Highly efficient synthesis of [3]rotaxane assisted by preorganisation of pseudorotaxane using bis(crown ether)s

Abstract

The tether-assisted synthesis of [3]rotaxane by olefin metathesis has been studied in detail. Bis(crown ether)s, in which two crown ethers are connected by a linker, were threaded onto ammonium salts bearing a terminal olefin to form pseudorotaxanes. The pseudorotaxanes were converted into tethered rotaxanes in the presence of Grubbs catalyst, followed by removal of the linkers to produce [3]rotaxanes in excellent yields. Preorganisation of the two reactive ends has led to a great improvement in the yield of [3]rotaxanes. The ring strain of the tethered rotaxanes and the flexibility of the pseudorotaxanes were responsible for the formation of the tethered rotaxanes.

Keywords:

• [3]rotaxane
• preorganisation
• bis(crown ether)
• olefin metathesis

Acknowledgements

This work was supported by Grant-in-Aid for Scientific Research for Young Scientists (B) (No. 19750035) from the Japan Society for the Promotion of Science (JSPS). We are grateful to the Izumi Science and Technology Foundation, the Electric Technology Research Foundation of Chugoku, the Kinki-chiho Hatsumei Center, the JGC-S Scholarship Foundation and the Uchida Energy Science Promotion Foundation for financial support.

Notes

^†Dedicated to the memory of Prof. Dmitry M. Rudkevich.

^1. [3]Rotaxane 1 √ 3b √ 1 is formed by the homo coupling reaction of complexed form 1 √ 2b; thus, on probability theory and statistics, the theoretical yield (88%) of [3]rotaxane 1 √ 3b √ 1 can be estimated from the square of the statistical distributions (94%) of complexed form 1 √ 2b. [2]Rotaxane 1 √ 3b is formed by the coupling reaction between complexed form 1 √ 2b and uncomplexed form 2b; thus, the theoretical yield (11%) of [2]rotaxane 1 √ 3b can be estimated from twice as much as the product of the statistical distributions (94%) of complexed form 1 √ 2b and those (6%) of uncomplexed form 2b.

^2. Isolated yields of tethered rotaxane 3b √ 4a-c are 54%, 61%, and 47%, respectively. The yield of the tethered rotaxanes 3b √ 4a-c might be reduced during the purification process.

^3. The olefin geometries of the [3]- and [2]rotaxanes, newly formed through olefin metathesis reaction, were not determined.