Thiazolium-Tethering Rotaxane-Catalyzed Asymmetric Benzoin Condensation: Unique Asymmetric Field Constructed by the Cooperation of Rotaxane Components

Abstract

Asymmetric benzoin condensation of aromatic aldehydes with two kinds of optically active rotaxanes possessing thiazolium salt moieties was studied. A binaphthyl group as the chiral auxiliary was introduced in either the wheel or the axle component of the rotaxanes. Rate of the catalytic benzoin condensation of benzaldehyde with a rotaxane catalyst without the binaphthyl moiety was compared with its axle component to understand the effect of wheel component. Among several solvents used, methanol was the best solvent, which showed the highest yield (98%) of benzoin in the presence of 5 mol% of either the rotaxane and the axle catalysts. The benzoin condensations of aromatic aldehydes catalyzed by the chiral rotaxanes were studied in detail and found to give optically active benzoins with 0–32% e.e. in 5–92% yield depending on the structure of the rotaxane and the reaction conditions employed. From the results, two intrarotaxane chirality transfers are confirmed: (i) through-space chirality transfer from wheel to axle and (ii) through-bond chirality transfer controlled with an achiral wheel. Because these asymmetric reaction fields are specific to the rotaxane structure, the importance and possibility of the “rotaxane field” as a particular reaction field are demonstrated.

Keywords:

• Asymmetric benzoin condensation
• binaphthyl
• rotaxane

Acknowledgments

Dedicated to Professor Naomichi Furukawa on the occasion of his 70th birthday.

N.K. is thankful for the Grant-in-Aid for Encouragement of Young Scientists (11750750) from the Ministry of Education, Science, Sports, and Culture. He is also grateful for the grants from The Association for the Progress of New Chemistry, The Japan Securities Scholarship Foundation, the Iketani Science and Technology Foundation, and the Yazaki Memorial Foundation for Science and Technology.

Notes

^a The reaction was carried out using benzaldehyde, Et₃N (20 mol%), and thiazolium catalyst at 50°C in solvent (0.70 mL) for 24 h.

^a The reaction was carried out with benzaldehyde, Et₃N (20 mol%), and catalyst in solvent (0.70 mL) for 24 h.

^b For 3 h.

^a The reaction was carried out with aldehyde (0.80 M), Et₃N (0.16 M), and thiazolium salt (0.04 M) in solvent (0.70 mL) for 24 h.