Metal-Free Borylation of Heteroarenes Using Ambiphilic Aminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C–H Bond Activation
journal contributionposted on 13.09.2017, 00:00 by Julien Légaré Lavergne, Arumugam Jayaraman, Luis C. Misal Castro, Étienne Rochette, Frédéric-Georges Fontaine
Two novel frustrated Lewis pair (FLP) aminoboranes, (1-Pip-2-BH2-C6H4)2 (2; Pip = piperidyl) and (1-NEt2-2-BH2-C6H4)2 (3; NEt2 = diethylamino), were synthesized, and their structural features were elucidated both in solution and in the solid state. The reactivity of these species for the borylation of heteroarenes was investigated and compared to previously reported (1-TMP-2-BH2-C6H4)2 (1; TMP = tetramethylpiperidyl) and (1-NMe2-2-BH2-C6H4)2 (4; NMe2 = dimethylamino). It was shown that 2 and 3 are more active catalysts for the borylation of heteroarenes than the bulkier analogue 1. Kinetic studies and density functional theory calculations were performed with 1 and 2 to ascertain the influence of the amino group of this FLP-catalyzed transformation. The C–H activation step was found to be more facile with smaller amines at the expense of a more difficult dissociation of the dimeric species. The bench-stable fluoroborate salts of all catalysts (1F–4F) have been synthesized and tested for the borylation reaction. The new precatalysts 2F and 3F are showing higher reaction rates and yields for multigram-scale syntheses.
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FLP-catalyzed transformationBHNEt 2reaction ratesheteroarenePipmultigram-scale synthesesC 6 H 4precatalysts 2 Ftheory calculationsLewis pair-2-Kinetic studiesTMP-2-BHborylation reactionMetal-Free BorylationAmbiphilic Aminoboranesdimeric species3 FNMe 2bulkier analogue 1bench-stable fluoroborate salts