Enantioselective Synthesis of Planar Chiral Zigzag-Type Cyclophenylene Belts by Rhodium-Catalyzed Alkyne Cyclotrimerization

Planar
chiral zigzag-type [8]- and [12]­cyclophenylene (CP) belts
have been synthesized in good yields with high ee values of 98% and
83%, respectively, by the rhodium-catalyzed enantioselective intramolecular
sequential cyclotrimerizations of the corresponding cyclic polyynes.
The observed high enantioselectivity arises from the regioselective
formation of a rhodacycle intermediate from an unsymmetric triyne
unit. The X-ray crystal structural analysis of the racemic planar
chiral zigzag-type [8]­CP belt revealed that the uneven molecules mesh
with each other to form a one-dimensional columnar packing structure,
in which one column contains single enantiomers, giving two types
of chiral columns [(S)- and (R)-form
columns] arranged alternately. The ring strain of the zigzag-type
[8]­CP belt was smaller than that of the armchair-type [8]­CPP belt
despite its smaller ring size, due to the presence of the strain-relieving m-terphenyl moieties. The effect of the number of the benzene
rings of the zigzag-type CP belts on absorption and emission peaks
was small due to interruption of π-conjugation at the m-phenylene moieties. However, the bending effect on the
absolute fluorescence quantum yield as well as absorption and emission
peaks was significant. Concerning chiroptical properties, the modest
anisotropy dissymmetry factors of ECD and CPL were observed in the
[8]­CP belt.