Syntheses, Structures and Dynamics of 9‐(Ferrocenylmethyl)anthracene and Related Molecular Gears: Phosphorus to the Rescue!

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 Ph 3 P/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.