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Formation, Structure and Conformational Dynamics of Highly Substituted Diphenylcarbonates
Author(s) -
Godt Adelheid,
Ünsal Ömer,
Enkelmann Volker
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20001002)6:19<3522::aid-chem3522>3.0.co;2-r
Subject(s) - triphosgene , chemistry , moiety , carbonate , pyridine , tetrahedron , crystallography , crystal structure , base (topology) , aryl , conformational isomerism , stereochemistry , molecule , medicinal chemistry , organic chemistry , mathematical analysis , alkyl , mathematics , phosgene
The symmetrical carbonates 5 – 8 were prepared from ethyl 4‐hydroxybenzoates with aryl‐, 4‐(arylethynyl)phenyl‐, arylethynyl‐ or arylbutadiynyl‐substituents in the 3‐ and 5‐positions, by reaction with triphosgene. The choice of 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 carbonates revealed that the substituents point towards the corners of a distorted tetrahedron with the carbonate group sitting in the middle of the tetrahedron and the two angular phenolic building blocks intersecting with an angle of 51–71°. In solution at room temperature, all four substituents are magnetically equivalent as a consequence of conformational flexibility. The two enantiomeric conformers of carbonate 5 a interconvert rapidly, probably via a perfect trans conformation with the plane of the Ar α perpendicular to the carbonate plane. In the case of carbonates 6 – 8 this process is inhibited by unfavourable interaction of the long substituents at the benzoate 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 the angular building blocks around the carbonate unit with the C aryl −O bonds as the axes of rotation.