Imidazol-2-ylidenes, stable singlet carbenes

Since this discovery, a few types of other stable singlet carbenes have been described and reviewed5 imidazol-2-ylidenes,6 l,2,4-triazol-3-yli-denes,7 imidazolidin-2-ylidenes,8 acyclic diaminocarbenes,9 thiazol-2-yli-denes,10 and acyclic aminooxy- and aminothiocarbenes.11... 

An extreme example in which the single state is stabilized by electron-donating substituents is seen in the recent isolation of singlet carbenes in 1988, phosphinocarbene (10) was isolated as a stable yellow oil which can be distilled at 80-85 °C/10"2 Torr [15] and in 1991, imidazole-2-ylidene (11) was isolated as stable crystals that melt above 100 °C [16] (Scheme 2). 

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