Synthesis and Keto-Enol Tautomerism of 1-Phenyl(1-thiophen-2-yl)-4-(diphenylphosphoryl)butane-1,3-diones

Novel 1-phenyl(1-thiophen-2-yl)-4-(diphenylphosphoryl)butane-1,3-diones were synthesized by the Claisen-type condensation of 1-(diphenylphosphoryl)propan-2-one with ethyl phenyl(thiophene-2-yl)carboxylates. The tautomeric equilibrium of diketone moiety of phosphine oxides was investigated by ¹H and ¹³C{¹H} NMR spectroscopy. Two species out of five possible tautomers (diketo and mono-enol) were observed in the solution of 1-phenyl(1-thiophen-2-yl)-4-(diphenylphosphoryl)butane-1,3-diones. The major tautomer was the mono-enol form with content 80–90%. Furthermore, the double bond position at C³ atom was determined by ¹³C{¹H} NMR spectroscopy.