Trifluoromethyl radicals, generation

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],... 

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]. 

Tellurium vapor4 or tellurium tetrabromide5 combined with trifluoromethyl radicals, generated from hexafluoroethane in an electrical discharge, to form bis[trifluoromethyl] tellurium. 

Ethyl or trifluoromethyl radicals generated from the corresponding sulfonyl radicals are able to abstract iodine or hydrogen atoms or add to thiocarbonyl derivatives, and in so doing to play the role of chain carrier [see Volume 1, Chapter 1.6]. Their involvement in cascade processes will be exemplified in Section 5.5.4.3. 

Tetrakistrifluoromethylcyclotetraphosphine has been prepared in small amounts by reaction of white phosphorus in the vapor pliase with trifluoromethyl radicals generated from trifluoromethane and benzoyl peroxide (20). 

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). 

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

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)... 

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-... 

Photochemically induced epoxidation of tetrafluoroethene by oxygen with improved yields (71-76%, conversion 21-62%) is achieved in the presence of radical generators such as tri-bromofluoromethane, 1,2-dibromotctrafluorocthane, ethyl nitrite or 1 H, /F,5//-octafluoropen-tyl nitrite.36 The oxidation of tetrafluoroethene with oxygen can also be catalyzed with bis(trifluoromethyl)diazene an undistillable viscous oil with peroxide composition is formed initially which can be quantitatively converted into carbonyl fluoride when heated.37-38... 

There are other sources of perfluoroalkyl radicals which have found occasional use, such as photolysis of hexafluoroacetone to generate trifluoromethyl radicals [75, 76], photolysis of perfluoroacyl halides [77], Umemoto s photolysis of... 

Tellurium, evaporated from a crucible, reacted with trifluoromethylthio radicals, generated from bis[trifluoromethyl] disulfane in a radio-frequency glow discharge, in a vessel equipped with a cold finger chilled with liquid nitrogen and evacuated to 0.001 torr. Trap-to-trap distillation of the material that condensed in the cold finger produced bis[trifluoromethylthio tellurium. The thermally unstable compound condensed in the - 45° trap4. 

The trifluoromethyl radical is electrophilic in nature (61JA4732) and may be generated from precursors photochemically, thermally, by chemical reactions, and electrochemically (92T6555). A large number of precursors... 

Trifluoromethyl radicals have been generated thermally from bis-(trifluoromethyl)tellurium [90JFC(46)265], iodotrifluoromethane [8IJFC-(17)345], bromotrifluoromethane, hexafluoroacetone, or A(-trifluoromethyl-N-nitrosotrifluoromethylsulfonamide. 

Trifluoromethyl radicals are generated from the photolysis of hexafluoroacetone. In the presence of trichlorosilane the hydrogen atom is abstracted with formation of trifluoromethane (2). The competitive abstraction of chlorine atoms does not occur. 

PerfluoroaUcyl cadmium compounds are also known. Bis(trifluoromethyl)cadmium has been prepared at -78 °C by cocondensing Cd atoms with -CFs radicals generated in a hexafluoroethane plasma (see Metal Vapor Synthesis of Transition Metal Compounds),and also by the unusual reaction of activated cadmium powder with difluorodihalomethanes, for example, CF2CI2, CF2Br2, or CF2BrCl, in DMF (equation 7). ... 

Trifluoromethyl radicals may also be generated by electrolysis of salts of trifluoro-methanesulphinic acid [222] (Figure 8.76). 

In a series of papers, Itoh and Mikami have reported the high yield, radical a-trifluoro-methylation of ketones using titanium euolates and Mefl/BEts as free radical generator and trifluoromethylating agent . The use of excess Ida and Ti(OPr-/)4 in the preparation of the titanium euolate represeuts the key to the success of the reaction (equation 41). Interest in this reaction stems from the fact that in general, the radical a-trifluoromethylation of ketones via lithium enolates is complicated by the easy defluorination of the a-CFs ketone by the parent enolate . For this reason, less reactive enolates such as silyl or germyl enolates have been initially used instead of lithium enolates" . 

Although a number of such radical reactions are known, few promise much synthetic potential, Examples include the 2-phenylation of imidazole and benzimidazole by benzoyl peroxide, but both products are more readily obtained by other routes. Homolytic alkylations of imidazole and benzimidazole also occur at C-2, but usually give indifferent yields [10]. A potentially useful reaction is the synthesis of 2- and 4-trifluoromethylimidazoles from imidazoles and photochemically generated trifluoromethyl radicals. 1-Substituted imidazoles are largely substituted at C-5 in these reactions benzimidazole reacts initially at the 4-position [11-14]. 

This radical then abstracts an iodide atom from trifluoroiodomethane to generate product and the chain-propagating radical, the trifluoromethyl radical. 

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