Helical Atropisomers of Strained Phenanthrenes by Photochemistry of Aromatic Pauson-Khand Cycloadducts

Norbornene and norbornadiene Pauson–Khand adducts of bis(3,5‐dimethylphenyl)acetylene and bis(3,4,5‐trimethylphenyl)acetylene were prepared. These compounds were subjected to a photochemical 6π‐electrocyclic oxidative aromatization reaction to give the corresponding phenanthrene compounds in satisfactory yield in the case of norbornene derivatives. The helical twist imposed by the methyl groups at the 3‐ and 5‐positions on the aromatic rings led to two atropisomers as a result of the non‐planar helical phenanthrene structure. The molecular structures and conformational stabilities of these atropisomers were examined by X‐ray crystallography and variable temperature NMR studies. Norbornene and norbornadiene Pauson–Khand adducts of bis(3,5‐dimethylphenyl)acetylene and bis(3,4,5‐trimethylphenyl)acetylene were subjected to a photochemical 6π‐electrocyclic oxidative aromatization reaction to give the corresponding phenanthrene compounds. The helical twist imposed by the methyl groups at the 3‐ and 5‐positions on the aromatic rings led to two atropisomers as a result of the non‐planar helical phenanthrene structure.