Synthesis and Properties of Partially Saturated Fluorenyl-Derived [n]helicenes Featuring an Overcrowded Alkene.

The straightforward, multigram-scale synthesis of the partially saturated H 6 -fluoreno[ n ]helicenes ( n = 5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5-diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i.e. the configurationally-stable ( R,R,P ) / ( S,S,M ) pair of enantiomers and an apparently-achiral compound resulting from the rapid interconversion between the ( R,S,P ) and ( S,R,M ) enantiomers. The partially saturated H 6 -fluoreno[ n ]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[ n ]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum-chemical calculations, confirming their helicoidal nature.