Copper(I)-Catalyzed Syntheses of Benzo[b]fluorenes by the Cascade Reactions of 2-Alkynylbenzaldehyde N-Tosylhydrazones and Aromatic Terminal Alkynes

Benzo[b]fluorenes were synthesized via CuI-catalyzed cascade reactions of 2-alkynylbenzaldehyde N-tosylhydrazones and aromatic terminal alkynes in the presence of base including the formation of the benzoenyne–allene intermediate in situ and its Schmittel cyclization. Density functional theory calculation was performed to give the energy difference for the formation of 5-phenyl-11H-benzo[b]fluorene and 1,2-diphenyl-1H-cyclobuta[a]indene from the proposed diradical intermediates.