Dibromodifluoromethane/Zinc

The use of dibromodifluoromethane with zinc [26] or lead metal [27] is a simple, economical method for generating difluorocarbene under mild conditions, but the reaction is limited to highly nucleophilic olefins (equation 8)... 

Difluoromethylenation (79,80) of aldonolactones may be readily accomplished by treatment of the lactone derivative with tris(dimethyl-amino)phosphine, dibromodifluoromethane, and zinc in refluxing tetrahy-... 

Methylene difluorocyclopropanes are relatively rare and their rearrangement chemistry has been reviewed recently [14]. In addition, electron deficient alkenes such as sesquiterpenoid methylene lactones may be competent substrates. Two crystal structures of compounds prepared in this way were reported recently [15,16]. Other relatively recent methods use dibromodifluoromethane, a relatively inexpensive and liquid precursor. Dolbier and co-workers described a simple zinc-mediated protocol [17], while Balcerzak and Jonczyk described a useful reproducible phase transfer catalysed procedure (Eq. 6) using bromo-form and dibromodifluoromethane [18]. The only problem here appears to be in separating cyclopropane products from alkene starting material (the authors recommend titration with bromine which is not particularly amenable for small scale use). Schlosser and co-workers have also described a mild ylide-based approach using dibromodifluoromethane [19] which reacts particularly well with highly nucleophilic alkenes such as enol ethers [20], and remarkably, with alkynes [21] to afford labile difluorocyclopropenes (Eq. 7). 

The carbene is easily obtained in situ by the reaction of dibromodifluoromethane with activated zinc dust. The reaction is run at room temperature without solvent. Only ketones give reasonable yields. Aromatic aldehydes react with poor yields aliphatic aldehydes form tars in this reaction. 

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