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.