Chromate ester oxidation — mild variant
Collins (Sarett) oxidation
How the Collins (Sarett) oxidation 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: CrO₃·2 pyridine (Collins/Sarett reagent). Conditions: Anhydrous CH₂Cl₂, 25 °C. Bonds formed: C=O; Cr(IV); bonds broken: O–H, α C–H, Cr–N (py). Stereochemistry: Neighbouring stereocentres retained. Rate and selectivity: Alpha C–H cleavage rate-limiting; use 6 equiv reagent.
At a glance
Step 1: CrO₃ binds two pyridines
CrO₃ complexed with two equivalents of pyridine forms a stable, crystalline, far milder chromium(VI) oxidant than the aqueous chromic acid used in the Jones oxidation — pyridine's coordination tempers chromium's reactivity considerably.
Step 2: Chromate ester, then E2-like elimination
An alcohol's oxygen displaces one pyridine ligand, forming a chromate ester exactly as in the Jones oxidation, followed by the identical E2-like elimination that transfers the alpha C–H's electrons into the Cr=O bond as chromium is reduced.
Step 3: Stops at the aldehyde
Because the reagent is used in anhydrous, non-acidic conditions (unlike Jones), a primary alcohol's resulting aldehyde is not further hydrated and over-oxidized to the carboxylic acid — this milder profile is exactly why Collins conditions are chosen whenever stopping cleanly at the aldehyde matters.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- py
- Pyridine. Weak amine base / nucleophilic catalyst and solvent.
- E2
- Bimolecular elimination. Concerted base removal of β-H and loss of the leaving group (anti-periplanar).
How it compares with related reactions
| Collins (Sarett) oxidation | Jones oxidation | Baeyer-Villiger oxidation of a ketone | |
|---|---|---|---|
| Reagents | CrO₃·2 pyridine (Collins/Sarett reagent) | CrO₃, H₂SO₄, acetone (Jones reagent: H₂CrO₄) | mCPBA or CH₃CO₃H (peracid) |
| Conditions | Anhydrous CH₂Cl₂, 25 °C | Acetone, 0–25 °C, strongly acidic | CH₂Cl₂, 25 °C; cat. acid helps |
| Stereochemistry | Neighbouring stereocentres retained | Retention of configuration at neighbouring centres; carbinol centre destroyed | Retention at the migrating carbon |
| Rate / selectivity | Alpha C–H cleavage rate-limiting; use 6 equiv reagent | Chromate ester formation fast; C–H cleavage rate-limiting (large kH/kD) | Migratory aptitude: tert > cyclohexyl ≈ sec > Ph > prim > Me |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Collins (Sarett) oxidation?
CrO₃·2 pyridine (Collins/Sarett reagent). Conditions: Anhydrous CH₂Cl₂, 25 °C.
Which bonds are formed and which are broken?
Formed: C=O; Cr(IV). Broken: O–H, α C–H, Cr–N (py).
What is the stereochemical outcome?
Neighbouring stereocentres retained
What controls the rate or the selectivity?
Alpha C–H cleavage rate-limiting; use 6 equiv reagent
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go