Addition, radical polyhalo compounds

The tributyltin hydride-mediated carbon-carbon bond formation via radical addition and cyclization of alkyl halides with alkenes has often been a choice for construction of various organic molecules [1], However, the requirement for high-temperature initiators or photo initiation and the difficulties associated with purification of the products from tributyltin halides tend to limit the widespread use of these methods, despite the efforts to make the methods easier [Ic, 2], Recently, nickel-mediated radical additions and cyclizations have been introduced as promising alternatives to the tributyltin hydride methods. These are the nickel powder-acetic acid method for cyclization of haloamides to y-lactams, y -lactams and in-dolones, the borohydride exchange resin-nickel boride method for radical addition, nickel-catalyzed electroreductive cyclization and nickel-catalyzed Kharasch addition of polyhalo compounds. 

All these reactions proceed most likely by initial activation of the iron(0) species by a ligand exchange with the activating additives, such as amines, benzonitrile or DMF (Fig. 7). Thus generated mononuclear iron complexes bearing a labile ligand are activated to form coordinatively unsaturated iron complexes 37A. These species reduce the polyhalo compounds to radicals 38A, which add to olefins 30 or 33. 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

The compounds CCI4, BrCCla, ICF3, and similar simple polyhalo alkanes add to alkenes in good yield. These are free-radical additions and require initiation, for... 

Traditionally, this radical addition reaction has been used for the preparation of new poly halo organic compounds by reaction of simple polyhalo alkanes with alkenes689 698. The products from these reactions have often been used as key intermediates for the synthesis of heterocyclic compounds690. In some cases it has been found that metal complexes significantly speed up reactions and/or increase yields. 

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