Polyhalomethanes

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane ... 

The estimation of reactivity of polyhalomethanes in the reactions with the same monomer shows that the quantity of halogen atoms in a molecule is the most essential factor affecting the easiness of homolysis of even one C— Br bond in molecule, and the influence of the halogen nature (chlorine or bromine) is of less significance. For instance, the analysis of the data on relative kinetics of some polyhalomethanes reactions with vinyl chloride allows us to grade the studied polyhalomethanes according to their reactivity, as follows ... 

Castro CE, WC Kray (1966) Carbenoid intermediates from polyhalomethanes and chromium (II). The homogeneous reduction of geminal halides by chromous sulfate. J Am Chem Soc 88 4447-4458. 

The overall transformation is the conversion of the carbon-sulfur bonds bond to a carbon-carbon double bond. The original procedure involved halogenation of a sulfide, followed by oxidation to the sulfone. Recently, the preferred method has reversed the order of the steps. After the oxidation, which is normally done with a peroxy acid, halogenation is done under basic conditions by use CBr2F2 or related polyhalomethanes for the halogen transfer step.92 This method was used, for example, to synthesize 1,8-diphenyl-1,3,5,7-octatetraene. 

If one takes out of this list the polyhalomethanes and tries to understand what the chemistry is about, certainly a simple-minded view is that this is simply a halogen atom abstraction reaction such as one finds in a gas phase reaction, like sodium atom with organic halides. There is a linear free energy correlation in that the logarithms of the rate constants bear a linear relation to one another. 

Radical addition of polyhalomethanes to olefins can be initiated by electro-chemically generated Mn(lll) species. The procedure offers an alternative method of a radical addition reaction with an easy control of the initiation via the current... 

The RuCl2(PPh3)3 catalyzed addition of polyhalomethanes, CCI4 and CBrCl3, to 3-acetyl-2(37f)-oxazolone 84 leads to the exclusive formation of the tmns-4-halo-5-(trichloromethyl)-3-acetyl-2-oxazolidinones 178." In the presence of a radical... 

Despite the weight of evidence implying that methyl radicals produced in the photodissociation of CH3I are unusually reactive, there remain certain weaknesses in the case presented. Certainly the radicals are produced initially with high energy contents, and it seems probable that a major fraction of the energy is concentrated in vibration (as has been established for a wide range of polyhalomethanes).10 24 However, it... 

Transient, Spectra Observed in the Flash Photolysis of Polyhalomethanes... 

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. 

The arylation reaction can be extended to bis- and tris-(dialkylamino)phosphines73, but in the case of iV-ethylanilinophosphine the reaction takes place with a rearrangement of the phosphasemidine type483. A structural transposition also occurs in the alkylation of some N-silylaminophosphines484-487 (reaction 141). Finally, it is noteworthy that, as far as the tris(dimethylamino)phosphine is concerned, many polyhalomethyltris-(dimethylamino)phosphonium salts have been prepared by the action of the appropriate polyhalomethanes (reaction 142). 

Alkylcyclohexanes, cycloheptane, and cyclooctane were also transformed into the esters of the corresponding tertiary cyclohexanecarboxylic acids in the presence of the superelectrophile CBr4—2AlBr3 in good yields.314 315 Such organic superacids generated from polyhalomethanes with aluminum halides are capable of promoting the selective and effective carboxylation of linear alkanes (propane, butane) and adamantane as well.316... 

Weakening of the Si—H bond favors radical reactions of 49 with a variety of organohalides and some reactive organic compounds. Thus, polyhalomethanes CH2CI2, CHCI3 and CHBr3 are reduced by 49 in the presence of organic peroxides (equation 66)308. 

Electrophiles which enable four substitutions at the same carbon atom include C13CN02 and CI3CSCI both compounds yield tetraalkyl orthocarbonates on treatment with alcoholates [150], In polyhalomethanes selective substitutions of the most reactive halide are possible [153] but not always easy to perform (Scheme 4.38). 

Nedelec and coworkers reported a manganese(III)-initiated cyanoacetate-catalyzed atom-transfer radical addition of polyhalomethanes or dibromomalonate 172 to alkenes 126 (Fig. 48) [272]. Since neither Mn(II) nor Mn(III) is useful to initiate Kharasch-type additions, an organocatalyst served this purpose. Thus, a short electrolysis of a mixture of 126,172,10 mol% of Mn(OAc)2, and 10 mol% of methyl cyanoacetate 171 led to initial oxidation of Mn(II) to Mn(III), which served to form the cyanoacetate radical 171A oxidatively. The latter is able to abstract a halogen atom from 172. The generated radical 172A adds to 126. The secondary... 

C. Halogenation with Polyhalomethanes under Basic Conditions. 567... 

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