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-1-OXIDATION AND REDUCTION
Introduction
In general, reducing agents are electron-rich systems and compounds – such as
carbonyl groups – which are easily attacked by nucleophiles and undergo easy
reductions. In contrast, oxidising agents are typically electron-deficient species, so
that compounds which undergo easy electrophilic attack will also be easily oxidised.
OXIDATION
Oxidation By Metal Ions
Metal oxidants can be considered mechanistically as either:
2 electron oxidants – Cr(VI), Mn(VII), Os(VIII), Tl(III), Pb(IV), or
1 electron oxidants – Fe(III), Cu(II), Ce(IV).
Oxidation of Alcohols and Related Species
The oxidation of alcohols by Cr(VI) goes by initial fast formation of a chromate(VI)
ester, followed by rate determining break down (shown by a primary kinetic isotope
effect) to the ketone and Cr(IV). The details of the chemistry are complex since
Cr(IV) disproportionates into Cr(V) and Cr(III) and many of these other species are 1,
2 or 3 electron oxidants.
Classic oxidation rates of axial versus equatorial alcohols:
Axial alcohols are oxidised faster than equatorial alcohols since the rate determining
breakdown of the axial intermediate A is accompanied by relief of 1,3-diaxial
interactions not present in the breakdown of E.
One problem is that carbonyl compounds tend to get oxidised further, usually by the
same mechanism on the hydrates or hemiacetals; thus aldehydes form acids
whereas ketones give esters. There are other mechanisms for further oxidation, for
example by the enol, but it is clear that the absence of water would remove the
above as a potential pathway for over-oxidation.
To this end a number of reagents have been devised which allow the Cr(VI) reagent
to be used in solvents such as dichloromethane from which water be excluded.
PCC (pyridinium chlorochromate) is the classic oxidant which is
frequently used in the presence of a dehydrating agent. It is less
effective in the presence of allylic substrates.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
, -2-
PDC is the less acidic form, with no chloro (pyridinum dichromate). This is good for
allylic substrates.
Collins reagents are CrO3-pyridine complexes, particularly CrO3.2C5H5N. This does
not attack alkenes.
In summary, the common mechanistic path A for chromium and many other metals
involves formation of a metal ester followed by rate determining breakdown with loss
of a proton. An alternative path B is sometimes followed – for example in some
Mn(VII) oxidations – in which the carbon-bound hydrogen is lost as a hydride ion.
A number of alternative reactions compete with the oxidation of the alcohol, such as
cleavage of the C-C bond by loss of the proton of the hydroxyl on a vicinal alcohol.
Milder two electron oxidants will also achieve this oxidation, including periodate and
lead tetra-acetate which usually goes via a cyclic Pb(IV) ester.
However, even rigid trans-diaxial diols are cleaved by lead tetra-acetate via a non-
cyclic mechanism.
Dess-Martin Periodinane
Hypervalent iodine. It is a mild and neutral reagent. It can be formed by adding
KBrO3 to 2-iodobenzoic acid in sulphuric acid, under heat with acetone or acetic acid.
It forms the following, which oxidises alcohols to aldehydes:
TPAP
A metal oxidant very commonly used for alcohol to carbonyl transformation is Tetra-
isopropyl ammonium perruthennate (TPAP) usually used catalytically in the presence
of an amine oxide as the re-oxidant – no mechanistic proposal on the course of the
reaction has yet been published.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
, -3-
Pt/O2
This can be regulated effectively, but requires heats up to 320oC. It is the reverse of
hydrogenation. Alkenes are unaffected. It preferentially favours primary alcohols (1o >
2o > 3o), and stops at the aldehyde.
Manganese (IV) Oxide
Selective for allylic and benzylic hydroxyls. It works in a neutral solvent at room
temperature. It proceeds to the aldehyde, unless cyanide is present.
Silver Carbonate (Fetizon)
Primary or Secondary alcohol reacts to form aldehyde or ketone. It is non-
destructive.
It is concerted and reversible. Adding NaOCl means that secondary alcohols are
oxidised before primary. Polar solvents generally will inhibit the reaction.
Oppenhauer
Al(OiPr)3 in acetone (or any H acceptor).
The reverse is Meerwein-Ponndorf-Varley mechanism, a REDUCTION.
Carboxylic Acids
As mentioned, it can sometimes be difficult to stop reactions proceeding past the
aldehyde stage and straight to the carboxylic acid. Most of the reagents above are
designed to stop at the aldehyde stage. The major ones for fully oxidising an alcohol
are shown below.
RuO4 is a very powerful oxidising agent and works at room temperature, in CCl4.
KMnO4 is similar.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
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