Stepwise fluorination

Boric acid and fluoride ion react to form a series of fluroborates where OH is displaced by (see Fluorine compounds, inorganic—Boron, FLUOROBORic acid). Stepwise formation of the ions fluorotrihydroxyborate [32554-53-3] BF(OH) 3, difluorodihydroxyborate [32554-52-2] BF2(OH) 2 trifluorotrihydroxyborate [18953-00-9] BF3(OH) 3, proceeds rapidly in acidic solutions, but tetrafluoroborate [14874-70-5], forms slowly (56). A... 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

The mechanism of attack of 1,3-dipolar reagents on fluoroalkenes can be considered to be either stepwise or concerted. Heteroaromatic N-imines react by a stepwise 1,3 addition to perfluoroalkenes and -alkynes to give fluorinated pyrazolo[l,5-a]pyridines [82JCS(P1)1593]. Pyridinium /-butoxycarbonylmethylide with fluoroalkenes gave pyrrolo[l,2-a]pyri-dines [86JCS(P 1) 1769] and indolizines (22) are obtained with pyridinium phenacylide [91JFC(51)407]. 

Stepwise fluorination of S02CH3 is thus seen to lead to a continuous increase in based substituent constants. 

The stepwise replacement of fluorine atoms in fluorocyclophosphazenes, (NPF2) n = 3—5), by methyl groups has been followed.and n.m.r. spectroscopy showed that methyl-lithium in diethyl ether generally effects a geminal replacement pattern. With N3P3F6, only mono- and di-methyl derivatives were obtained, but with N4P4F8 the dimethyl (52), trimethyl (53), and octamethyl derivatives were noted. The formation of... 

The best carbonyl components for these reactions are highly electrophilic compounds such as glyocylate, pyruvate, and oxomalonate esters, as well as chlorinated and fluorinated aldehydes. Most synthetic applications of the carbonyl-ene reaction utilize Lewis acids. Although such reactions may be stepwise in character, the stereochemical outcome is often consistent with a cyclic TS. It was found, for example, that steric effects of trimethylsilyl groups provide a strong stereochemical influence.28... 

Some of the fluorinated hydrocarbons—e.g., trifluoromethane 44), 1,1-difluoroethane 45), and difluorodichloromethane 82)—are known to be chemically inactive and physiologically inert. Stepwise replacement of the chlorine atoms in chloroform by fluorine yields the dichlorofluoromethane and chlorodifluoromethane, neither of which is as toxic as the original substance (7). 

Heats of formation for a complete set of Group VILA fluorides are unavailable, but a set of xenon fluoride cations, isoelectronic with iodine fluorides, exhibits the alternating pattern expected for odd- and even-electron molecules. The original energy-level diagram for stepwise fluorine dissociation is shown in Fig. 5. The tabulated values were derived from the ionization energies of XeF and the threshold values for XeFJ — XeF, - + F, where n is even (27), together with heats of formation obtained by reaction calorimetry (137). 

Platinum(iv).—Group VII Donors. Simple syntheses of hexafluoro-complexes of some noble metals have been reported, including K2[PtF ]. The method involved the melt reaction between KHFj and K2[PtXg] (X = Cl, Br, or I), which has the advantage of not requiring corrosive reagents such as fluorine or BF3- The stepwise stability constants K have been measured for the bromoaquo-complexes [PtBr , (H20)-7 ] "" (n = 4, 5, or 6) formed in... 

Stepwise fluorination. Haimson and Knanth (1983) and Matheney and Knauth (1989) noted that the first fractions of oxygen were 0-depleted compared with oxygen recovered in later fractions, snggesting that the water-rich components of hydrous silica react preferentially in the early steps of fluorination. 

The replacement of chlorine in the dithietane by fluorine occurs stepwise. As a consequence, small amounts of the partially fluorinated dithietanes II, III, and IV are ordinarily obtained. Since efforts are usually made to fluorinate... 

In a very recent publication [70] Rudiger has summed up these preceding results to propose a persuasive mechanism in which fluorination proceeds stepwise via partially fluorinated intermediates which are repeatedly adsorbed on the anode, fluorinated, and desorbed. 

Further evidence against the ECbECN carbocation mechanism is claimed by Sartori and co-workers [186] from their work on the fluorination of N,N-dime-thyltrifluoromethanesulphonamides at different temperatures (-15°C and + 5°C). According to the carbocation mechanism, it is stated, introduction of fluorine atoms into the sulphonamide should take place stepwise, first in one methyl group and then into the other however, detailed examination of the reaction mixture by 19F NMR spectroscopy showed no trace of compounds containing the moieties... 

Fullerenes can be derivatized by various means. For example, reaction with fluorine gas proceeds stepwise to the formation of colorless CeoFeo, which, according to the 19F nuclear magnetic resonance (NMR) spectrum, contains just one type of F site and so evidently retains a high degree of symmetry.9 In view of the low adhesion typical of fluorocarbons, this spherical molecule is expected to have extraordinary lubricant properties. Curiously, bromination of Ceo is reversible on heating otherwise, the reactions of fullerenes resemble those of alkenes or arenes (aromatic hydrocarbons). 

By the end of this period, aromatic fluorine chemistry was established.9 Though C —F bonds were difficult to make, basic features of the reactivity of compounds were understood, and simple stepwise synthetic sequences could be devised, making available a range of products. 

At the beginning of the 20th century, arenes carrying fluorinated side chains were made from (trichloromethyl)benzene (benzotrichloride, 12). Unexpectedly perhaps, this was a facile exchange. Proceeding stepwise, it went to completion, and full fluorination could be achieved even using antimony(III) fluoride alone. 

---