Transmission of Internal Rotations: Correlated, Uncorrelated, and Localized Disrotatory Rotations in Propeller Chains

The stereochemistry of the propeller chain systems 5 and 6 is analyzed. These systems possessing two or three partially overlapping diarylmethyl moieties, respectively, exist in several conformations arising from the different possible helicities and chain arrangements. System 5 was prepared by acid-catalyzed reaction of pentamethylbenzyl acetate with durene. The ¹H NMR spectrum of 5 indicates, even at 150 K, that there is a rapid rotation of the rings on the NMR time scale. The relative energies of the conformations and the barriers for the internal rotation of the systems were calculated using molecular mechanics, and the results were compared with the parent system decamethyldiphenylmethane (4). The rotational mechanism of lowest activational energy of 5 and 6 is a localized disrotation in which only two neighboring rings in the chain rotate in unison. This results in a helicity reversal of the two rings and a change in the conformation of the chain. Enantiomerization of any conformation of 6 is achieved by at least three successive localized disrotatory rotations. The calculations show that the longer the propeller chain, the larger the energy barrier for a correlated rotation pathway involving all rings and this results in a smaller energetic preference of the correlated over uncorrelated (180°) rotations. This poses a limitation on the transmission of internal rotation of coupled rotors along a chain. Only systems with tightly meshed rotors and with a large energy gap between correlated and uncorrelated rotations may have efficient transmission of correlated rotations.