Synthesis of 9,10-bis(9-triptycyloxy)triptycenes : Molecular design of a system with doubly correlated internal rotation

Abstract

Torsional motions around the two C—C and C—O bonds in di(9-triptycyl)methanes, Tp2CH2, and di(9-triptycyl) ethers, Tp2O, respectively, have a high barrier to uncorrelated rotation and a very low barrier to coupled disrotation. As a result, new stereoisomerism is generated due to different phase relationships between appropriately labeled benzene rings, at least one on each Tp unit. To extend the concept and further demonstrate the high correlation in the torsional motions for these systems, a doubly geared molecule, 9,10-bis(3-chlorotriptycyloxy)triptycene (1), was conceived and constructed. Bis(3-chloro-9-triptycyl) 9,10-triptycenebis(peroxycarboxylate) was prepared. The meso and dl isomers of 1 were separated by HPLC on microsilica. The structures were confirmed by high resolution 13C-NMR spectra which revealed an interesting stercochemical feature : one benzene ring of the middle unsubstituted triptycene moiety is diastereotopic to the other two. The rates of isomerization were measured in diphenylmethane solution to give the activation parameters for the gear slipping process: ΔH‡ = 42.1±1.3 kcal mol-1 and ΔS‡ = -3.2±2.3 e.u. The significance of these findings as an extreme case for the dynamics of molecular chains is discussed.