Expanding Water/Base Tolerant Frustrated Lewis Pair Chemistry to Alkylamines Enables Broad Scope Reductive Aminations

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• 作者：Valerio Fasano and Dr. Michael J. Ingleson
• 刊名：Chemistry - A European Journal
• 出版年：2017
• 出版时间：February 10, 2017
• 年：2017
• 卷：23
• 期：9
• 页码：2217-2224
• 全文大小：1483K
• ISSN：1521-3765

文摘

Lower Lewis acidity boranes demonstrate greater tolerance to combinations of water/strong Brønsted bases than B(C₆F₅)₃, this enables Si−H bond activation by a frustrated Lewis pair (FLP) mechanism to proceed in the presence of H₂O/alkylamines. Specifically, BPh₃ has improved water tolerance in the presence of alkylamines as the Brønsted acidic adduct H₂O–BPh₃ does not undergo irreversible deprotonation with aliphatic amines in contrast to H₂O–B(C₆F₅)₃. Therefore BPh₃ is a catalyst for the reductive amination of aldehydes and ketones with alkylamines using silanes as reductants. A range of amines inaccessible using B(C₆F₅)₃ as catalyst, were accessible by reductive amination catalysed by BPh₃ via an operationally simple methodology requiring no purification of BPh₃ or reagents/solvent. BPh₃ has a complementary reductive amination scope to B(C₆F₅)₃ with the former not an effective catalyst for the reductive amination of arylamines, while the latter is not an effective catalyst for the reductive amination of alkylamines. This disparity is due to the different pK_a values of the water–borane adducts and the greater susceptibility of BPh₃ species towards protodeboronation. An understanding of the deactivation processes occurring using B(C₆F₅)₃ and BPh₃ as reductive amination catalysts led to the identification of a third triarylborane, B(3,5-Cl₂C₆H₃)₃, that has a broader substrate scope being able to catalyse the reductive amination of both aryl and alkyl amines with carbonyls.