Kinetic studies of the fluorene series. Part VI. The reaction of 3-substituted fluorenones with sodium borohydride and the acidcatalysed solvolysis of 3-substituted 9-diazofluorenes

Abstract

The rates of the sodium borohydride reduction of some 3-substituted fluorenones in anhydrous isopropyl alcohol, and of the perchloric acid-catalysed solvolysis of some 3-substituted 9-diazofluorenes in 93·8 % w/w ethanol–water have been determined. The former reaction is hindered by electron-releasing groups and facilitated by electron withdrawal, whilst for the latter process the opposite is true. In both series the well known compensating relationship between the energies and entropies of activation is found.

Treatment of the data by Hammett type equations indicates that, for the 9-diazofluorene series, the extent of transmission of T effects to the 9-position is essentially that which would be expected for a normal para-position, whereas, for the fluorenone series, resonance effects are slightly but significantly smaller. This result is discussed in terms of possible loss of conjugation in the transition state and differences in aromaticity of the central five-membered ring.

The results for both series agree satisfactorily with the predictions of molecular orbital calculations, and a simple relationship between the theoretical quantities and the extent of operation of resonance effects is developed which is shown to hold for substituents in the 2-, 3-, and 4-positions.