Ring rotation, phenyl rings

Shinkai and coworkers have capitalized on this conformation flexibility in their designs of several pyrene-derivatized calixarene chemosensors. The rotated phenyl ring of the partial cone conformer of 37 allows two pyrene units to more easily interact by decreasing steric hindrance at the lower rim [376], Addition of Li+, Na+, and K+ ions enforces cone formation and the disruption of the initially formed excimer. Accordingly, the metal ions are detected by a decrease in pyrene excimer emission and concomitant increase in the pyrene fluorescence. 

Because of the steric overcrowding, the zwitterionic intermediate is able to grow (propagation step) only in very concentrated solutions. The exclusive formation of the 1,2-polymer appeared to be justified by the fact that the 2-position (Scheme 16) is evidently less sterically hindered than the 4-position (the freely rotating phenyl ring at the 4-position occupies more space than the CH2= at the 2-position, which does not occupy space outside the plane of the bicyclic system) and the zwitterionic dimer, or the growing zwitterionic intermediate, sufficiently selective in reacting with the less crowded position of BFl. 

Figure 18.12 The electron-density map is interpreted by fitting into it pieces of a polypeptide chain with known stereochemistry such as peptide groups and phenyl rings. The electron density (blue) is displayed on a graphics screen in combination with a part of the polypeptide chain (red) in an arbitrary orientation (a). The units of the polypeptide chain can then be rotated and translated relative to the electron density until a good fit is obtained (b). Notice that individual atoms are not resolved in such electron densities, there are instead lumps of density corresponding to groups of atoms. [Adapted from A. Jones Methods Enzym. (eds. H.W. Wyckoff, C.H. Hirs, and S.N. Timasheff) 115B 162, New York Academic Press, 1985.]...

Ethyl dibromodihydrocinnamate (14), for example, can form the three staggered conformers 14a-c by rotation around the CC single bond a to the phenyl ring. 

A variable-temperature NMR spectroscopic study of the titanium(IV) complex 43 also indicated free rotation of the five-membered rings, but, as in the ferrocene derivative 38 allowed the determination of the activation barrier for the phenyl ring rotation (AG (-90 °C) = 9.8 0.5 kcal mol1). 

At the same time, Ciajolo et al, [136] presented the crystal structure of azinobis(ethylidyne-p-phenylene)dipropionate. The crystal structure contains two crystallographically independent molecules with an almost planar conformation, the phenyl rings being slightly rotated with respect to the average molecular plane. The molecules are arranged in layers. 

Figure 7 Plot of energy vs torsion angle from an energy profiling study resulting from rotating the oxazoline ring of the 5 isomer of WIN52084 about the phenyl ring.

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