Lecture 7 Pericyclic -...

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LECTURE 7

Dr Ali El-Agamey

CHEM-405:

PERICYCLIC REACTIONS

DAMIETTA UNIVERSITY

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  (1) Sigmatropic Reactions

  (2) Migration of Hydrogen

[1,3] hydrogen shift



Degenerate rearrangements

  (3) Migration of Carbon

LEARNING OUTCOMES

LECTURE 7

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Pericyclic Reaction III A σ bond is broken in the reactant, a new σ bond is formed in the product, and the π bonds rearrange

  A sigmatropic rearrangement produces a new sigma bond at the expense of a sigma bond, so this reaction is the most inherently reversible of all pericyclic reactions. The position of the equilibrium depends on the relative thermodynamic and kinetic stabilities of the starting material and products.1

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Sigmatropic Reactions   Sigmatropic reactions: A concerted reaction in which a group migrates with its sigma bond within a pi framework, an ene or a polyene, and the migration is accompanied by a shift in pi bonds.1

  Since in these reactions a change in the position of one sigma bond takes place, Woodward and Hofmann coined the term “sigmatropic shifts” to describe them.2-4

  In a sigmatropic reaction, a sigma bond that breaks is bonded to an allylic carbon.2-3

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Sigmatropic Reactions1   To identify the type of a particular sigmatropic reaction, the two atoms forming the bond being broken are both numbered as atom 1.

  Then the atoms in each direction from the bond being broken, up to and including the atoms that form the new sigma bond in the product, are numbered consecutively as atoms 2,3, and so on.

  The numbers assigned to the atoms forming the new bond, separated by commas, are placed within brackets to designate the reaction type.

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Sigmatropic Rearrangements

  In the designations [1,3] and [1,5] the “3” and “5” refer to the number of the carbon to which migrating group is moved to (the migration terminus). The “1” does not refer to the migration source; instead it specifies that in both reactant and product bonding is to the same atom (number 1) in the migrating group.1

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Homework   Indicate the order of each of the sigmatropic shifts shown in the equations below.

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  It is important to note the number of electrons involved in sigmatropic rearrangements.1

  The [3,3] and [1,5] rearrangements are six-electron reactions, the cationic [1,2] rearrangement is a two-electron reaction, and [1,3] rearrangement is a four-electron reaction.1

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  For all sigmatropic rearrangements, the size of the cyclic transition state is given by the sum of the two numbers in the square brackets. For example; [3,3] and [1,5]-sigmatropic rearrangements have six-membered cyclic transition states. Also, [2,3]-sigmatropic rearrangements have five-membered cyclic transition states.1

  Sigmatropic rearrangements have cyclic transition states.3

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Sigmatropic Rearrangements   In the TS of sigmatropic reaction, the migrating group is bonded to both the migration source and the migration terminus.

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  The HOMO of an allylic radical depends on the number of carbons in the pi framework. The migrating group is passed from one end of the allylic radical to the other, and so it is the end carbons that we are concerned with.1

  In the TS, there is overlap between the HOMO of one component and the HOMO of the other. Each HOMO is singly occupied, and together they provide a pair of electrons.1

  We consider the bonding in the TS for sigmatropic reactions to arise from overlap between an orbital of an atom or free radical (G) and an orbital of an allylic free radical (the pi framework).

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Sigmatropic Rearrangements Migration of Hydrogen

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  In the TS, a three-center bond is required, and this must involve overlap between the s orbital of the hydrogen and the lobes of p orbitals of the two terminal carbons.1

For [1,5] hydrogen shift:

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  The phase relationships between the HOMOs of the two “radicals” then determine whether the reaction is suprafacial or antarafacial.2

  [1,3] and [1,5] antara shifts should be extremely difficult, since they would require the pi framework to be twisted far from the planarity that it requires for delocalization of electrons.1

  Whether a sigmatropic reaction actually take places, depends not only on the symmetry requirements but also on the geometry of the system.1

  For larger pi frameworks, both supra and antara shifts should be possible on geometric grounds.1

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Sigmatropic Rearrangements Migration of Hydrogen1

Ψ1

Ψ3

Ψ2

Ψ1

Ψ3 Ψ2

Ψ5 Ψ4

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  [1,3] sigmatropic shifts of hydrogen are not known, whereas [1,5] shifts are well known. For example:1

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  As a general rule: for [1,x] hydrogen shift, where x = 2, 3, 4, 5, 6, 7,….

Suprafacial When the total number of electrons = 4n + 2

Antarafacial When the total number of electrons = 4n

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  A thermal, concerted [1,7]-H shift is sometimes observed in acyclic systems because the 1,3,5-triene system is floppy enough to allow the H to migrate from the top face to the bottom face (making the triene the antarafacial component).


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  A very important [1,7] sigmatropic rearrangement occurs in the human body. Precalciferol (provitamin D2) is converted to ergocalciferol (vitamin D2) by a thermal [1,7] sigmatropic H shift. This shift must be antarafacial.1,2

  In cyclic compounds like cycloheptatriene, geometric constraints prevent the H from migrating from the top to the bottom face of the seven-carbon pi system, and the shift does not occur.1

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Sigmatropic Rearrangements Degenerate rearrangements1

  Degenerate rearrangements: It is a process in which a reactant rearranges to itself.

  On heating, 1,3-pentadiene rearranges to itself via a [1,5]-H shift.

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  How can we establish the degenerate rearrangements?

  This can be confirmed by using isotopically labeled molecules or suitably substituted molecules.

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Migration of Carbon1

  As compared to a hydrogen atom, which has only one lobe (1s orbital), the carbon free radical has two lobes of opposite phases (p orbital). Thus carbon can simultaneously interact with the migration source and the migration terminus using either one of its lobes or both of them.1

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Sigmatropic Rearrangements Migration of Carbon1

  Bonding through the same lobe on carbon means attachment to the same face of the atom, that is to say, retention of configuration in the migrating group.1

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  Bonding through different lobes of a p orbital- these lobes are on opposite faces of carbon- leads to an inversion of configuration in the migrating group.1

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Sigmatropic Rearrangements Migration of Carbon1 [1,3] alkyl shift:

  Suprafacial migration with retention of configuration of the migrating group would again be forbidden.

  An antarafacial migration with retention, while theoretically allowed would be very unlikely to occur.

  A suprafacial migration forming a new bond to the "back" lobe of the migrating orbital would also be theoretically allowed. This process would result in inversion of the configuration of the migrating group.

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Migration of Carbon1 [1,3] alkyl shift:

  Not surprisingly, there do not seem to be any examples of [1,3] alkyl shifts in open-chain alkenes. However, [1,3] shifts of alkyl groups can occur if the reactions involve expansions of strained three-membered or four-membered rings.1

  It would obviously require grossly distorted bonds in the TS and would be an unlikely process, although perhaps not quite so unlikely as a migration to the opposite face of the pi system.1

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Migration of Carbon1

[1,3] alkyl shift

[1,5] alkyl shift

[1,7] alkyl shift

ar or si

ai or sr

ar or si

Where s and a refer to suprafacial and antarafacial, and r and i to retention and inversion of configuration at the migrating center.2

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These prediction have been confirmed by experiments:

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  This reaction proceeds by a [1,3] migration and with complete inversion of configuration in the migrating group.1

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The Woodward-Hoffmann rules for [1,n] sigmatropic rearrangements1

  If the migrating group is hydrogen, the pathways involving inversion must be ignored.

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(4n + 2) electrons

(4n) electrons

e.g. 2 or 6 or 10 electrons

e.g. 4 or 8 or 12 electrons

(4n + 2) electrons

suprafacial shift with retention (or antarafacial shift with inversion but impossible in practice)

(4n + 2) electrons

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(4n) electrons

suprafacial shift with inversion (or antarafacial shift with retention)

  (4n) electrons are the opposite

  Photochemical reactions follow the reverse of the thermal rule.

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