Investigation of aromatic properties by proton magnetic resonance

• Main Author: Lewis, P. M. E.
• Published: University of Surrey 1968
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.751721

The possibility of determining the aromaticity of certain pseudo-aromatic compounds from chemical shift measurements, following the original procedure of Elvidge and Jackmann 7 has been investigated. The compounds studied in the present work were two isomeric dimethyl-4-pyridones, three isomeric dimethyl-4-quinolones , four isomeric dimethyl-2-quinolones, and various thio-analogues. The results for the 4-pyridones (16% aromaticity) are in agreement with theoretical calculations, which predict a lower value than for. the 2-pyridones (found by Elvidge and Jackmann to be about 35%). The aromaticity of 40% calculated for the 4-thiopyridones is also of the expected order, since sulphur is more polarizable than oxygen, and is therefore likely to facilitate pi-delocalisation. The values of 70-75% aromaticity obtained for the pyridone rings of the 2-and 4-quiholones and their thio-analogues are, however, much higher than expected, since earlier workers have found that aromaticity values for fused heterocyclic rings calculated in this manner normally agree quite well with those for the isolated heterocyclic ring. Amongst the new compounds prepared the thiones of various dihydro-trimethylnaphthalenes were of particular interest. Compounds of this type have not been described in the literature because of their instability and inherent tendency to polymerise, but in this work their fleeting presence was established spectroscopically. Other notable features encountered were a) the unexpected formation of 5-methyl-2-tetralone as the major product in the synthesis of 8-methyl-2-tetralone from o-tolylacetyl chloride and ethylene, and b) the ready aromatisation occurring in reactions involving the carbonyl group of 1, 4-dihydro-4, 4, 7-trimethyl-l-oxonaphthalene.