Thiourea-Catalyzed Transfer Hydrogenation of Aldimines

Abstract

The present letter reports on the thiourea-catalyzed transfer hydrogenation of imines through hydrogen-bonding activation with Hantzsch 1,4-dihydropyridine as the hydrogen source. A ­variety of aromatic as well as aliphatic aldimines can be reduced to give the respective amines under acid- and metal-free reaction conditions.

Key words

Hantzsch dihydropyridine - hydrogen bonds - organo­catalysis - thiourea - transfer hydrogenation

References and Notes

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13

Since it is difficult to avoid the formation of carboxylic acids generated from aldehydes during the reaction, 10 mol% of NaHCO₃ was added.

14

General Procedure for the Thiourea-Catalyzed Transfer Hydrogenation of Aldimines
In a typical experiment the aldimine 1 (1.0 mmol), thiourea 3 (1 mol%) and Hantzsch dihydropyridine 2 (1.1 equiv) were suspended in anhyd CH₂Cl₂ (5 mL) in a flask. The resulting mixture was allowed to stir at r.t. for 15 h. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel using mixtures of PE and Et₂O to afford the pure corresponding amine 4. All compounds were fully characterized on the basis of IR, ¹H NMR, ¹³C NMR and HRMS.
Compound 4a: light yellow oil. IR (film): 3418, 1699, 1602, 1505, 1179, 1064 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 4.20 (s, 2 H), 6.50-6.52 (d, 2 H, J = 8.71 Hz), 6.59-6.63 (t, 1 H, J = 7.41 Hz), 7.04-7.09 (t, 2 H, J = 7.40 Hz), 7.14-7.27 (m, 5 H). ¹³C NMR (100 MHz, CDCl₃): δ = 48.2, 112.8, 117.5, 127.1, 127.4, 128.5, 129.2, 139.4, 148.1. HRMS: m/z calcd for C₁₃H₁₃N: 183.10425; found: 183.10347.