A Rationally Designed Prototype of a Molecular Motor.

A proof of principle of the first rationally designed, chemically powered, molecular-scale motor is described. The thermodynamic considerations leading to the choice of 6a and 7a as the initial prototypes are provided, and the synthesis of 6a and 7a and the separation of them from their atropisomers are detailed. The phosgene-powered unidirectional rotation of 6a to its rotamer 6b is demonstrated. It is further established that shortening the length of the tether (→7a) changes the rate-limiting step and accelerates the speed of rotation.

Progress toward a rationally designed, chemically powered rotary molecular motor.

Building on prototype 1, which achieves 120 degrees of phosgene-powered unidirectional rotation to rotamer 6 (see Figure 5 in the full article), 7 was designed to accomplish repeated unidirectional rotation (see Scheme 7). 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 (1 --> 4), 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.