Formation, structure and conformational dynamics of highly substituted diphenylcarbonates

The symmetrical carbonates 5-8 were prepared from ethyl 4-hydroxybenzoates with aryl-, 4-(arylethynyl)-phenyl-, arylethynyl- or arylbutadiynyl-substit uents in the 3- and 5-positions, by reaction with triphosgene. The choice o f base (pyridine, DMAP, NaH) had a strong influence on the conversion: For the synthesis of the carbonates 5 and 6, it was sufficient to use pyridine as the base. However, for the synthesis of the carbonates 7 and 8, NaH had to be used instead. Single-crystal X-ray structure analysis of these carbon ates revealed that the substituents point towards the corners of a distorte d tetrahedron with the carbonate group sitting in the middle of the tetrahe dron and the two angular phenolic building blocks intersecting with an angl e of 51-71 degrees. In solution at room temperature, all four substituents are magnetically equivalent as a consequence of conformational flexibility The two enantiomeric conformers of carbonate 5a interconvert rapidly, proba bly via a perfect trans conformation with the plane of the Ar-alpha perpend icular to the carbonate plane. in the case of carbonates 6-8 this process i s inhibited by unfavourable interaction of the long substituents at the ben zoate moiety. The dynamic process, which has an energy barrier of 8-10 kcal mol(-1), is described in a simplified manner as a continuous rotation of t he angular building blacks around the carbonate unit with the C-aryl-O bond s as the axes of rotation.