Boraindenes as versatile precursors to benzannulated boron heterocycles

Abstract

Five-membered boroles and their doubly benzannulated 9-borafluorene derivatives are established precursors for synthesizing various boron-doped heterocycles of broad applied interest. Extending this chemistry, we show that a series of 1-aryl-substituted boraindenes – the mono-benzannulated analogues of boroles – can similarly generate diverse boracycles via insertion of various substrates into their five-membered BC₄ ring. Insertion of chalcogens (O, S, Se), alkynes and an aryl azo compound yields boraindene-derived products that are distinguished from earlier examples by their unique ring-fusion patterns, thereby broadening the structural diversity within this class of boron heterocycles. Notably, these transformations provide access to unprecedented boron–sulfur and boron–selenium naphthalene analogues, while azobenzene insertion into the boraindene framework yields two distinct diazaborepin isomers.