Reactions of Free Radicals : Substitution Reactions

The alkyl halide reduction described above is one example of a radical substitution reaction. Two other substitution reactions are shown in the diagram below. The first two equations illustrate a useful procedure for reducing alcohols to alkanes, known as "Barton-McCombie deoxygenation". A thionoester, such as a xanthate, is first prepared from the alcohol, and then treated with tributyltin hydride and a radical initiator. Phenylsilane may be substituted for the stannane as a radical carrier. In either case a radical (X• )adds to the C=S function, and the resulting •C(OR)–S–X species fragments to R• and XSC=O. This reduction is particularly useful for 2º-alcohols, as in the second equation. Deoxygenation of 1º-alcohols is often effected by LiAlH₄ reduction of tosylate derivatives, a technique that is less satisfactory for 2º-alcohols.
The second equation shows an interesting substitution of sulfur for bromine. Here the phenyl radical intermediate bonds to sulfur, followed by homolysis of the tert-butyl substituent.

A variation of the Barton-McCombie, called "Barton decarboxylation" will be displayed above. Here advantage is taken of e weak N–O bond to generate a carboxyl radical, which rapidly decarboxylates to an alkyl radical. The N-hydroxy-2-thiopyridone derivative of a carboxylic acid or acid chloride is readily prepared. Photolysis in the presence of a thiol as the hydrogen transfer agent initiates the chain reaction shown above the light blue line. The alkyl radical thus prepared (in pink brackets) may then accept a hydrogen or halogen atom, depending on other reagents chosen for the reaction. The reaction shown below the blue line illustrates the versatility of this procedure.