Radical trifluoromethyl

The rates of these two reactions have been studied for the attack of trifluoromethyl (51) and methyl radicals (52) in isoprene that has been dissolved in 2,3-dimethylbutane and isooctane, respectively. The rate constants for the reactions with isoprene are much greater than those for the attack on the solvent. The ratio between the two rates for the attack of trifluoromethyl radicals varies from 1090 at 65°C to 233 at 180°C. For the corresponding reaction involving methyl radicals, the ratio is 2090 at 65°C. 

The reaction of trifluoromethyl radicals, generated in a radio-frequency discharge process, with elemental mercury f/7i], mercury halides [174], dime-thylmercuiy [24], or HgO [175] has been used for the preparation of CFjHgX and (CF3)2Hg. Direct fluorination of dimethylmercury with elemental fluorine gives (CF3)2Hg [176],... 

Electrochemically generated trifluoromethyl radicals add to 1-hexyne to give a 1 4 mixture of ( )- and (Z)-l,l,l-trifluoro-2-heptene [22] Kinetic data on the addition of photochemically generated trifluoromethyl radicals to acetylene and substituted acetylenes were reported [2J]. Alcohols and aldehydes add to hexa-fluoro-2-butyne in the presence of peroxide and y-ray initiation [24] (equations 16 and 17). 

First examine the geometry of methyl radical. Is it planar or puckered Examine the geometries of 2-methy 1-2-propyl radical, trifluoromethyl radical, trichloromethyl radical and tricyanomethyl radical. Classify each of the substituents (methyl, fluoro, chloro and cyano) as a n-electron donor or as a Tt-electron acceptor (relative to hydrogen). Does replacement of the hydrogens by 7t-donor groups make the radical center more or less puckered Does replacement by Jt-acceptor groups make the radical center more or less puckered Justify your observations. 

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

The fraction of head-to-head linkages in the poly(fluoro-olefms) increases in the series PVF2 < PVF PVF3 (Tabic 4.2). This can be rationalized in terms of the propensity of electrophilic radicals to add preferentially to the more electron rich end of monomers (i.e, that with the lowest number of fluorines). This trend is also seen in the reactions of trifluoromethyl radicals wilh the fluoro-olefins (see 2.3). 

Kolbe radicals can also be trapped by oxygen to yield dialkylperoxides, aldehydes, and ketones [97]. Furthermore methyl and trifluoromethyl radicals from acetic acid and trifluoroacetic acid are trapped, although inefficiently, by pyridine (3-20%) [234], benzotrifluoride and benzonitrile[ 235]. 

II. Plasma-Generated Trifluoromethyl Radicals as a Synthetic Reagent.. . 181... 

F. Reactions of Trifluoromethyl Radicals with Sulfur Vapor. 188... 

Although belonging to a slightly different class of reactions, the reaction of trifluoromethyl radicals with sulfur vapor has been shown to provide a route to trifluoromethyl polysulfide compounds (20). Instead of using sulfur halides, which undoubtedly would also give positive results, elemental sulfur (Ss) was vaporized and dissociated into atomic and polyatomic sulfur species. 

For completeness, it should be mentioned that the reaction of trifluoromethyl radicals to replace halogens is extremely general, and not confined solely to metal species. Plasma-generated trifluoromethyl radicals will react with halocarbons according to the reaction 21)... 

Specifically, the reaction of trifluoromethyl radicals with carbon tetra-iodide produces perfluoro-tert-butyl iodide and perfluoroneopentane in the ratio of 3 1. Incomplete substitution is presumably due to steric factors around the crowded, central carbon atom. 

Ideally, it would be desirable to determine many parameters in order to characterize and mechanistically define these unusual reactions. This has been an important objective that has often been considered in the course of these studies. It would be helpful to know, as a function of such parameters of the plasma as the radio-frequency power, pressure, and rate of admission of reactants, (2) the identity and concentrations of all species, including trifluoromethyl radicals, (2) the electronic states of each species, (3) the vibrational states of each species, and (4) both the rotational states of each species and the average, translational energies of, at least, the trifluoromethyl radicals. 

With the exception of the reactions of trifluoromethyl radicals with sulfur vapor, which is really a separate class of reactions, if the power supplied to the load coil surrounding the reactor (see Fig. 2) was maintained at, or near, the minimum amount needed to support the discharge, in only two cases were compounds found that clearly resulted from reactions other than replacement of halogen by trifluoromethyl. The reaction of tellurium tetrabromide (or the chloride) gave, in addition to the products just reported, very small proportions of such species as BrCF2TeCF2Br and (C2F5)2Te, which were isolated in yields of... 

C. Synthesis of Trifluoromethyl Organometallic Compounds by Cocondensation of Trifluoromethyl Radicals and Metals... 

When colloidal selenium was heated with mercuric trifluoroacetate or silver trifluoroacetate, bis(trifluoromethyl)diselenide was formed (43). Later work with selenium/silver carboxylate, RC02Ag (R = CF3, C2F5, or C3F7), mixtures at 280° C in a vacuum produced a mixture of the bis(perfluoroalkyl)selenide and the bis(perfluoroalkyl)diselenide (44). Formation of a polyselenium trifluoroacetate, which decarboxylates to produce the trifluoromethylselenides, was the proposed mechanism for R = CF3 (44). However, silver trifluoroacetate is a source of trifluoromethyl radicals when heated above 260° C (21), hence the trifluoromethylselenides may be formed by reaction of trifluoromethyl radicals with selenium, as in the reaction of CF3I with selenium [Eq. (34)] (45). 

Mercuric carboxylates, which decarboxylate by a chain mechanism when initiated by peroxides, also decarboxylate under UV irradiation (123,128,129,131-140,142,144-146,153-155). In addition, decarboxylation was observed for mercuric benzoate and mercuric a-naphthoate (123). Side reactions [Eqs. (24), (25), (109)] observed in peroxide initiated reactions also occurred on UV irradiation, and mercurous salt formation [Eq.(24)] was more extensive under the latter conditions. Decarboxylation giving methylmercuric acetate occurred on irradiation of mercuric acetate in aqueous solution and is considered to be of environmental significance (156,157). Stepwise decarboxylation giving (CF3)2Hg occurred on irradiation of solid mercuric trifluoroacetate at -196° C (158), but, at 20° C, trifluoromethyl radicals diffused from the solid and dimerized (158). No other diorganomercurial has been formed by radical decarboxylation, and the reaction is not preparatively competitive with the thermal decarboxylation synthesis of (CF3)2Hg (26,27) (Section III,A). 

Absolute rates for the addition of the methyl radical and the trifluoromethyl radical to dienes and a number of smaller alkenes have been collected by Tedder (Table l)3. Comparison of the rate data for the apolai4 methyl radical and the electrophilic trifluoromethyl radical clearly show the electron-rich nature of butadiene in comparison to ethylene or propene. This is also borne out by several studies, in which relative rates have been determined for the reaction of small alkyl radicals with alkenes. An extensive list of relative rates for the reaction of the trifluoromethyl radical has been measured by Pearson and Szwarc5,6. Relative rates have been obtained in these studies by competition with hydrogen... 

TABLE 1. Absolute rate constants for the addition of methyl and trifluoromethyl radicals to simple alkenes at 164 °C3... 

TABLE 2. Relative rate constants for the addition of the trifluoromethyl radical and the diethyl a-benzylmalonyl radical to simple alkenes and dienes... 

The synthesis of Sn(CF3)4 can be achieved by the reaction of Snk with the trifluoromethyl radical in a radio-frequency discharge315. 

Electrooxidative generation of trifluoromethyl radicals (CF3-) and their synthetic application have been developed since the early 1970s because trifluor-oaeetic acid (TFA) is readily available and one of the most economical starting materials for trifluoromethylation [61]. Heteroaromatics as well as olefins have been employed as substrates for the trifluoromethylation (Scheme 7.1) [62]. 

Recently, Uneyama et al. have systematically investigated the anodic generation of CF3 radicals and their utilization (Scheme 7.3) [68-72], They have clarified that trifluoromethyl radicals can be generated almost quantitatively in the oxidation of TFA at 0 °C in an aq. MeOH/Pt system using an undivided cell [70]. They have also found that the trifluoromethylation of electron-deficient olefins can be controlled by the current density, reaction temperature, and the substituents of the olefins. Interestingly, anodic trifluoromethylation of fumar-... 

Eq. 19c). The mechanism of this transformation involves either a final iodine transfer step or an electron transfer process that give back the trifluoromethyl radical. 

A type iii-d reaction leads to the formation of (69). Trifluoromethyl radicals generated electrochemically from triflu-oroacetate can attack electron-deficient olefins leading to trifluoromethylated carbon radicals whose chemical and electrochemical follow-up reactions can be controlled by current density, reaction temperature, and substituents of the olefins. With fumaronitrile (86) at 50 °C the monotri-fluoromethylated compound (87) was obtained in 65% yield (Scheme 31) [110]. 

The anodic oxidation of trifluoroacetic acid affords the trifluoromethyl radical CFj. Addition of this radical to unsaturated heterocycles can produce bis(trifluoromethylated) compounds (Scheme 119) [225]. 

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