Helical atropisomers of strained phenanthrenes by photochemistry of aromatic Pauson-Khand cycloadducts

Article abstract of DOI:10.1002/ejoc.201200735

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.