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 diarylme thyl moieties, respectively, exist in several conformations arising fr om the different possible helicities and chain arrangements. System 5 was prepared by acid-catalyzed reaction of pentamethylbenzyl acetate w ith durene. The H-1 NMR spectrum of 5 indicates, even at 150 K, that t here is a rapid rotation of the rings on the NMR time scale. The relat ive energies of the conformations and the barriers for the internal ro tation of the systems were calculated using molecular mechanics, and t he results were compared with the parent system decamethyldiphenylmeth ane (4). The rotational mechanism of lowest activational energy of 5 a nd 6 is a localized disrotation in which only two neighboring rings in the chain rotate in unison. This results in a helicity reversal of th e two rings and a change in the conformation of the chain. Enantiomeri zation of any conformation of 6 is achieved by at least three successi ve localized disrotatory rotations. The calculations show that the lon ger the propeller chain, the larger the energy barrier for a correlate d rotation pathway involving all rings and this results in a smaller e nergetic preference of the correlated over uncorrelated (180-degrees) rotations. This poses a limitation on the transmission of internal rot ation of coupled rotors along a chain. Only systems with tightly meshe d rotors and with a large energy gap between correlated and uncorrelat ed rotations may have efficient transmission of correlated rotations.