Enantioselective Helicene Synthesis by Rhodium-Catalyzed [2+2+2] Cycloaddition

Helicenes have excellent chiroptical properties derived from their helical chirality and so attract the attention of scientists for their potential application to chiral organic functional materials. Thus, developing an enantioselective synthetic method for helicenes is an important goal in organic synthesis. Here we describe the enantioselective synthesis of helicene and helicene-like molecules by cationic rhodium(I)/bisphosphine complex-catalyzed [2+2+2] cycloaddition reactions. The enantioselective intramolecular [2+2+2] cycloaddition reactions of 2-naphthol-linked triynes afford [7]helicene-like molecules in good yields and ee. [9]Helicene-like molecules and fully aromatic carbo[7]helicenes can also be synthesized enantioselectively by intramolecular [2+ 2+2] cycloaddition reactions. The enantioselective sequential intramolecular [2+2+2] cycloaddition reactions of variously linked hexaynes afford either a [11]helicene-like molecule or an S-shaped double helicene-like molecule, both in high ee. Cationic rhodium(I)/biaryl bisphosphine complexes can catalyze not only intramolecular reactions, but also intermolecular ones between tetraynes and 1,4-diynes to afford [7]-[9]helicene and helicene-like molecules. We also describe some representative examples of the enantioselective synthesis of helicene and helicene-like molecules by [2+2+2] cycloaddition reactions using transition metal catalysts other than rhodium (iridium, nickel, and palladium). © 2022 Society of Synthetic Organic Chemistry. All rights reserved.