Catalytic Mitsunobu chemistry with recyclable azo reagents
Ethyl arylazocarboxylates plus iron phthalocyanine and air replace stoichiometric DEAD in Mitsunobu esterifications with high inversion.
Source
Advances and mechanistic insight on the catalytic Mitsunobu reaction using recyclable azo reagents
Study at a glance
- Design
- Animal / in-vitro — Catalytic ethyl arylazocarboxylates reoxidized with O2 and iron phthalocyanine
- N
- Substrate-scope esterifications rather than a single analytic N
- Population
- Secondary alcohols and nucleophiles under Mitsunobu conditions
- Outcome
- Recyclable azo reagents with air as oxidant give high yield and stereochemical inversion, replacing stoichiometric DEAD
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Inspect source passages
Catalytic Mitsunobu chemistry with recyclable azo reagents
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Heating THF gave ester 5 in 50% yield and 97% inversion.
The reaction between 3 and 4 in heating THF (65 °C) gave ester product 5 in 50% yield and in 97% inversion (, entry 1).
Toluene at room temperature gave 88% yield and perfect inversion.
To our delight, the reaction in toluene at room temperature provided product 5 in an excellent yield (88%) and in a perfect inversion ratio.
What they did
Authors screened ethyl 2-arylhydrazinecarboxylates as catalytic azo sources, reoxidized hydrazines with O2 and iron phthalocyanine, optimized solvent, molecular sieves, and concentration, and measured yield/inversion on secondary alcohols and other nucleophiles.
What they found
Catalyst 1a (3,4-dichlorophenyl) is best for carboxylic acids and stereochemical inversion; 1j (4-cyanophenyl) is best for other nucleophiles. Toluene at RT gave 88% yield with perfect inversion; flame-activated 5 Å sieves gave 94% yield. Air is the sacrificial oxidant.
The limits
What it doesn't show
Plant-scale process metrics (E-factor on multi-kilo batches) are not provided; some chlorinated solvents still erode inversion, and PPh3 remains stoichiometric.
Key terms
- Mitsunobu reaction
- Redox condensation that substitutes alcohols, typically with inversion, using an azo oxidant and PPh3.
- DEAD
- Diethyl azodicarboxylate, the classic stoichiometric, hazardous azo reagent.
- Iron phthalocyanine
- Cheap aerobic oxidation catalyst that recycles the hydrazine to the azo form.
- Organocatalyst 1a
- Ethyl 2-(3,4-dichlorophenyl)hydrazinecarboxylate, best for acid nucleophiles/inversion.
- Walden inversion
- Stereochemical inversion at a secondary alcohol carbon during substitution.
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Catalytic azo recycling uses:
Common questions
What replaces stoichiometric DEAD?
Catalytic ethyl arylazocarboxylates reoxidized by O2/FePc.
Which catalyst inverts secondary alcohols best?
1a (3,4-dichlorophenyl).
Why molecular sieves?
The azo–PPh3 intermediate is moisture sensitive.
Best simple solvent result cited?
Toluene, RT, 88% yield, perfect inversion.
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