Evidence for equilibrium between N-substituted 3-phosphorylidenepyrrolidine-2,5-diones and N-substituted maleimides and phosphines:: Thermal instability of the N-substituted phosphoranes

The formation of N-substituted 3-phosphorylidenepyrrolidine-2,5-diones from N-substituted maleimides and phosphines is reversible and at elevated temperature the phosphoranes behave as a source of maleimide derivatives which give the corresponding cycloadducts with nitrones and 1,3-dienes; a consequence of this equilibrium is the thermal instability of the ylides.