Addition fluoroalkenes

Difluoroethanol is prepared by the mercuric oxide cataly2ed hydrolysis of 2-bromo-l,l-difluoroethane with carboxyHc acid esters and alkaH metal hydroxides ia water (27). Its chemical reactions are similar to those of most alcohols. It can be oxidi2ed to difluoroacetic acid [381-73-7] (28) it forms alkoxides with alkaH and alkaline-earth metals (29) with alkoxides of other alcohols it forms mixed ethers such as 2,2-difluoroethyl methyl ether [461-57-4], bp 47°C, or 2,2-difluoroethyl ethyl ether [82907-09-3], bp 66°C (29). 2,2-Difluoroethyl difluoromethyl ether [32778-16-8], made from the alcohol and chlorodifluoromethane ia aqueous base, has been iavestigated as an inhalation anesthetic (30,31) as have several ethers made by addition of the alcohol to various fluoroalkenes (32,33). Methacrylate esters of the alcohol are useful as a sheathing material for polymers ia optical appHcations (34). The alcohol has also been reported to be useful as a working fluid ia heat pumps (35). The alcohol is available ia research quantities for ca 6/g (1992). 

Perfluoroepoxides have also been prepared by anodic oxidation of fluoroalkenes (39), the low temperature oxidation of fluoroalkenes with potassium permanganate (40), by addition of difluorocarbene to perfluoroacetyl fluoride (41) or hexafluoroacetone (42), epoxidation of fluoroalkenes with oxygen difluoride (43) or peracids (44), the photolysis of substituted l,3-dioxolan-4-ones (45), and the thermal rearrangement of perfluorodioxoles (46). 

Generally, additions of halogens to fluoroalkenes are less stereoselective than the analogous reactions with nonfluorinated systems. The stereochemical mode of addition can be either anti or syn Partitioning between these paths is determined... 

Conjugated, cross-conjugated, and homoconjugated fluoroalkenes react with halogens to yield predominantly 1,4-adducts Results from the reactions of a series of conjugated fluoroalkenes with elemental halogens are summarized in Table 4 In nearly all cases, the fraMs-l,4-addition products are formed exclusively [Il ... 

Addition of hydrogen halide across fluoroalkenes and fluoroalkylalkenes is an important route to halogen-containing fluoroorganics Both lontc and free radical... 

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

The mechanism of attack of 1,3-dipolar reagents on fluoroalkenes can be considered to be either stepwise or concerted. Heteroaromatic N-imines react by a stepwise 1,3 addition to perfluoroalkenes and -alkynes to give fluorinated pyrazolo[l,5-a]pyridines [82JCS(P1)1593]. Pyridinium /-butoxycarbonylmethylide with fluoroalkenes gave pyrrolo[l,2-a]pyri-dines [86JCS(P 1) 1769] and indolizines (22) are obtained with pyridinium phenacylide [91JFC(51)407]. 

Perfluoroalkyl carbanions, generated by reversible nucleophilic addition of a fluoride anion to fluoroalkenes, react with dry benzenediazonium chloride in dimethyl formamide, giving phenylazoperfluoroalkanes in 41-53% yield (Dyatkin et al., 1972). The dianion obtained from 1,2-dinitrobenzene with dipotassium cyclo-octatetraenide reacts in a complex way with arenediazonium salts, forming 4-aryl-azo-2-nitrophenol in 46-58% yield (Todres et al., 1988). 

Diradical intermediates are found more commonly in thermal 2 + 2 cycloadditions. The best substrates for such additions are fluoroalkenes and allenes. Thus P.D. Bartlett and his coworkers have demonstrated that the cycloaddition of l,l-dichloro-2,2 difluroethylene to stereoisomeric hexa-2, 4 dienes yields a mixture of isomeric products in a non stereospecific way. 

Even simple fluoroalkenes display exploitable electrophilic character. Bailey described an asymmetric synthesis of protected difluoroglycines initiated by nucleophilic addition/elimination at the difluorocentre of chlorotrifluoro-... 

More recently, radical additions to fluoroethenes have attracted attention. Eguchi et al. [125] applied the Barton decarboxylation procedure to add a range of alkyl radicals to l,l-dichloro-2,2-difluoroethene. Addition was regioselective and the terminal carbon could be hydrolysed to a carboxyl group with silver(I) mediation (Eq. 39). The fluoroalkene is effectively an equivalent for either difluoroacetyl anion or cation synthons, because the adding radical can be approached from either polarity manifold. 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

More often, however, fluorocarbanions are intermediates in reactions of fluoroalkenes,35 e.g. by addition of nucleophiles, especially fluoride. Deprotonation of hydrofluorocarbons also yields the anion. Whereas a-fluorination destabilizes the anion, /J-fluorination is essentially stabilizing. The latter effect may be assisted by so-called fluorine-nonbond resonance (negative anionic hyperconjugation) which has been discussed in diverse ways supported by some,3 but criticized by others.2 31... 

Fluorinated radicals play a significant role in synthetic organo-fluorine chemistry, for example, in electrophilic radical addition to alkenes, single-electron transfer reactions (SET), telomerization of fluoroalkenes with perfluoroalkyl iodides, polymerization to fluoropolymers and copolymers, and thermal, photochemical and radiation destruction of fluorocarbons. Furthermore, such free radicals are of interest for studying structures, reaction kinetics and ESR spectroscopic parameters.38... 

An intensive study of fluoroalkenes was initiated by the Environmental Protection Agency (EPA) in 1981.3,1 In addition to acute toxicities, a lot more toxic information is included in recent literature.34 36 Metabolic studies of alkenes, acting as nephrotoxins have been reported.37... 

The mixtures bromine trifluoride/bromine and iodine pentafluoride/iodine can be successfully employed to prepare bromo- and iodo-substituted fluoroalkanes by addition to poly-fluoroalkenes. The reactions are carried out, in the presence of catalysts aluminum, aluminum/ aluminum triiodide9 or without a catalyst,10 in steel reactors at room temperature or with heating. In most cases the addition products are obtained in high yield (see Table 1). 

The direction of iodine monofluoride addition to hexafluoropropene and, 1,1-di-fluoroethene suggests an electrophilic reaction mechanism. However, chlorotrifluoroethene gives a mixture of isomers both with iodine monofluoridc and with bromine monofluoride .1 0 The reactions of fluoroalkenes with bromine monofluoride proceed too quickly and for this reason they are conducted in l,l,2-trichloro-l,2,2-trifluoroethane though this hampers the isolation of addition products. 

F-Teda BF4 is effective for the selective addition of fluorine to steroids in good yield, re-gioselectively and, in many cases, stereoselectively at the 6- and 16-positions, under very mild reaction conditions (Table 7).92 Further, 6 will also efficiently fluorinate silyl and alkyl enolates, enamides, carbanions, a-alkenes and actived aromatic compounds (Table 8). As an extension of this method F-Teda BF+ has been used for the electrophilic fluorination of (fluorovinyl)tin compounds affording terminal fluoroalkenes (see Table 9).88... 

---