Stabilization by fluorination

The thallium trinitrate-mediated ring contraction of frani-decal-2-ones has opened up a new route to the hydrindane system, and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and EtsN-HF. A possible mechanism is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product. 

A carbocation is strongly stabilized by an X substituent (Figure 7.1a) through a -type interaction which also involves partial delocalization of the nonbonded electron pair of X to the formally electron-deficient center. At the same time, the LUMO is elevated, reducing the reactivity of the electron-deficient center toward attack by nucleophiles. The effects of substitution are cumulative. Thus, the more X -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. As an extreme example, guanidinium ion, which may be written as [C(NH2)3]+, is stable in water. Species of the type [— ( ) ]1 are common intermediates in acyl hydrolysis reactions. Even cations stabilized by fluorine have been reported and recently studied theoretically [127]. 

A fluorine substituent, however, has the opposite effect on geometry. Pyramidal ions are stabilized by fluorine and planar ions destabilized conjugation with the filled orbitals on fluorine is unfavorable. See A. Streitwieser, Jr., and F. Mares, J. Amer. Chem. Soc., 90, 2444 (1968). Chlorine, bromine, and iodine apparently stabilize an adjacent carbanion more than does fluorine, presumably because the destabilizing orbital overlap is less effective with the larger halogens (see Section 5.2, p. 227). J. Hine, N. W. Burske, M. Hine, and P. B. Langford, J. Amer. Chem. Soc., 79, 1406 (1957). 

Table 12.5 shows evidence for the trends that we have come to expect. The highest oxidation number (+6) is stabilized by fluorine only in the cases of selenium and tellurium PoF is not known. There are no low oxidation number fluorides, apart from F—Se—Se—F and Se=SeF2, and these can only be trapped at low temperatures. Selenium forms no iodides, but the more electropositive tellurium and polonium do. 

In attempts to further improve the stability of fluorine-containing elastomers Du Pont developed a polymer with no C—H groups. This material is a terpolymer of tetrafluoroethylene, perfluoro(methyl vinyl ether) and, in small amounts, a cure site monomer of undisclosed composition. Marketed as Kalrez in 1975 the polymer withstands air oxidation up to 290-315°C and has an extremely low volume swell in a wide range of solvents, properties unmatched by any other commercial fluoroelastomer. This rubber is, however, very expensive, about 20 times the cost of the FKM rubbers and quoted at 1500/kg in 1990, and production is only of the order of 1 t.p.a. In 1992 Du Pont offered a material costing about 75% as much as Kalrez and marketed as Zalak. Structurally, it differs mainly from Kalrez in the choice of cure-site monomer. 

Fluorinatton of graphite with fluorine gives graphite fluorides that have interesting properties, as recently reviewed [63]. Pyridine and its derivatives add elemental fluorine to form unstable N-fluoro adducts [14, 26, 64, 65]. These may decompose to 2-fluoropyndines [65] or be stabilized by treatment with inflate salts to form useful electrophilic fluonnating agents [64]... 

F3CIO was discovered in 1965 but not published until 1972 because of US security classification. It has low kinetic stability and is an extremely powerful fluorinating and oxidizing agent. It can be made in yields of up to 80% by fluorination of CI2O in the presence of metal fluorides, e.g. NaF ... 

Now we can say why the chemical bond forms between two fluorine atoms. First, the electron affinity of a fluorine atom makes it energetically favorable to acquire one more electron. Two fluorine atoms can realize a part of this energy stability by sharing electrons. All chemical bonds form because one or more electrons are placed so as to feel electrostatic attraction to two or more positive nuclei simultaneously. 

Electrochemical fluorination is an important technique for obtaining saturated perfluoroheterocycles. Incorporation of a partially fluorinated group into hydrocarbon ethers enhances their stability toward fluorination with CoF3 and this approach has been extended toward electrochemical fluorination [89MI1 90JFC(49)409]. Adduct (6) obtained by free radical... 

The three structures of type 4 are unstable for two reasons the presence of electric charges of the same sign on adjacent atoms, and the use by the nitrogen atom of only three orbitals. The contribution of these structures to the normal state of the molecule is accordingly small, and we may take it to be zero. Also, structures of type 3, with a double bond and the transfer of the positive charge to a fluorine atom, are stabilized by the formation of an additional covalent bond with use of the fourth orbital and may accordingly make a greater contribution to the normal state moreover, there is an extra factor 2 for the six structures of type 3 over the three of type 1. 

In the reaction of 1 with alkynes possessing electron-withdrawing substituents, the corresponding silacyclopropene derivatives 66 and 67 are formed, as described in Scheme 23.29 An unexpected pathway was observed in the reaction with the electron-poor hexafluorobutyne(2) the X-ray characterized heterocycle 68 was most likely obtained by nucleophilic attack of 1 at the triple bond. A subsequent shift of a fluorine atom from carbon to silicon creates an allene-type molecule which was stabilized by a [2 + 2] cycloaddition process involving a double bond from the pentamethylcyclopentadienyl unit, as described in Scheme 24.33... 

The kinetics of these rearrangements have been reviewed in detail 1 the most consistent measurements provide kc 1 x 106 s 1 for 55-C1, with 3 kcal/mol and log A ss 8 s 1 (AS —20 to —24 e.u.).109 The comparable data for 55-F are kc = 1.4 x 10s s-1, Ea = 4.2 kcal/mol, and log A = 8.3 s 1,1,96 Cy-clopropylfluorocarbene ring expands about 7 times more slowly than its chloro analogue. This difference, due to a slightly higher a ( 1 kcal/mol), reflects the larger spectator substituent stabilization by the better electron donor, fluorine. 

The Na-Na distances of 3.076(3) (Nal-Na2) and 3.202(3) A (Nal-Nal ) reflect Na-Na bonds (3.82 A in elemental Na), whereas the Nal-Na2 distance of 3.530(3) A suggested less attractive interactions. The Na4-dication cluster is embedded between two silyl(fluoro-silyl)phosphanide counterions. The relatively low-coordinated Na centers are remarkably stabilized by the -fluorine atoms and by... 

In the toluidines (methylanilines) the n MO is destabilized by 0.2-0.3 eV, while in the corresponding monofluoroanilines it is stabilized by about 0.1 eV75. The effect of several fluorine atoms seems to be additive, as indicated by the IP( n) value of pentafluoroaniline42,71 which is 0.7 eV greater than that of aniline. 

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