The Role of Substituents and Solvents in Promoting "Medium-Size" Ring-Chain Tautomerism

Article abstract of DOI:10.1021/jo00299a036

Two series of o-hydroxyalkoxybenzaldehydes have been examined by 1H and 13C NMR for their tendency to exhibit ring-chain tautomerism.All members of the diethylenoxy family exist as chain tautomers rather than partially or wholly as the 10-membered rings.The ethylenoxy compounds in solution are mixtures of chain and 7-membered ring tautomers in every case where a substituent is present on the aromatic ring ortho to the alkoxy group.Ring-chain equilibrium constants, determined by 1H NMR integration of methine and formyl peaks, varied as much as 100-fold with substituents and solvent.A good correlation is noted between percent ring tautomer and the size of the ortho substituent.A second substituent para to the alkoxy group enhances K considerably, even though K = 0 in cases where there is only a para substituent.