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Lecture 7 Pericyclic DAMIETTA UNIVERSITY CHEM-405: PERICYCLIC REACTIONS LECTURE 7 Dr Ali El-Agamey 1 LEARNING OUTCOMES LECTURE 7 (1) Sigmatropic Reactions (2) Migration of Hydrogen [1,3] hydrogen shift [1,5] hydrogen shift [1,7] hydrogen shift Degenerate rearrangements (3) Migration of Carbon 2 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 1 thermodynamic and kinetic stabilities of the starting material and products. 3 4 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 5 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. 6 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.7 1 Homework Indicate the order of each of the sigmatropic shifts shown in the equations below. 8 Sigmatropic Rearrangements 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 9 Sigmatropic Rearrangements Sigmatropic rearrangements have cyclic transition states.3 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 10 Sigmatropic Rearrangements In the TS of sigmatropic reaction, the migrating group is bonded to both the migration source and the migration terminus. 11 Sigmatropic Rearrangements 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). 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 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 12 Sigmatropic Rearrangements Migration of Hydrogen 13 Sigmatropic Rearrangements Migration of Hydrogen 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: 14 Sigmatropic Rearrangements Migration of Hydrogen For [1,3] hydrogen shift: 15 Sigmatropic Rearrangements Migration of Hydrogen The phase relationships between the HOMOs of the two “radicals” then determine whether the reaction is suprafacial or antarafacial.2 Whether a sigmatropic reaction actually take places, depends not only on the symmetry requirements but also on the geometry of the system.1 [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 For larger pi frameworks, both supra and antara shifts should be possible on geometric grounds.1 16 Sigmatropic Rearrangements Migration of Hydrogen1 Ψ3 Ψ2 Ψ1 Ψ5 Ψ4 Ψ3 Ψ2 Ψ1 17 Sigmatropic Rearrangements Migration of Hydrogen [1,3] sigmatropic shifts of hydrogen are not known, whereas [1,5] shifts are well known. For example:1 18 Sigmatropic Rearrangements Migration of Hydrogen1 As a general rule: for [1,x] hydrogen shift, where x = 2, 3, 4, 5, 6, 7,…. When the total number of electrons = 4n + 2 Suprafacial When the total number of electrons = 4n Antarafacial 19 Sigmatropic Rearrangements Migration of Hydrogen1 For [1,7] hydrogen shift: 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). 20 21 Sigmatropic Rearrangements Migration of Hydrogen1 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 22 Sigmatropic Rearrangements 23 Sigmatropic Rearrangements Degenerate rearrangements1 On heating, 1,3-pentadiene rearranges to itself via a [1,5]-H shift. Degenerate rearrangements: It is a process in which a reactant rearranges to itself. 24 Sigmatropic Rearrangements Degenerate rearrangements1 How can we establish the degenerate rearrangements? This can be confirmed by using isotopically labeled molecules or suitably substituted molecules. 25 Sigmatropic Rearrangements Degenerate rearrangements1 26 Sigmatropic Rearrangements 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 27 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 28 Sigmatropic Rearrangements Migration of Carbon1 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 29 Sigmatropic Rearrangements 1 [1,3] alkyl shift: Migration of Carbon 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. 30 Sigmatropic Rearrangements Migration of Carbon1 [1,3] alkyl shift: 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 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 31 Sigmatropic Rearrangements Migration of Carbon1 [1,3] alkyl shift ar or si [1,5] alkyl shift ai or sr [1,7] alkyl shift 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 32 Sigmatropic Rearrangements Migration of Carbon1 These prediction have been confirmed by experiments: 33 Sigmatropic Rearrangements Migration of Carbon1 This reaction proceeds by a [1,3] migration and with complete inversion of configuration in the migrating group.1 34 The Woodward-Hoffmann rules for [1,n] sigmatropic rearrangements1 If the migrating group is hydrogen, the pathways involving inversion must be 35ignored . The Woodward-Hoffmann rules for [1,n] sigmatropic rearrangements1 (4n + 2) electrons e.g. 2 or 6 or 10 electrons (4n) 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) 36 The Woodward-Hoffmann rules for [1,n] sigmatropic rearrangements1 (4n) electrons are the opposite (4n) electrons suprafacial shift with inversion (or antarafacial shift with retention) Photochemical reactions follow the reverse of the thermal rule. 37 .
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