20.21:
Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride
Radical substitution reactions are useful for the halogenation of alkanes and alkenes.
Similarly, radical substitutions can remove halogen from alkyl halides using tributyltin hydride, known as the hydrogenolysis of alkyl halides.
Notably, tin hydride has a weak Sn–H bond and can react with alkyl halides to replace the halogen with hydrogen, forming alkane and tin halide.
The reaction is energetically favored as the new bonds formed are stronger than the bonds broken.
The mechanism of hydrogenolysis follows a chain reaction.
Initially, tin hydride produces a tributyltin radical in the presence of light, which then abstracts halogen from an organic halide producing a radical intermediate.
Next, the radical intermediate abstracts hydrogen from tin hydride to finally form an alkane and a tributyltin radical, which propagates the reaction.
The reactivity of alkyl halides towards hydrogenolysis decreases from iodide to fluoride, where the alkyl fluorides are unreactive.
In particular, the reduction of alkyl chlorides requires a higher concentration of tributyltin radicals, which is achieved by adding an initiator such as AIBN to the reaction.
20.21:
Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride
Radical substitution reactions can be used to remove functional groups from molecules. The hydrogenolysis of alkyl halides is one such reaction, where the weak Sn–H bond in tributyltin hydride reacts with alkyl halides to form alkanes. Here, the reagent Bu3SnH yields tributyltin halide as a byproduct.
The bonds formed in this reaction are stronger than the bonds broken, making it energetically favorable. The reaction follows a radical chain mechanism similar to radical halogenation reactions, where Bu3SnH undergoes homolytic cleavage in the presence of light to form a tributyltin radical. This radical abstracts the halogen from an alkyl halide, forming an alkyl radical intermediate and tributyltin halide. The radical intermediate further abstracts hydrogen from tributyltin hydride and produces an alkane and a tributyltin radical, which propagates the reaction.
As C–Br and C–I bonds are weaker than C–Cl bonds, daylight is sufficient to initiate the hydrogenolysis of alkyl bromides and iodides. In contrast, alkyl fluorides are unreactive because of the strong C–F bonds. The hydrogenolysis of alkyl chlorides using Bu3SnH requires a high concentration of tributyltin radicals, which is achieved by adding an initiator to the reaction mixture. AIBN is the most widely used initiator, as it undergoes thermal homolysis above 60 °C to form nitrile-stabilized radicals. These radicals abstract hydrogen from tributyltin hydride, forming the tributyltin radical, which propagates the reaction.
Peroxides are not used as initiators because peroxide radicals are highly reactive and can abstract hydrogen from organic halides leading to unwanted side reactions.