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

Valerio Fasano,Michael J Ingleson

doi:10.1002/chem.201605466

Valerio Fasano, Michael J Ingleson

Open Access

https://doi.org/10.1002/chem.201605466

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Abstract

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.

Highlights

Considerable progress in frustrated Lewis pair (FLP) chemistry has been achieved in the last decade principally using tris(pentafluorophenyl)borane, B(C6F5)3.[1]
The Brønsted acidity of H2O–B(C6F5)3 was determined by Parkin and co-workers to be comparable to that of HCl (8.5 in MeCN).[7a]. This poses a limit to the water tolerance of these fluorinated arylboranes in the presence of certain Brønsted bases because irreversible deprotonation of the borane–water adduct yields an inactive hydroxytriarylborate anion
Coordination of BnNH2 to BPh3 is reversible at room temperature as addition of benzaldehyde led to rapid imine formation, the absence of any observable [HO–BPh3]À is attributed to the lower Brønsted acidity of H2O–BPh3

Summary

Introduction

Considerable progress in frustrated Lewis pair (FLP) chemistry has been achieved in the last decade principally using tris(pentafluorophenyl)borane, B(C6F5)3.[1]. BPh3 does still possess sufficient hydridophilicity to be useful as a catalyst in FLP-type reactions as recently demonstrated.[13,14] While H2O–B(C6F5)3 is well documented,[7] the corresponding H2O–BPh3 adduct is less studied, its ability to act as a Brønsted acid.[16,17,18,19] we report an extension to the water and base tolerance of boranes to strong amine bases, focusing, in particular on the triarylborane-catalysed reductive amination of aldehydes/ketones with alkylamines using silanes as reducing agents.

Results

Conclusion