lecture 19 21

7/30/2019 Lecture 19 21

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10/11/2010 – Lecture 19 Huckel Molecular Orbital Theory

Conjugated Dienes, Hückel Molecular Orbitals and Electrophilic addition to conjugated dienes

• Polyene systems that have alternating single and double bonds are called conjugated.

•

CH2=CH-CH=CH-CH3 is conjugated: the double and single bonds alternate allowing forπ-orbital overlap.

• CH2=CH-CH2-CH=CH2 is not conjugated: the CH2 group interposed between the double

bonds prevents π-orbital overlap.

• The π-orbital overlap results in a slight increase in stability referred to as Resonance

Stabilization

• To better understand the effects of resonance and also the reactions of conjugatedsystems we will introduce a simplified molecular orbital description based on the work of

the German mathematician and physicist, Erich Hückel.

Hückel Molecular Orbitals

• In the Hückel molecular orbital treatment only the π-bonding is considered. The

underlying σ-framework is ignored.• For each atom involved in the π-bonding a p-orbital is assigned.

• There are as many π-orbitals as there are p-orbital components: a system with two p-orbitals (such as a simple enes) has two π-orbitals; a system with three p-orbitals (such as

a allylic cations, enolate or carboxylate anions) has three π orbitals; a system with four p-

orbitals (conjugated dienes or conjugated enones) has four π-orbitals, and so on.

• The energy levels of the orbitals increases with the introduction of nodes – changes in thesign (+/-) or phase of the orbitals.

• Depending upon the position of the nodes and the signs (or phases) of the orbitals, the π-

orbitals are designated as: bonding; non-bonding; or anti-bonding.

Hückel MO’s for a simple ene

• For a simple ene there are two sp2 carbon atoms and each one has a p-orbital.

• These are combined to make two π-orbitals: π-bonding (p1 + p2), and π*-antibonding (p1- p2).

• For Hückel MO’s it is more convenient to just draw pictures:

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Hückel MO’s for a three p-orbital system – the allyl system

• The allyl system is one of the more important systems: as it describes allyls, enols andenolates, enamines, amides, and carboxylates.

Reactions involving allylic cations

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Reactions involving allylic radicals

Reactions involving allylic type anions

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Hückel MO’s for conjugated dienes and enones

Hückel MO’s for pentadienyl systems

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For Electrophilic Aromatic Substitution the Wieland intermediate is a pentadienyl cation.

For β-dicarbonyl enolates the intermediate is a “pentadienyl” anion.

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The “pentadienyl” anion is also the Meisenheimer intermediate in Nucleophilic Aromatic

Substitution

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10/13/2010 – Lecture 20 Diels Alder - [4]+[2] cycloaddition

1) The Diels-Alder cycloaddition reaction is classified as a pericyclic reaction.

a) Pericyclic reactions take place by the concerted cyclic redistribution of bonding electrons.

b) In pericyclic reactions neither ions nor radicals are involved as intermediates, however,

many pericyclic reactions are affected by resonance and inductive effects present in thereacting partners.

c) The basic Diels-Alder Reaction

d) The formation of the Diels-Alder adduct is an exothermic reaction: three pi bonds lost,

two sigma bonds and one pi bond gained. At high temperatures, entropic contributionsbecome more significant and retro Diels-Alder reactions are favored.

G = H - T Se) However, many cycloaddition reactions require moderate heating to overcome the

activation energy. So a cycloaddition may require heating to make the reaction "go," but

if it is heated too much the equilibrium will favor retrocycloaddition.2) The compound cyclopentadiene slowly undergoes cycloaddition with itself: one molecule of

cyclopentadiene acts as a 4 pi-electron diene and the other as a 2 pi-electron dieneophile. The

product is a Diels-Alder "adduct" called dicyclopentadiene. This dimeric material can be

cracked back to cyclopentadiene by heating at 150°C for an hour and then distilling off thediene monomer

3) In the Diels-Alder reaction there are two reaction components: a DIENE (must be a

conjugated diene); and a DIENOPHILE (an ene usually with electron withdrawing

substituents).

a) The DIENE Component is usually the electron rich component in the reaction.b) The diene component must be able to achieve a “cisoid” conformation in order to react.

This is called the s-cis conformer – the s is for the single bondc) The DIENOPHILE Component is usually the electron poor component in the reaction.

i) Typical dienophiles are enes that have electron withdrawing groups:

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4) Dienophile without EWG are slower to react. a) Consider the following reactions. In each reaction, the dieneophile differs in substituent:

Acrylaldehyde contains an aldehyde electron withdrawing substituent; ethylene has no

substituents; and methoxyethylene contains and Electron donating group. In terms of reactivity,

notice the reaction conditions and reaction yields below:

i) These differences may be attributed to the energy gap between the diene HOMO and

the dienophile LUMO – electron withdrawing groups on the dienophile reduces the

energy of the LUMO and thereby reduces the energy difference between the reacting

partners. Experimentally, the energy of the HOMO is the negative of the ionizationpotential of the molecule; the energy of the LUMO is the electron affinity of the

molecule

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b) Stereochemistry is maintained in the Diels Alder reaction. i) Consider the dienophile

ii) at the diene, trans/trans configurations will undergo Diels Alder as well as some trans/cis

arrangements; however, cis/cis dienes are usually too sterically hindered to achieve the s-cis

conformation necessary for Diels-Alder reactions

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c) Diels Alder reactions are good ways for forming bicyclic systems (1) concept of endo and exo

5) Orbital Correlations for the Diels-Alder Reaction. Use the Huckel MOs to map toethylene (dieneophile) and butadiene (diene)

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a) More advanced correlations for acrolein and butadiene

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10/15/2010 – Lecture 21 Diels Alder - [4]+[2] cycloaddition Part II

1) The Diels-Alder reaction proceeds stereospecifically with respect to both component reactants. A

stereospecific reaction is one in which one stereoisomer of a reactant gives exclusively onestereoisomer in the product, and in which another stereoisomer of the reactant gives a different

stereoisomer of the product. A good example of a stereospecific reaction is the SN2 substitution reaction:

because it proceeds with inversion of configuration, one enantiomer of the reactant gives only one

enantiomer of the product. A stereoselective reaction is one in which one stereoisomer of the reactant

gives predominantly, but not exclusively, one stereoisomer of the product.

a) When the Diels-Alder reaction between ( E , E )-2,4-heptadiene and maleic anhydride is actually

carried out, only two diastereomeric racemates are isolated, and one (designated as the endo

isomer) is formed in great preponderance over the other (designated as the exo isomer). The

observed stereochemistry of these two products can be rationalized only in terms of a suprafacial

addition with respect to both participating reactants. Consequently, the stereochemistry of the

reactants is mirrored in the products: groups which are trans to each other in the dienophile are

trans to each other in the product, and groups that are trans to the center C-C bond of the diene

are cis to each other in the product. The difference between the endo and exo products arises

from the different relative orientations of the diene and the dienophile as they approach eachother to establish the initial HOMO-LUMO overlap

b) The terms endo and exo were first used by Bredt in his work with the bicyclo[2.2.1]heptane(norbornane) ring system. This ring system has both a convex and a concave surface. The

convex surface, from which all substituents project out from the surface, is called the exo surface.

The concave surface, from which all substituents project into the cavity, is called the endo surface

c) The endo product is favored in Diels Alder reaction due to secondary orbital interactions between

the carbonyl group of the dienophile (colored blue, below) and the newly forming bond

(colored red below). Only an endo approach allows the orbitals on the carbonyl to interact with

this new system, thereby reducing the energy of the endo transition state.

O

O

O

H

H

CH3

H

CH2

CH3

H

endo

CH3

H

CH2

CH3

H

O

H

H

O

O

exo

end

ex

convex

concave

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i. For bridged structures, endo and exo is somewhat easy to visualize. For example

ii. For non-bridged partners, it is more difficult to see the endo products. Here the

hydrogen atoms on the diene replace the bridge from the previous picture, so they will

point “up” like the bridge. With the hydrogen atoms pointing up, the substituents must

point down in the endo transition state.

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2) Secondary orbital interactions favor the “endo” transition state in which the electronwithdrawing group of the dienophile comes in on the side of the diene rather than away from

it, however, in some cases you may have multiple endo products

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i) Consider the following reaction

ii) How to predict which endo product is favored? The relative orientations of

substituents on the diene and the dienophile can be predicted based upon

stereoelectronic contributions in the transition state.iii) Consider the individual electronic contributions from the diene and the dieneophile.

Consider either bond polarization or resonance – it doesn’t matter. Any electron

withdrawing group on the dieneophile (such as an aldehyde) will polarize the attached

carbon - whereas any electron donating group (such as the methoxy) will polarize

the attached carbon +

iv) Next, consider the two possible endo transition states:

(1) Note that the favored transition state aligns opposite partial charges. By orienting

the + and - charges in the major resonance contributors the major products can

be predicted

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3) Summary

a. stereochemistry at the diene and the dienophile:

b. bicyclic endo / exo (Facial selectivity)

c. regioselectivity - acyclic endo / exo

d. enatoselectivity. Cycloaddition products are chiral if they do not contain anyelements of symmetry including axes of symmetry. Without asymmetric

induction, both enantiomers are formed in equal parts

4) Scope

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5) Practice Problems

a)

b)

c)

d)

type

type

type

type

Diels-Alder Reaction

O

O

OO O

O

H

H

CN

Cl

H3C H

ClCN


CO2Me

Diels-Alder ReactionCH3

CH3

H CH3

CO2Me

H

O OO

O

(singlet state)

CH2Cl2


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