SYNTHESIS AND PROPERTIES OF 5-BROM- AND 5-CHLOROSUBSTITUTED trans-ACENAPHTHENE-1,2-DIOLS

Abstract

We carried out transformations that describe the preparation of an unsubstituted trans-acenaphthene-1,2-diol using 5-bromo- and 5-chloro-substituted starting materials. Thus, by the reflux of trans-1,2,5-tribromacenaphthene and 1,2-dibromo-5-chlorocenaphthene in water we obtained the corresponding 1,2-diols, which are considered to be cis-isomers by melting points and NMR 1H-spectra. So hydrolysis of trans-1,2,5-tribromo- and 1,2-dibromo-5-chloroacenaphthenes leads to very moderate yields of 1,2-diols (25-26%) with undesired cis-stereoselectivity. By the reaction of 5-bromo- and 5-chlorocenaphthylenes with iodine and silver benzoate in benzene followed by hydrolysis, we obtained mixtures of cis- and trans-1,2-diols which were separated chromatographically and trans-isomers of 5-bromo- and 5-chloro-acenaphthene-1,2-diols were firstly isolated. The yield of trans-diols was low (10-15%), which led us to the search for a convenient method for preparation of trans-isomers. We found this method using the reduction reaction of 5-bromoacenaphthenequinone, replacing the explosive and non-selective reductant lithium aluminum hydride by a more selective and easy-to-handle sodium borohydride in isopropanol. Reduction of 5-bromocenaphthenequinone with sodium borohydride leads to a substantially pure trans-5-bromoacenaphthen-1,2-diol in a rather high yield (57% after re-crystallization).trans-5-Bromo- and 5-chloroacenaphthene-1,2-diols were analyzed by the complex of physical and chemical methods. It is shown that the trans-5-bromo- and 5-chloroacenaphthen-1,2-diols have essentially the same mass spectra with cis isomers, but substantially differ from them by melting points, IR and 1H NMR spectra. Melting points of trans-diols are generally lower than of cis-diols. In IR spectra cis-doils have two characteristic bands of O-H groups (due to association via intramolecular and intermolecular hydrogen bonds) whereas transisomers have only one broad band (only intermolecular hydrogen bonds are possible). In 1H NMR spectra significant differences in the region of the methyne protons at C1 and C2 are noticeable – their signals in the spectrum of the trans-diol are shifted to a weak field in comparison with the cis-isomer.