Cyclometalated Iridium Complexes as Highly Active Catalysts for the Hydrogenation of Imines

Abstract

A robust cyclometalated iridium catalyst has been developed for highly effective hydrogenation of imines. With very low catalyst loading (down to 0.005%), good to excellent yields have been achieved for a range of substrates in a short reaction time under mild conditions, providing an easy, efficient protocol for making amines.

Key words

cyclometalated complexes - hydrogenation - imines - iridium - iridicycles

• References and Notes


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• 8 General Procedure for the Synthesis of the Cyclometalated Iridium Complexes: An oven-dried Schlenk tube containing a magnetic stirrer bar was charged with [Cp*IrCl₂]₂ (1 equiv), imine ligand⁵ (2 equiv) and NaOAc (10 equiv). Following degassing with N₂ (3 ×), freshly distilled CH₂Cl₂ was injected. The resulting mixture was stirred at r.t. overnight. The reaction mixture was then filtered through Celite^®, washed with CH₂Cl₂ and the combined organic solvents were concentrated in vacuo. The resulting solid was washed with Et₂O–hexane and recrystallised from CH₂Cl₂–hexane. C1: orange powder (90.5 mg, 98%). ¹H NMR (400 MHz, CDCl₃; 258 K): δ = 7.79 (br, 1 H), 7.62–7.64 (d, J = 7.6 Hz, 1 H), 7.12–7.16 (m, 1 H), 6.92–6.99 (m, 3 H), 6.76–6.78 (d, J = 7.2 Hz, 1 H), 3.85 (s, 3 H), 2.63–2.97 (m, 4 H), 1.87–1.88 (m, 2 H), 1.43 (s, 15 H). ¹³C NMR (100 MHz, CDCl₃; 258 K): δ = 182.9, 168.4, 157.4, 144.6, 143.4, 143.0, 132.7, 132.4, 125.2, 123.3, 121.2, 115.0, 112.3, 88.9, 55.7, 30.4, 29.2, 23.8, 15.5, 8.9. Anal. Calcd for C₂₇H₃₁ClIrNO: C, 52.88; H, 5.10; N, 2.61. Found: C, 52.69; H, 5.12; N, 2.09. C2: pale orange powder (31.7 mg, 98%). ¹H NMR (400 MHz, CDCl₃; 258 K): δ = 7.76–7.79 (m, 1 H), 7.15–7.16 (d, J = 1.6 Hz, 1 H), 6.82–6.93 (m, 3 H), 6.33 (s, 1 H), 3.86 (s, 3 H), 3.84 (s, 3 H), 2.56–2.93 (m, 4 H), 1.84–1.85 (m, 2 H), 1.42 (s, 15 H). ¹³C NMR (100 MHz, CDCl₃): δ = 181.4, 170.7, 162.3, 157.5, 144.7, 143.5, 138.4, 124.8, 117.6, 114.2, 113.8, 113.5, 106.9, 88.7, 55.6, 55.0, 30.2, 29.5, 23.9, 8.7. Anal. Calcd for C₂₈H₃₃ClIrNO₂: C, 52.28; H, 5.17; N, 2.18. Found: C, 52.43; H, 5.48; N, 1.94. Procedure for the Hydrogenation of Imines with Cyclometalated Iridium Complex C1: A glass liner containing a stirrer bar was charged with the requisite imine (0.3 mmol) and TFE (0.75 mL). The mixture was stirred until the imine was dissolved and catalyst C1 was then added (10 μL stock solution, made by dissolving C1 in TFE: 0.03 mmol C1 in 10 mL TFE). The glass liner was then placed into an autoclave followed by degassing with H₂ (3 ×). The hydrogenation was carried out at 20 bar H₂ with stirring at 85 °C for 1 h. After the reaction was finished, the autoclave was allowed to cool to r.t. The hydrogen gas was then carefully released in the fume hood, the solution transferred to a flask and concentrated in vacuo to afford the crude product. Flash chromatographic purification with a column of silica gel eluted with petroleum ether–EtOAc (20:1 → 5:1) yielded the desired amine product.