Difluorocarbene dibromodifluoromethane

Condensatron between lithium acetylides and dibromodifluoromethane [124] or dichlorofluoromethane [125] leads to fluorohaloacetylenes (equation 107) Sodium alkyl malonates are also alkylated by dihalogenodifluoromethanes [124] (equation 108) These reactions involve difluorocarbene as intermediate (for the mechanism of the Cp2Br2 condensation, see equation 15)... 

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)... 

On the contrary, the cathodic reduction of dibromodifluoromethane generates difluorocarbene, which was successfully trapped with reactive olefines yielding 1,1-difluorocyclopropanes (Scheme 3.3.) [13]. 

Difluorocarbene cannot be generated (<1%) under liquiddiquid phase-transfer catalytic conditions [29] owing to the rapid hydrolysis of the carbene at the interface [30], although it has been indicated that it is possible to obtain low yields of 1,1-difluorocyclopropanes under soliddiquid conditions [1]. More successful is the reaction of dibromomethane and dibromodifluoromethane under basic conditions. It is assumed that the initially formed dibromomethyl anion is transported into the organic phase where an equilibrium reaction with dibromodifluoromethane produces the bromodifluoromethyl anion and, subsequently, the difluorocarbene [31]. 

In general, halodifluoroniethanessuch as dibromodifluoromethane, difluorodiiodomethane and dichlorodifluoromethane are very poor sources of difluorocarbene for the synthesis of cyclopropanes.Cyclic systems 5-8 contain a difluoromclhyl group which can be added to multiple bonds. 

Elimination of bromine from dibromodifluoromethane is readily achieved, and various trapping experiments have been carried out with difluorocarbene from this source [55] (Figure 6.39). 

Other electrophiles that react with in situ-generated perfluorocarbanions in elude epoxides [226] equation 47), carbon dioxide [227] (equation 47) acyl halides [228, 229, 230, 20, 232, 233] (equation 48), fluoroformates [23d] car bonyl fluonde [23S, 236, 237] hexafluorothioacetone (generated from its dimei) [238] (equation 48), an a-fluoroalkylamine f2J9] (equation 48), cyanuric fluoride [240], and reactive alkyl halides [247, 242 243, 244, 245] (equation 49) Interestingly, an in situ-generated carbanion will also react with dibromodifluoromethane ia a mechanism involving difluorocarbene [246] (equation 50)... 

The lipophilic ion pair produced after the deprotonation of dibromomethane, HCBr2l U4, undergoes halophilic reaction with dibromodifluoromethane within the organic phase to form tribromomethane and the ion pair CBrF2NBu4. The latter splits into tetrabutylammonium bromide and difluorocarbene, which reacts with alkene to form 1,1-difluorocyclopropane (for an alternative possibility, see ref 17). 

Reduction (electrochemical or metals ) of dibromodifluoromethane produces difluorocarbene (carbenoid) which reacts with alkenes to form 1,1-difluorocyclopropanes. Electrochemical reduction was carried out on a platinum electrode at 20 C, while metal reduction with lead in the presence of tetrabutylammonium bromide (TBAB) in various solvents was carried out at 40-60°C. In both cases 1,1-difluorocyclopropanes were formed in moderate yield (Houben-Weyl, Vol. E19b, p 1475). 

In contrast, cathodic reduction of dibromodifluoromethane generates difluorocarbene, which is successfully trapped with olefins to yield gem-difluorocyclopropane derivatives (Eq. 10). ... 

Recent work has described the generation of difluorocarbenes from a mixture of bromoform and dibromodifluoromethane under basic condi-... 

Imines 80 react with difluorocarbene in the presence of dimethyl acetylenedi-carboxylate (DMAD) producing 2-fluoropyrroles 83 in 11-78 % yields. This domino process was assumed to occur via difluorocarbene attack on the nitrogen lone pair resulting in formation of azomethine ylides 81, 1,3-dipolar cycloaddition of the latter one to DMAD, and dehydrofluorination of pyrroUnes 82 thus formed [44]. Difluorocarbene can be generated by reduction of dibromodifluoromethane with lead powder in the presence of tetrabutylanunonium bromide (Method A) or using active lead obtained by reduction of aqueous lead acetate with sodium borohydride (Method B). This reaction can be also performed as intramolecular version to form substituted 2-fluoropyrroles 85,86 [45]. 

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