Difluorocarbene, singlet carbenes

In the photolysis of difluorodiazirine (218) a singlet carbene was also observed (65JA758). Reactions of the difluorocarbene were especially studied with partners which are too reactive to be used in the presence of conventional carbene precursors, such as molecular chlorine and iodine, dinitrogen tetroxide, nitryl chloride, carboxylic acids and sulfonic acids. Thus chlorine, trifluoroacetic acid and trifluoromethanesulfonic acid reacted with difluorodiazirine under the conditions of its photolysis to form compounds (237)-(239) (64JHC233). 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

Despite the negative inductive effect of fluorine, because of its electronegativity, a-fluorinated carbenes are stabilized in their singlet state by -donation from the fluorine to the carbon. This combination of destabilizing and stabilizing effects renders difluorocarbene a moderately electrophilic species [35]. 

---