Practical Synthesis of [n]Cycloparaphenylenes (n=5, 7–12) by H2SnCl4‐Mediated Aromatization of 1,4‐Dihydroxycyclo‐2,5‐diene Precursors

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. ¹¹⁹Sn 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.