Rotational Isomerism in Fluorene Derivatives XII. : Conformational Equilibria of 9-Substituted 9-(2-methoxymethylphenyl) fluorenes

A series of 9-(2-methoxymethylphenyl) fluorenes [4] (9-hydro-[4a], 9-hydroxyl-[4b]. 9-methyl-[4c], 9-acetyl-[4d], 9-benzoyl-[4e], 9-carboxyl-[4f], 9-methoxycarbonyl-[4g] and 9-methoxyl-9-(2-methoxymethylphenyl) fluorene [4h] were prepared, and their conformational equilibria between ap and sp were investigated and compaired with that of known 9-substituted 9-(2-methoxyphenyl) fluorenes [1]. It was confirmed that [4a] existed as predominant sp conformer (ap/sp=1/4.1) at room temperature, and [4b] as approximately equal proportions of two conformers (ap/sp=1.5/1) at -58℃ on their NMR spectra, respectively. In the case of [4b], we inferred that the formation of intramolecular hydrogen bond would be difficult, because the degree of freedom of 2'-substituent was increased than that of [1b]. Further, the existence of attractive interaction between methoxyl-oxygen and carbonyl-carbon in [4e] was recognized on its NMR spectrum at low temperature.