Metal-free catalytic C-H bond activation and borylation of heteroarenes

A metal-free catalyst born of frustration Boron (a Lewis acid) and nitrogen or phosphorus fragments (both Lewis bases) tend to pair up. Keeping them separated on opposite ends of the same molecule creates a “frustrated” Lewis pair. Such molecules can manifest powerful reactivity, such as scission of the hydrogen-hydrogen bond in H2. Légaré et al. now extend this reactivity to the cleavage of carbon-hydrogen bonds in heteroaromatic compounds such as furans and pyrroles (see the Perspective by Bose and Marder). Their frustrated Lewis pair complex catalyzed borylation of these compounds. The selectivity pattern of the reaction complemented that seen with the metal catalysts conventionally used. Science, this issue p. 513; see also p. 473 Boron and nitrogen centers cooperatively catalyze a reaction that has previously relied on transition metal catalysts. [Also see Perspective by Bose and Marder] Transition metal complexes are efficient catalysts for the C-H bond functionalization of heteroarenes to generate useful products for the pharmaceutical and agricultural industries. However, the costly need to remove potentially toxic trace metals from the end products has prompted great interest in developing metal-free catalysts that can mimic metallic systems. We demonstrated that the borane (1-TMP-2-BH2-C6H4)2 (TMP, 2,2,6,6-tetramethylpiperidine) can activate the C-H bonds of heteroarenes and catalyze the borylation of furans, pyrroles, and electron-rich thiophenes. The selectivities complement those observed with most transition metal catalysts reported for this transformation.