Perfluoroalkyl derivatives lithium

Perfluoropropynyl [15, 26, 27] and perfluorovinyl [16, 28-30]-lithium and -magnesium are considerably more stable than perfluoroalkyl derivatives, whilst the corresponding perfluoroaryl derivatives may be used as effectively as in the hydrocarbon series, and direct syntheses involving iodo compounds are also possible (Figure 10.3). 

Outstandingly different in character from the perfluoroalkyl derivatives described above are the lithium and magnesium compounds derived from highly fluorinated bicyclo-[2.2.1]heptanes [37, 38]. Some reactions of the lithio derivative are shown in Figure 10.8, illustrating that it can be utilised in the normal synthetic procedures with much less competition from elimination. 

The Grignard or lithium route is of limited value for the preparation of perfluoroalkyl derivatives. Much of the early work was concerned with the addition of silanes to fluorinated alkenes [88, 89], leading to the preparation of important fluorinated poly-siloxanes, manufactured by Dow Coming Co. (Figure 10.39). 

Silyl enol ethers have also been used as a trap for electrophilic radicals derived from a-haloesters [36] or perfluoroalkyl iodides [32]. They afford the a-alkylated ketones after acidic treatment of the intermediate silyl enol ethers (Scheme 19, Eq. 19a). Similarly, silyl ketene acetals are converted into o -pcriluoroalkyl esters upon treatment with per fluoro alkyl iodides [32, 47]. The Et3B/02-mediated diastereoselective trifluoromethylation [48,49] (Eq. 19b) and (ethoxycarbonyl)difluoromethylation [50,51] of lithium eno-lates derived from N-acyloxazolidinones have also been achieved. More recently, Mikami [52] succeeded in the trifluoromethylation of ketone enolates... 

Perfluoroalkyl-lithium derivatives are thermally unstable and their use in organic synthesis has been limited by competing (3-elimination processes [29]. Pentafluoroethyl-lithium has a half-life of around 8 h at —78° C [30]. In complete contrast, perfluorinated bridgehead lithio derivatives are much more stable since, in these cases, elimination of LiF would contravene Bredt s rule and would, therefore, be a higher-energy process. Consequently, perfluoroadamantyl-lithium is stable at 0° C for several days [31] (Figure 6.22). 

Reactions which partly compensate for the unsuitability of the perfluoroalkyl-lithium route involve addition of fluoride ion to an unsaturated site giving corresponding carb-anions (Cs or K derivatives) that may be used in synthesis (see Chapter 7, Section IIC,... 

The allyltin compounds (CH2 CH CH8)aSn-CaF7-n, (CHa CH-CHj)a-Sn-CaF, and (CHj CH-CHa)aSn(C2F,)j have been synthesized from allyltin bromides and the appropriate perfluoroalkyl-lithium, prepared from the corresponding iodide and methyl-lithium arguments based on comparisons between u.v. and i.r. spectral data for these compounds and those for their alkyl counterparts have been advanced in support of hyperconjugation in allyltin derivatives ... 

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