Radical chain — deoxygenation
Barton-McCombie deoxygenation
How the Barton-McCombie deoxygenation 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: 1) NaH, CS₂, CH₃I 2) Bu₃SnH, AIBN (or (TMS)₃SiH). Conditions: Toluene, 80–110 °C, radical chain. Bonds formed: C–H; C=O (S-stannyl dithiocarbonate); bonds broken: C–O; Sn–H. Stereochemistry: Stereocentre lost (planar radical). Rate and selectivity: Driven by strong C=O/Sn–S bonds; chain propagation.
At a glance
Step 1: Alcohol → xanthate
An alcohol is first converted to a xanthate ester (via CS₂ and methyl iodide) — this installs a C=S group that radicals add to easily, unlike a plain C–OH.
Step 2: Sn• adds to S, C–O fragments
A tin or silicon radical (from Bu₃SnH or (TMS)₃SiH, generated by a radical initiator) adds to the xanthate's sulfur, and the resulting radical fragments, expelling a stable carbon radical at the original alcohol position.
Step 3: H-atom transfer, chain continues
That carbon radical abstracts a hydrogen atom from another equivalent of the tin or silicon hydride, giving the fully deoxygenated C–H product and continuing the radical chain.
Abbreviations and terms
- TMS
- Trimethylsilyl. –Si(CH₃)₃; a small silicon group used to protect or activate.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- NaH
- Sodium hydride. Insoluble, irreversible base: removes H⁺ and bubbles off H₂.
- 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.
How it compares with related reactions
| Barton-McCombie deoxygenation | Kharasch addition (peroxide-mediated HBr addition) | Radical hydrostannylation (Bu₃SnH reduction) | |
|---|---|---|---|
| Reagents | 1) NaH, CS₂, CH₃I 2) Bu₃SnH, AIBN (or (TMS)₃SiH) | HBr, ROOR (peroxide initiator) or light, alkene | Bu₃SnH (cat. or stoich.), AIBN, alkyl bromide/iodide |
| Conditions | Toluene, 80–110 °C, radical chain | Heat or hν, non-polar solvent | Benzene or toluene, 80 °C (AIBN thermolysis) |
| Stereochemistry | Stereocentre lost (planar radical) | Anti-Markovnikov; radical chain | Radical is planar; stereocentre lost |
| Rate / selectivity | Driven by strong C=O/Sn–S bonds; chain propagation | Chain propagation; H-abstraction from HBr is fast | Chain propagation; Sn–Br bond strength drives halogen abstraction |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Barton-McCombie deoxygenation?
1) NaH, CS₂, CH₃I 2) Bu₃SnH, AIBN (or (TMS)₃SiH). Conditions: Toluene, 80–110 °C, radical chain.
Which bonds are formed and which are broken?
Formed: C–H; C=O (S-stannyl dithiocarbonate). Broken: C–O; Sn–H.
What is the stereochemical outcome?
Stereocentre lost (planar radical)
What controls the rate or the selectivity?
Driven by strong C=O/Sn–S bonds; chain propagation
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make