Substituent-Controlled and Rhodium-Catalyzed Intramolecular [2+2+2] and [2+1+2+1] Cycloadditions of Electron-Deficient Triynes

It has been established that the ratio of [2+2+2] and [2+1+2+1] cycloaddition products in the reactions of electron-deficient 2-phenylnaphthalene-linked triynes, possessing carbonyl groups at the alkyne termini, with a cationic rhodium(I)/bisphosphine complex are dominated by substituents on the carbonyl groups rather than the ligands used. Thus, a triyne, possessing the bulky and electron-withdrawing isobutanoyl and pivaloyl groups at the alkyne termini, exclusively afforded the [2+1+2+1] cycloaddition products, on the contrary, a triyne, possessing the highly coordinating dimethylcarbamoyl groups at the alkyne termini, exclusively afforded the [2+2+2] cycloaddition product. Additionally, helicity stability of the [2+2+2] cycloaddition product, dibenzo[7]helicenes, possessing two adjacent carbonyl groups, was examined. The dibenzo[7]helicenes showed lower racemization barrier than [7]helicenes presumably due to low aromaticity of the two benzene rings in the middle of the two triphenylene skeletons, which may allow flexible bending of two terminal phenanthrene moieties to form the parallel transition state in the racemization process. Furthermore, steric repulsion between the substituents on the carbonyl groups and the benzene rings outside the helicene core affects the helicity stability, which may also affect the ee values of the reaction products.