Computational Studies on the Racemization Barriers of Winding Vine-Shaped Heterobiaryls with Molecular Asymmetry

Synthesis and Racemization Studies of Winding Vine‐Shaped Biphenyl Derivatives

“…Enantiomerically enriched 8 c thus separated as enantiomeric ratio of 13 : 87 was found to be hardly racemized in a solution of chlorobenzene at 100 °C after 24 h. Racemization of 8 c slowly took place as shown in Figure 5 by heating the solution at 131 °C and the enantiomeric ratio was decreased to 38 : 62 (24 % ee) after heating for 4 h. The energy barrier of racemization was experimentally calculated as 134 kJmol −1 , which well corresponded to the calculated one by DFT (126.34 kJmol −1 ). By contrast, biphenyl 10 c cyclized by the ring‐closing metathesis was found to hardly racemize under similar conditions to that of 8 c at 131 °C even after 14 h, which results reasonably agreed with the calculation study suggesting the energy barrier for the isomerization of 10 c as 211.6 kJmol −1 by DFT [10] . The remarkably high racemization barrier of 10 c toward 8 c is ascribed to the difficulty of free rotation of the carbon‐carbon double bond in the vine moiety, while that of 8 c is composed of freely rotating single bonds [21] .…”

“…By contrast, biphenyl 10 c cyclized by the ring-closing metathesis was found to hardly racemize under similar conditions to that of 8 c at 131°C even after 14 h, which results reasonably agreed with the calculation study suggesting the energy barrier for the isomerization of 10 c as 211.6 kJmol À 1 by DFT. [10] The remarkably high racemization barrier of 10 c toward 8 c is ascribed to the difficulty of free rotation of the carbon-carbon double bond in the vine moiety, while that of 8 c is composed of freely rotating single bonds. [21] The structural difference of the benzene ring from thiophene (8 c/10 c vs. 1/2) is the presence of the CÀ H bond at the 6,6'-positions of biphenyl, which significantly inhibit the rotation of the CÀ C bond between phenyl rings to result in a higher racemization barrier compared with less sterically congested five-membered thiophene bearing no substituent on the sulfur atom.…”

“…We have preliminarily studied the DFT calculation on the racemization behavior [10] of thus cyclized heterobiaryls 2 and found that the isomerization barrier of 2 was much lower because of the ease of the free rotation of the carbon‐carbon single bond. It was also shown to be calculated that the energy barrier of the related derivative 1 bearing C−C double bond was slightly higher but 1 still slowly racemized experimentally even at room temperature [6] .…”

“…It was also shown to be calculated that the energy barrier of the related derivative 1 bearing C−C double bond was slightly higher but 1 still slowly racemized experimentally even at room temperature [6] . Although the related non‐heteroaromatic analogs 3 composed of the benzene ring was suggested to racemize with a higher energy barrier by DFT calculation, [10] the actual synthesis of such biphenyl 3 has not been successful to date. Accordingly, we envisaged to synthesize the vine‐shaped biphenyl derivatives and to study their racemization behaviors.…”

Studies on the stereochemical behaviors of a winding vine-shaped molecular wire of a bithiophene dimer with molecular asymmetry

“…7 h) within a reasonable time scale. [19][20][21][22] We planned to synthesize a molecular wire 6,23,24 of bithiophene, in which the linkage of each chirality unit consists of molecular asymmetry and its chirality is detectable spectroscopically as a diastereomer. Herein, we report our initial efforts as a study of the isomerization behavior of a winding vine-shaped bithiophene wire, which showed a unique diastereomeric change between meso and racemic forms in solution and solid states.…”