Restricted Rotation around the C(11)-Aryl Single Bond in 11-Arylbenzo[a]fluorenest

Abstract

11-Arylbenzo[a]fluoren-11-ols [2] 11- (o-methylphenyl) -[2a], 11- (m-methylphenyl) -[2b], 11- (p-methylphenyl) -[2c] and 11- (2-methyl-1-naphthyl) benzo [a] fluoren-11-ol [2d] were synthesized by reaction of benzo[a]fluorenone [1] with a variety of arylmagnesium bromides or aryllithiums. Reduction of [2] with HI in acetic acid gave the corresponding 11-arylbenzo [a]fluorenes [3]. Both rotational isomers, ap-form of [2d] and 11 R, sc (or 11 S, + sc)- form of 11- (2-methyl-1-naphthyl) benzo[a]fluorene (3d) were isolated as stable crystals at room temperature.
The rotational barriers (δG) around the C_5P3-C_SP2 single bond in 11- (p-methylphenyl) benzo [a]fluorene [3a] were determined by means of the DNMR method R, sc (or 11 S, + sc)→11R, +ac (or 11S, ac) δG₁₄₀°C=19.7kcal/mol=19.4 kcal/mol, 11 R, +ac, 11 R, sc: δG₁₄₀°C=19.7 kcal/mol. Furthermore, the rates of isomerization for the process of [2d]ap→[2d]sp and [3d]+11R, ac (or [3d]11S, +sc)[3d] 11 R, +ac (or [3d] 11 S, ac) were measured at various temperatures on their NMR spectra, and the free energy values of activation for each process were obtained as δG₁₄₀°C=19.7kcal/mol=24.6 kcal/mol and δG₅₀°C=32.8kcal/mol=, respectively.