Boraindenes as versatile precursors to benzannulated boron heterocycles

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 <i>via</i> 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.