Substitution reactions of arylated oxadiazoles and related compounds
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The nitrations of 2,5-diphenyl-1,3,4-oxadiazole and 2,5-diphenyloxazole are investigated. Previously, there have been problems associated with the separation of isomeric products in such reactions. We have developed separation methods for the products by using high performance liquid chromatography. In the nitration of 2,5-diphenyl-1,3,4-oxadiazole, considerable variations in the isomer ratios are found when the acid strength of the nitrating medium is altered. In fuming nitric acid alone the orientation is mainly para/meta but with fuming nitric acid in concentrated sulphuric acid a large enhancement in meta-orientation occurs. With nitronium tetrafluoroborate an enhancement in ortho-substitution is found. In an attempt to explain these results 2-nitrophenyl-5-phenyl-1,3,4-oxadiazole isomers were nitrated under the same conditions as above. With fuming.nitric acid the three mono-nitro isomers give approximately 45% para-substitution. With fuming nitric acid in sulphuric acid the p:m:o ratio is approximately 1:3:1. With nitronium tetrafluoroborate there is a large increase in ortho -substitution. The first and second nitrations were found to be consecutive for the nitration in fuming nitric acid and p:m:o ratios for the first nitration are calculated. Possible reasons for the variations in isomer ratios are discussed. It is proposed that the oxadiazole ring is protonated in concentrated sulphuric acid and that the enhancement in meta-nitration in the second nitration is due to a positive pole effect. It is proposed that N-nitration accounts for the enhancement in ortho-nitration in nitronium tetrafluoroborate. Analogous systems do not provide a good comparison e.g. 2,5-diphenyloxazole gives mainly parasubstitution. The brominations of some 2,5-diphenyl-1,3,4- oxadiazoles, in oleum, are investigated. The products were identified by hydrolysis of the brominated oxadiazole and inspection of the esterified hydrolysis product by h.p.l.c. Most of the esters were found to contain a bromine atom in the meta-position. Mono- , di-, tri-,tetra-and pentabromo species were identified by mass spectroscopy.
Thesis, PhD Doctor of Philosopy
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