Metal-Free Borylation of Heteroarenes Using Ambiphilic Aminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C–H Bond Activation

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.