Radical chain — reductive dehalogenation
Radical hydrostannylation (Bu₃SnH reduction)
How the Radical hydrostannylation (Bu₃SnH reduction) works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Bu₃SnH (cat. or stoich.), AIBN, alkyl bromide/iodide. Conditions: Benzene or toluene, 80 °C (AIBN thermolysis). Bonds formed: C–H; Sn–Br; bonds broken: C–Br; Sn–H. Stereochemistry: Radical is planar; stereocentre lost. Rate and selectivity: Chain propagation; Sn–Br bond strength drives halogen abstraction.
At a glance
Step 1: AIBN initiates; Bu₃Sn• forms
AIBN, decomposed by heat, generates a radical that abstracts a hydrogen atom from Bu₃SnH, forming a tin radical (Bu₃Sn•) — the chain-carrying species for this entire family of reductions, including Barton-McCombie deoxygenation covered earlier.
Step 2: Bu₃Sn• abstracts Br from R–Br
The tin radical abstracts a halogen atom from an alkyl halide, forming a carbon radical and a tin halide — tin's exceptionally strong affinity for halogens is what drives this abstraction forward thermodynamically.
Step 3: H-atom transfer, chain continues
That carbon radical abstracts a hydrogen atom from another equivalent of Bu₃SnH, giving the reduced (dehalogenated) product and regenerating Bu₃Sn• to continue the chain — this simple reduction is the same tin-radical chain machinery that underlies Barton-McCombie, just applied directly to a C–X bond instead of a xanthate.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- X
- Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
- Bu3SnH
- Tributyltin hydride. Radical H-atom donor (weak Sn–H bond).
- AIBN
- Azobisisobutyronitrile. Radical initiator: on heating releases N₂ and two 2-cyano-2-propyl radicals.
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Radical hydrostannylation (Bu₃SnH reduction) | Barton-McCombie deoxygenation | Kharasch addition (peroxide-mediated HBr addition) | |
|---|---|---|---|
| Reagents | Bu₃SnH (cat. or stoich.), AIBN, alkyl bromide/iodide | 1) NaH, CS₂, CH₃I 2) Bu₃SnH, AIBN (or (TMS)₃SiH) | HBr, ROOR (peroxide initiator) or light, alkene |
| Conditions | Benzene or toluene, 80 °C (AIBN thermolysis) | Toluene, 80–110 °C, radical chain | Heat or hν, non-polar solvent |
| Stereochemistry | Radical is planar; stereocentre lost | Stereocentre lost (planar radical) | Anti-Markovnikov; radical chain |
| Rate / selectivity | Chain propagation; Sn–Br bond strength drives halogen abstraction | Driven by strong C=O/Sn–S bonds; chain propagation | Chain propagation; H-abstraction from HBr is fast |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Radical hydrostannylation (Bu₃SnH reduction)?
Bu₃SnH (cat. or stoich.), AIBN, alkyl bromide/iodide. Conditions: Benzene or toluene, 80 °C (AIBN thermolysis).
Which bonds are formed and which are broken?
Formed: C–H; Sn–Br. Broken: C–Br; Sn–H.
What is the stereochemical outcome?
Radical is planar; stereocentre lost
What controls the rate or the selectivity?
Chain propagation; Sn–Br bond strength drives halogen abstraction
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make