Utente:Grasso Luigi/sanbox1/Ossidazione di Riley

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L' ossidazione di Riley è una reazione di ossidazione mediata dal diossido di selenio di gruppi metilenici adiacenti ai gruppi carbonili.Fu prodotta per la prima volta da Riley e collaboratori nel 1932.[1] In the decade that ensued, selenium-mediated oxidation rapidly expanded in use, and in 1939, Guillemonat and co-workers disclosed the selenium dioxide-mediated oxidation of olefins at the allylic position.[2] Today, selenium-dioxide-mediated oxidation of methylene groups to alpha ketones and at the allylic position of olefins is known as the Riley Oxidation.[3]

Meccanismo[modifica | modifica wikitesto]

The mechanism of oxidation of -CH2C(O)R group by SeO2 has been well investigated. [4] [5][6][7] The oxidation of carbonyl alpha methylene positions beings with attack by the enol tautomer at the electrophilic selenium center. Following rearrangement and loss of water, a second equivalent of water attacks the alpha position. Selenic acid is liberated in the final step to give the 1,2-dicarbonyl product.

Allylic oxidation using selenium-dioxide proceeds via an ene reaction at the electrophilic selenium center. A 2,3-sigmatropic shift, proceeding through an envelope-like transition state, gives the allylselenite ester, which upon hydrolysis gives the allylic alcohol. The (E)- orientation about the double bond, a consequence of the envelope-like transition state, is observed in the penultimate ester formation, is retained during the hydrolysis step giving the (E)-allylic alcohol product.[4]

Scopo[modifica | modifica wikitesto]

The Riley Oxidation is amenable to a variety of carbonyl and olefinic systems with a high degree of regiocontrol based on the substitution pattern of the given system.

Ketones with two available alpha methylene positions react more quickly at the least hindered position.:[1]

Allylic oxidation can be predicted by the substitution pattern on the olefin. In the case of 1,2-disubstituted olefins, reaction rates follow CH > CH¬2 > CH3:

Geminally-substiuted olefins react in the same order of reaction rates as above:[2]

Trisubstiuted alkenes experience reactivity at the more substituted end of the double bond. The order of reactivity follows that CH2 > CH¬3 > CH:

Due to the rearrangement of the double bond, terminal olefins tend to give primary allylic alcohols:

Cyclic alkenes prefer to undergo allylic oxidation within the ring, rather than the allylic position at the sidechain. In bridged ring systems, Bredt’s rule is followed and bridgehead positions are not oxidized:

Applicazioni[modifica | modifica wikitesto]

In their landmark synthesis of strychnine, R.B. Woodward and co-workers leveraged the Riley Oxidation to attain the trans-glyoxal. Epimerization of the alpha hydrogen led to cis-glyoxal, which spontaneously underwent cyclization with the secondary amine to yield dehydrostryninone.[8]

Selenium-dioxide mediated oxidation was used in the synthesis of the diterpenoid ryanodol.[9]

Selenium dioxide mediated allylic oxidation to access ingenol.[10]

Note[modifica | modifica wikitesto]

  1. ^ a b (EN) Harry Lister Riley, 255. Selenium dioxide, a new oxidising agent. Part I. Its reaction with aldehydes and ketones, in Journal of the Chemical Society (Resumed), vol. 0, 1º gennaio 1932, pp. 1875–1883, DOI:10.1039/jr9320001875.
  2. ^ a b Guillemonat, Oxidation of ethylenic hydrocarbons using selenium dioxide, su chemport.cas.org, 1939.
  3. ^ Laszlo Kurti, Strategic Applications of Named Reactions in Organic Synthesis, pp. 380–381, ISBN 978-0-12-429785-2.
  4. ^ a b (EN) Trachtenberg Edward N.; Nelson Charles H.; Carver Jane R, Mechanism of selenium dioxide oxidation of olefins, in J. Chem. Soc., vol. 35, n. 5, 1970, pp. 1653–1658, DOI:10.1021/jo00830a083, ISSN 0022-3263 (WC · ACNP).
  5. ^ K. Barry Sharpless, Selenium dioxide oxidation of ketones and aldehydes. Evidence for the intermediacy of .beta.-ketoseleninic, in Journal of the American Chemical Society, vol. 98, n. 1, 1º gennaio 1976, pp. 300–301, DOI:10.1021/ja00417a083.
  6. ^ M. A. Warpehoski, Selenium dioxide oxidation of endocyclic olefins. Evidence for a dissociation-recombination pathway, in The Journal of Organic Chemistry, vol. 47, n. 15, 1º luglio 1982, pp. 2897–2900, DOI:10.1021/jo00136a017.
  7. ^ (EN) Cynthia M. Shafer, Mechanism of SeO2 promoted oxidative rearrangement of 2-substituted oxazolines to dihydrooxazinones: Isotopic labeling and kinetic studies, in Tetrahedron, vol. 52, n. 46, 1996, pp. 14475–14486, DOI:10.1016/0040-4020(96)00902-7.
  8. ^ R. B. Woodward, THE TOTAL SYNTHESIS OF STRYCHNINE, in Journal of the American Chemical Society, vol. 76, n. 18, 1º settembre 1954, pp. 4749–4751, DOI:10.1021/ja01647a088.
  9. ^ (EN) Kangway V. Chuang, A 15-step synthesis of (+)-ryanodol, in Science, vol. 353, n. 6302, 26 agosto 2016, pp. 912–915, DOI:10.1126/science.aag1028.
  10. ^ (EN) Lars Jørgensen, 14-Step Synthesis of (+)-Ingenol from (+)-3-Carene, in Science, vol. 341, n. 6148, 23 agosto 2013, pp. 878–882, DOI:10.1126/science.1241606.
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