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

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, BPh₃has improved water tolerance in the presence of alkylamines as the Brønsted acidic adduct H₂O–BPh₃does not undergo irreversible deprotonation with aliphatic amines in contrast to H₂O–B(C₆F₅)₃. Therefore BPh₃is a catalyst for the reductive amination of aldehydes and ketones with alkylamines using silanes as reductants. A range of amines inaccessible using B(C₆F₅)₃as catalyst, were accessible by reductive amination catalysed by BPh₃via an operationally simple methodology requiring no purification of BPh₃or reagents/solvent. BPh₃has a complementary reductive amination scope to B(C₆F₅)₃with 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_avalues of the water–borane adducts and the greater susceptibility of BPh₃species towards protodeboronation. An understanding of the deactivation processes occurring using B(C₆F₅)₃and BPh₃as 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.