Keto-Enol Tautomerism in the Thiophene Analogues of Naphthacen-5-one

Article abstract of DOI:10.1021/jo00343a021

A study of the synthesis and keto-enol tautomerism in a naphtho and several anthrathiophene systems was undertaken with the objective of determining the structural features which influence the position of equilibrium in these fused systems.Thus 4,11-dihydroanthra<2,3-b>thiophen-4-one (3) and 4,11-dihydroanthra<2,3-b>thiophen-11-one (4) were synthesized and studied spectroscopically.Difficulties in the attempted synthesis of 4,11-dihydroanthra<2,3-c>thiophen-4-one (5) prompted the synthesis of 1,3-dimethyl-4,11-dihydroanthra<2,3-c>thiophen-4-one (6).This modificationnecessitated the synthesis of 1,3-dimethyl-4,9-dihydronaphtho<2,3-c>thiophen-4-one (7) so that comparisons with the naphtothiophene system could be made.It was found that the c-fused ketones 6 and 7 gave no evidence of the presence of enolic material when their NMR spectra were studied in CDCl3 or Me2SO-d6.A solution of 3 in CDCl3 or CDCl3-CF3CO2H gave no NMR spectroscopic evidence of enolic material on standing.In Me2SO-d6 only enolic material appeared to be present.In the case of 4 a freshly prepared solution in CDCl3 showed only keto form, but when the solution was allowed to stand for 96 h, the NMR spectrum of the solution showed a 33percent enol content.In CDCl3-CF3CO2H at equilibrium a 39percent enol content was observed.It is concluded that in these systems c fusion of a thiophene ring predisposes the compound to exist exclusively in the keto forms (6k, 7k) since this avoids the intermediacy of the unstable anthra- and naphthothiophenes. b fusion of a thiophene ring permits varying amounts of enol in the cases of 3 and 4, depending on the solvent.