Halogen transfer

Stereospecificity of this reaction reaches 15 1 for telomer T3. Telomer T3 is a crystalline product, this allowed the authors to use X-ray diffraction analysis for studying stereochemistry. Stereoselectivity observed in the formation of T3 shows that both addition step and the step of halogen transfer to the growing radical proceed stereoselectively in this case. 

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. 

Bartlett et al.62 on the other hand have found that mainly exo-2-norbornyl halides 28 are obtained from 2-norbornyl radicals27 and halogen transfer agents. 

The product ratio of exo-halide 28 and endo-isomer 29 was largest for large halogen transfer agents XY. XY apparently approaches 2 7 preferentially from the less shielded exo-side. The torsional effect5discussed before is probably also of importance. Similar results were obtained more recently for the transfer of hydroxy groups from peracids to 2763). 

An investigation of the competing halogen transfer from BrCCl3 and CCl45, 79 has shown that steric effects are also of importance in atom transfer reactions to alkyl and aryl radicals. Giese80 investigated very carefully the temperature depen-... 

Fig. 2. Linear free enthalpy relationship between the difference in enthalpy of activation for the halogen transfer from CCI4 and BrCCl3 to alkyl radicals and the steric substituent parameters of alkyl radicals83 ...

When a trialkyl phosphite reacts with PX3, there is halogen transfer, as illustrated in the following equation ... 

Halogen Transfer Reactions from bis-Amino Halonium Ions to Acceptor Olefins Mechanism and Strategies for Chiral Halogenation... 

Cavell and Dobbie (214-216) have suggested that halogen transfer rearrangements in trifluoromethylphosphines arise from interactions of nonbonding fluorine p orbitals with vacant d orbitals on phosphorus. Such an explanation is consistent with observations for the Groups IV and V pentafluorophenyl derivatives, exclusive of carbon and nitrogen, and similarly fits the behavior of boron with its vacant p orbital. 

About 5% of the ethyl radical adduct was isolated in the reaction with r-butyl iodide and about 20% of the ethyl radical adduct was formed in the reaction with i-propyl iodide. These ratios compare quite well with those calculated by using the rate constants for halogen transfer in the above reference and the rate constants for addition of a primary radical to methyl vinyl ketone. 

Fig. 3. Correlation of AH — AHgr [for the halogen transfer from CCI4 and B1CCI3 to radicals] and the steric substituent constants S f84, 8 ...

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