Syntheses, Structures and Dynamics of 9‐(Ferrocenylmethyl)anthracene and Related Molecular Gears: Phosphorus to the Rescue!
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abstract
The rotational barriers of the anthracenyl and triptycyl units in 1‐(9‐anthracenyl)‐1′‐(9‐triptycyl)ferrocene were independently measured as 11.6 and 17.6 kcal mol–1, respectively. Attempts to reduce 9‐ferrocenoylanthracene to 9‐(ferrocenylmethyl)anthracene, 3, in which the sandwich moiety and the planar aromatic are linked through an angular methylene unit, were thwarted by reduction of the anthracene moiety to form 9‐ferrocenylmethyl‐9,10‐dihydroanthracene, 5, and 9‐ferrocenoyl‐9,10‐dihydroanthracene, 6. However, treatment of phenyl(ferrocenyl)methanol, 9, with triphenylphosphine and triflic acid gave [(phenyl)(ferrocenyl)methyl]phosphonium triflate, 10; subsequent hydrolysis yielded benzylferrocene (93 %). Analogously, (9‐anthracenyl)(ferrocenyl)methanol, 11, and Ph3P/TfOH furnished 3 (up to 92 %) together with various amounts of its anthracenylidene tautomer, 15, depending on the hydrolysis conditions. Surprisingly, during the chromatographic purification of 11, partial dehydration occurred to give di[(9‐anthracenyl)(ferrocenyl)methyl] ether, 12. The fluxional dynamics of 3, 11 and 12 are very different: anthracenyl rotation of the group in 3 and 11 is facile (barriers of < 9 and ≈ 9.6 kcal mol–1, respectively), but is arrested in 12 due to mutual steric locking of the two anthracenes. Diels–Alder addition of benzyne to 3 furnished 9‐(ferrocenylmethyl)triptycene, 16. The molecular structures of 3, 5, 9, 10, 11, 12 and 16 were determined by X‐ray crystallography.
