[5]Cycloparaphenylene: The Smallest Nanohoop

Significance

Cycloparaphenylenes (CPPs), cyclic carbon nanohoop-shaped molecules composed of para-linked benzene rings, have been of particular interest owing to their aesthetic architecture, highly strained aromatic systems, and novel properties. [5]CPP (5), the smallest nanohoop and highly strained, is synthetically challenging, and a synthesis had not been realized to date. Yamago and co-workers started the synthesis of [5]CPP from compound 1a (J. Xia, R. Jasti Angew. Chem. Int. Ed. 2012, 51, 2474), which has four methoxy groups. Although the next metal-mediated cyclization was successful, the resulting nanohoop 2a (with methoxy groups) did not undergo reductive aromatization to 5. The authors’ solution was to replace the low reactive methoxy groups by hydroxyl groups and to start from 1b. The Jasti group synthesized [5]CPP with a slightly different method.

Comment

The Yamago group started with 1b (with TES ethers), ring closure and deprotection of the TES ethers afforded 4 (with four hydroxyl groups). The final reductive aromatization was achieved by using SnCl₂·H₂O, which produced the dark-purple solid [5]CPP. Jasti and co-workers observed homocoupling product 2a using bisboronate 1c during their synthesis of [10]CPP. However, 2a was not fully reduced to 5 by using sodium naphthalenide, as was possible for the synthesis of [6]CPP– [12]CPP and [18]CPP. Instead, after quenching the reduction reaction with methanol, 4 was observed, indicative of a dianion intermediate. Finally, E1cB elimination of 4 with LDA afforded [5]CPP (5). The authors obtained an X-ray crystal structure of [5]CPP.