Exhaustive Chemoselective Reduction of Nitriles by Catalytic Hydrosilylation Involving Cooperative Si–H Bond Activation

Published as part of the Cluster Silicon in Synthesis and Catalysis

Abstract

A chemoselective method for the catalytic hydrosilylation of nitriles to either the imine or amine oxidation level is reported. The chemoselectivity is controlled by the hydrosilane used. The usefulness of the nitrile-to-amine reduction is demonstrated for a diverse set of aromatic and aliphatic nitriles, and the amines are easily isolated after hydrolysis as their hydrochloride salts. This exhaustive nitrile reduction proceeds at room temperature.

Key words

chemoselectivity - cooperativity - hydrosilylation - reduction - silicon

• References and Notes

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• 20 General Procedure for Nitrile-to-Amine Reduction In a glove box, a flame-dried GLC vial equipped with a magnetic stir bar is charged with [3a]⁺[BAr^F ₄]^– (1.0 mol%) and Me₂PhSiH (2a, 2.1 or 5.0 equiv). The indicated nitrile is added either in the glove box (for solid starting materials) or by microsyringe outside the glove box, and the resulting reaction mixture is maintained at r.t. for the indicated time. The reaction is quenched by the addition of a mixture of cyclohexane and tert-butyl methyl ether (90:10) containing 4% Et₃N (0.5 mL), and the resulting solution is filtered through a pad of Celite^® coated by a small layer of silica gel with a solution of cyclohexane and tert-butyl methyl ether (90:10) containing 4% Et₃N (3–4 mL) as eluent. Solvents are removed under reduced pressure, and the residue is dissolved in Et₂O (1 mL) followed by addition of HCl (2 M in Et₂O, 1.0 mL, 2.0 mmol, 10 equiv). The resulting suspension is stirred for 1 h and filtered, affording the amines as hydrochloride salts as white to yellow solids.
• 21 General Procedure for Nitrile-to-Imine Reduction In a glove box, a flame-dried GLC vial equipped with a magnetic stir bar is charged with [3a]⁺[BAr^F ₄]^– (1.0 mol%), Et₃SiH (2b, 2.0 equiv), and mesitylene (10 μL, 8.7 mg, 0.20 mmol, internal standard). The indicated nitrile is added either in the glove box (for solid starting materials) or by microsyringe outside the glove box, and the resulting reaction mixture is maintained at r.t. for 18 h. The mixture is then dissolved in CD₂Cl₂ (0.6 mL) and transferred into a NMR tube. ¹H NMR spectroscopy is used to determine the yield with reference to mesitylene.