Gallium(III) Triflate Catalyzed Direct Reductive Amination of Aldehydes
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Gallium(III) Triflate Catalyzed Direct Reductive Amination of Aldehydes G. K. Surya Prakash • Clement Do Thomas Mathew • George A. Olah
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Received: 18 March 2010 / Accepted: 9 April 2010 / Published online: 28 April 2010 Ó Springer Science+Business Media, LLC 2010
Abstract Direct hydroamination of aldehydes and ketones provides one-step entry into desired a-aminoalkane derivatives which are important synthons for many biologically active molecules. The reductive amination of aldehydes in the presence of silanes has been effectively promoted by Ga(OTf)3 as a catalyst. Mild conditions, easy work-up and high purity of products with excellent yields are the major advantages of this method. Keywords Gallium triflate Hydroamination Organosilanes Green chemistry Organic synthetic transformations
1 Introduction Amines and their derivatives act as important synthetic precursors in organic chemistry. They are highly versatile building blocks for various organic target molecules and are essential precursors to a variety of biologically active compounds, such as pharmaceuticals [1] and agrochemicals [2]. Due to their importance, numerous methods have been developed for the preparation of amines. Reductive amination of aldehydes and ketones, in which a mixture of an aldehyde or ketone and an amine is treated with a reductant in a one-pot fashion, is one of the most useful methods for the
G. K. Surya Prakash (&) C. Do T. Mathew (&) G. A. Olah Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, CA 90089-1661, USA e-mail: [email protected] T. Mathew e-mail: [email protected]
preparation of secondary or tertiary amines and related functional compounds in biological and chemical systems [3–6]. The major advantage of this reaction is that there is no need to isolate intermediate imines, in particular, in cases where condensation of aromatic amines with aliphatic carbonyls would give expectedly unstable imines. The choice of the reducing agent is very crucial for the success of the reaction, since the selective reduction of imines (or iminium ions) by the reducing agent over aldehydes or ketones is required under the reaction conditions [7, 8]. The two most commonly used direct reductive amination methods differ in the nature of the reducing agent. The first method is catalytic hydrogenation with platinum, palladium, or nickel catalysts. This is an economical and effective reductive amination method, particularly in large scale reactions. However, hydrogenation has limited use with compounds containing carbon–carbon multiple bonds and in the presence of reducible functional groups such as nitro and cyano groups. Inhibition of the catalytic activity of the catalyst by sulfur compounds has also been observed. The second method utilizes hydride based reducing agents [8]. Among the hydride based reducing agents [5–9], different borohydrides are frequently used to carry out this transformation, mainly sodium cyanoborohydride (NaBH3CN) and sodium triacetoxyborohydride
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