Rhodium-Catalyzed Enantioselective Synthesis, Structures, and Properties of Single and Double Azahelicene-Like Molecules.

The enantioselective synthesis of aza[6] and [7]helicene-like molecules have been achieved by the cationic rhodium(I)/axially chiral biaryl bisphosphine complex-catalyzed intramolecular [2+2+2] cycloaddition of cyanodiynes. This protocol was successfully applied to the diastereo- and enantioselective synthesis of an S-shaped double aza[6]helicene-like molecule with a high ee value of 89 %. Although no epimerization and racemization were observed in the double carbo[6]helicene-like molecule at 80 °C, epimerization and racemization of the double aza[6]helicene-like molecule proceeded at 80 °C. This double aza[6]helicene-like molecule showed good fluorescent quantum yields and chiroptical responses under both neutral and acidic conditions.

Synthesis, Structures, and Properties of Highly Strained Cyclophenylene-Ethynylenes with Axial and Helical Chirality.

Single and double cyclophenylene-ethynylenes (CPEs) with axial and helical chirality have been synthesized by the Sonogashira cross-coupling of di- and tetraethynyl biphenyls with a U-shaped prearomatic diiodoparaphenylene followed by reductive aromatization. X-ray crystallographic analyses and DFT calculations revealed that the CPEs possess highly twisted bent structures. Bend angles on the edge of the paraphenylene units were close to the value of [5]cycloparaphenylene (CPP)-the smallest CPP to date. The double and single CPEs possessed stable chirality despite flexible biphenyl structures because of the high strain in the diethynyl-paraphenylene moiety. In both the single and double CPEs, orbital interactions along the biphenyl axis were observed by DFT calculations in LUMO and LUMO+2 of the single CPE and LUMO+1 of the double CPE, which likely cause lowering of these orbital energies. Concerning chiroptical properties: boosting of the gabs value was observed in the biphenyl-based double CPE, as well as the binaphthyl-based single CPE, compared to the biphenyl-based single CPE.

Synthesis of a Strained Spherical Carbon Nanocage by Regioselective Alkyne Cyclotrimerization.

The smallest spherical carbon nanocage so far, [2.2.2]carbon nanocage, has been synthesized by the cationic rhodium(I)/H8 -binap complex-catalyzed regioselective intermolecular cyclotrimerization of a cis-1-ethynyl-4-arylcyclohexadiene derivative followed by the triple Suzuki-Miyaura cross-couplings with 1,3,5-triborylbenzene and reductive aromatization. This cage molecule is highly strained, and its ring strain is between those of [6] and [5]cycloparaphenylenes. A significant red-shift of an emission maximum was observed, compared with that of known [4.4.4]carbon nanocage. The sequential cyclotrimerizations of a cis-1,4-diethynylcyclohexadiene derivative with the same rhodium(I) catalyst followed by reductive aromatization failed to afford [1.1.1]carbon nanocage; instead, a ß-graph-shaped cage molecule was generated.

Rhodium-Catalyzed Synthesis, Crystal Structures, and Photophysical Properties of [6]Cycloparaphenylene Tetracarboxylates.

The synthesis of C2-symmetrical [6]cycloparaphenylene (CPP) tetracarboxylates has been achieved via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-alkyne cyclotrimerization and subsequent reductive aromatization. The 1H NMR spectra of the thus-obtained C2-symmetrical [6]CPP-tetracarboxylates revealed that the rotation of unsubstituted benzene rings is slow at room temperature. These [6]CPPs formed columnar packing structures, and their absorption maxima were significantly blue-shifted compared to that of nonsubstituted [6]CPP due to the presence of four electron-withdrawing ester moieties.

Synthesis of [8]Cycloparaphenylene-octacarboxylates via Rh-Catalyzed Stepwise Cross-Alkyne Cyclotrimerization.

The synthesis of C4- and C2-symmetrical [8]cycloparaphenylene (CPP)-octacarboxylates has been achieved via macrocyclization by the rhodium-catalyzed intermolecular stepwise cross-cyclotrimerization and subsequent reductive aromatization. The C4-symmetrical octa-tert-butyl [8]CPP-octacarboxylate forms a dimer in which eight ester moieties face each other. The dimers are aligned so as to make a one-dimensional column with a channel structure inside. Both absorption and fluorescence maxima of [8]CPP-octacarboxylates in CHCl3 were significantly blue-shifted compared to those of [8]CPP due to the presence of eight electron-withdrawing ester moieties.