Pinnick oxidation

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Pinnick oxidation
Named after Harold W. Pinnick
Reaction type Organic redox reaction

The Pinnick oxidation is an

sterically hindered groups. This reaction is especially useful for oxidizing α,β-unsaturated aldehydes, and another one of its advantages is its relatively low cost.[4][5]

PinnickOxidationReaction
PinnickOxidationReaction

Mechanism

The proposed reaction mechanism involves chlorous acid as the active oxidant, which is formed under acidic conditions from chlorite.

ClO2− + H2PO4− ⇌ HClO2 + HPO42−

First, the chlorous acid adds to the aldehyde. Then resulting structure undergoes a pericyclic fragmentation in which the aldehyde hydrogen is transferred to an oxygen on the chlorine, with the chlorine group released as hypochlorous acid (HOCl).[6]

Side reactions and scavengers

The HOCl byproduct, itself a reactive oxidizing agent, can be a problem in several ways.[6] It can destroy the NaClO2 reactant:

HOCl + 2ClO2− → 2ClO2 + Cl− + OH−

making it unavailable for the desired reaction. It can also cause other undesired

halohydrin formation reaction
.

To prevent interference from HOCl, a scavenger is usually added to the reaction to consume the HOCl as it is formed. For example, one can take advantage of the propensity of HOCl to undergo this addition reaction by adding a sacrificial alkene-containing chemical to the reaction mixture. This alternate substrate reacts with the HOCl, preventing the HOCl from undergoing reactions that interfere with the Pinnick reaction itself. 2-Methyl-2-butene is often used in this context:

Resorcinol and sulfamic acid are also common scavenger reagents.[6][7]

Hydrogen peroxide (H2O2) can be used as HOCl scavenger whose byproducts do not interfere in the Pinnick oxidation reaction:

HOCl + H2O2 → HCl + O2 + H2O

In a weakly

acidic
condition, fairly concentrated (35%) H2O2 solution undergoes a rapid oxidative reaction with no competitive reduction reaction of HClO2 to form HOCl.

HClO2 + H2O2 → HOCl + O2 + H2O

Chlorine dioxide reacts rapidly with H2O2 to form chlorous acid.

2ClO2 + H2O2 → 2HClO2 + O2

Also the formation of oxygen gives good indication of the progress of the reaction. DMSO has been used instead of H2O2 to oxidize reactions that do not produce great yields using only H2O2. Mostly electron rich aldehydes fall under this category.[7] (See Limitation below)

Also, solid-supported reagents such as phosphate-buffered

organic solvents.[8]

Scope and limitations

The reaction is highly suited for substrates with many group functionalities. β-aryl-substituted α,β-

iodides and even stannanes are quite stable in the reaction.[7][9][10][11]
The examples of the reactions shown below also show that the stereocenters of the α carbons remain intact while double bonds, especially trisubsituted double bonds do not undergo E/Z–isomerization in the reaction.

Scope

Lower yields are obtained for reactions involving

t-BOC, are a viable solution to these problems.[12]

Thioethers are also highly susceptible to oxidation. For example, Pinnick oxidation of thio

anisaldehyde gives a high yield of carboxylic acid products, but with concomitant conversion of the thioether to the sulfoxide or sulfone.[7]

See also

References

  1. doi:10.3891/acta.chem.scand.27-0888
    .
  2. doi:10.1021/jo01312a004
    .
  3. doi:10.1021/jo01294a058
    .
  4. ^
    doi:10.1016/S0040-4020(01)97963-3
    .
  5. ISBN 978-0-471-22854-7
    .
  6. ^
    ISBN 9780124297852
    .
  7. ^
    doi:10.1021/jo00354a037
    .
  8. doi:10.1055/s-2001-17448
    .
  9. ^
    doi:10.1002/1521-3897(200006)342:6<605::aid-prac605>3.0.co;2-i
    .
  10. doi:10.1016/S0040-4039(00)00013-7
    .
  11. doi:10.1016/S0040-4039(99)01512-9
    .
  12. PMID 11672125
    .
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