Goradia, M. T. (1973)
Synthesis and stereochemistry of some 2, 3, 2', 3'-tetrasubstituted biphenyls.
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Two series of 2,3,2',3'-tetrasubstituted biphenyls having chlorine or bromine in the 3 and 3' positions have been synthesised from substituted benzenes. Initially, the 2 and 2' substituents were carboxylic groups and from them both unbridged and 2, 2'-bridged biphenyls have been prepared. The latter are the first of their kind to be synthesised and studied.
The effect of the 3 and 3 substituents on the chemical behaviour of the 2 and 2' substituents has been studied. In many cases, the yields of the compounds formed by nucleophilic substitution reactions were found to have decreased in comparison with those for the unbuttressed analogues, the decrease usually being larger for bromine than chlorine.
The buttressing effect in biphenyls has been studied for the first time by UV and NMR spectroscopy. Study of the biphenyl conjugation band in the UV region has shown that the buttressed compounds have a bigger dihedral angle than their unbuttressed analogues. The NMR studies, particularly of the methylene protons, has shown that the rate of change of enantiomeric conformations is slower in the buttressed compounds than in the unbuttressed analogues. Both these observations demonstrate the buttressing effect. The energy of inversion AE for 1',4"-dibromo-2,7-dihydro-3:4>5:6-dibenzoxepin has been calculated from its NMR spectra at various temperatures as 10.6 kcal mole. The buttressing effect in this compound has been calculated to be 1.4 kcal mole.
3,3'-Dibromo-2,2'-diphenic acid has been obtained in optically active form. The double brucine salt of the acid underwent second-order asymmetric transformation and its decomposition gave the dextro-acid. The optical activation of the above acid also demonstrates the buttressing effect.
4,6,4',6'-Tetrachloro-2,2,-diphenic acid has been synthesised and resolved into its enantiomers; optically active 4, 6, 4', 6'-tetrachloro-2,2'-bishydroxymethylbiphenyl and 2',4',1",3"-tetrachloro-2,7"dihydro-3:4,5:6-dibenzoxepin have been prepared from the optically active acids. ORD and CD studies have been made of the (+)-acid, the (-)-diol and the (+)-oxepin and these have been assigned the S-configuration. A comparison of their UV and NMR data with those of 6,6'-dichloro-2,2'-disubstituted biphenyls has given information about the effect of extra chlorine atoms in the 4 and 4' positions.
The IR spectra were taken of all compounds synthesised in order to identify them and mass spectra of some azepinium compounds were obtained to study their pattern of fragmentation.
NMR, IR and polarimetric studies on diols have revealed the presence of H-bonding. The IR spectra of (-)-4,6,4',6'-tetrachloro-2,2'-bishydroxymethylbiphenyl in solution has shown that it prefers the cis conformation. Whereas the IR studies show the presence of intramolecular OH...O and OH...pi bondings, the presence of OH...pi bonding in diols is indicated for the first time from the NMR studies.
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Institution: University of London, Bedford College (United Kingdom).