Restricted Rotation Involving the Tetrahedral Carbon. XXXIII. Restricted Rotation about a Csp3-Csp2 Bond in 10,10-Disubstituted 9-(2,6-Xylyl)-9,10-dihydroanthracene Derivatives

Article abstract of DOI:10.1246/bcsj.53.2977

A series of 10,10-disubstituted 9-(2,6-xylyl)-9,10-dihydroanthracene derivatives have been prepared and their conformations and barriers to internal rotation about the C_xylyl-C₉ bond have been examined.These compounds exist predominantly as a pseudo-equatorial form in the ground state for rotation.A large effect is inserted by the substituents in 10 position on barriers to rotation and is interpreted in terms of a rotation-inversion mechanism rather than a simple rotation mechanism for the methyl-exchange process which is observed by 1H NMR spectra.The highestbarrier to rotation has been found in the compound having two peri-chloro groups and a hydroxyl group in 9-position.The high barrier is ascribed to destabilization of the transition state, where 9-aryl group takes axial conformation, due to the three electronegative groups which are forced to be arranged parallelly in quite a near region.The change of 9-OH to 9-H thus decreases the barrier to rotation when peri-chlorines are present, while the increase in barrier is observed as in the case of 9-aryfluorenes and 9-arylxanthenes when they are absent.