Selective deoxygenative bis- and monoborylation of carboxylic esters via boryloxy chromium-carbenes.

Fischer-type metal carbenes are versatile intermediates that mediate a wide array of molecular transformations. However, their classical synthesis typically requires stoichiometric amounts of chromium and hazardous reagents such as diazo compounds or organolithium species, posing significant safety and operational concerns. A recent copper- and palladium-catalyzed Brook rearrangement of aroylsilanes opened a catalytic strategy to alleviate these problems. Herein, we report a complementary strategy that enables catalytic deoxygenative borylation of carboxylic esters under conditions consistent with the intermediacy of boryloxy chromium-carbene species, enabled by a combination of pinacolborane, magnesium metal, and a chromium catalyst. The reaction converts the ester to either bis- or monoboryl alkanes-valuable intermediates for further synthetic diversification-depending on the choice of bipyridine or terpyridine ligand. Multiple lines of evidence, including cyclopropanation, deuterium quenching, and DFT analysis, support a low-energy pathway involving chromium-carbene intermediates. The ligand-controlled bifurcation arises from subtle energy differences between quintet and triplet pathways, elucidated by the DFT calculations.