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A Sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one sigma bond changed to another sigma bond.[1] In this type of rearrangement reaction, a substituent moves from one part of a pi-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the pi system.

Sigmatropic rearrangements are classified by the substituent that moves and the order of the rearrangement, which is given in brackets [i,j] with i and j the number of atoms that each sigma terminus has moved. For example, in a [1,5]hydride shift, a proton moves over 5 carbon positions.

Sigmatropic Hydride Shifts
Sigmatropic Hydride Shifts

Overview: Theory of Sigmatropic Shifts edit

Woodward-Hoffman edit

Main Article: Woodward-Hoffmann rules

Frontier Molecular Orbital Theory edit

Main Articles: HOMO

Dewar-Zimmerman Method - Orbital Topology edit

Classes of Sigmatropic Rearrangements edit

[1,3] Shifts edit

Thermal Hydride Shifts edit

In a thermal [1,3] hydride shift, a hydride moves three atoms. The Woodward-Hoffman rules dictate that it would proceed in an antarafacial shift. Although such a shift is symmetry allowed allowed, the Mobius topology required in the transition state prohibits such a shift because it is geometrically impossible, which accounts for the fact that enols do not isomerizes without an acid or base catalyst[2].

 
Impossible Shift

Pasto and Brophy, however, show that alkyl substituted allenes, due to the geometry of the pi orbitals of the central sp-hybridized carbon, do undergo [1,3] hydride shifts.[3]:

 
allene exception to [1,3] hydride impossiblity

Thermal Alkyl Shifts edit

Thermal alkyl [1,3] shifts, similar to [1,3] hydride shifts, must proceed antarafacially. Here the geometry of the transition state is prohibitive, but an alkyl group (due to the nature of its orbitals) can invert (form a new bond with the back lobe of its sp3 orbital picture) its geometry and therefore proceed via a suprafacial shift [2]. These reactions are still not common in open chain systems[2] because of the highly ordered nature of the transition state, which is more readily achieved in cyclic molecules.

 
[1,3] Alkyl Shifts

Photochemical [1,3] Shifts edit

Photochemical [1,3] shifts should show proceed through suprafacial shifts, however, most show non-concertedness because they proceed through a triplet state, i.e.: have a diradical mechanism [2], and therefore the Woodward-Hoffmann rules do not apply.

[1,5] Shifts edit

Hydride Shifts edit

A [1,5] shift involves the shift of 1 substituent (-H, -R or -Ar) down 5 atoms of a pi system. Hydrogen has been shown to shift in both cyclic and open chain systems at temperatures at or above 200˚C[2]. These reactions are predicted to proceed suprafacially, via a Huckel-topology transition state.

 
[1,5] Hydride shift in a cyclic system

Photoirradiation would require an antarafacial shift of hydrogen. Although rare, there are examples where antarafacial shifts are favored[4]:

 
Antarafacial [1,5] Hydride Shift

In Contrast to hydrogen [1,5] shifts, there have never been any observed [1,5] alky shifts in an open-chain system[2]. Several studies have been done and determined preferences for [1,5] shifts[5][6][7]: carbonyl and carboxyl> hydride> phenyl and vinyl>> alkyl.

Alkyl groups undergo [1,5] shifts very poorly, usually requiring high temperatures, however, on cyclohexadienes, the temperature for alkyl shifts isn’t much higher than that for carbonyls, the best migratory group. A study showed that this is because alkyl shifts on cyclohexadienes proceed through a different mechanism. First the ring opens, followed by a [1,7] shift, and then the ring reforms:

 
alkyl shift on cyclohexadiene

[1,7] Shifts edit

[1,7] sigmatropic shifts are predicted by the Woodward-Hoffmann rules to proceed in an antarafacial fashion, via a Mobius topology transition state. Antarafacial [1,7] shifts are observed in the conversion of lumisterol to vitamin D and in walk (link to below) reactions of bicyclic nonatrienes[8].

 
conversion of lumisterol to vitamin D
 
walk rearrangement of bicycle nonatriene

[3,3] Shifts edit

[3,3] sigmatropic shifts are well studied sigmatropic rearrangements. The Woodward-Hoffman rules predict that these six electron reactions would proceed suprafacially, Hückel topology transition state.

Claisen Rearrangement edit

Main article: Claisen rearrangement

The Claisen rearrangement is a powerful carbon-carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of an allyl vinyl ether will initiate a [3,3]-sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl.

 
The Claisen rearrangement

Discovered in 1912, the Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement.[9][10][11]


Cope Rearrangment edit

Main article: Cope rearrangement

The Cope rearrangement is an extensively studied organic reaction involving the [3,3]-sigmatropic rearrangement of 1,5-dienes [12][13][14][15]. It was developed by Arthur C. Cope. For example 3-methyl-1,5-hexadiene heated to 300°C yields 1,5-heptadiene.

 
The Cope rearrangement of 3-methyl-1,5-hexadiene

Carroll Rearrangement edit

Main article: Carroll rearrangement

The Carroll rearrangement is a rearrangement reaction in organic chemistry and involves the transformation of a β-keto allyl ester into a α-allyl-β-ketocarboxylic acid.[16] This organic reaction is accompanied by decarboxylation and the final product is a γ,δ-allylketone. The Carrol rearrangement is an adaptation of the Claisen rearrangement and effectively a decarboxylative Allylation.

Fischer Indole Synthesis edit

Main article: Fischer indole synthesis

The Fischer indole synthesis is a chemical reaction that produces the aromatic heterocycle indole from a (substituted) phenylhydrazine and an aldehyde or ketone under acidic conditions.[17][18] The reaction was discovered in 1883 by Hermann Emil Fischer. Today antimigraine drugs of the triptan class are often synthesized by this method.

 
The Fischer indole synthesis

The choice of acid catalyst is very important. Bronsted acids such as HCl, H2SO4, polyphosphoric acid and p-toluenesulfonic acid have been used successfully. Lewis acids such as boron trifluoride, zinc chloride, iron chloride, and aluminium chloride are also useful catalysts.

Several reviews have been published.[19][20][21]

[5,5] Shifts edit

Similar to [3,3] shifts, the Woodward-Hoffman rules predict that [5,5] sigmatropic shifts would proceed suprafacially, Hückel topology transition state. These reactions are rarer than [3,3] sigmatropic shifts, but this is mainly a function of the fact that molecules that can undergo [5,5] shifts are rare than molecules that can undergo [3,3] shifts[2].

 
[5,5] shift of phenyl pentadienyl ether

Walk Rearrangements edit

The migration of a divalent group, such as O, S, NR or CR2, which is part of a three-membered ring in a bicyclic molecule, is a commonly referred to as a walk rearrangement. This can be formally characterized according to the Woodward-Hofmann rules as being a (1, n) sigmatropic shift[22]. An example of such a rearrangement is the shift of substituents on tropilidenes (1,3,5-cycloheptatrienes). When heated, the pi-system goes through an electrocyclic ring closing to form bicycle[4,1,0]heptadiene (norcaradiene). Thereafter follows a [1,5] alkyl shift and an electrocyclic ring opening.

 
norcaradiene rearrangment

Proceeding through a [1,5] shift, the walk rearrangement of norcaradienes is expected to proceed suprafacially with a retention of stereochemistry. Experimental observations, however, show, that, “there is no significant bonding in the transition state between C7 and C2 or C6 and that the 1,5-shifts of norcaradienes are diradical processes, but do not involve any diradical minima on the potential energy scale.[23].

See also edit


References edit

  1. ↑ F.A. Carey, R.J. Sundberg, Advanced Organic Chemistry Part A ISBN 0-306-41198-9
  2. ↑ a b c d e f g Miller, Bernard. Advanced Organic Chemistry. 2nd Ed. Upper Saddle River: Pearson Prentice Hall. 2004.
  3. ↑ Pasto, Daniel J. and John E. Brophy. J. Org. Chem.,1991, 56 (14), 4554-4556
  4. ↑ .F. Kiefer and C. H. Tana. J. Chem. Soc., 1969, 91, 4478.
  5. ↑ D.J. Fields, D.W. Jones and G. Kneen, Chem. Comm 1976. 873.
  6. ↑ L.L. Miller, R. Greisinger and R.F. boyerJ. Am. Chem. Soc.1969. 91. 1578
  7. ↑ C. Manning, M.R. McClary and J.J. McCullough J. Org. Chem. 1981. 46. 919.
  8. ↑ F.G. Klaerner. Agnew. Chem. Intl. Ed. Eng., 1972, 11, 832.
  9. ↑ Claisen, L.; Ber. 1912, 45, 3157.
  10. ↑ Claisen, L.; Tietze, E.; Ber. 1925, 58, 275.
  11. ↑ Claisen, L.; Tietze, E.; Ber. 1926, 59, 2344.
  12. ↑ Arthur C. Cope; et al.; J. Am. Chem. Soc. 1940, 62, 441.
  13. ↑ Rhoads, S. J.; Raulins, N. R.; Org. React. 1975, 22, 1-252. (Review)
  14. ↑ Hill, R. K.; Comp. Org. Syn. 1991, 5, 785-826.
  15. ↑ Wilson, S. R.; Org. React. 1993, 43, 93-250. (Review)
  16. ↑ Carrol, M. F. "131. Addition of α,β-unsaturated alcohols to the active methylene group. Part I. The action of ethyl acetoacetate on linalool and geraniol". J. Chem. Soc. 1940, 704–706. doi:10.1039/JR9400000704.
  17. ↑ Fischer, E.; Jourdan, F. Ber. 1883, 16, 2241.
  18. ↑ Fischer, E.; Hess, O. Ber. 1884, 17, 559.
  19. ↑ Van Orden, R. B.; Lindwell, H. G. Chem. Rev. 1942, 30, 69-96. (Review)
  20. ↑ Robinson, B. Chem. Rev. 1963, 63, 373-401. (Review)
  21. ↑ Robinson, B. Chem. Rev. 1969, 69, 227-250. (Review)
  22. ↑ F. Jensen. J. Chem. Soc., 1989, 111 (13), 4643 – 4647.
  23. ↑ A. Kless, M. Nendel, S. Wilsey, and K.N. Houk. J. Chem. Soc., 1999, 121, 4524.
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