Tandem Synthesis of 9,10-Dihydro-9,10-diboraanthracenes via Elusive ortho-Lithiated Phenylboronates

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 tBuLi. 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 nBuLi. 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.