text: Examples of tetrazol-5-ylidenes based on tetrazole have been prepared by Araki.  The N1 and N3 positions are substituted with alkyl or aryl groups.  Transition metal complexes of these carbenes have been generated in situ.  Mesoionic carbenes based on isoxazole and thiazole have been reported by Albrecht and Bertrand respectively.  The isoxazol-4-ylidenes are trisubstituted in the N2, C3, and C5 positions with alkyl groups.  The thiazol-5-ylidenes are trisubstituted in the C2, N3, and C4 positions with aryl groups.  Transition metal complexes of both types of carbenes have been generated in situ.  Bertrand also reported a 1,3-dithiol-5-ylidene based on 1,3-dithiolane, but it can only be isolated as a transition metal complex.