Rotational Isomerism in Fluorene Derivatives

Abstract

Although it has been reported that 9, 9-bis (9-fluorenyl) fluorene (1) which belong to a class of compounds of Csp³-Csp³-Csp³ system has two stable rotational isomers at room temperature, we confirmed that this compound 1 existed only in the gauche-gauche form at room temperature by DNMR method. On the derivatives of 9-aryl-9, 9'-bifluorenyl (2) (belong to Csp³-Csp³-Csp² system) and 9, 9-diarylfluorene (3) (belong to Csp²-Csp³-Csp² system), it was able to presume that their two C-C single bonds rotated as gear bit each other, respectively. On 9, 9'-bifluorenyl derivatives (4) (belong to Csp³-Csp³ system) which have substituents at 1 or 1, 8-positions of fluorene ring, we observed a remarkable rotational barrier in the C-C single bond. As the compounds of Csp³-Csp² system (5), many 9- (2-substituted phenyl) fluorene derivatives and 1-substituted or, 1, 8-disubstituted 9- (2-methylphenyl) fluorene derivatives were chosen to investigate their conformational equilibrium between anti and syn conformers which was revealed by the restricted rotation of the C-C single bond. In these cases, the conformational equilibrium was able to explain due to their electronic and steric interaction between the two substituents or the substituent and the fluorene ring.