From BN‐Naphthalenes to Benzoborole Dianions

The synthesis of benzoborole dianions by alkali metal reduction of BN‐naphthalene derivatives via a ring‐contraction strategy has been developed. Reduction of 1‐alkynyl 2,1‐benzazaborine 1 a in Et 2 O led to the elimination of alkynyllithium with the formation of 1‐amino‐1‐benzoborole trilithium salt 2 a , whereas reduction of 1‐phenyl 2,1‐benzazaborine 1 c in THF yielded 1‐phenyl‐1‐benzoborole dilithium salt 2 c with the elimination of ArNHLi. The trilithium and dilithium salts 2 a and 2 c have been fully characterized. Treatment of trilithium salt 2 a with Et 3 NHCl led to the selective protonation of the amino lithium to afford the dilithium salt 2 aH , which could be cleanly oxidized to 1‐amino‐1‐benzoborole 3 in an excellent yield. Reaction of 1‐phenyl‐1‐benzoborole dilithium salt 2 c with MeI yielded the lithium borate 4 c , which is luminescent both in solution and in the solid state.