Conjugation in Alkadienes and Allylic Systems

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:

Requirements: Continuous _____ systems with adjacent ___ orbitals overlapping.

Bonding Energy: Extra bonds between

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

Types of Dienes

Continuous, overlapping p-orbitals.

Isolated:

_________ stable than conjugated.

Requirements: ____ systems separate and are isolated by an ________ center.

Bonding Energy: _______ bonding.

Reactivity: __________simple alkenes.

Types of Dienes

sp3 center

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

Cumulated:

_________stable.

Requirements: Double bonds _____________hybridization of middle carbon.

Bonding Energy:

Reactivity: Same as simple alkynes.

Types of Dienes

CC C

Name Line Diagram π system Type Resonance

Propene

1,2-propadiene

1,3-butadiene

1,4-pentadiene

Dienes

CH2 C CH2

Name Line Diagram π system Type Resonance

1,3-cyclopentadiene

1,3-cyclohexadiene

1,4-cyclohexadiene

Benzene

Dienes

sp 2sp

Bonding in Allene

sp 2

The Double Bond as a Substituent

carbocation

C+C

C

radical

•CC

C

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)

Allylic Carbocations Stability

CCl

CH3

CH3

CH2C CH

Provides good evidence that allylic carbocationsare __________________________________.

CH3

stabilizes C+ better than does

Allylic Carbocations Stability

C C

CH3

CH3

H2C CH + +

CH3

CH3

-Must have π systems –

-__________change positions in resonance contributors shown by ______________.

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

-More resonance leads to __________ stability:

Resonance

C+

C+

Delocalization of electrons in the doublebond stabilizes the carbocation.

Stabilization of Allylic Carbocations

Resonance Model

CH3

CH3

H2C CH +C

C

CH3

CH3

H2C CH

C

CH3

CH3

H2C CH

Allylic Free Radicals are Stabilized byElectron Delocalization

CC

C • CC

C

Vinylic versus Allylic

CC

C

Vinylic versus Allylic

CC

C

H

H

H

Allylic hydrogens are attached to allylic carbons.

Vinylic versus Allylic

CC

C

Vinylic versus Allylic

CC

C

Vinylic substituents are attached to vinylic carbons.

Vinylic versus Allylic

CC

C

Allylic substituents are attached to allylic carbons.

Resonance

Molecular Orbitals

Resonance Hybrid

Allylic Carbocations

C+

C+

+ +

1/2+ 1/2+

Resonance

Molecular Orbitals

Resonance Hybrid

Allylic Radicals

Allylic Carbocations/Radicals

Either _____________ by a radical.

Either ____________ by nucleophiles

Reaction Site

_______ radical is stabilizing.

________ of charge is stabilizing.

Delocalized

On _______l C’s,

never on a ______ C.

On __________l C’s,

never on a ______ C.

Position

Double bonds ______ electron density.

Stabilization

RadicalsCarbocations

Allylic Carbocations/Radicals

Allylic bonds are often ________ and are

_______ broken.

Bond Dissociation Energies

One π= ____ R groups

~ ____-propyl radical

One π= _____ R groups ~ _____-propyl cation

Stabilization

_________ energy than alkyl

Radical intermediates.

_________ energy then alkyl

Carbocation intermediates.

Intermediates

RadicalsCarbocations

+ +

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

Radical Bond Energies

H+ H

H

KJ/mol

KJ/mol+ H

ClCH2CHCH3

Cl

500 °C

CHCH3H2C + Cl2

CHCH2ClH2C

+ HCl

Chlorination of Propene

Reaction Type:

Overall Reaction: Alkene

Reactivity Order:

Regioselectivity: Substitution at the

______position due to the stability of the ______

radical (resonance).

Stereoselectivity:

Requirements: Br2 or Cl2 with ________, 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

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+

heat+

O

O

NH

all of the allylic hydrogens are ____________

and

the resonance forms of allylic radicalare ________________.

Limited Scope

Allylic halogenation is only used when:

Example

Cyclohexene satisfies both requirements.

H

H

H

•

H

H

H•

H H

HH

Example

2-Butene CH3CH CHCH3

But

•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

Thermodynamic Factors: Corresponds to the relative

____________of the products.

Kinetic Factors: Is the ______ 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

SM Reaction 2 (dash) generates ______.

P2 is the ________ stable product.

P2 has ________ energy than P1

P2 is the _______________ product.

Increase in temperature: Average energy of the molecules increases.

Low Temperatures:

Preferred Path: Path similar to ______

(on previous slide.)

Reaction 1:

Reaction 2:

Product Ratio: Is determined by the

Control:

Control and Temperature

Intermediate Temperatures:

Preferred Path: Path similar to __________

Reaction 1:

Reaction 2:

Product Ratio: Dependent on ______________

(a ________ of reaction results in more product ____

______ forms initially then over time goes back

to starting material, then forms the ____________.

Major product: Depends on time of reaction

Short (time):

Long (time):

Control: Variable

Control and Temperature

High Temperatures:

Preferred Path: ___________ is preferred, but then goes through ___________.

Reaction 1:

Reaction 2:

Product Ratio: Dependent on _______________

between P1 and P2

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

Short (time):

Long (time):

Control:

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

Dienes undergo electrophilic addition reactions similar to alkenes:

Isolated dienes: Double bonds react ___________ one another, and therefore react like ___________.

Cumulated dienes: React more ___________

Conjugated dienes: Conjugated C=C changes the reactivity.

Dienes act as ____________, reacting with _______________.

Reactions of Dienes

Nu E

Three types of electrophilic addition of dienes:

Reaction with H-X:

Reaction with X2:

Reactions of Dienes

HX ++

X2 ++

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 ________ of diene system.

Carbocation formed is __________.

H X

H

+

Introduction to 1,2 and 1,4 Addition

Example: H

H

H

H

H

H

HCl HH

H

H

HH

?

H

HH

H

HH

?

via: HH

H

H

HH

H

H

H

H

H

H

H X HH

H

H

HH

Protonation of the end of

the diene unit gives an

________________.

and: HH

H

H

H

H HH

H

H

H

H

HH

H

H

H

H

H

HH

H

H

H

1,2-addition of XY 1,4-addition of XY 1,2-Addition versus 1,4-Addition

Via resonance

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

+

HBr

-80 C

+

20 C

Addition of Hydrogen Halides to Dienes

Structure

Control

Determination

Reversibility

Control

Room TempLow TempConditions

Reaction Type:

Overall Reaction: Diene + Dienophile (alkene)

Stereoselectivity: Syn and Endo or Exo

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

____________ process:

This makes the reaction very __________ and ____________selective.

Thermodynamically favorable:

Aromatic like transition state.

Simple Diels Alder Examples:

1,3-butadiene + ethene

Diels-Alder Reaction

1,3-butadiene + ethyne

Diels-Alder Reactivity

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

Typical E___Gs

E___G

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

Example

+

solvent 100°C

O

O

O

Example

CHC CH2H2C

CH3

Example Diels-Alder Questions

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

CH3

CO2Me OO

iiiiiiiiiiii

Example Diels-Alder Questions

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

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

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 ‘_________’ diene.

___________________ product.Exo: Dienophile is ________.

__________________ product.

______ conformations are generally the major product with _______ being a minor product.

Reactions with Cyclic Dienes

O

O

OO

O

O+

O

O

O

+

Reactions with Cyclic Dienes

____________________________favors the endo transition state.

H

OR

C

H

OR

C

H

O

H

O

R

R

Diels-Alder: Both the diene and the dienophile are _______

Cis-dienophile: __________ substituted product.

Trans-dienophile: __________ substituted product.

Both diene and alkene are Z (or E) both on the _________side of the product.

Dienes and alkene are E and Z Are on ________ 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

+H2C CHCH CH2

Example

C C

H

H

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

Diels-Alder Reaction is Stereospecific Examples

Product has the two ___________groups ___________

– Dienophile has to be _______________

Predict the reactants:

Regiochemistry

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

Common EDG groups include ethers, amines, sulfides

(Using the the nonbonding electron pair).

CH3OCH3O

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).

CH3O

HO

+

Example Problems

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

H2N H2N

O O O

Example Problems

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

H2N

O

+