Troubleshooting a molecular motor: a remarkably stable N-acyl pyridinium salt

With the objective of establishing why reaction of the proposed molecular motors 7 and 22 with carbonyldiimidazole and phosgene does not result in unidirectional rotation, N-ethyl-2-[4-(N,N-dimethylamino)-2-pyridinyl]benzenamine [28, 2-(2-(ethylamino)phenyl)DMAP] was examined as a model substrate. The synthesis of 28 is described. Compound 28 was found to react with phosgene to give the unexpectedly stable N-acyl pyridinium salt 30. The latter (30) is so stable that it is effectively inert to reaction with methanol. At room temperature the two methyls in the Me2N-group of 30 are nonequivalent (NMR) and the barrier to rotation around the Me2N–pyridinium bond is 18.5 kcal/mol. To the authorsʹ knowledge, that is the first quantitative determination of the barrier to rotation around the bond between a 4-(N,N-dimethylamino) group and an N-acyl pyridinium unit. The implications of the findings regarding 30 as to troubleshooting the proposed molecular motor 7, and possible strategies to follow, are discussed.