Halomethyl radicals

The behavior of methyl and halomethyl radicals in their reactions with the fluoro-olefms (Table 1.2), can thus be rationalized in terms of a more dominant role of polar factors and the nucleophilic or electrophilic character of the radicals involved." Methyl radicals are usually considered to be slightly nucleophilic, trifluoromcthyl and triehloroincthyl radicals arc electrophilic (Tabic 1,4). 

Cyclic a-halomethyl or a-phenylselenenylmethyl (3-ketoesters undergo one-carbon ring expansion via transient cyclopropylalkoxy radicals.360... 

In general, if condensation polymers are prepared with methylated aryl repeat units, free radical halogenatlon can be used to introduce halomethyl active sites and the limitations of electrophilic aromatic substitution can be avoided. The halogenatlon technique recently described by Ford11, involving the use of a mixture of hypohalite and phase transfer catalyst to chlorinate poly(vinyl toluene) can be applied to suitably substituted condensation polymers. 

Carbon-carbon bonds may be formed by the addition, among other things, of halomethyl radicals to alkenes. The -CX3 (X = Br, Cl) may be generated by the action of peroxides on, or by photolysis of, CX4 ... 

Radical-mediated ring expansion of 2-halomethyl cycloalkanones. 

Resists based on chloromethyl substitution have been extensively studied in the past few years (7-7). Halogen and halomethyl groups have been introduced by a variety of methods. Choong and Kahn (7) synthesized polychloromethylstyrene (PCMS) by free radical polymerization of chloromethylstyrene and reported a sensitivity (D 5) of 0.4 / 2 at 20 kV with a contrast of 1.5 for materials with molecular weight of about 400,000 (7). The molecular weight distribution of these polymers, all of which contained one chloromethyl group per repeat unit, was about 2. Fractionation of the polymer resulted in improved contrast as a result of narrowing the distribution. 

Unfortunately, it is not possible to determine the maximum vibrational energy content of the methyl radicals produced in the photolysis of methyl iodide. On the basis of the arguments outlined above, strong excitation of the C-H stretching vibration would probably not be anticipated. The methyl radical is planar, however,36 so that its deformation vibration would be excited. On the other hand, the halomethyl radicals are believed to be pyrimidal,37 and unless the grouping has a planar configuration in the excited parent polyhalomethane, deformation would not make any appreciable contribution to the vibrational excitation of the radical. 

Radical reduction of l-(halomethyl)cyclopropenes can however lead to complex products 288). 

It may be of interest that for carbon radicals (but not for halomethyl ones) a continuous transition appears between liquid and gas phase data suggesting some generality in the compensation phenomenon of homogeneous reactions. There are gas-phase data indicating decreased A values with decrease of E. The line drawn gives the suggested trend. At... 

More recently, the related indium-mediated radical reactions have been widely studied (Scheme 7.13).19 Indium iodide-mediated radical cyclisation was first reported by Cook et al.20 The indium-mediated 1,4-addition of alkyl radicals to (F)-but-2-enenitrile was investigated by using 1-ethylpiperidinum hypophosphite (EPHP) as a hydrogen donor in aqueous media (Scheme 7.13).21 Atom transfer radical cyclisation and reductive radical cyclisation were studied using indium and iodine.22 Indium mediated alkyl radical addition to dehydroamino acid derivatives was also reported.23 The indium-mediated radical ring expansion of a-halomethyl cyclic (3-keto esters, shown in Scheme 7.13, was achieved in aqueous alcohols.24... 

Bicyclo[n.l.O]alkan-l-oxyl radicals are fleeting intermediates in homolytic reactions of 2-(halomethyl)cycloalkanones and related radical precursors. Unrearranged 2-methyl-cyclohexanone (19) was obtained from treatment of 2-[(phenylselanyl)methyl]cyclohexanone (18) with tributyltin hydride. ... 

The methyl group at the 4-position of selenazoles can be further functionalized by free-radical bromination to the bromomethyl intermediate (Scheme 6) <89JHC709> this provides an alternative to incorporating the halomethyl group as part of the ring synthesis <81JHC789,86JHC861>. 

Comparative rate data for the addition of some halomethyl radicals to ethylene... 

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