Head-to-Tail Selective [2 + 2 + 2] Cycloadditions of Bisallenes Enabled by Ligand-Free Nickel(0)

A ligand-free Ni(0) catalyst enables highly chemo- and regioselective [2 + 2 + 2] cycloadditions between 1,5-bisallenes and acetylenes, affording 6,6-fused hexahydroisoquinoline frameworks under mild conditions. DFT studies reveal that head-to-tail selectivity arises from competing oxidative cyclization pathways with metallacycle geometry dictating product formation. This work highlights how simple nickel complexes can achieve precise molecular construction without the need for ancillary ligands.