Triple Helicene Cage: Three-Dimensional π-Conjugated Chiral Cage with Six [5]Helicene Units.

A three-dimensional π-conjugated chiral cage with six [5]helicene units (a triple helicene cage) was synthesized for the first time. Taking advantage of the Yamamoto coupling reaction, the triflate-substituted triple [5]helicene, a strained and preorganized precursor, was dimerized to afford the target compound. Single-crystal X-ray diffraction analysis revealed the unique structural features of the triple helicene cage: a cage-shaped rigid structure with outer helical grooves and an inner chiral cavity. All-P and all-M enantiomers were separated successfully by HPLC over a chiral column and their chiroptical properties were characterized by circular dichroism spectra.

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.

Air-Driven Potassium Iodide-Mediated Oxidative Photocyclization of Stilbene Derivatives.

A new method has been developed for the potassium iodide-mediated oxidative photocyclization of stilbene derivatives. Compared with conventional iodine-mediated oxidative photocyclization reactions, this new method requires shorter reaction times and affords cyclized products in yields of 45-97%. This reaction proceeds with a catalytic amount of potassium iodide and works in an air-driven manner without the addition of an external scavenger. The radical-mediated oxidative photocyclization of stilbene derivatives using TEMPO was also investigated.