Imidoyl fluorides

A Chapman-like rearrangement has been observed in an attempt to prepare imidoyl fluoride 9. Instead, carbamoyl fluoride 10 is formed by an oxygen to nitrogen 1,3-alkyl migration."... 

A fluoro-Beckmann rearrangement has been reported for perfluorinated /V-fluoro-subsliluted ketimines 6. which undergo a 1,2-perfluoroalkyl shift to give perfluorinated imidoyl fluorides 7 when heated with antimony(V) fluoride.2... 

Definitive evidence for the imidate intermediate was provided by Glikmans in 1966. Reaction between isobutene and acrylonitrile in anhydrous acetic acid containing 100% sulfuric acid allowed isolation and characterization of (7), which was readily hydrolyzed to the corresponding amide by dehydration of acetic acid (equation 1). Subsequent work by NorelF on reactions of alkenes with nitriles In anhydrous liquid hydrogen fluoride allowed isolation of imidoyl fluorides such as (8) for the first time. Such materials, obtained pure and fully characterized, react instantly with water to yield the amide (Scheme 4). 

Though very stable thermally, aziridine 31 slowly decomposes at 220°C, producing imidoyl fluoride 195 and amine 196 in a 1 1 ratio in 95% yield. A likely course of events entails reversible ring opening to azomethine ylide 197, which abstracts a fluoride ion from the starting aziridine to form anion 198 and cation 199 (akin to 179 above). Ring closure of 199 yields 200, and then transfer of a proton from 200 to 198 gives the final products. 

The reaction of perfluorinated cyclic amines with strong Lewis acids, such as SbFs or aluminum chlorofluoride (ACF), at elevated temperature results in rapid cleavage of amine, leading to the formation of cyclic perfluorinated imidoyl fluorides (Fig. 9.23). In contrast to AICI3 reactions, this process is catalytic. The outcome of the reaction depends on the structure of amine. For example, the reaction of A-alkyl piperidines or morpholines leads to the elimination of perfluoroalkane... 

The proposed mechanism of the reaction involves abstraction by Lewis acid of F from activated by nitrogen a-CF2 group of heterocycle, leading to the formation of cyclic immonium cation, which is followed by its ring opening and the formation of imidoyl fluoride 84 or 85 (Fig. 9.24). " ... 

The ability of Al-fluoro amines to undergo reductive defluorination was discovered by Mitsch in 1965. " The treatment of amines 30, 31, and 67 by ferrocene results in the formation of the corresponding cyclic imidoyl fluorides 81, 83, and... 

This process has a high selectivity for N F bond and SF4 group in compound 67 does not undergo reduction under reaction conditions. The abihty of Al-fluoro amines selectively convert triphenyl- phospine, arsine, and stibine into the corresponding difluorides (reported in 1967 by Haszeldine group ) was utilized in an effective laboratory scale synthesis of imidoyl fluorides 81 and 83 involving the reaction of amines 30 and 31 with triphenylphosphine. ... 

The mechanism of the reductive defluorination of perfluorinated N F amines may involve a relatively low laying LUMO of N—F bond. The injection of an electron on this orbital leads to the formation of radical anion D, which can further lose fluoride anion, forming radical E. The reduction of E to azaanion F followed by second fluoride anion elimination may lead to the corresponding cyclic imidoyl fluoride as a result of reductive defluorination (Scheme 9.1). 

The formation of 83 in this reaction is not surprising, since secondary amines (Rf)2NH are known to be prone to HF elimination, which can be caused by such week base as a glass. Obviously, the formation of diacids 92 in reaction with KI/H2O/ acetone (or diethyl ester of perfluoroglutaric acid in the reaction of 31 with ethanol ) is the result of ring opening hydrolysis of the corresponding imines 81 or 83, formed as intermediates in this process (for detailed mechanism see Ref. 87). It should be pointed out that the conversion of amine 31 into imidoyl fluoride 81 at elevated temperature in the presence of Fe or Pt metals was also reported. ... 

One of the drawbacks of amine 30 as fluorinating agent is a side reactions of highly reactive imidoyl fluoride 81, formed as by-product in fluorination process. This problem can be eliminated by the use of N—F amine, which in the course of the reaction will be converted into stable, unreactive azaanion. Indeed, higher yields of fluoride 96 were reported in the reaction of fluorinating reagents 97 and 98 with salt 95. ... 

Several methods are available for the preparation of perfluorinated cyclic imidoyl fluorides. For example, reductive defluorination of cyclic A-fluoro compounds... 

Fig. 9.26) by ferrocene or PPh3 was used for the preparation of imidoyl fluorides 81,83, and 91. The cleavage of perfluorinated tertiary amines under the action of SbFs (see Fig. 9.23) is another convenient method for the synthesis of cyclic imines, such as 81-83. 

The presence of highly electrophilic C=N bond defines the reactivity of this class of compounds. Similar to other perfluorinated imidoyl fluorides, cyclic analogues are sensitive to the action of nucleophiles. For example, compounds 81 and 124 react with metal fluoride anion in polar solvents with the formation of the salt 125, which can react with a strong electrophiles (Fig. 9.38). 

It is noteworthy, that in contrast to noncyclic perfluorinated imidoyl fluorides RfN = CFR f, cyclic imines 124 and 81 undergo the dimerization upon treatment with fluoride anion (Fig. 9.40). 

FIGURE 9.40 Dimerization of cyclic perfluorinated imidoyl fluorides. 

The reaction of with nucleophilic reagents is the most studied group of chemical transformations involving cyclic imidoyl fluorides. Some representative reactions of compound 81 with secondary amines, pentachlorophenoxide and (CF3)2NONa leading to products 135-138 are shown below (Fig. 9.41). ... 

The chloro groups in XLVI can also be displaced by fluoride ion under mild conditions, to afford the perfluorinated imidoyl fluoride LII... 

Yields are only moderate (20—49%) and the ketenimines are accompanied by resinous by-products. Alternative routes to bis(fluoroalkyi)ketenimines involve dehydrofluorination of 2Sf-imidoyl fluorides, or treatment of the dangerously toxic perfluoroisobutene with mixtures of primary and tertiary... 

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