Dynamic NMR study of the rotation around "biphenyl-type" bonds in polycyclic sulfoxides

In a recent paper, we reported on the base-catalyzed rearrangement of bis-propargylic sulfoxides that eventually leads to polycyclic products featuring an unsaturated, cyclic substituent such as cyclohexenyl or phenyl. Due to steric constraints, the latter is positioned roughly perpendicularly to the tricyclic core, and in most cases, two rotamers can be observed in the ground state. In the present work, we report on the synthesis and the products of both symmetrical and asymmetrical starting materials. We also measure, by NMR techniques, the rotation rate of the side chain for several such polycyclic sulfoxides. The barriers for this process, which is similar to a biphenyl rotation, are very strongly dependent on the nature of the substrate, ranging between <7 and 21.0 kcal·mol^-1 for sulfoxides with two five-membered rings and two seven-membered rings, respectively. These barriers can be successfully simulated by molecular-mechanics calculations, and the geometries of the transition states are discussed.