Meshed tert-butyl gears on a quasirigid backbone

MM3-based calculations showed that bicycles and polycycles with four- to six-membered rings-components of the bi- and polycyclic backbone are sufficiently rigid to keep a syn-periplanar orientation of vicinal tert-butyl substituents. As a result of the spatial proximity of these groups, their rotation occurs in a concerted manner as demonstrated by conformational schemes that are built using MM3-derived methodology. Only correlated disrotation in saturated systems with four- to five-membered rings-components and in the adamantane system leads to isochronism for Me groups of the tert-Bu substituents (i.e., to dynamic gearing in Mislow's terms). Moreover, correlated rotation of these substituents is coupled with a change of the backbone geometry (pseudorotation) except in the most rigid bicyclo[2.1.1]hexa-2-ene system. Thus, a new type of dynamic gearing, correlated rotation-rotation-pseudorotation, is predicted for quasirigid bi- and polycycles with syn-periplanar oriented tert-Bu substituents.