N-Heterocycle-Ligated Borocations as Highly Tunable Carbon Lewis Acids.

James E Radcliffe,Jay J Dunsford,Jessica Cid,Valerio Fasano,Michael J Ingleson

doi:10.1021/acs.organomet.7b00779

Abstract

The relative (to BEt3) hydride ion affinity (HIA) of a series of acridine borenium salts has been calculated, with some HIAs found to be similar to that for [Ph3C]+. The HIA at the acridine C9 position is controlled by both acridine and the boron substituents, the latter presumably affecting the strength of the B=N bond in the acridane-BY2 products from hydride transfer. Through a range of hydride abstraction benchmarking reactions against organic hydride donors the experimental HIA of [F5acr-BCat]+ (cat = catechol, F5acr = 1,2,3,4,7-pentafluoroacridine) has been confirmed to be extremely high and closely comparable to that of [Ph3C]+. The high HIA of [F5acr-BCat]+ enables H2 and alkene activation in a FLP with 2,6-di-tert-butylpyridine. Finally, the HIA of pyridine and quinoline borenium cations has been determined, with the HIA at boron in [PinB(amine)]+ (pin = pinacol, amine = pyridine or quinoline) found to be relatively low. This enabled the hydroboration of pyridine and quinoline by HBPin to be achieved through the addition of 5–10 mol % of bench-stable cationic carbon Lewis acids such as 2-phenyl-N,N-dimethylimidazolium salts.

Highlights

Borenium cations are three-coordinate boron compounds possessing a unit positive charge which often results in significant Lewis acidity at the boron center
These cations have been applied in numerous stoichiometric and catalytic transformations (e.g., as chiral Lewis acid catalysts6 and in frustrated Lewis pair (FLP) mediated reductions)
The electrondeficient nature of the {BY2}+ fragment in an [L→BY2]+ borenium cation results in considerable modulation of the electronic structure of the neutral donor ligand L

Summary

■ INTRODUCTION

Borenium cations are three-coordinate boron compounds possessing a unit positive charge which often results in significant Lewis acidity at the boron center. Recently, these cations have been applied in numerous stoichiometric (including hydroboration, C−H borylation, and carboboration5) and catalytic transformations (e.g., as chiral Lewis acid catalysts and in frustrated Lewis pair (FLP) mediated reductions). These applications exploit the boron center as the locus of electrophilic reactivity. Upon further heating no changes were observed, again indicating that the reaction had reached equilibrium A F5acr-AlCl3/ Ph3CH reaction mixture was subjected to conditions identical with those for the partially successful [4][AlCl4]/Ph3CH hydride transfer reaction, but in this case using 6 no hydride transfer was observed This is consistent with calculated HIA values and indicates that the borenium [4]+ is required in order to abstract hydride from triphenylmethane but that its considerable electrophilicity at boron and carbon results in currently unidentified side reactions.

■ ACKNOWLEDGMENTS

Findings

■ REFERENCES