Flash Communication: Arene η6‑Coordination to Chromium Tricarbonyl Promotes Cobalt-Catalyzed C-H Borylation of Electron-Rich Arenes.

The cobalt-catalyzed borylation of electron-rich arenes η6-coordinated to [Cr-(CO)3] has been demonstrated using the pyridine-(dicarbene) pincer-supported dicobalt alkoxide (iPrACNC)-Co2(tBuO)4 as the precatalyst. Metal coordination accelerated the borylation of the arene by 20-200 fold, enabling reactivity with substrates that were otherwise sluggish or inactive for cobalt-catalyzed C-H functionalization. The cobalt-catalyzed borylation of η6-coordinated electron-rich arenes proceeded at ambient temperature in the absence of HBPin scavengers. Arene coordination to chromium also proved to be an effective strategy for reversal of the traditional chemoselectivity of metal-catalyzed C-(sp2)-H borylation as 2-fluorobiphenyl was exclusively functionalized at the more electron-rich ring.