Cobalt-Catalyzed Borylation of Aryl Methoxides and Sulfides via Formation of Grignard Reagents

Abstract

The functionalization of carbon-oxygen bonds leads to a new methodology for various organic transformation as C−O electrophiles have been utilized as a benign and attractive partner for general cross-coupling reactions. For the purpose, activated C−O electrophiles such as aryl esters, carbamates, or sulfonates have been routinely used as a coupling partner. However, C−O electrophiles such as aryl methoxides have been less developed due to the high activation barrier originated from strong bond dissociation energy of C−O bond in aryl methoxides. An additional factor of a poor leaving ability of the corresponding methoxide also clogs the development of direct activation of aryl methoxides. Nevertheless, C−C bond formation utility aryl methoxides have been successfully demonstrated by several research groups and however, most examples require harsh conditions or a stoichiometric amount of reactive organometallic reagents. Herein, we report the cobalt-catalyzed C–O bond borylation of aryl methoxides with HBpin in the presence of Mg. Importantly, this method is also applicable to C–S bond borylation of aryl sulfides. Both methods can be operated under the mild condition and show high selectivity against C–H bond borylation. Furthermore, this system also affords the desired borylated products even under aerobic conditions. The new cobalt-based system operates Mechanistic studies support in situ formation of a Grignard reagent from the aryl methoxides, which is captured by not only borylating source but various carbon electrophiles. This is very surprising that aryl methoxides (acyclic ether) are usually well-known to stabilize the organomagnesium reagents rather than to be cleaved by transition metal complex. To the best of our knowledge, this is the first example of the Grignard formation of aryl methoxide via transition metal catalysis.