title: Practical synthesis of [n]cycloparaphenylenes (n = 5, 7-12) by H2SnCl4-mediated aromatization of 1,4-dihydroxycyclo-2,5-diene precursors.
abstract: 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.
authors.affiliation: Institute for Chemical Research, Kyoto University, Uji 611-0011 (Japan), Fax: (+81) 774-38-3060; Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, Tokyo 102-0076 (Japan).
authors.forename: Vijay Kumar
authors.initials: VK
authors.lastname: Patel
authors.forename: Eiichi
authors.initials: E
authors.lastname: Kayahara
authors.forename: Shigeru
authors.initials: S
authors.lastname: Yamago
keywords: aromatization; conjugation; cycloparaphenylene; synthetic methods; tin