Gas-phase rates of heme-model olefin epoxidation
FT-ICR and DFT show iron(IV)–oxo porphyrin cation radicals epoxidize olefins with rates that track substrate ionization energy.
Source
A comprehensive test set of epoxidation rate constants for iron(iv)-oxo porphyrin cation radical complexes
Study at a glance
- Design
- Other — Gas-phase FT-ICR bimolecular rate constants for [FeIV(O)(TPFPP+·)]+ with olefins plus DFT
- N
- Physical-organic kinetics panel of olefins — not a sample-N study
- Population
- Gas-phase iron-oxo porphyrin cation and olefin substrates
- Outcome
- Epoxidation rate constants and efficiency versus olefin ionization potential
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
Authors generated [FeIV(O)(TPFPP+·)]+ from iodosylbenzene at low temperature, measured bimolecular rate constants in an FT-ICR cell against a panel of olefins, and computed epoxidation pathways with DFT on porphine models.
What they found
All olefins undergo oxygen-atom transfer; electron-rich substrates also do hydride and charge transfer. Efficiencies run from 0.001–0.08% for terminal olefins up to about 50% for electron-rich monoterpenes. kH/kD ≈ 1 rules out rate-limiting H-atom transfer, and rates scale with ionization potential.
The limits
What it doesn't show
These are naked ions in the gas phase, not protein-bound P450 Compound I in water; axial-ligand and solvent effects are only treated computationally, and absolute rate errors are estimated at ±30%.
Key terms
- Compound I
- Iron(IV)–oxo porphyrin cation radical, the reactive oxidant of cytochrome P450.
- Oxygen-atom transfer (OAT)
- Transfer of the oxo ligand to an olefin to make an epoxide.
- FT-ICR MS
- Fourier-transform ion cyclotron resonance mass spectrometry used to time ion–molecule reactions.
- TPFPP
- meso-Tetrakis(pentafluorophenyl)porphyrin, an electron-deficient porphyrin ligand.
- Kinetic isotope effect (KIE)
- kH/kD ratio; near 1 here argues against rate-limiting C–H abstraction.
Flashcards
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Quiz yourself
Gas-phase rates of this Compound I model track:
Common questions
What is the rate-determining event?
Electron transfer from olefin to the iron–oxo oxidant, tracking ionization energy.
Does cyclohexene react by H-atom transfer?
No: kH/kD is very close to 1.
How was the oxidant made?
Iodosylbenzene plus [FeIII(TPFPP)]Cl at −40 °C, then electrospray into the ICR cell.
OAT vs hydroxylation?
The gas-phase chemistry is regioselective for epoxidation over hydroxylation.
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