A carbon-13 NMR study of 2-substituted-5-N′,N′-dimethylaminophenyl-N,N-dimethylcarbamates:: Correlations of substituent-induced chemical shifts with substituent parameters

The Carbon-13 NMR spectra of seven 2-substituted-5 -N', N'- d i m e t hy la m i n o p he n y l N, N-dimethylcarbamates and of the parent compound were recorded, and unequivocal assignments of the chemical shifts through the use of HETCOR and COLOC sequences were performed. Large intramolecular interaction chemical shifts (ICS) were observed for C-1 (-9.3 to +3.4 ppm) and moderate values for C-2 to C-6 (-3.0 to +3.8), mostly for the oxygen-bearing derivatives [OCH3, NO, and C(O)CH3]. Correlation. analyses for the substituent-induced chemical shifts (SCS) with several sets of substituent parameters were carried out, through SSP (Single Substituent Parameter), DSP (Dual Substituent Parameter) and DSP-NLR (Dual Substituent Parameter Non-Linear Resonance) treatments, as well as through multiple regression analysis. Good correlations were found for C-1, C-4, C-5 and C-6 (r>0.97), while for C-2, C-3 and C-10 (the carbonyl carbon) just rough trends were observed (r>0.89). The resonance contribution is predominant for C-4 and C-6 (unsubstituted carbons) and for C-5, while for C2 and C-3 and C-10) the electronegativity parameter is the prevailing component and for C-1 both contributions are relevant. Negative rho values were found for several correlations (reverse substituent effect).