Conjugation in Alkadienes andConjugation in Alkadienes andAllylic SystemsAllylic Systems

conjugare is a Latin verb meaning conjugare is a Latin verb meaning

"to link or yoke together" "to link or yoke together"

Isolated – p system on a single pair of adjacent atoms.

Extended – p system on a longer series of atoms. This gives extended chemical reactivity.

Classification of Allylic Systems

Conjugated:

Most stable

Requirements: Continuous π systems with adjacent “p” orbitals overlapping.

Bonding Energy: Extra bonds between p systems.

Reactivity: Reactivity differs depending on specific diene and other chemicals involved.

Types of Dienes

Continuous, overlapping p-orbitals.

Isolated:

Less stable than conjugated.

Requirements: π systems separate and are isolated by an sp3 center.

Bonding Energy: Standard bonding.

Reactivity: Same as simple alkenes.

Types of Dienes

sp3 center

Alkene p-orbital overlap.Alkene p-orbital overlap.

Cumulated:

Least stable.

Requirements: Double bonds share the sp hybridization of middle carbon.

Bonding Energy: Wrong angle, there is no overlap.

Reactivity: Same as simple alkynes.

Types of Dienes

CC C

Linear arrangement of carbons causes a nonplanar geometry.

Learning Check

Which of the following have conjugated double bonds:

A. B.

C. D.

Name Line Diagram π system Type Resonance

Propene Isolated No

1,2-propadiene Cumulated No

1,3-butadiene Conjugated Yes

1,4-pentadiene Isolated No

Dienes

CH2 C CH2

Name Line Diagram π system Type Resonance

1,3-cyclopentadiene Conjugated Yes

1,3-cyclohexadiene Conjugated Yes

1,4-cyclohexadiene Isolated No

Benzene Conjugated Yes

Dienes

sp 2sp

Bonding in Allene

sp 2

The Double Bond as a Substituent

allylic carbocation

C+C

C

allylic radical

•CC

C

conjugated diene

CCC

C

The fact that a tertiary allylic halide undergoessolvolysis (SN1) faster than a simple tertiary

alkyl halide…

Cl

CH3

CH3

CH3

relative rates: (ethanolysis, 45°C)

123 1

Allylic Carbocations Stability

CCl

CH3

CH3

CH2C CH

Provides good evidence that allylic carbocationsare more stable than other carbocations.

CH3

H2C=CH— stabilizes C+ better than does CH3—

Allylic Carbocations Stability

C C

CH3

CH3

H2C CH + +

CH3

CH3

-Must have π systems – double or triple bonds must be present.

-π electrons change positions in resonance contributors curved arrows.

-Molecular structure is composite of all the resonance contributors, with the most favorable contributing the most character.

-More resonance leads to more stability:

Resonance

C+

C+

Delocalization of electrons in the doublebond stabilizes the carbocation.

Stabilization of Allylic Carbocations

Resonance Model

CH3

CH3

H2C CH +C

CH3

CH3

H2C CH+

C

C

CH3

CH3

H2C CH ++

Allylic Free Radicals are Stabilized byElectron Delocalization

CC

C • CC

C•

Vinylic versus Allylic

CC

C

vinylic carbonsallylic

carbon

Vinylic hydrogens are attached to vinylic carbons.

Vinylic versus Allylic

CC

C

H

H

H

Allylic hydrogens are attached to allylic carbons.

Vinylic versus Allylic

CC

C

H

H

H

Vinylic versus Allylic

CC

C

X

X

X

Vinylic substituents are attached to vinylic carbons.

Vinylic versus Allylic

CC

CX

X

X

Allylic substituents are attached to allylic carbons.

Learning Check

How many allylic and vinylic hydrogens respectively are in cyclohexene?

A) 2 and 4

B) 4 and 2

C) 2 and 2

D) 6 and 2

Resonance

Molecular Orbitals

Resonance Hybrid

Allylic Carbocations

C+

C+

+ +

1/2+ 1/2+

Resonance

Molecular Orbitals

Resonance Hybrid

Allylic Radicals

Allylic Carbocations/Radicals

Either radical C’s are attacked by a radical.

Either δ+ C’s attacked by nucleophiles

Reaction Site

Delocalizing radical is stabilizing.

Delocalizing of charge is stabilizing.

Delocalized

On Terminal C’s, never on a middle C.

On Terminal C’s, never on a middle C.

Position

Resonance.Double bonds donates electron density.

Stabilization

RadicalsCarbocations

Allylic Carbocations/Radicals

Allylic bonds are often weaker and are easily

broken.

Bond Dissociation Energies

One π= 2 R groups ~ 2-propyl radical

One π= 2 R groups ~ 2-propyl cation

Stabilization

Lower energy.

Radical intermediates.

Lower energy.

Carbocation intermediates.

Intermediates

RadicalsCarbocations

+ +

A comparison of bond energies associated with radicals and allylic radicals:

Radical Bond Energies

H+ H

H

410 KJ/mol

368 KJ/mol+ H

ClCH2CHCH3

Cl

addition

500 °C

substitution

CHCH3H2C + Cl2

CHCH2ClH2C

+ HCl

Chlorination of Propene

Reaction Type: Radical Substitution.

Overall Reaction: Alkene Allyl halide and HX.

Reactivity Order: 3 > allyl > 2 > 1 > methyl

Regioselectivity: Substitution at the allylic position

due to the stability of the allylic radical

(resonance).

Stereoselectivity: None.

Requirements: Br2 or Cl2 (with light), or

N-bromosuccinimide (NBS) which can act as a source of Br2

Allylic Halogenation

N

O

O

Br

Step 1 (Initiation):

First step in radical halogenation of an allylic system is to perform homolytic cleavage of a diatomic halogen by heat or UV light.

Mechanism, Step 1

Br Br Br Br

Step 2 (Propagation):

Step 2 has two steps.

The first is the radical abstraction of H by Br

The second step adds Br to the radical and creates another Br radical.

Mechanism, Step 2

Br C

HH Br C

CBr Br BrC

Br

Step 3 (Termination):

Step 3 has three steps which ends the radical reaction. Three different products are made.

The first product forms Br2 again.

The second product forms the expected allyl bromide.

The third product is a byproduct of the two radical carbons linking together

Mechanism, Step 3

Br Br

CBr

CC

C

Br

Br Br

CC

N-Bromosuccinimide

O

O

NBr

CCl4

Br

+heat

(82-87%)

+

O

O

NH

all of the allylic hydrogens are equivalent

and

the resonance forms of allylic radicalare equivalent.

Limited Scope

Allylic halogenation is only used when:

Example

Cyclohexene satisfies both requirements.

Both resonance forms are equivalent.

H

H

H

•

H

H

H•

All allylichydrogens areequivalent.

H H

HH

Example

2-ButeneAll allylichydrogens areequivalent.

CH3CH CHCH3

Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.

But

•CH3CH CH CH2

•CH3CH CH CH2

Learning Check

Which radical would be more stable?

•CH3CH CH CH2

•CH3CH CH CH2

Example

2-ButeneAll allylichydrogens areequivalent.

CH3CH CHCH3

Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.

forms

CH3CH CH CH2 CH3CH CH CH2

Br Br

and

What is the other product formed in the reaction shown here?

A) B)

C) D)

Learning Check

Thermodynamic Factors: Corresponds to the relative stability of the products.

Kinetic Factors: Is the rate at which the product is formed.

It is possible to start off with the same material and receive two different products via different pathways.

Kinetic vs. Thermodynamic Control

Pathway 1 vs. Pathway 2

Kinetic vs. Thermodynamic Control

Reaction Coordinate

Energy

TS1

SM

P1

TS2

P2

Reaction 1 (solid) generates P1.

Faster reaction: More stable transition state.

Transition State 1 (TS1) has a lower activation barrier (ΔHact)

Product 1 (P1) is the kinetic product.

Reaction 2 (dash) generates P2.

P2 is the more stable product.

P2 has lower energy than P1

P2 is the thermodynamic product.

Increase in temperature: Average energy of the molecules increases.

Low Temperatures:

Preferred Path: Path similar to P1 on previous slide.

Reaction 1: Irreversible – it lacks the energy to go back to starting material.

Reaction 2: Is also irreversible.

Product Ratio: Is determined by the rates of formation for P1 and P2, where the rates are k1:k2

Control: Kinetic control

Control and Temperature

Intermediate Temperatures:

Preferred Path: Path similar to P1 (lower transition state), but less favored.

Reaction 1: Is reversible.

Reaction 2: Is irreversible.

Product Ratio: Dependent on time of reaction (a longer time of reaction results in more product 2 (P2).

P1 forms initially then over time goes back to starting material, then forms the more stable P2.

Major product: Depends on time of reaction

Short (time): P1

Long (time): P2

Control: Variable

Control and Temperature

High Temperatures:

Preferred Path: Path 1 is preferred, but then goes through Path 2.

Reaction 1: Is reversible.

Reaction 2: Is reversible.

Product Ratio: Dependent on equilibrium constants between P1 and P2 (K1:K2)

Major product: Depends on time of reaction, but end result is more of P2

Short (time): P1.

Long (time): P2.

Control: Variable.

Control and Temperature

Dienes can be prepared by elimination reactions of unsaturated alkyl halides and alcohols.

Elimination favors the most stable product.

Conjugated dienes major product are more stable than isolated dienes unless structure doesn’t allow.

Preparation of Conjugated Dienes

OH BrKOH

heatKHSO4

heat

1:

2:

Predict the Products

++

KOH

heat

Br2

CH2Cl2

N Br

O

O1,3-butadiene

KOH

heat

Dienes undergo electrophilic addition reactions similar to alkenes:

Isolated dienes: Double bonds react independently of one another, and therefore react like alkenes.

Cumulated dienes: React more like alkynes

Conjugated dienes: Conjugated C=C may change the reaction.

Dienes act as nucleophiles, reacting with electrophiles.

Reactions of Dienes

Nu E

Nu

EE

Nu

Three types of electrophilic addition of dienes:

Reaction with H-X:

Reaction with X2:

Reactions of Dienes

HX +

H

X

+X

H

1 42 31 42 3 1 3 42

Direct addition (1,2)

Conjugate addition (1,4)

X2 +

X

X

+X

X

1 42 31 42 3 1 3 42

Direct addition (1,2)

Conjugate addition (1,4)

Note the numbering scheme from the previous slide. The 1,2 and 1,4 addition will be discussed in detail in upcoming notes.

Third Reaction type:

Reaction with other C=C (Diels Alder):

Reactions of Dienes

+heat

Proton adds to end of diene system.

Carbocation formed is allylic.

H X

H

+

Introduction to 1,2 and 1,4 Addition

Example: H

H

H

H

H

H

HCl

Cl

HH

H

H

H

HH

?

H

H

HH

H

Cl

HH

?

HCl

Example: H

H

H

H

H

H

Cl

HH

H

H

H

HH

via: HH

H

H

H

HH

+

H

H

H

H

H

H

H X H

H

H

H

H

HH

+

Protonation of the end of

the diene unit gives an

allylic carbocation.

and: HH

H

H

H

HH

+ HH

H

H

H

HH

+

Cl–

Cl

HH

H

H

H

HH

H

HH

H

H

HH

Cl

3-Chlorocyclopentene

1,2-addition of XY

XX

YY 1,4-addition of XY

XX

YY 1,2-Addition versus 1,4-Addition

Via resonance XX

++

Two types of addition:

Direct: H-X adds directly across the ends of a C=C (1,2-addition)

Conjugate: H-X adds across the ends of a conjugated system (1,4-addition).

Distribution of product depends on conditions:

Addition of Hydrogen Halides to Dienes

81% 9%

44% 56%

BrBr+

HBr

-80 C

+ H H+

+

Br- Br-

2o 1o

20 C

Addition of Hydrogen Halides to Dienes

More substituted alkene

With secondary cationStructure

Product – more stable system

Rate of the fastest reaction and the more

stable carbocation

Control

EquilibriumRateDetermination

Reversible conditionsIrreversible conditionsReversibility

ThermodynamicKineticControl

Room TempLow TempConditions

Learning Check

Which is the product when the reaction shown is carried out under thermodynamic control?

A) B)

C) D)

Heated samples:

When heating pure samples, the major product is the direct product (1,2 addition)

PLACEHOLDER

Major product depends on conditions of the conjugate. More stable E?

Addition of Hydrogen Halides to Dienes

Reaction Type: Conjugate addition/electrophilic addition.

Overall Reaction: Diene + Dienophile (alkene) Cyclic Alkene.

Stereoselectivity: Syn and Endo or Exo addition to a cyclic compound.

Requirements: Diene + Dienophile, high temp or EDG on diene/EWG on dienophile.

Diels-Alder Reaction

W

Z

B

DC

AX

Y

W X

Y Z

A

BC

D

+ E

W X

Y Z

B

AD

C

+ E+

Mechanism

Concerted process: Happens all at once. This makes the reaction very regio- and stereoselective.

Thermodynamically favorable: 2 pi-bonds 2 new stronger sigma bonds.

heat

Aromatic like transition state.

Simple Diels Alder Examples:

1,3-butadiene + ethene cyclohexene

Diels-Alder Reaction

diene alkenediene dienophile

1,3-butadiene + ethyne 1,4-cyclohexadiene

diene dienophile diene alkyneheat

heat

Diels-Alder Reaction Forms

PLACEHOLDER SLIDE

Diels-Alder Reaction Forms

PLACEHOLDER SLIDE

Learning Check

Which diene is shown in its s-cis conformation?

A) B)

C) D)

Diels-Alder Reactivity

The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.

Typical EWGs

C O

C N

C C

EWG

Diels-Alder Reactivity

The most reactive dienes have an electron-donating group (EDG) directly attached to nucleophilic diene.

Typical EDGs

EDG -NH2

-OH

Effect of Electron Donor/Acceptors

A molecular orbital look at the effect of electron donor/acceptors

HOMO

Orbital energy

Better Acceptor Groups

Diene Dieneophile

LUMO

Better Donor Groups

+

solvent 100°C

H2C CHCH CH2 H2C CH CH

O

(100%)

CH

O

Example

CH

Ovia:

+

solvent 100°C

O

O

O

Example

CHC CH2H2C

CH3

(100%)

H3C

O

O

O

H3C

via:O

O

O

Example Diels-Alder Questions

1. Rank the relative reactivity towards 1,3-cyclopentadiene of the following:

CH3

CO2Me OO

#1#2#3

Reason: EWG (C=O) provide resonance, thereby stabilizing the reaction.

iiiiiiiiiiii

Example Diels-Alder Questions

2. Rank the relative reactivity towards dimethyl cis-butendioate of the following:

#2#1#3

Reason: II is locked in the s-cis conformation, III can change to the s-cis, but I is locked in the s-trans conformation.

O

O

O

O

CH3

CH3

ii iiii iiiiii

Example Diels-Alder Questions

3. Rank the order of the relative reactivity towards 3-buten-2-one of the following

#2#1#3

Reason: Benzene is not a diene and would lose aromaticity (a bad thing), so is unreactive, III is sterically destabilized by the reactive s-cis conformation, making is less favorable than II.

CH3CH2

O

iii iii

Common Diels-Alder Reactants

Common Dienes:

Common Dienophiles:O

CO2Me

CO2Me

O

O

O

O

O

CN CO2Me

MeO2C

CO2Me

CO2Me

Two different conformations are possible:Endo: Dienophile is ‘under’ diene. Kinetic product.Exo: Dienophile is exposed or out. Thermodynamic product.

Endo conformations are generally the major product with exo being a minor product.

Reactions with Cyclic Dienes

O

O

OO

O

O+

O

O

O

+

Reactions with Cyclic Dienes

Secondary overlap favors the endo transition state.

H

OR

C

H

OR

C

H

O

H

O

R

R

ENDO EXO

Secondary interactions

New bonds

Diels-Alder: Both the diene and the dienophile are syn.

Cis-dienophile: cis substituted product.

Trans-dienophile: trans substituted product.

Both diene and alkene are Z both on the same side of the product.

Dienes and alkene are E and Z Are on opposite side of the product.

Diels-Alder Reaction is Stereospecific*

*A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.

+H2C CHCH CH2

Example

C C

C6H5 COH

H H

O

only product

H

C6H5

H

COH

O

+H2C CHCH CH2

Example

C C

C6H5

COHH

H

O

only product

H

C6H5

H

COH

O

Diels-Alder Reaction is Stereospecific Examples

+

+

+

+2

CO Me

2CO Me2

CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

2CO Me

Cis dienophile

Trans dienophile

Both Z on diene

E and Z on diene

Diels-Alder Reaction is Stereospecific Examples

+

Product has the two ester groups TRANS – Dienophile has to be TRANS.

Predict the reactants:

Regiochemistry

Determined by the position of the electron donating group (EDG) on the diene.

Common EDG groups include ethers, amines, sulfides (the nonbonding electron pair).

Resonance structures help explain why this occurs.

CH3O -CH3O

CH3O

H

O

H

O

H

O

Regiochemistry

Determined by the position of the electron donating group (EDG) on the diene.

Common EDG groups include ethers, amines, sulfides (the nonbonding electron pair).

Resonance structures help explain why this occurs.

CH3O

HO

+CH

3O

H

O

Example Problems

1. What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene?

Resonance:

H2N

e- rich

H2N

O

e- poor

O O

Example Problems

1. What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene?

H2N

O

+

H2N

O

Major product

H2N

ONone

+