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

Abstract

Lower Lewis acidity boranes demonstrate greater tolerance to combinations of water/strong Brønsted bases than B(C ₆F ₅) ₃, this enables Si−H bond activation by a frustrated Lewis pair (FLP) mechanism to proceed in the presence of H ₂O/alkylamines. Specifically, BPh ₃ has improved water tolerance in the presence of alkylamines as the Brønsted acidic adduct H ₂O–BPh ₃ does not undergo irreversible deprotonation with aliphatic amines in contrast to H ₂O–B(C ₆F ₅) ₃. Therefore BPh ₃ is a catalyst for the reductive amination of aldehydes and ketones with alkylamines using silanes as reductants. A range of amines inaccessible using B(C ₆F ₅) ₃ as catalyst, were accessible by reductive amination catalysed by BPh ₃ via an operationally simple methodology requiring no purification of BPh ₃ or reagents/solvent. BPh ₃ has a complementary reductive amination scope to B(C ₆F ₅) 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 p K _a values of the water–borane adducts and the greater susceptibility of BPh ₃ species towards protodeboronation. An understanding of the deactivation processes occurring using B(C ₆F ₅) ₃ and BPh ₃ as reductive amination catalysts led to the identification of a third triarylborane, B(3,5‐Cl ₂C ₆H ₃) ₃, that has a broader substrate scope being able to catalyse the reductive amination of both aryl and alkyl amines with carbonyls.