Synthesis, Structures, and Properties of Hexapole Helicenes: Assembling Six [5]Helicene Substructures into Highly Twisted Aromatic Systems

Hexapole
helicenes 1, which contain six [5]­helicene
substructures, were synthesized by Pd-catalyzed [2+2+2]­cycloadditions
of aryne precursor 6. Among the possible 20 stereoisomers,
which include ten pairs of enantiomers, HH-1 was obtained
selectively. Density functional theory (DFT) calculations identified HH-1 as the second most stable isomer that quantitatively
isomerizes under thermal conditions into the most stable isomer (HH-2). Both enantiomers of HH-2 can be separated
by chiral HPLC. Single-crystal X-ray diffraction analyses revealed
a saddle-like structure for (P,M,P,P,M,P) HH-1 and a propeller-like structure for
(P,M,P,M,P,M) HH-2. Because of the helical assembly and the resulting steric repulsion,
the structure of HH-1 is significantly distorted and
exhibits the largest twisting angle reported so far (up to 35.7°
per benzene unit). Electrochemical studies and DFT calculations indicated
a narrow HOMO–LUMO gap on account of the extended π-system.
Kinetic studies of the isomerization from HH-1 to HH-2 and the racemization of enantiomerically pure HH-2 were conducted based on 1H NMR spectroscopy, HPLC analysis,
and DFT calculations.