Restricted Rotation Involving the Tetrahedral Carbon. XX. Barriers to Rotation and 13C NMR Spectra of 9-(2-Alkylphenyl)fluorene Derivatives

Abstract

Abstract A series of 9-(2-alkylphenyl)fluorene derivatives are prepared and their internal rotation about the C9–Car bond is examined. Those which have a methyl, an ethyl, or an isopropyl group in 2′ position exist as a mixture of sp and ap forms, showing coalescence phenomena in the NMR spectra at high temperatures. The barriers to rotation of these compounds are obtained by the usual DNMR method. 9-(2-t-Butylphenyl)fluorene, in contrast, is found to exsist exclusively as an sp form and thus the barrier to rotation must be determined by measuring the rates of conversion from the unstable ap form which is successfully obtained by the protonation of the corresponding lithium fluorenide at low temperatures. The barriers to rotation of the compounds are alike except for the t-butyl compound. The difference is explained in terms of the conformation of the alkyl group in the ground and the transition state for rotation. The 13C NMR data also exhibit a large steric interaction between the alkyl group and the fluorene ring.