Inorganic — metal-hydride radical chemistry
MHAT hydrofunctionalization
How the MHAT hydrofunctionalization 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: Co(salen) or Mn(dpm)₃ cat., PhSiH₃ (hydride source), oxidant (O₂, TBHP) or radical trap. Conditions: EtOH or iPrOH, 25 °C, mild. Bonds formed: C–H (at CH₂ terminus); then C–O / C–N / C–C; bonds broken: M–H; alkene π. Stereochemistry: Planar radical: stereochemistry set in the trapping step. Rate and selectivity: H-atom transfer from M–H is rate-limiting; Markovnikov selective.
At a glance
Step 1: M–H transfers H• to the alkene
A metal-hydride complex (commonly a Co or Mn catalyst regenerated by a silane) transfers a hydrogen atom directly to an alkene's less-hindered carbon in a single step — metal-hydrogen atom transfer, generating the more stable, more substituted carbon radical, mechanistically parallel to how Markovnikov selectivity arises in ionic addition.
Step 2: Radical adds to a Michael acceptor
This carbon radical is now a nucleophilic species that can add to a radical trap — commonly a Michael acceptor (Giese-type addition, covered earlier) or be captured by a second equivalent of the metal to continue a catalytic cycle.
Step 3: Different traps, different products
Depending on the trap used, this single MHAT step can deliver hydration, hydroamination, or radical conjugate addition products directly from an unactivated alkene — this strategy has become one of the most important modern tools for installing functionality at the more substituted carbon of simple alkenes under remarkably mild conditions.
Abbreviations and terms
- M
- Metal. A generic metal centre.
- EtOH
- Ethanol. Polar protic solvent.
- TBHP
- tert-Butyl hydroperoxide. Oxidant/oxygen donor (Sharpless epoxidation).
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
How it compares with related reactions
| MHAT hydrofunctionalization | Associative ligand substitution at a square-planar Pt(II) complex | CuAAC | |
|---|---|---|---|
| Reagents | Co(salen) or Mn(dpm)₃ cat., PhSiH₃ (hydride source), oxidant (O₂, TBHP) or radical trap | K₂[PtCl₄] + NH₃ (aq) | Terminal alkyne, organic azide, Cu(I) (CuSO₄ + sodium ascorbate, or CuI), base/ligand (TBTA) |
| Conditions | EtOH or iPrOH, 25 °C, mild | H₂O, 25 °C | tBuOH/H₂O or DMSO, 25 °C |
| Stereochemistry | Planar radical: stereochemistry set in the trapping step | Trigonal-bipyramidal intermediate; retention of geometry | 1,4-regioisomer only (thermal gives 1,4 + 1,5 mixture) |
| Rate / selectivity | H-atom transfer from M–H is rate-limiting; Markovnikov selective | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | 10⁷–10⁸ × faster than uncatalysed Huisgen cycloaddition |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the MHAT hydrofunctionalization?
Co(salen) or Mn(dpm)₃ cat., PhSiH₃ (hydride source), oxidant (O₂, TBHP) or radical trap. Conditions: EtOH or iPrOH, 25 °C, mild.
Which bonds are formed and which are broken?
Formed: C–H (at CH₂ terminus); then C–O / C–N / C–C. Broken: M–H; alkene π.
What is the stereochemical outcome?
Planar radical: stereochemistry set in the trapping step
What controls the rate or the selectivity?
H-atom transfer from M–H is rate-limiting; Markovnikov selective
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam