Selenium dioxide (SeO2) and Raney nickel are both useful reagents in organic synthesis. SeO2 can be used to oxidize alkenes to allylic alcohols or carbonyls. It also oxidizes carbonyls to 1,2-dicarbonyls and internal alkynes to 1,2-dicarbonyls. Raney nickel catalyzes hydrogenation of aromatics and reduction of carbonyl groups by cleaving C-S bonds. Both reagents have applications in functional group transformations.

1. 13. SeO2
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2. 2
...SeO2: Structure
Ø Selenium dioxide is a colorless solid. It exists as one dimensional
polymeric chain with alternating selenium and oxygen atoms.
Ø It sublimes readily and hence the commercial samples of SeO2 can be
purified by sublimation.
Ø SeO2 is an acidic oxide and dissolves in water to form selenous acid,
H2SeO3.

3. 3
...Selenium dioxide
Ø Selenium dioxide, SeO2 is an oxidizing agent generally employed in the
allylic oxidation of alkenes to furnish allylic alcohols, which may be
further oxidized to conjugated aldehydes or ketones.
Ø It is also used to oxidize the α-methylene group adjacent to a carbonyl
group to give a 1,2-dicarbonyl compound. However selenium dioxide can
perform several common types of oxidations, such as alcohols to ketones
or aldehydes.
Ø The oxidations of methylene groups using Selenium dioxide are referred to
as Riley oxidations.

4. 4
...Selenium dioxide
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5. 5
...SeO2: Properties
Reaction conditions:
Ø Compounds of selenium are very poisonous and smelly. Hence the reaction setup
must be maintained under a fuming cupboard.
Ø Use of acetic acid as solvent stops the reaction at allylic alcohol stage due to
formation of acetate esters.
Ø A convenient way to carry out the reaction is to use only a catalytic amount of SeO2
along with an oxidizing agent like t-butyl hydroperoxide, that reoxidizes the
selenium(II) compounds after each cycle of the reaction. This eliminates the need to
get rid of large amounts of selenium compounds, which are toxic and usually smelly.
Ø It also ensures the principal product is allylic alcohol by reducing the chances of
further oxidation to conjugated carbonyl compounds.
Workup:
Ø The final workup involves precipitation of selenium or selenium compounds, which
can be filtered off before isolation of product from the reaction mixture.

6. 6
...SeO2: Applications
1. Allylic oxidation of alkenes:
Selenium dioxide oxidizes allylic positions to alcohol or carbonyl groups. It starts with
Alder-ene like 4+2 cycloaddition of SeO2 to give an allylic selenic acid that further
undergoes [2,3]-sigmatropic rearrangement to give an unstable compound that may
decompose to allylic alcohol or an allylic carbonyl compound as shown below.
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7. 7
...SeO2: Examples
i. Catalytic amount of selenium dioxide and t-BuOOH can be employed in the
allylic oxidation of cyclohexene to cyclohex-2-en-1-ol, an allylic alcohol.
ii. Trisubstituted alkenes are oxidized selectively at more substituted end of
double bond by giving E-allylic alcohols or conjugated carbonyl
compounds predominantly.
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8. 8
...SeO2: Examples

9. 9
...SeO2: Examples
It is because the initial ene type 4+2 cycloaddition involves preferential attack of the
more nucleophilic end of double bond at selenium. In this step, alkene uses the π-
HOMO to attack the π*-LUMO of Se=O. Meanwhile the π-HOMO of Se=O attacks the
σ*-LUMO of C-H of the allylic system.
The E-selectivity is due to cyclic nature of final [2,3] sigmatropic step in which
the alkyl substituent adopts pseudoequatorial position.
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10. 10
...SeO2: Examples
iii. The allylic oxidation occurs predominantly at most nucleophilic double
bond. In the following example, no allylic positions of double bond nearer to
electron withdrawing acetyl group are oxidized.
iv. It also oxidizes benzylic methylene, CH2 group to C=O.
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11. 11
...SeO2: Examples

12. 12
...SeO2: Mechanism
2. Formation of 1,2-dicarbonyl compounds from carbonyls:
SeO2 can also oxidize α-methylene group on a carbonyl compound to furnish
1,2-dicarbonyl compound. The mechanism involves steps similar to allylic
oxidation.
adichemistry.com

13. 13
...SeO2: Examples
i. Acetophenone can be oxidized with SeO2 to oxo(phenyl)acetaldehyde, a
1,2-dicarbonyl compound.
Semperviridine: Gribble, G. W.; Barden, T. C.;
Johnson, D. A. Tetrahedron Lett. 1988, 44, 1988
ii. Dicarbonyls;

14. 14
...Selenium dioxide
3. The internal alkynes are converted to 1,2-dicarbonyl compounds,
whereas terminal alkynes are oxidized to glyoxylic acids: Selenium oxide
can also be used to oxidize alkynes in presence of acids.
adichemistry.com

15. 15
...SeO2: Summary
Applications
1. Allylic oxidation of alkenes
2. Formation of 1,2-dicarbonyl compounds from carbonyls
3. Internal alkynes are converted to 1,2-dicarbonyl compounds, whereas
terminal alkynes are oxidized to glyoxylic acids.

16. 16
…Raney nickel

17. 17
…Raney nickel
Ø Raney Ni (Ni-Al) is produced when a block of Ni-Al alloy is treated with
concentrated NaOH. This treatment, called "activation", dissolves most of the
Al out of the alloy. The porous structure left behind has a large surface area,
which gives high catalytic activity.

18. 18
…Raney nickel
1. It is one of the common catalysts used for the hydrogenation of aromatic
compounds.

19. 19
2. Removal of carbonyl groups: It is harder, although there are several possible
methods. C–O bonds are strong, but C–S bonds are much weaker and are
often easily reduced with Raney nickel. We can get rid of aldehyde and
ketone carbonyl groups by making them into thio-acetals, sulfur analogues
of acetals, formed in a reaction analogous to acetal formation but using a
dithiol with a Lewis acid catalyst. Freshly prepared Raney nickel carries
enough H2 to reduce the thioacetal without added hydrogen.
…Raney nickel
Jonathan Clayden (University of Manchester), Nick Greeves (University of Liverpool), Stuart Warren (University of Cambridge); Organic Chemistry, 2nd edition.

20. 20
…Raney nickel
Raney Ni is also used for the reduction of a series of functional groups. For
example, Raney Ni is particularly useful for the cleavage of C-S bond
Jonathan Clayden (University of Manchester), Nick Greeves (University of Liverpool), Stuart Warren (University of Cambridge); Organic Chemistry, 2nd edition.

21. 21
…Raney nickel
Jonathan Clayden (University of Manchester), Nick Greeves (University of Liverpool), Stuart Warren (University of Cambridge); Organic Chemistry, 2nd edition.

22. 22
Ø A slightly more vigorous method, known as the Wolff–Kishner reduction, is
driven by the elimination of nitrogen gas from a hydrazone.
Ø Hot concentrated sodium hydroxide solution deprotonates the hydrazone,
which can then lose nitrogen to form an alkyl anion, which is immediately
protonated by water.
…Raney nickel
Jonathan Clayden (University of Manchester), Nick Greeves (University of Liverpool), Stuart Warren (University of Cambridge); Organic Chemistry, 2nd edition.

23. 23
...SeO2: Summary
…Raney Ni: Summary
1. Hydrogenation of aromatic compounds
2. Removal of carbonyl groups (cleavage of C-S bond)
Applications
1. Allylic oxidation of alkenes
2. Formation of 1,2-dicarbonyl compounds from carbonyls
3. Internal alkynes are converted to 1,2-dicarbonyl compounds, whereas
terminal alkynes are oxidized to glyoxylic acids.