfer Reductive Amination of Aldehydes Catalysed by Nickel Nanoparticles

Francisco Alonso*, Paola Riente, Miguel Yus*
Departamento de Química Orgánica, Facultad de Ciencias and Instituto de Química Orgánica (ISO), Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
Fax: +34(96)5903549; e-Mail: falonso@ua.es; e-Mail: yus@ua.es;
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Publication History

Received 12 February 2008
Publication Date:
07 May 2008 (online)

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Abstract

Nickel nanoparticles have been found to catalyse the reductive amination of aldehydes by transfer hydrogenation with isopropanol at 76 °C.

Key words

reductive amination - aldehydes - imines - nickel nanoparticles - hydrogen transfer - amines

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General Procedure for the Hydrogen-Transfer Reductive Amination of Aldehydes Catalysed by Nickel Nanoparticles
A solution of the aldehyde (5 mmol) and the amine (5 mmol) was prepared in i-PrOH (10 mL) and stirred for about 1 h. Meanwhile, NiCl2 (130 mg, 1 mmol) was added over a suspension of lithium (14 mg, 2 mmol) and DTBB (13 mg, 0.05 mmol) in THF (2 mL) at r.t. under argon. The reaction mixture, which was initially dark blue, changed to black indicating that nickel(0) was formed. After 10 min, the initially prepared solution of the aldehyde and amine was added to the nickel suspension. The reaction mixture was warmed up to 76 °C and monitored by GLC-MS. The resulting suspension was filtered through a pad containing Celite and the filtrate was dried over MgSO4. The residue obtained after removal of the solvent (15 Torr), when necessary, was purified by column chromatography (SiO2, hexane-EtOAc) to give the pure secondary amine.
N-Benzylaniline, N-benzyl-2-phenylethanamine, dibenzylamine, and N-benzyl-1-phenylethanamine were characterized by comparison of their physical and spectroscopic analyses with those of commercially available samples (Aldrich). N-Benzyl-4-methylaniline, [17] N-benzyl-2-methylaniline, [17] N-benzyl-3,5-dimethoxyaniline, [18] N-benzyloctan-1-amine, [19] N-(4-methylbenzyl)aniline, [20] N-(4-methylbenzyl)octan-1-amine, [21] N-(4-methylbenzyl)-2-phenylethanamine, [22] N-benzyl-4-methylbenzylamine, [23] N-(4-methoxybenzyl)aniline, [24] N-(4-methoxybenzyl)-4-methylaniline, [25] N-(4-methoxybenzyl)octan-1-amine, [21] N-(4-methoxybenzyl)-2-phenylethanamine, [26] N-[(furan-2-yl)methyl]octan-1-amine, N-(n-decyl)aniline, [27] and N-(cyclohexylmethyl)aniline [28] were characterised by comparison of their physical and spectroscopic data with those described in the literature.
Spectroscopic Data of New Compounds
N -(4-Methylbenzyl)-2-methylpropan-1-amine Yellow oil; R f = 0.18 (hexane-EtOAc, 1:1). IR (neat): 3345 cm-1. 1H NMR (300 MHz, CDCl3): δ = 0.91 (d, J = 6.7, 6 H, 2 × CH 3CH), 1.69 (br s, 1 H, NH), 1.76-1.85 (m, 1 H, CHCH3), 2.34 (s, 3 H, CH3C), 2.42 (d, J = 7.0, 2 H, CH 2CH), 3.71 (s, 2 H, CH2C), 7.15, 7.19 (AB system, J = 8.2, 4 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 20.2, 20.8 (3 × CH3), 27.5 (CHCH3), 53.1, 56.6 (2 × CH2), 128.0, 129.0 (4 × ArCH), 136.0, 137.0 (2 × ArC). MS (70 eV): m/z = 177 [M+], 134, 106, 105, 77. HRMS: m/z [M+] calcd for C12H19N: 177.1517; found: 177.1516.
N -[(Furan-2-yl)methyl]octan-1-amine Yellow oil; R f = 0.24 (hexane-EtOAc, 1:1). IR (neat): 3325 cm-1. 1H NMR (300 MHz, CDCl3): δ = 0.86 (t, J = 6.2, 3 H, CH3), 1.26 [br s, 10 H, (CH 2)5CH3], 1.48 (m, 2 H, CH 2CH2N), 1.98 (br s, 1 H, NH), 2.59 (t, J = 6.9, 2 H, CH2CH 2N), 3.77 (s, 2 H, CH2C), 6.16, 6.30, 7.35 (3 s, 3 H, ArH). 13C NMR (75 MHz, CDCl3): δ = 14.0 (CH3), 22.6, 27.3, 29.2, 29.4, 29.9, 31.7, 46.2, 49.1 (8 × CH2), 106.7, 110.0, 142.0 (3 × ArCH), 154.0 (ArC). MS (70 eV):
m/z = 209 [M+], 110, 81. HRMS: m/z [M+] calcd for C13H23NO: 209.1779; found: 209.1777.

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In principle, deactivation of the catalyst cannot be attributed to particle agglomeration as confirmed by transmission electron microscopy (TEM) analysis of a sample of the reused suspension. Further studies to understand the properties, reactivity, and deactivation mechanism of the catalyst are under way.