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Development of Amine-Boranes and Derivatives for Reductive Amination
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Development of Amine-Boranes and Derivatives for Reductive Amination
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Development of Amine-Boranes and Derivatives for Reductive Amination
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Dissertation
Development of Amine-Boranes and Derivatives for Reductive Amination
2023
نظرة عامة
Reported herein is a summary of the reductive amination of carbonyls as a facile strategy for the synthesis of amines. The history, applications, and chemical properties of various frequently utilized reductive amination reagents have been described providing an insight into the significance of chemoselectivity of the reducing agent in achieving an effective protocol. The key to a successful reductive amination is the ability of the reducing agent to selectively reduce imines in the presence of the carbonyl starting material. Utilizing the recent advances in synthetic methodologies for obtaining amine-boranes and their versatility for selective reductions, amine-boranes and their derivatives have been explored for developing improved reductive amination procedures. First, Ammonia-borane is described as an accessible, air-and moisture-stable, and atom-economical reducing agent which provides amines in high-to-excellent yields in the presence of sub-stoichiometric trimethyl borate as a Lewis acid promoter. This solvent-free route is environment-friendly with a low process mass intensity and has been demonstrated to perform at a mole scale. While the substrate scope of this procedure extends to the amination of aldehydes and ketones with aliphatic and aromatic amines, a two-step indirect approach is necessary in the case of challenging substrates. These include (i) aryl ketones, (ii) deactivated anilines, and (iii) aldehydes with 2°-amines due to the sluggish generation of imines and competitive reduction of carbonyl. To address these limitations, amine-boranes are modified via reaction with trifluoroacetic acid to provide novel monotrifluoroacetoxyborane-amine (TFAB-amine) complexes. This new class of reagents serves as mild and highly chemoselective agents for the direct reductive amination of aldehydes and ketones. Among these, TFAB-NEt3 is established as superior, compared to widely used reductive amination reagents, providing improved yields even for challenging substrates. The application of the monotrifluoroacetoxyborane-derivatives has been extended for the synthesis of lactams from keto-acids and amino-acids via a tandem reductive amination-amidation protocol.
