λ5-Phospha[7]helicenes: synthesis, properties, and columnar aggregation with one-way chirality.

Helical extension of p-conjugated systems is of great interest for the production of novel molecules and materials with unusual properties and applications. A typical example is helical polyacetylenes, in which continuous p conjugation is achieved through covalent bonds. Another example is the assembly of short helical fragments, such as helicenes. 4] For instance, helicenebisquinone derivatives were reported to form one-dimensional columnar aggregates with the aid of long alkyl side chains. Unique optical properties, such as high nonlinear optical susceptibility and circularly polarized luminescence, were reported for these aggregates. Single crystals would also be suitable for creating such architectures if the p-conjugated helical molecules are appropriately stacked. Nevertheless, our analysis of the Cambridge Crystallographic Database revealed that structures with one-dimensional columnar helicene packing that is driven by p–p stacking interactions are rather uncommon, and in the majority of structures with helicene packing, the packing is noncolumnar and mostly driven by CH-p interactions. We hypothesized that the use of dipole–dipole interactions in addition to inherent p–p stacking interactions might produce a one-dimensional columnar arrangement of helicenes. Dipole–dipole interactions have been used as part of systems that contain cooperative interactions for the formation of one-dimensional molecular arrangements. Theoretical studies on the corannulene dimer have also demonstrated that dipole–dipole interactions should be a significant part of the binding energy of bowl-shaped molecules. Previously, we have reported the synthesis of oxaand aza[7]helicenes 3 and 4 by using palladium-catalyzed reactions. These helicenes do not possess dipole-moment vectors that are parallel to their helical axes, and no onedimentional columnar stack was formed. Herein, we report the preparation of l-phospha[7]helicenes 1 and 2 as a new family of helicenes. 12] The phosphole oxide and phosphole sulfide moieties give these helicenes a dipole-moment vector that is parallel to their helical axes, and thus the columnar stacking of 1 and 2 was achieved. More importantly, a racemate of phosphole sulfide 2 crystallized with a unique packing motif: the columns with one dipole direction consist of a single enantiomer and the columns with the opposite dipole direction consist of the other enantiomer (Figure 1). l-Phospha[7]helicenes 1 and 2 were synthesized as shown in Scheme 1. A racemic 4,4’-biphenanthryl-3,3’-diyl bis(trifluoromethanesulfonate, rac-6), from which we previously synthesized aza[7]helicene, was selected as the starting compound. The palladium-catalyzed cross-coupling of rac-6 with ethyl phenylphosphinate gave a 46% yield of the monophosphrous compound rac-7 as a mixture of diastereomers (axial chirality and P-centered chirality). l-Phospha[7]helicene rac-9, which has a phosphole moiety, was obtained by the reduction of rac-7 with LiAlH4 [13c,14] and a subsequent palladium-catalyzed intramolecular P-arylation.