CLAISEN R.......REACTION AND MECHANISM

A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecularprocess.^[1] The name sigmatropic is the result of a compounding of the long-established sigma designation from single carbon–carbon bonds and the Greek word tropos, meaning turn. In this type of rearrangement reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. True sigmatropic reactions are usually uncatalyzed, although Lewis acid catalysis is possible. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangement, Claisen rearrangement,Carroll rearrangement and the Fischer indole synthesis.









 3,3-sigmatropic reaarangment after that decarboxylation




The mecahnism involves the abstraction of proton from the active methylene group of the keto-ester followed by intramolecular Michael addition and decarboxylation to yield the unsaturated ketone compound

Firstly the ketoester tautomerize to give a enol ester and then a [3,3]-sigmatropic rearrangement occur (or simply called Claisen Rearrangement). After that, the substrate become a keto-acid and it undergoes decarboxylation under heat.

Claisen rearrangement

Main article: Claisen rearrangement

Discovered in 1912 by Rainer Ludwig Claisen, the Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement.^[10][11][12] This rearrangement is a useful carbon-carbon bond-forming reaction. An example of Claisen rearrangement is the [3,3] rearrangement of an allyl vinyl ether, which upon heating yields a γ,δ-unsaturated carbonyl. The formation of a carbonyl group makes this reaction, unlike other sigmatropic rearrangements, inherently irreversible.

Aromatic Claisen rearrangement

The ortho-Claisen rearrangement involves the [3,3] shift of an allyl phenyl ether to an intermediate which quickly tautomerizes to an ortho-substituted phenol.

When both the ortho positions on the benzene ring are blocked, a second ortho-Claisen rearrangement will occur. This para-Claisen rearrangement ends with the tautomerization to a tri-substituted phenol.

Cope rearrangement

Main article: Cope rearrangement

The Cope rearrangement is an extensively studied organic reaction involving the [3,3] sigmatropic rearrangement of 1,5-dienes.^[13][14][15] It was developed by Arthur C. Cope. For example 3,4-dimethyl-1,5-hexadiene heated to 300 °C yields 2,6-octadiene.

Oxy-Cope rearrangement

In the Oxy-Cope rearrangement, a hydroxyl group is added at C3 forming an enal or enone after Keto-enol tautomerism of the intermediate enol:^[16]

References

1. Jump up^ Carey, F.A. and R.J. Sundberg. Advanced Organic Chemistry Part A ISBN 0-306-41198-9
2. ^ Jump up to:^a b Woodward, R.B.; Hoffmann, R. The Conservation of Orbital Symmetry. Verlag Chemie Academic Press. 2004. ISBN 0-89573-109-6.
3. ^ Jump up to:^{a b c d e f g h} Miller, Bernard. Advanced Organic Chemistry. 2nd Ed. Upper Saddle River: Pearson Prentice Hall. 2004. ISBN 0-13-065588-0
4. Jump up^ Kiefer, E.F.; Tana, C.H. J. Am. Chem. Soc., 1969, 91, 4478. doi:10.1021/ja01044a027
5. Jump up^ Fields, D.J.; Jones, D.W.; Kneen, G. Chem. Comm 1976. 873 – 874. doi:10.1039/C39760000873
6. Jump up^ Miller, L.L.; Greisinger, R.; Boyer, R.F. J. Am. Chem. Soc. 1969. 91. 1578. doi:10.1021/ja01034a076
7. Jump up^ Schiess, P.; Dinkel, R. Tetrahedron Lett., 1975, 16, 29, 2503. doi:10.1016/0040-4039(75)80050-5
8. Jump up^ Carey, Francis A; Sundberg, Richard J (2000). Advanced Organic Chemistry. Part A: Structure and Mechanisms (4th ed.). New York: Kluwer Academic/Plenum. p. 625. ISBN 0-306-46242-7.
9. Jump up^ Klaerner, F.G. Agnew. Chem. Intl. Ed. Eng., 1972, 11, 832.doi:10.1002/anie.197208321
10. Jump up^ Claisen, L.; Ber. 1912, 45, 3157. doi:10.1002/cber.19120450348
11. Jump up^ Claisen, L.; Tietze, E.; Ber. 1925, 58, 275. doi:10.1002/cber.19250580207
12. Jump up^ Claisen, L.; Tietze, E.; Ber. 1926, 59, 2344. doi:10.1002/cber.19260590927
13. Jump up^ Cope, A. C.; et al. J. Am. Chem. Soc. 1940, 62, 441. doi:10.1021/ja01859a055
14. Jump up^ Hoffmann, R.; Stohrer, W. D. J. Am. Chem. Soc. 1971, 93, 25, 6941–6948. doi:10.1021/ja00754a042
15. Jump up^ Dupuis, M.; Murray, C.; Davidson, E. R. J. Am. Chem. Soc. 1991, 113, 26, 9756–9759. doi:10.1021/ja00026a007
16. Jump up^ Berson, Jerome A.; Jones, Maitland. J. Am. Chem. Soc. 1964, 86, 22, 5019–5020. doi:10.1021/ja01076a067
17. Jump up^ Carrol, M. F. J. Chem. Soc. 1940, 704–706. doi:10.1039/JR9400000704.
18. Jump up^ Fischer, E.; Jourdan, F. Ber. 1883, 16, 2241.doi:10.1002/cber.188301602141
19. Jump up^ Fischer, E.; Hess, O. Ber. 1884, 17, 559. doi:10.1002/cber.188401701155
20. Jump up^ van Orden, R. B.; Lindwell, H. G. Chem. Rev. 1942, 30, 69–96. doi:10.1021/cr60095a004
21. Jump up^ Robinson, B. Chem. Rev. 1963, 63, 373–401. doi:10.1021/cr60224a003
22. Jump up^ Robinson, B. Chem. Rev. 1969, 69, 227–250. doi:10.1021/cr60262a003
23. Jump up^ Jensen, F. J. Am. Chem. Soc., 1989, 111, 13, 4643 – 4647. doi:10.1021/ja00195a018
24. Jump up^ Klarner, F.G. Topics in Stereochemistry, 1984, 15, 1–42. ISSN 0082-500X
25. Jump up^ Kless, A.; Nendel, M.; Wilsey, S.; Houk, K. N. J. Am. Chem. Soc., 1999, 121, 4524. doi:10.1021/ja9840192

PRISMANE 棱晶烷

PRISMANE

650-42-0 cas

Tetracyclo[2.2.0.02,6.03,5]hexane

Prismane is a polycyclic hydrocarbon with the formula C₆H₆. It is an isomer of benzene, specifically a valence isomer. Prismane is far less stable than benzene. The carbon (and hydrogen) atoms of the prismane molecule are arranged in the shape of a six-atomtriangular prism. Albert Ladenburg proposed this structure for the compound now known as benzene.^[1] The compound was not synthesized until 1973.^[2]

Prismane
IUPAC name[hide] Tetracyclo[2.2.0.02,6.03,5]hexane
Identifiers
CAS number	650-42-0
ChemSpider	16736515
Jmol-3D images	Image 1
SMILES [show]
InChI [show]
Properties
Molecular formula	C6H6
Molar mass	78.11 g mol−1

History

In the mid 19th century, investigators proposed several possible structures for benzene which were consistent with its empirical formula, C₆H₆, which had been determined by combustion analysis. The first, which was proposed by Kekulé in 1867, later proved to be closest to the true structure of benzene. This structure inspired several others to propose structures that were consistent with benzene's empirical formula; for example, Ladenburg proposed prismane, Dewar proposed Dewar benzene, and Koerner and Claus proposedClaus' benzene. Some of these structures would be synthesized in the following years. Prismane, like the other proposed structures for benzene, is still often cited in the literature, because it is part of the historical struggle toward understanding the mesomeric structures and resonance of benzene. Some computational chemists still research the differences between the possible isomers of C₆H₆.^[3]

Properties

Prismane is a colourless liquid at room temperature. The deviation of the carbon-carbon bond angle from 109° to 60° in a triangle leads to a high ring strain, reminiscent of that of cyclopropane but greater. The compound is explosive, which is unusual for a hydrocarbon. Due to this ring strain, the bonds have a low bond energy and break at a low activation energy, which makes synthesis of the molecule difficult; Woodward and Hoffmann noted that prismane's thermal rearrangement to benzene is symmetry-forbidden, comparing it to "an angry tiger unable to break out of a paper cage."^[4]

The substituted derivative hexamethylprismane (in which all six hydrogens are substituted by methyl groups) has a higher stability, and was synthesized by rearrangement reactionsin 1966.^[5]

Synthesis

The synthesis starts from benzvalene (1) and 4-phenyltriazolidone, which is a strong dienophile. The reaction is a stepwise Diels-Alder like reaction, forming a carbocation as intermediate. The adduct (2) is then hydrolyzed under basic conditions and afterwards transformed into a copper(II) chloride derivative with acidic copper(II) chloride. Neutralized with a strong base, the azo compound (3) could be crystallized with 65% yield. The last step is a photolysis of the azo compound. This photolysis leads to a biradical which forms prismane (4) and nitrogen with a yield of less than 10%. The compound was isolated by preparative gas chromatography.

SYNTHESIS

MeLi, CH2Cl2, 
Et2O
-45 °C, 45 %

+

Et2O, Dioxane
0 °C to RT, 60 min, 50-60 %

KOH, 
MeOH, H2O
Reflux, 24 h

CuCl2, HCl,
H2O
65 % (2 steps)

hν, 
PhMe
30 °C, 5 h, 8 %

References	J. Am. Chem. Soc. 1973, 95, 2738. J. Am. Chem. Soc. 1971, 93, 3782.

https://www.beilstein-journals.org/bjoc/single/printArticle.htm?publicId=1860-5397-7-30

http://chemistry.stackexchange.com/questions/8898/does-benzene-have-isomers-and-resonance-structures

References

1. Ladenburg A. (1869). "Bemerkungen zur aromatischen Theorie". Chemische Berichte 2: 140–2. doi:10.1002/cber.18690020171.
2. Katz T. J., Acton N. (1973). "Synthesis of Prismane". Journal of the American Chemical Society 95 (8): 2738–2739. doi:10.1021/ja00789a084.
3.  UD Priyakumar, TC Dinadayalane, GN Sastry (2002). "A computational study of the valence isomers of benzene and their group V hetero analogs". New J. Chem. 26 (3): 347–353.doi:10.1039/b109067d.
4. R. B. Woodward and R. Hoffmann, Angew. Chem., Int. Ed. Engl., 8, 789, (1969)
5.  Lemal D. M., Lokensgard J. P. (1966). "Hexamethylprismane". Journal of the American Chemical Society 88 (24): pp 5934–5935. doi:10.1021/ja00976a046.