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Chemical Bonding

Lecture 15: “Simple” Symmetry-Controlled Reactions

Spring 2012

6/15/2012

Symmetry-Controlled Reactions

• A chemical reaction’s overall course may depend on the symmetry characteristics of molecular orbitals.

• For orbital-symmetry to play a role, it requires: single-step reaction & at least one persistent symmetry element.

• Symmetry helps to determine whether a reaction pathway is allowed or forbidden (high Ea) and to predict product selectivity.

[TS]

Roger DeKock & Harry Gray

Roger DeKock & Harry Gray, Chemical Structure and Bonding, University Science Books, 1989

http://www.nobelprize.org/

The Reaction of Lewis acids and bases

• The frontier-orbital concept: In Lewis acid-base reactions, electron density is transferred from the HOMO of the Lewis base to the LUMO of the Lewis acid


Reactions of CO:

Proton affinity vs. ionization energy

Product geometry

Site selectivity: nucleophiles attack the atom that contributes the most to HOMO, while electrophiles attack the atom that contributes the most to LUMO.


Symmetry-Controlled Reactions

• Simple reactivity determined by HOMO/LUMO interactions between reactants:

H2 + D2 2HD Symmetry-forbidden

Na + F2 Symmetry-allowed

H2 + C2H4 ??

Pericyclic Reactions

• “Concerted” reaction: all bonding changes occur at the same time in a single step, and no intermediate is involved.

• “Pericyclic reaction”: a reaction that occurs by a concert process through a cyclic transition stste.

• Major pericyclic reactions: – Electrocyclic reactions – Cycloadditions – Sigmatropic rearrangements

Symmetry-Controlled Pericyclic Reactions

• Key questions:

• We will focus on the MO explanations of electrocyclic & cycloadditions in this lecture.

• Reference:

– B.E. Douglas & C.A. Hollingsworth, Symmetry in Bonding and Spectra, Chapter 11.

heat

heat heat

Electrocyclic Reactions

• Ring closure of polyolefins and the reverse ring opining process.

Electrocyclic Reactions

• Product dependent on the number of π electrons in the polyolefin, and whether thermally or photochemically initialized.

• Two pathways to make a new σ bond from two p orbitals:

or

Thermal product Photochemical product

C2

σ

persistent symmetry

Frontier Orbital Method for Electrocyclic Reactions

• Consider frontier orbital to predict reaction mechanism:

Allowed!!

heat stereospecificity

• Use the persistent symmetry to label orbitals and make correlation diagrams – no-crossing rule

• Electrons can not move to another orbital with a different symmetry

Correlation Diagrams

Conrotatory (C2) Disrotatory (σ)

A

S

A

S

S

S

A

A

Electron configuration conserved allowed Electron configuration not conserved forbidden

Presenter

Presentation Notes

Electrons can not move to another orbital with a different symmetry No-crossing rule!!

Photon-induced Processes Ground- and Excited-State Electronic configurations

Note that this is under the single-determinantal approximation.

Photochemical Electrocyclic Reactions

• If the reaction proceeds on the excited state (i.e. photochemically driven), then disrotation becomes the allowed pathway.

Correlation Diagrams

Conrotatory (C2) Disrotatory (σ)

A

S

A

S

S

S

A

A

Electron configuration not conserved forbidden Electron configuration conserved allowed

Cyclization of Allyls • Allyl cation: thermal symmetry-allowed

Allyl anion: thermal symmetry-forbidden

S

A

σ σ

S

A

S

S

C2: conrotatory σ: disrotatory

Symmetry in Bonding and Spectra: An Introduction Bodie E. Douglas and Charles A. Hollingsworth

Woodward-Hoffmann Rules • A thermal electrocyclic reaction involving 4n π

electrons (n=1,2,3,…) proceeds with conrotatory motion; in this case the photochemical reaction proceeds with disrotatory motion.

• A thermal electrocyclic reaction involving (4n+2) π electrons (n=1,2,3,…) proceeds with disrotatory motion; the photochemical reaction proceeds with conrotatory motion.

Cycloadditions • A cycloaddition reaction is a reaction in which two

unsaturated molecules add to one another, yielding a cyclic product

• Controlled by orbital symmetry • Major examples:

– Diels-Alder reaction (thermal) – [2+2] cycloaddition (photochemical)

Mechanism of Diels-Alder Reaction • A cycloaddition takes place when a bonding interaction occurs

between the HOMO of one reactant and the LUMO of the other • In Diels-Alder reactions, each HOMO-LUMO combination of

orbitals is with the same symmetry, resulting in stabilization of the bonding orbitals.

Stereochemistry: 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

2001 Brooks/Cole Thomson Learning

Stereochemistry: 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 Diels-Alder Reaction

• Suprafacial condition allows the Diels-Alder reaction to occur, accounting for the reaction being thermal


Diels-Alder Reaction

[4+2] cycloaddition Is thermally allowed

Lewis Acid Catalysis • Electron-withdrawing groups on the dienophile can lower the energy of

the dienophile LUMO, leading to amore favorable FMO interaction.

Potential Thermal [2+2] Cycloaddition


[2+2] Cycloadditions -- Photoactive

Ethylene dimerization requires photoexcitation

Ethylene MO Ground-state interactions

Excited-state interactions

[2+2] Cycloadditions

Thermal symmetry-forbidden

MO interaction diagram


[2+2] Cycloadditions -- Examples


[2+2] Cycloadditions – Metal Catalysis

Orientation of two olefins relative to the coordinate axes and to the metal dyz and dxz orbitals. The electron transfer from b1 → dxz to form dyz → b2 to form σ (b2) are shown. Occupied orbitals are shaded and empty orbitals are open Symmetry in Bonding and Spectra: An Introduction

Bodie E. Douglas and Charles A. Hollingsworth

If an empty metal d orbital can accept the electron pair from the π orbital of b1 symmetry and an filled metal d orbital can donate the electron pair to the π* orbital of b2 symmetry to break the π bond and form a b2 σ bond, then the [2+2] cycloaddition can become thermal symmetry-allowed.

[2+2] Cycloadditions – Metal Catalysis


d8, d10 complexes are good catalysis for olefin polymerization

Metal-Catalyzed Addition Reactions

• The reaction N2 + H2 N2H2 is symmetry forbidden, but it can be achieved by metal catalysis

• The two hydrogen atoms can be transferred to ethylene for net ethylene hydrogenation


Metal-Catalyzed Addition Reactions • Metal-catalyzed chlorination of ethylene


Orbitals involved in the atom transfer for the chlorination of C2H4 by PbCl4

Summary on Symmetry-Controlled Reactions

• Many chemical reactions are controlled by orbital symmetries. • Symmetry rules can help to determine mechanism and

stereospecificity of reactions. – Symmetry of frontier orbital (LUMO/HOMO) interactions – Correlation diagram approach

• For Pericyclic reactions, the thermal symmetry-selection rule is often opposite to the photochemical symmetry-selection rule.

• Here, we only provided a “simplified” introduction to the symmetry rules (Nobel Prize 1981: Kenichi Fukui & Roald Hoffmann).




© B.E. Douglas & C.A. Hollingsworth, Symmetry in Bonding and Spectra, Chapter 11


©

B.E. Douglas & C.A. Hollingsworth, Symmetry in Bonding and Spectra, Chapter 11






Yun-Chen Chien


© B.E. Douglas & C.A. Hollingsworth, Symmetry in Bonding and Spectra


Yun-Chen Chien


©

B.E. Douglas & C.A. Hollingsworth, Symmetry in Bonding and Spectra






©


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