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 kcalmol(-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 approximate to 9.6 kcalmol(-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.