An analysis of 13C nuclear magnetic resonance substituent chemical shifts in 3-substituted thiophene-2-carboxylic and 2-substituted benzoic acids by linear free energy relationships

Abstract

The I3C n.m.r. chemical shifts of the title compounds and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been analysed by means of crosscorrelations and of single and dual substituent parameter linear free energy relationships. Different results have been obtained for thiophene and benzene derivatives using DSP analysis: for the first ring system the results have been rationalized in terms of separate resonance and inductive contributions of substituent effects (pointing out the occurrence of an apparent reverse polar effect); in contrast the same treatment of the data for the benzene system gave poor results, confirming once again the different proximity effects in six-and five-membered ring derivatives.From studies of the reactivity of many classes of ortho-dior poly-substituted thiophene derivatives and the application of linear free energy (l.f.e.) ortho-correlations to reactivity and equilibrium data, we have shown that in thiophene compounds it is possible to provide evidence for many examples of good 1.f.e. ortho-correlations, both in reactions on the ring'"" and on the side-chain,Id-' at variance with the results obtained for six-membered ring derivatives (e.g. benzene, na~hthalene).~The aim of the present work is to ascertain whether an ortholike substituent induces on the carbon of the side-chain in thiophene series a 3C n.m.r. substituent chemical shift (SCS) which could give linear correlations by dual substituent parameter (DSP) treatment of data, duplicating the situation observed in reactivity and equilibrium studies. It must be noted that SCSs induced on side-chain carbon atoms in benzene derivatives by ortho-substituents are usually excluded from DSP treatment due to steric and/or magnetically anisotropic effects. We have chosen as probe compounds a series of 3substituted thiophene-2-carboxylic acids (1) t and the corresponding anions (1-), whose 13C chemical shifts have been measured in deuteriated methanol.In order to enable a comparison between the effects of orthosubstituents in thiophene-and benzene-carboxylic acids we have also collected ' 3C n.m.r. data of ortho-substituted benzoic acids (2) and the corresponding anions (2-) under similar experimental conditions.Recently: we measured the 13C n.m.r. chemical shifts of some

Effect of ortho substituents on carbonyl carbon 13C NMR chemical shifts in substituted phenyl benzoates

“…The influence of the ortho effect on the 13 C NMR chemical shift of the side‐chain functional group was studied for substituted methyl benzoates,17, 20, 21 ethyl benzoates,26 benzoic acids,21, 23–25 thiocyanatobenzenes,27 arylacetamides,28–30 acetonitriles,31 acetophenones, and benzaldehydes 32–36. Guy27 obtained excellent correlations for the 13 C NMR chemical shift in ortho ‐substituted thiocyanatobenzenes with the inductive, resonance, and the steric substituent parameters.…”

Solvent sensitivity of ortho substituent effect on 13C NMR chemical shift of the carboxyl carbon (δco) in benzoic acid

“…However, this trend is exactly the reverse of that for log k 1 (toluene) of the acids. This reverse substituent effect is well known and has been observed for different sets of ortho , meta , para and disubstituted benzoic acids and esters in polar and apolar media, although the substituents chosen were neither all aprotic nor all free from intramolecular H‐bonding, cyclisation and other secondary interactions. Evidently, for a proper analysis of the reverse ortho effect, both the reactivity parameter (log k 1 ) and the δ co should be measured in very similar solvents.…”

“…Hence, it is of significant interest to probe the effect of ortho substituents, the ortho effect on δ co in benzoic acid in an apolar aprotic solvent, for ensuring a virtual freedom from specific solvation effects and to see how well does δ co correlate with log k 1 of acids. Studies so far reported for the ortho effect on δ co in benzoic acids, to the best of authors' knowledge, are restricted mainly to specific solvation inducing polar protic CD 3 OD and dipolar aprotic DMSO‐d 6 for several mono‐ ortho derivatives and to apolar aprotic CDCl 3 for mono‐ and di‐ ortho fluoro/chloro‐derivatives only . The coefficient of correlation of δ co with a physicochemical reactivity parameter was found negative in either case in a way similar to those for meta or para isomers .…”