American Chemical Society
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Progress toward a Rationally Designed, Chemically Powered Rotary Molecular Motor

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journal contribution
posted on 2007-01-17, 00:00 authored by T. Ross Kelly, Xiaolu Cai, Fehmi Damkaci, Sreeletha B. Panicker, Bin Tu, Simon M. Bushell, Ivan Cornella, Matthew J. Piggott, Richard Salives, Marta Cavero, Yajun Zhao, Serge Jasmin
Building on prototype 1, which achieves 120° of phosgene-powered unidirectional rotation to rotamer 6 (see Figure in the full article), 7 was designed to accomplish repeated unidirectional rotation (see Scheme ). Compound 7 contains an amino group on each blade of the triptycene and a 4-(dimethylamino)pyridine (DMAP) unit to selectively deliver phosgene (or its equivalent) to the amine in the “firing position”. The synthesis of 7 is described:  the key constructive steps are a benzyne addition to an anthracene to generate the triptycene, a stilbene photocyclization to construct the helicene, and a Stille coupling to incorporate the DMAP unit. The DMAP unit was shown to regioselectively relay 1,1‘-carbonyldiimidazole (but not phosgene) to the proximal amino group, as designed, but rotation of the triptycene does not occur. Extensive attempts to troubleshoot the problem led to the conclusion that the requisite intramolecular urethane formation, as demonstrated in the prototype (14), does not occur with 7 (to give 85) or 97 (to give 100). We speculate that either (i) hydrogen bonding between the hydroxypropyl group and functionality present in 7 but absent from 1 or (ii) a Bürgi−Dunitz (or similar) interaction involving the DMAP (see 106) prevents achievement of a conformation conducive to intramolecular urethane formation.