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Electronic effects on 13C NMR chemical shifts of substituted 1,3,4-thiadiazolium salts

✍ Scribed by Ana Cristina Souza dos Santos; Aurea Echevarria

Publisher
John Wiley and Sons
Year
2001
Tongue
English
Weight
122 KB
Volume
39
Category
Article
ISSN
0749-1581

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✦ Synopsis

A series of 3-and 4-X-cinnamoyl-1,3,4-thiadiazolium-2-phenylamine chlorides were prepared and fully characterized by spectroscopic techniques. The 13 C NMR chemical shifts of the a and b side-chain carbons were compared with those of the corresponding cinnamic acid precursors and more significant transmission electronic effects were observed for mesoionic compounds. The ratio r R /r I for the a and b side-chain carbons in mesoionic derivatives reflects a more important inductive contribution whereas the value of r R /r I for the b side-chain carbons indicates a higher resonance contribution. The chemical shifts of C-2 and C-5 of the heterocyclic ring were correlated with substituent constants and showed that the more extensive conjugation of the side-chain with an exocyclic moiety is favoured by electron-withdrawing groups. The correlation analysis of chemical shifts with dual parameters indicated a significant contribution of a resonance effect for C-2 (r R /r I = 1.70) when compared with C-5 (r R /r I = 1.03).

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