Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 139, Issue 51, Pages 18512-18521Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.7b07113
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Funding
- JSPS (Japan) [JP16H01039]
- MEXT (Japan) [15H0370, 16K13956]
- MEXT-supported Program for the Strategic Research Foundation at Private Universities [51201034]
- Grants-in-Aid for Scientific Research [15H03780, 16H07126, 16K13956, 16H01039] Funding Source: KAKEN
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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 degrees per benzene unit). Electrochemical studies and DFT calculations indicated a narrow HOMO-LUMO gap on account of the extended pi-system. Kinetic studies of the isomerization from HH-1 to HH-2 and the racemization of enantiomerically pure HH-2 were conducted based on H-1 NMR spectroscopy, HPLC analysis, and DFT calculations.
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