J. Burdon

Fluorinations with high valency metal fluorides and by the electrochemical method : Are they oxidation processes?

Abstract It is suggested that fluorination with high valency metal fluorides (e.g. AgF2, CoF3, MnF3, CeF3) involves the initial oxidation of the substrate by the metal ion either to a radical-cation or, less commonly, by the abstraction of a hydrogen atom. This is essentially the same as many aqueous oxidations, the difference coming in the next step where the first-formed intermediate is quenched by fluorine (as an ion or atom) instead of some hydroxylic species. It is possible to rationalize many features of the fluorinations of benzene and some of its derivatives, thiophen and tetrahydrofuran in this way. The fluorinations of olefins, hydrocarbons and hetero-atom compounds are also discussed as oxidation processes. The fluorinating powers of the reagents vary with the oxidation potentials of the metal ions in accord with the oxidation hypothesis, i.e., AgF2 > CoF3 > MnF3. More tentatively, it is also suggested that electrochemical fluorination is an anodic oxidation process. Fluorination with xenon difluoride is also discussed in terms of radical-cation intermediates.

A rationalization of orientation and reactivity in the nucleophilic replacement reactions of aromatic polyhalo-compounds

Abstract C 6 F 5 X compounds react with nucleophiles at the position para to X in some cases (X = H, CH 3 , NMe 2 , SMe, CF 3 ), meta in others (X = NH 2 , O − ) and occasionally at both meta and para in comparable amounts (X = OMe, NHME); significant amounts of ortho -replacement also occur (X = Cl, OMe). These orientations, and those in polychloro-aromatics and polyfluoro-polynuclear aromatics, can be rationalized by considering the relative stabilities of the Wheland-type intermediates involved (I, II, III), provided the halogens are taken as electron-repelling in π-systems. Reactivities can be rationalized if ground state stabilities are considered as well. A number of predictions have been made on the basis of these arguments.

The consequences of cation radical fluorination theory—I : A comparison of various theoretical scenarios with experiment for carbocyclic-aromatics, using CNDO/2 and INDO calculations

Abstract Various of the possible scenarios (Table 1) derived from cation radical fluorination theory are examined critically in the light of experiment. The predictions of the theory are made using CNDO/2 and some INDO calculations, and these are compared with the known experimental results for benzene, benzotrifluoride, naphthalene, anthracene and phenanthrene. It is concluded that cation radical theory must be broadly correct; it works well for monocyclic hydrocarbon compounds, assuming product control to be via spin or charge density in the intermediates (cations, radicals, cation-radicals), with sequence 3 (Table 1) preferred. In the absence of any modifying hypothesis, however, control of products by spin or charge density alone cannot explain the experimental results for the polycyclic substrates. The most plausible modifications appear to be product control via Wheland intermediate stability (sequence 7, Table 1) and/or rapid rearrangements.

Trifluoromethanesulphonate esters and their alkylating properties

Abstract Trifluoromethanesulphonate esters, even the 2,2,2-trifluoroethyl, will alkylate pyridine and alkoxides to give pyridinium salts and ethers, respectively. These esters are probably the most powerful alkylating agents of this type.

The sodium-promoted Claisen ester condensations of ethyl perfluoroalkanecarboxylates

Abstract Sodium has been used to promote the Claisen ester condensation of ethyl trifluoroacetate and ethyl acetate to give ethyl γγγ-trifluoroacetoacetate and then 1,1,1-trifluoroacetone by acidic hydrolysis. Sodium has also been used in the preparation of the α-methyl-, α-ethyl, and α-ethoxy-γγγ-trifluoroacetoacetate esters and of the 4,4,5,5,6,6,6-heptafluorobutyrylacetate ester from which the corresponding ketones have been prepared by hydrolysis. Some comments are made on the mechanism of Claisen condensations involving perfluoroesters and of the role of sodium in promoting them.

Aromatic polyfluoro-compounds—XXXII : Isomer distributions in the nucleophilic replacement reactions of the pentafluorohalogenobenzenes

Abstract The pentafluorohalogenobenzenes react with nucleophiles mainly at the position para to the halogen; ortho replacement occurs to a lesser extent and diminishes in the order C6F5Cl > C6F5Br > C6F5I∼C6F5H. This is rationalized in terms of an electronic effect, which involves electron repulsion by halogens in π-electron systems and also by steric factors.

The fluorination of Butane over cobalt trifluoride

Abstract The fluorination of butane over cobalt trifluoride has given a complex mixture of partially fluorinated compounds: 51 of these have been identified, comprising over 99% of the products. Most were polyfluorobutanes but 1-2% were polyfluoro-2-methylpropanes. The reaction has no synthetic utility. There was some selectivity in the fluorination: secondary C-H was converted into C-F more easily then primary, and the ease of replacement of a particular H was reduced by geminal and vicinal fluorines. A computer of the fluorination was only partially successful, perhaps because the fluorination proceeded in part by simple F for H replacement and in part via alkenes: the model only allowed for the former.

A simple stereoselective synthesis of biologically important 2-halo-2,3-dideoxy-arabinose derivatives from 1,1,1,2-tetrafluoroethane (134a) and 1-chloro-2,2,2-trifluoroethane (133a)

Abstract A simple stereoselective three step synthesis of 2-fluoro- and 2-chloro-2,3-dideoxy arabinose derivatives from the readily available starting materials, ( R )-glycidol and either 1,1,1,2-tetrafluoroethane (134a) or 1-chloro-2,2,2-trifluroethane (133a), is described. This compares very favourably with the multistep syntheses previously described in the literature. A mechanistic interpretation of the reaction via an unfavourable 5-endo-trig cyclisation is given.

Reactions of trifluorovinyllithium and 1-chloro-2,2-difluorovinyllithium: the synthesis of fluorinated heterocycles

Abstract Trifluorovinyllithium (from 1,1,1,2-tetrafluoroethane [HFC-134a]) reacted with 2-trifluoromethylaniline at −78 °C to give 1,2,3-trifluoroquinolin in moderate to good yield. In a similar reaction, 1 -chlorodifluorovinyllithium (from 1-chloro-2,2,2-trifluoroethane [HCFC-133a] ) yielded 2-chloro-1,3-difluoroquinoline.

Fluorocyclohexanes. Part XVI. The six H-trifluoromethyldecafluoro- and two of the 2H,4H-trifluoromethylnonafluoro-cyclohexanes

Abstract Fluorination of benzotrifluoride by cobaltic fluoride at 260–280° gave, besides the fluorocarbon, the six possible tridecafluoromethylcyclohexane isomers, and two 2H,4H-dodecafluorides. Of the C 7 HF 13 compounds, only the cis−2H-, and the cis- and trans-4H-isomers could be isolated pure. The two 3H-isomers were made by pyrolysis of perfluoro(1-methyl-3-isopropylcyclohexane) in the presence of toluene. The trans-2H-isomer was made by further fluorination of one of the 2H,4H-dodecafluorides. Aqueous potash and the cis-2H-tridecafluoride gave 1-trifluoromethylnonafluorocyclohex-1-ene, which with stronger alkali hydrolysed to 1-carboxynonafluorocyclohex-1-ene. The cis- and trans- 2H and 4H-tridecafluorides were dehydrofluorinated by sodium fluoride at 320–380° (the cis-isomer of each pair reacted faster than the trans- : axial versus equatorial hydrogen) to give, respectively, the 1-trifluoromethylnonafluoro-ene and the 4-trifluoromethyl-isomer. The latter was isomerised to the former by sodium fluoride at 500–600°. The 1−CF 3 -ene gave hexafluoroglutaric acid on oxidation with alkaline potassium permanganate.