Pericyclic Reaction. Molecular Orbitals in Conjugated Systems (Review)

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1 Pericyclic Reaction - reaction that occurs by a concerted process through a cyclic transition state - concerted means that all bonding changes occur at the same time and in a single step (no intermediates) - usually involves a conjugated diene or polyene Example: Diels-Alder Reaction [42]-π-electrons diene dienophile - two theories: 1) Woodward-offman rules, 2) frontier orbital theory Molecular rbitals in onjugated Systems (Review) 1

2 Woodward-offman Rules and Frontier rbital Theory Woodward-offman Rules - pericyclic reactions can only take place if the symmetries of the reactant molecular orbitals are the same symmetries as the product molecular orbitals (lobes of reactant M s must be of the correct algebraic sign for bonding to occur in the transition state leading to the product) Frontier rbital Theory - we need to consider only two molecular orbitals, the M and LUM, to predict the structure of the product; called frontier M s Three types of pericyclic reactions: 1) electrocyclic, 2) cycloaddition, 3) sigmatropic rearrangement Electrocyclic Reaction - an electrocyclic reaction is a pericyclic process that involves the cyclization of a conjugated polyene a conjugated triene a cyclohexadiene a conjugated diene a cyclobutene Stereochemisty of Electrocyclic Reactions (2E,4Z,6E)- octatriene 3 3 cis-5,6-dimethyl- 1,3-cyclohexadiene cis-3,4-dimethylcyclobutene (2E,4Z)-hexadiene 3 trans-5,6-dimethyl- 1,3-cyclohexadiene 3 (2E,4E)-hexadiene - outermost π lobes must rotate so that bonding is achieved - 2

3 Two Types of Rotation - according to frontier orbital theory, the stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene M Thermal Electrocyclic Reactions = (2E,4Z,6E)-octatriene cis-5,6-dimethyl-1,3-cyclohexadiene = cis-3,4-dimethylcyclobutene (2E,4Z)-hexadiene - ote: conjugated diene and triene react in opposite stereochemical senses Ground- and Excited-State Electronic onfigurations 6 5 LUM 4 LUM LUM 4 M LUM 3 M M ψ 3 Mψ 2 ψ 2 ψ 1 ψ 1 Ground state Excited state Ground state Excited state onjugated diene onjugated triene 3

4 Photochemical Electrocyclic Reactions M (excited) M (ground) 3 3 (2E,4E)-hexadiene 3 3 cis-3,4-dimethylcyclobutene M (excited) 3 M (ground) 3 3 (2E,4Z,6E)-octatriene 3 trans-5,6-dimethyl-1,3- cyclohexadiene Stereochemical Rules for Electrocyclic Reactions Electron pairs (double bonds) even number odd number Thermal Reaction Photochemical Reaction ycloaddition Reactions - a cycloaddition reaction is a reaction in which two unsaturated molecules add to one another, yielding a cyclic product - controlled by orbital symmetry - takes place when a bonding interaction occurs between the M of one reactant and the LUM of the other - major examples: 1) Diels-Alder reaction (thermal) 2) [22] cycloaddition (photochemical) 4

5 Diels-Alder Reaction [22] ycloaddition Reaction o reaction a cyclobutane - how do reactants approach each other? Suprafacial - when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant 5

6 Antarafacial - when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the opposite face of the other reactant Stereochemistry of ycloadditions - according to frontier molecular orbital theory, a cycloaddition reaction takes place when a bonding interaction occurs between the M of one reactant and the LUM of another reactant (i.e. one reactant donates electrons to the other) - electrons in the M of the first reactant are least tightly held and most likely to be donated - electrons must be donated into a vacant orbital - LUM - selection of which reactant provides the M and which reactant provides the LUM is arbitrary Stereochemistry of Diels-Alder Reaction - only conditions to allow the Diels-Alder reaction to occur, accounting for the reaction being thermal 6

7 Potential Thermal [22] ycloaddition Stereochemistry of the [22] ycloaddition Example: 2-cyclohexenone 2-methylpropene 40% 7

8 Example: Me 2 2 Me 2 2 Me head-to-tail (45%) a trans-stilbene Me 2 Me 2 head-to-head (5%) - control of reaction beyond the molecule? Supramolecular hemistry - supra means beyond, therefore supramolecular chemistry is chemistry beyond the molecule - the chemistry is controlled using noncovalent forces (e.g. hydrogen bonds, van der Waals interactions) - supramolecular chemistry is the chemistry of life (e.g. DA, enzymes) - can the [22] photodimerization be controlled supramolecularly?? 1,3-benzenediol trans-1,2-bis(4-pyridyl)ethylene Template-Directed rganic Synthesis crystal solid template reactants template product 2(1,3-benzenediol) 2(trans-1,2-bis(4-pyridyl)ethylene) - reaction proceeds in the solid state in quantitative yield (100% yield), where the diol functions as a linear template (compare to DA) - applications: - stereo- and regiocontrol of reactivity - construct molecules that cannot be made using traditional synthesis - Synthetic hemistry, Green hemistry, Materials Science 8