Conformational analysis of organic carbonyl compounds. Part 4. A 1H and 13C nuclear magnetic resonance study of formyl and acetyl derivatives of benzo[b]furan

Abstract

A conformational study of formyl and acetyl derivatives of benzo[b] furan provides evidence that in the case of 2- and 7-derivatives the E,Z-conformational mixture is solvent dependent, the Z-form prevailing in solvents with higher polarity. The presence of the same substituents in the 3- and 4-position gives compounds with predominantly Z-conformation and no change is caused by solvents. These results show an interestingly different behaviour by carbonyl derivatives of benzo[b] furan with respect to the same benzo[b] thiophene derivatives. The conformational analysis was carried out by the n.m.r. method by employing ¹H and ¹³C chemical shifts, long-range proton–proton coupling constants, and lanthanide-induced shifts (LIS) simulation. For the derivatives substituted in position 7, the LIS method is not a suitable approach for the determination of conformer populations since, in solution, a chelate is formed where bonding of Eu to the oxygen atoms of the carbonyl and of the heterocyclic ring stabilizes the Z-form. A quantitative approach for determining the relative conformer populations of benzo[b] furan-7-carbaldehyde was set up by statistical manipulation of long-range proton–proton coupling constants, proton chemical shifts, and the classical theory of dielectrics.