Dehydrofluorination reaction

The fluorination of CF3CH2CI into CF3CH2F over chromium oxides is accompanied by a dehydrofluorination reaction (formation mainly of CF2=CHC1). This dehydrofluorination is responsible for the deactivation of the catalyst. A study of the dehydrofluorination reaction of CF3CH2CI proves that the reaction is favoured when the degree of fluorination of chromium oxide increases. Consequently it would be favoured on strong acid sites. Adding nickel to chromium oxide decreases the formation of alkenes and increases the selectivity for fluorination while the total activity decreases. Two kinds of active sites would be present at the catalyst surface. The one would be active for both the reactions of dehydrofluorination and of fluorination, the other only for the fluorination reaction. 

The aim of our work is to study, under adequate operating conditions, the dehydrofluorination reaction of CF3CH2CI so as to determine the nature of the sites involved in the 6uorination and the dehydrofluorination of CF3CH2CI. Thus a selective poisoning of dehydrofluorination sites would allow to increase the selectivity for the fluorination reactions. 

With this in view, we studied the development of the dehydrofluorination reaction of CF3CH2CI as function of the degree of fluorination of chromium oxide. Moreover, nickel and chromium oxide catalysts were prepared and tested for the dehydrofluorination reaction. Nickel oxide, a basic compound [S], could poison selectively the tes involved during the dehydrofluorination reaction. 

The transformation of CF3CH2CI was studied at 320 C in a pulse flow reactor. Indeed, in a dynamic reactor, the agnificant alkene formation leads to a rapid deactivation of the catalyst. The reaction is carried out in absence of HF in order to favour the dehydrofluorination reaction. Products distribution is shown in Fig. 1. 

The main product of the reaction was CF2=CHC1. Moreover, two secondary alkenes CFC1=CHC1 (Z and E) were formed from CF2 = CHCl by successive chlorine-fluorine exchanges [10], After a fast initial deactivation, the catalyst stabilized. The catalytic activity for the dehydrofluorination reaction was estimated after a 5 hours reaction from the sum of the amount of alkenes. 

However the dehydrofluorination reaction compared to the fluorination reaction is favoured. The ratio between the catalytic activity for the fluorination reaction and the... 

Two Ni-Cr catalysts containing one 5 % nickd atomic and the other 10 % were tested for fluorination and dehydrofluorination reactions of CF3CH2CI. The results were compared to those obtained with chromium oxide alone. These reactions are performed only with the non fluorinated catalysts (table 2). 

Moreover a significant decrease in the formation of alkenes could be observed for the Ni-Cr catalysts. Consequently the presence of nickel dis voured the dehydrofluorination reaction while increasing the selectivity for the fluorination reaction. 

We observed the dehydrofluorination reaction was favoured when the degree of fluorination of the catalyst increased. Indeed, we showed that the ratio between fluorination activity and dehydrofluorination activity decreased when the degree of fluorination of the catalyst increased. 

Thus when the degree of fluorination increased, the selectivity for the fluorination reaction decreased. The dehydrofluorination reaction required the rupture of the C-F bond of the CF3CH2CI molecule while the fluorination reaction involved the rupture of the C-Cl bond. The C-F bond being harder to split than the C-Cl bond [11], the dehydrofluorination reaction require stronger adsorption sites than the fluorination reaction. 

The fluorination of chromium oxide caused an increase of surface site Lewis acidity. Kemnitz and al.[12] as well as Peri [13], showed that on fluorinated alumina the progressive substitution of F for O and OH led, thanks to inductive attracting effect of fluorine, to an increase of the Lewis acidity of a sites. Hence the dehydrofluorination reaction was ftivoured on strong acide sites. 

The presence at the catalyst s surface of active sites which made possible the dehydrofluorination reaction (C-F rupture) and the chlorine-fluorine substitution (C-Cl rupture) was then supposed There would also exist active sites which would only allow the second reaction (C-Cl nipture). The addition of nickel, by suppressing the sites which allow the two reactions (fluorination and dehydrofluorination) decreases the total activity. 

Fig. 13. Dehydrofluorination reaction catalyzed by a MIP imprinted with the TSA AT-benzyl-N-isopropylamine...

This effect cannot be explained by simply considering differences in frontier orbital energies. A useful monofluorinated dienophile has been prepared [364] using metallated difluoroenol carbamate chemistry (Eq. 143) cycloaddition occurred smoothly with a range of dienes, and desulfination could be achieved under mild conditions without loss of the fluorine atom. Wakselman and co-workers [365] synthesised a rare competent difluorinated dienophile. Lewis acid catalysed Diels-Alder reaction with furan afforded an acceptable yield of (unfortunately) unstable cycloadduct which decomposed to a phenolic product via a dehydrofluorination reaction, circumscribing its utility (Eq. 144). 

This technique has also been employed for the preparation of a catalytic imprinted membrane by coating a cellulose membrane with a polymer incorporating particles imprinted with the transition-state analogue of a dehydrofluorination reaction [264]. The application of such an MIP composite membrane as the recognition element in an optical sensor has been reported for digitoxin analysis in serum samples by embedding digitoxin-MIP particles in polyvinyl chloride film in presence of plasticizer by the dry inversion process [265],... 

Generation of several other allylic and polyenylic cations as a result of a rather unusual dehydrofluorination reaction of the corresponding polyfluoro-alkane by SbF5 has been recently reported [76] ... 

Slade and Vulfson have shown that the catalytic activity of native BSA in the dehydrofluorination reaction in aqueous media is greater than that reported for catalytic antibodies and molecularly imprinted polymers [30]. These authors therefore imprinted ]S-lactoglobulin and papain using A-isopropyl-4-nitrobenzyl amine as the transition state analogue. The catalytic activity of the imprinted proteins was evaluated in the dehydrofluorination reaction using acetonitrile as the reaction medium. A three fold rate enhancement in the A eat value vis-d-vis non-imprinted proteins was observed. 

These dehydrofluorination reactions are particularly interesting in view of reports of the oxidation of certain fluorophosphines to derivatives of the hexafluorophosphate ion by bifluoride ion (235, 238, 249), namely,... 

Scheme 12 The MIP-catalyzed dehydrofluorination reaction developed by Shea et al Reagents and conditions (i) ethyleneglycol dimethacrylate (EGDMA), methyl methacrylate (MMA), azobisisobutyronitrile (AIBN), and dimethylformamide (DMF).

We believe that these dehydrofluorination reactions involve initial isomerizations of the isomeric mixture of 1,4-dihydropyridines and 3,4-dihydropyridines to 1,2-dihydropyridines 15 followed by dehydrofluorinations of 15 to the unstable intermediates 2-(difluoromethylene)-l,2-dihydropyridines 16. Rearrangements of 16 yield the 2-difluoromethylpyridines 17. In fact, if the 1,4-dihydropyridine is held at reflux in a weaker base, pyridine, for less than 2 hours, the 1,2-dihydropyridine 15 can be isolated in 60% yields. This 1,2-dihydropyridine reacts with a non-nucleophilic amine such as DBU or triethylamine at room temperature to give the expected 2-(difluoromethyl)pyridine 17. 

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