TRANSMISSION OF SUBSTITUENT EFFECTS IN HETEROCYCLIC-SYSTEMS BY C-13 NUCLEAR MAGNETIC-RESONANCE - BENZOTHIAZOLES

The carbon-13 NMR spectra of 57 benzothiazoles including 6-substituted 2-aminobenzothiazoles (1), 6-substituted 2-methylbenzothiazoles (2), 6-substituted benzothiazoles (3), 5-substituted 2-methylbenzothiazoles (4) and 2-substituted benzothiazoles (5) were determined in Me2SO-de6 The chemical shift assignments were made based upon the ones previously reported for benzothiazole, 2-aminobenzothiazole, and 2-methylbenzothiazole, chemical shift and signal intensity arguments, and by interpretation of their proton-coupled spectra. The chemical shift data for carbon 2 and carbon 9 for series 1-3 and carbon 2 and carbon 8 for series 4 gave good correlations with simple Hammett constants, and slightly better correlations with a linear combination of σm and σp, Swain-Lupton, and Taft-DSP treatments. Results from the dual parameter approaches indicate that resonance effects are primarily responsible for the substituent effect on chemical shifts at the carbons in question. The data also indicate that transmission ofsubstituent effects by the sulfur atom is limited and that the primary path of transmission of substituent effects to carbon 2 is through nitrogen. The data from series 3 and 5 suggest that transmission of substituent effects by substituents on carbon 2 to carbon6 is approximately one-third less effective than transmission by substituents on carbon 6 to carbon 2. © 1979, American Chemical Society. All rights reserved.