Difluorine

The heterocyclic system bimane produces a difluorinated product on fluonnation with acetyl hypofluorite [34] (equation 11)... 

The fluorination of P-diketones and p-ketoesters with N-/luorobis(trifluo-romethanesulfonyl)imide (Table 3a, B) can be controlled to give either mono-fluorination or difluorination. Monofluorination occurs when the strong acid, bis(trifluoromethanesulfonyl)imide, a reaction product, is removed by addition of water, which prevents further enolization and fluormation of the monofluoro adduct [83] (equation 38)... 

The extent of mono- versus difluorination of etiolates may also be controlled through the use of N-fluorosultam (Table 3a, G). With a base/W-fIiioro com-pound/substrate ratio of 1.2/1.3-1 6/1, the reaction yields inonofluoro products, but with a ratio of 2.4-3.6/2.6-3 6/1, the difluorinated compounds are obtained [68] (equation 42)... 

Geminal difluorination ot diethyl diazomalonate can be effected with molecu lar fluorine (10% in nitrogen) to give diethyl difluoromalonate m 70% yield [90] (equation 27)... 

A difluorinated analogue was found to be somewhat more stable to acid than was the BSB derivative but overall it showed no major advantage to the original Benzostabase. ... 

Many aldehydes and ketones have been converted to g m-difluoro compounds with sulfur tetrafluoride (SF4), " including quinones, which give 1,1,4,4-tetra-fluorocyclohexadiene derivatives. With ketones, yields can be raised and the reaction temperature lowered, by the addition of anhydrous HF. Carboxylic acids, acyl chlorides, and amides react with SF4 to give 1,1,1-trifluorides. In these cases the first product is the acyl fluoride, which then undergoes the ge i-difluorination reaction ... 

The (a-sulfanylalkyl)phosphonates and their difluorinated derivatives are also, by easy radical cleavage of the C-S bond, useful precursors of phospho-nomethyl or phosphonodifluoromethyl radicals, which can be added to double bonds and so, introduced in a variety of structures. Besides, the use of phospho-nodithioformates as radical trapping agents and their use as RAFT reagent for controlled polymerization open a new interesting field of appHcation for these dithioesters. 

GL4] [R 5] [P 5] A yield of 62% was achieved at 90% conversion [15], The amount and nature of difluorinated products was also specified in [16], Besides this performance in single micro-channels, conversion and yield for a fhree-micro-chan-nel reactor given [16],... 

A rather complex microwave-assisted ring-opening of chiral difluorinated epoxy-cyclooctenones has been studied by Percy and coworkers (Scheme 6.131) [265]. The epoxide resisted conventional hydrolysis, but reacted smoothly in basic aqueous media (ammonia or N-methylimidazole) under microwave irradiation at 100 °C for 10 min to afford unique hemiacetals and hemiaminals in good yields. Other nitrogen nucleophiles, such as sodium azide or imidazole, failed to trigger the reaction. The reaction with sodium hydroxide led to much poorer conversion of the starting material. 

Scheme 6.131 Ring-opening of chiral difluorinated epoxycyclooctenones.

Dichlorine shortens the induction period of autoxidation of paraffin wax [187] and accelerates the oxidation of hydrocarbons [109]. Difluorine is known as very active initiator of gas-phase chain reactions, for example, chlorination [188,189]. 

Another fast reaction of radical initiation is the difluorine addition to the double bond of the unsaturated compounds [192-194] ... 

For the values of the rate constants and the activation energies of difluorine with olefins calculated by the IPM method [303], see Table 3.14. 

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

In fluorine thermochemistry, two key heat values frequently occur. They are the dissociation energy of difluorine, required for evaluation of fluorine bond energies and the heat of formation of hydrogen fluoride, a product in hydrolysis, hydrogenation, fluorine combustion, or neutralization reactions. These values have been difficult to measure and have changed considerably over the years. A recommended set of values has been reported in recent CODATA bulletins (60) which are collected in Table I together with older values and corrections to update them. 

Photoionization of difluorine and chlorine monofluoride appeared to give an even lower value (68), but a reexamination showed that one of the dissociation processes had been ignored (29). In this method fluorine is exposed to monochromatic UV radiation, and the positive and negative ions are observed in a mass spectrometer. 

Nitrogen trifluoride is a kinetically inert gas, but when sparked or heated can fluorinate substrates and release nitrogen. Sulfur, for example, is quantitatively converted to SF using NF3 at 5 atm (231). Similarly, boron is converted to BF3 (159) and CF3CN to CF4 (232). Nitrogen trifluoride has some advantages over difluorine. It is safer to handle, and double-compartment bombs to avoid self-ignition are not required. However, some self-dissociation has to be measured and corrections made. 

Difluorine dissociation provides the simplest example of symmetric cleavage at comparatively low temperatures. The facile dissociation... 

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

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