Sydnone Methides: Intermediates between Mesoionic Compounds and Mesoionic N‐Heterocyclic Olefins

Sydnone methides represent an almost unknown class of mesoionic compounds which possess exocyclic carbon substituents instead of oxygen (sydnones) or nitrogen (sydnone imines) in the 5-position of a 1,2,3-oxadiazolium ring. Unsubstituted 4-positions give rise to the generation of anionic N-heterocyclic carbenes by deprotonation. Preparations of new sydnone methides are described here. They can be represented by mesomeric structures with either exocyclic carbanionic groups like -C(CN)2-, -C(CN)(COOMe)-, -C(CN)(CONH2)-, and -C(CN)(SO2Me)-, or with the corresponding exocyclic C=C double bonds as a common feature with mesoionic N-heterocyclic olefins. An X-ray single structure analysis revealed a length of 140.7(3) pm of the exocyclic bond in the solid state. From the coalescence temperature (55 °C) determined by a series of 13C NMR experiments (150 MHz) at various temperatures, an energy of rotation of 18.5 Kcal/mol was calculated for this bond. The 13C NMR signals of the anionic N-heterocyclic carbenes, from which the 2-mesityl-substituted anionic NHC proved to be stable up to 10 °C, are highly shifted upfield (δcarbene=157.9 ppm−-160.5 ppm). The carbenes can be reacted in situ with elemental selenium and chlorophosphanes to yield sydnone methide selenoethers after methylation and 4-phosphanylsydnone methides in good to excellent yields, respectively.