An Analysis of Substituent Effects on the Carbon-13 and Oxygen-17 NMR Chemical Shifts of Some Phenylthiophen-2-ylmethanones by Linear Free Energy Relationships

The 13C chemical shifts of the title compounds (1) were determined in CDCl3 and in CD3OD as well as in D2SO4 solutions. Moreover 17O chemical shifts of 1 in CDCl3 were measured. The SCS (substituent-induced chemical shift) values were analyzed by means of linear free energy (LFE) relationships and also by crosscorrelations with those of both the corresponding 5-substituted 2-acetylthiophenes (2) and 4-substituted acetophenones (3). The 13C SCS values indicate a transmission pattern of the substituent effect which is more complex than those observed in 2 and 3. On the other hand, the 17O SCS values are more significant and reliable indicators of the transmission of the electronic effects on to the carbonyl group of 1, clearly underlining the different propensities of the phenyl and the thiophen-2'-yl groups in accomplishing such a transmission. Some apparent discrepancies observed between protonation and 17O NMR data of compounds 1, 2 and 3 are also discussed. Copyright Ó 1999 John Wiley & Sons, Ltd.