Hindered Rotation around Csp2–Csp3 Bonds in the Enol Forms of α-(9-Fluorenyl)-β-diketones and in 3,5-Disubstituted 4-(9-Fluorenyl)pyrazoles. An 1H NMR Study

Slow rotation around the Csp2–Csp3 bonds of the enol forms of 3-(9-fluorenyl)-2,4-pentanedione, 2-(9-fluorenyl)-1-phenyl-1,3-butanedione, and the corresponding bonds of the derived pyrazoles, was studied by variable temperature 1H NMR. For the pyrazoles, the values Tc=315–320 K (ΔGTc=15.1±0.1 kcal mol−1) and Tc=338±5 K (corresponding to ΔGTc(sp→ap)=18.1±0.2 kcal mol−1 and ΔGTc(ap→sp)=16.1±0.2 kcal mol−1) were found, respectively.