Configurational Stability of [5]Helicenes.

A series of [5]helicenes difunctionalized in the fjord region with either fluoro, methoxy, or methyl groups was synthesized via photochemical and benzylic coupling route. Resolution of each compound into enantiomers and determination of the Gibbs activation energies of enantiomerization (ΔG(T)) revealed high configurational stability in all three cases. The ΔG(T) values of difunctionalized [5]helicenes were compared with those of their monofunctionalized analogues and the parent [5]helicene. Within this series, an exponential correlation between the torsional twist and ΔG(T) was found. The dimethyl derivative exhibits one of the highest configurational stabilities among [n]helicenes reported to date, comparable to that of [9]helicene.