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 CDCl<INF>3</INF> and in CD<INF>3</INF>OD as well as in D<INF>2</INF>SO<INF>4</INF> solutions. Moreover ¹⁷O chemical shifts of 1 in CDCl<INF>3</INF> were measured. The SCS (substituent-induced chemical shift) values were analyzed by means of linear free energy (LFE) relationships and also by cross-correlations 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 © 1999 John Wiley & Sons, Ltd.