The addition of dialkyl acetylenedicarboxylates to isatin, a strong N–H acid, in the presence of triphenylphosphine leads to stable crystalline phosphorus ylids in excellent yields. Some of these ylid derivatives exist as a mixture of two rotational isomers. Interconversion between these two forms was investigated by variable temperature NMR spectroscopy.
HudsonH.R. in The Chemistry of Organophosphorus Compounds, Vol. 1. Primary, Secondary and Tertiary Phosphines, Polyphosphines and Heterocyclic Organophosphorus (III) Compounds, (Ed. HartleyF. R.), Wiley, New York, 1990, pp. 386–472.
