The synthesis of highly strained [5]cycloparaphenylene ([5]CPP), a structural unit of the periphery of C60 and the shortest possible structural constituent of the sidewall of a (5,5) carbon nanotube, was achieved in nine steps in 17% overall yield. The synthesis relied on metal-mediated ring closure of a triethylsilyl (TES)-protected masked precursor 1c followed by removal of the TES groups and subsequent reductive aromatization. UV-vis and electrochemical studies revealed that the HOMO-LUMO gap of [5]CPP is narrow and is comparable to that of C60, as predicted by theoretical calculations. The results suggest that [5]CPP should be an excellent lead compound for molecular electronics.

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Cyclic precursors of cycloparaphenylenes (CPPs) containing 1,4-dihydroxy-2,5-cyclohexadien-1,4-diyl units are prepared by modifying a synthetic method developed by Jasti and co-workers for the synthesis of corresponding 1,4-dimethoxy derivatives. Reductive aromatization of the diyl moieties by SnCl2/2 HCl takes place under mild conditions and affords the CPPs in good yields, incorporating 5 or 7-12 phenylene units. Highly strained [5]CPP is synthesized in greater than 0.3 g scale. (119)Sn NMR spectroscopy clarifies the in situ formation of an ate complex, H2SnCl4, upon mixing a 2:1 ratio of HCl and SnCl2, which serves as a highly active reducing agent under nearly neutral conditions. When more than 2 equivalents of HCl, in relation to SnCl2, are used, acid-catalyzed decomposition of the CPP precursors takes place. The stoichiometry of HCl and SnCl2 is critical in achieving the desired aromatization reaction of highly strained CPP precursors.