Tandem Synthesis of 9,10‐Dihydro‐9,10‐diboraanthracenes via Elusive <i>ortho</i>‐Lithiated Phenylboronates

Abstract

Abstract A simple general protocol is reported for the synthesis of functionalised 9,10‐dihydroxy‐9,10‐dihydro‐9,10‐diboraanthracenes by treatment of the appropriate substituted (2‐halophenyl)boronates with t BuLi. The reaction apparently occurs through the formation of ortho ‐lithiated phenylboronates. These elusive species possess a trivalent Lewis‐acidic boron atom in the vicinity of a strongly nucleophilic lithiated carbon atom. The results of DFT calculations indicate that Br/Li exchange is kinetically favored over nucleophilic alkylation of the boron atom, which contrasts with the behaviour of the sterically less hindered n BuLi. The results are consistent with lower yields of 9,10‐dihydro‐9,10‐diboraanthracenes obtained by using the latter reagent. Subsequent steps of the cascade reaction sequence involve trapping of the reactive lithium/boron intermediate in situ with the starting boronate, followed by a second halogen/lithium exchange and final ring closure. Isolation of 9,10‐dihydroxy‐9,10‐dihydro‐9,10‐diboraanthracenes was further aided by their complexation with 8‐hydroxyquinoline.