Selenoxide elimination

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Selenoxide elimination (also called α-selenation)

carbonyl compounds from the corresponding saturated analogues.[2] It is mechanistically related to the Cope reaction
.

Mechanism and stereochemistry

After the development of

nucleophilic carbonyl derivatives (enols and enolates),[4]
selenoxide elimination has grown into a general method for the preparation of α,β-unsaturated carbonyl compounds.

(1)

Mechanism

Elimination of selenoxides takes place through an intramolecular syn elimination pathway. The carbon–hydrogen and carbon–selenium bonds are co-planar in the transition state.[5]

(2)

The reaction is highly

catalyzed and occurs readily) on the elimination reaction is nearly unknown. In one example, separation and warming of selenoxides 1 and 2 revealed that 2 decomposes at 0 °C, while 1, which presumably has more difficulty accessing the necessary syn conformation for elimination, is stable to 5 °C.[6]

(3)

Kinetic isotope effect studies have found a ratio of pre-exponential factors of AH/AD of 0.092 for sulfoxide elimination reactions, indicating that quantum tunneling plays an important role in the hydrogen transfer process.[7][8]

Scope and limitations

Selanylating and oxidizing reagents

α-Selanylation of carbonyl compounds can be accomplished with

nucleophilic selanylating reagents. Usually, simple phenylseleno compounds are used in elimination reactions; although 2-nitrophenylselenides react more quickly, they are more expensive to prepare, and phenylselenides typically react in minutes. Electrophilic selanylating reagents can be used in conjunction with enols, enolates, or enol ethers
. Phenylselanating reagents include:

  • Diphenyl diselenide
  • Benzeneselanyl chloride
  • Benzeneselanyl bromide
  • Benzeneselinyl chloride
  • Sodium benzeneselenolate
  • Trimethylsilyl phenyl selenide

The most common

Baeyer-Villiger reaction, for instance) has also been observed.[10]

(4)

For substrates whose product

Buffering with an amine base is necessary before warming to avoid acid-mediated side reactions.[11]

(5)

thermolysis or extreme care is necessary during workup. Quinones can be synthesized from the corresponding cyclic unsaturated carbonyl compounds using this method.[12]

(6)

Substrates

α-Phenylseleno aldehydes, which are usually prepared from the corresponding enol ethers, are usually oxidized with mCPBA or ozone, as hydrogen peroxide causes over-oxidation. α-Phenylseleno ketones can be prepared by kinetically controlled enolate formation and trapping with an electrophilic selanylating reagent such as benzeneselenyl chloride. A second deprotonation, forming a selenium-substituted enolate, allows alkylation or hydroxyalkylation of these substrates.[13]

(7)

Base-sensitive substrates may be selanylated under acid-catalyzed conditions (as enols) using benzeneselenyl chloride.

tautomerization.[14]

(8)

The seleno-

Pummerer reaction is a significant side reaction that may occur under conditions when acid is present.[15] Protonation of the selenoxide intermediate, followed by elimination of hydroxide and hydrolysis, leads to α-dicarbonyl compounds. The reaction is not a problem for more electron-rich carbonyls—generally, fewer side reactions are observed in eliminations of esters and amides.[15]

(9)

A second significant side reaction in reactions of ketones and aldehydes is selanylation of the intermediate selenoxide. This process leads to elimination products retaining a carbon-selenium bond,[16] and is more difficult to prevent than the seleno-Pummerer reaction. Tertiary selenoxides, which are unable to undergo enolization, do not react further with selenium electrophiles.

(10)

Comparison with other methods

Analogous sulfoxide eliminations are generally harder to implement than selenoxide eliminations. Formation of the carbon–sulfur bond is usually accomplished with highly reactive sulfenyl chlorides, which must be prepared for immediate use. However, sulfoxides are more stable than the corresponding selenoxides, and elimination is usually carried out as a distinct operation. This allows thermolysis conditions to be optimized (although the high temperatures required may cause other thermal processes). In addition, sulfoxides may be carried through multiple synthetic steps before elimination is carried out.[17]

(11)

The combination of

stoichiometric amounts of Pd(OAc)2 and thus is not amenable to large-scale synthesis.[18] Catalytic variants have been developed.[19]

(12)

For β-dicarbonyl compounds, DDQ can be used as an oxidizing agent in the synthesis of enediones. Additionally, some specialized systems give better yields upon DDQ oxidation.[20]

(13)

See also

References

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  2. ISBN 978-0-471-26418-7
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  3. doi:10.1021/jo01277a026
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  5. doi:10.1016/S0040-4039(01)96098-8
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  6. doi:10.1039/C29700000086
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  7. ISSN 0002-7863
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  8. ISSN 0001-4842
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  9. doi:10.1016/S0040-4020(01)81500-3
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  10. doi:10.1021/np50043a017
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  11. doi:10.1016/S0040-4020(01)97962-1
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  12. doi:10.1039/P19850000631
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  13. doi:10.1021/jo00256a029
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  14. doi:10.1002/cber.19781110528
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  15. ^
    doi:10.1021/jo00928a042
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  16. doi:10.1021/ja00852a019
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  17. doi:10.3987/R-1986-02-0309 (inactive 2024-03-07).{{cite journal}}: CS1 maint: DOI inactive as of March 2024 (link
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  18. doi:10.1021/jo00399a052
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  19. doi:10.1016/S0040-4039(01)81518-5
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  20. doi:10.1021/jo01294a046
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