Chapter 5 Dienes and Conjugated Systems5.1 Class and Nomenclature of Dienes5.2 Structure of Conjugated Dienes Molecular obitals5.3 Allylic Carbocations Hyperconjugation σ-p Hyperconjugation σ-πHyperconjugation5.4 Reaction of conjugated Dienes5.4.1 Electrophilic Attack on Conjugated Dienes: 1,4-addition Mechanism of conjugate addition

Kinetic versus thermodynamic control of reactions5.5 The Diels-Alde Cycloaddition Reaction

5.1 Classes and Nomenclature of DienesAn alkadiene

(二烯烃 ):

Hydrocarbon containing twocarbon-carbon double bonds

Alkadienes

Isolated dienes 1,4-pentadiene

1,5-Cyclo-octadiene

Separated by oneor more sp3-C atom.Separated by oneor more sp3-C atom.

Conjugated dienes:1,3-butadiene

1,3-cyclo- hexadiene

Double bonds andsingle bonds alternatealong the chain.

Double bonds andsingle bonds alternatealong the chain.

Cumulated dienes

Allene(丙二烯 )The C atom is commonfor two double bondsThe C atom is commonfor two double bonds

C C C C C

C C C C

H2C C CH2

Nomenclature:

cis,cis –2,4-hexadiene

(2Z,4Z)-2,4-hexadiene顺 ,顺 -2,4-己二烯(Z,Z)- 2,4-己二烯

(2Z,4E)-2,4-hexadiene

顺 ,反 -2,4-己二烯(Z,E)-

cis,trans-

P127.4.10P127.4.10• Suffix: ne diene• Cis-trans isomers:

C CC C

CH3H3C

H

HH

HC C

C CCH3

H3C

HH

H

H

CH

H

C CC

H

HH

H

5.2 Structure of Conjugated Dienes1,3-Butadiene:

4 C atoms are sp2-hybridized.

C2-C3 σbond: sp2-sp2overlap

πbond： 2p-2p overlapC2-C3 partially overlap

by 2p-2p orbital4 πelectrons are delocalized

over 4 C atomsDelocalization of πelectronslowers the energy.

4 C atoms are coplanar

P128P128

CH

H

C CC

H

HH

H

Two possible planar conformation of 1,3-butadiene:

s-Cis conformation s-Trans conformation

Antibonding molecularorbitals(反键轨道 )

The four p orbitals combine to form a set of πmolecularobitals:

Two of the πmolecular orbitals 1,3-butadiene are bonding molecular orbitals.

Bonding molecularorbitals(成键轨道 )

Ch. P94.Ch. P94.

En

ergy

π1

π2

π3*

π4*

(+)

(-)

(+)

(-) (+)

(-)

(+)

(+)

(+)

(-)

(-)

(-)

(-)(-)

(-)

(+)

(+)

(+)

CC C C

CC C C(-)

(-)(+)

(-) (+)

CC C C

CC C C

(+)

Molecular orbitals of 1,3-butadiene

node

π2 - HOMO (The highest occupied orbital)π3* - LUMO (the lowest unoccupied orbital)

π- πConjugate system:

The alternate system by single bond and double bond.

Character of conjugate system:πelectrons delocalization .

5.3 Allylic Carbocations

Relative Order of carbocation stability:

Allylic(烯丙型 )

> 3°> 2°> 1° >VinylAllyl(烯丙基 )

>

P129, 4.11P129, 4.11

Allyl cation (烯丙基正离子 )Allylic cation(烯丙基型正离子 )

H2C CH CH2 CH3CH CH CHCH3

CH3CH CH CHCH3 > C

C

C > H2C CH CH2

C

> C

H

C >

C

C

H

C >

H

CH2 CH3

C atom with positive chargeis sp2-hybridized. p orbital is vacant.

p -π overlap.

πelectrons delocalization

p -πconjugation

Resonance hybrid

CCC C

CCδ+

δ-

(CH3)2C CH CH2 (CH3)2C CH CH2

C C C

Hyperconjugation (超共轭效应 ) Ch.P96Ch.P96

σ-p Hyperconjugation:Obital overlap here

Vacant p orbital

sp2 hybride

The order of stability of carbocations:

C-H bond attached a positive carbon delocalize the positive charge

sp3 hybrideCR

H

H

CH2

H +C CR

H

H

H

σ-πHyperconjugationC-H bond atttached a Carbon-carbon double bond

Delocalization of electrons

Orbital overlap here

The stability of alkene is increasedby enhance of density of electronic cloud

CR

H

H

CH CH2

H

C CRH

CH

H

5.4 Reaction of conjugated Dienes5.4.1 Electrophilic Attack on Conjugated Dienes: 1,4-addition

1,3-butadiene

1-Bromo-2-butane (19%)

Mechanism of the addition:3-Bromo-1-butane (81%)

Step1. 1,2-addition

1,4-addition

The formation ofallylic carbocation

(烯丙基型正离子的生成 )

H2C CH CH CH2HBr H2C CH CH CH2

HBr

+-80 C°

CH2 CH CH CH2

Br H

Secondary allylic cation

(I) Primary Allylic cation (II)

Step 2. Cation-anion combination

Stability of allylic cations:(I) > (II)

1,4-addition

1,2-addition

H2C CH CH CH2HBr H2C CH CH CH3

H2C CH CH CH3 CH2 CH CH CH3

H2C CH CH CH2

HBr

CH2 CH CH CH2

Br H

H2C CH CH CH3 + Br δ+ δ+

Kinetic versus thermodynamic controlof reactions:

ΔG1,4 ΔG1,2

En

ergy

Reaction coordinate

Active energy:ΔG1,4 > ΔG1,2

Stability of product:1,2-addition < 1,4-addition

CH2 CH CH CH3

BrCH2 CH CH CH3 Br

H2C CH CH CH3

H2C CH CH + HBrBr

CH2

CH2

H2C CH CH CH3 + CH CH CH3

Br

Br

CH2H2C CH CH CH3 + CH CH CH3

Br

(44%) (56%)

40℃-80℃

r.t

(81%) (19%)

1,2-Addition product: Rate control or kineticcontrol product

1,4-Addition product:Equilibrium control or thermodynamiccontrol product

5.5 The Diels-Alder Cycloaddition Reaction

The conjugate addition of an alkene to diene

Otto Diels and Kurt Alder(Germany) receive the

1950 Nobel Prize in chemistry

Otto Diels and Kurt Alder(Germany) receive the

1950 Nobel Prize in chemistry

Diene Dienophile Adduct(双烯体 ) (亲双烯体 ) (加成物 )

Pericyclic reaction(周环反应 )

A B

X Y

BA

XY

Diels was the scientific mentor of Kurt Alder (see portrait) and together they discovered the reaction which bears their names, the Diels-Alder (DA) reaction. For this they jointly received the 1950 Nobel Prize in Chemistry. Diels was born in Hamburg, Germany into an academic family (his father was a professor of classical philology in Berlin, and two brothers alsobecame professors). He obtained the Ph.D. (1899)at the University of Berlin with the great Emil Fischer(see portrait), after which he was appointed to the faculty there. In 1907 his textbook "Einführung in die Organische Chemie“was published; it became one of the most popular texts in its field and,by 1962, had gone through 19 editions. In 1916 Diels became director of the Institute of Chemistry at the University of Kiel, where he remained until his retirement.Besides the DA reaction, Diels is also known for having discovered carbon suboxide (C3O2) and for his early research on the structure of cholesterol, especially for the use of selenium (rather than sulfur) for dehydrogenations, and for Diels' hydrocarbon (3'-methyl-1,2-cyclopentenophenanthrene) which he obtained from cholesterol by that technique.

Otto Paul Hermann Diels1876-1954

Alder is best known for his discovery, with Otto Diels (see portrait)of the cycloaddition reaction that bears their names, the Diels-Alder (DA) reaction.For this discovery they received the 1950 Nobel Prize in Chemistry.The reaction, which involves 1,4-addition of a dienophile(examples they studied included maleicanhydride,ethyl azodicarboxylate and benzoquinone)to a conjugated diene (their examples included cyclopentadiene, acyclic 1,3-dienes, furans and anthracene) is still one of the most general methods for constructing 6-membered rings.Born in Königshütte, Germany (now in Poland), Alder took his Ph.D. degree with Diels at theUniversity of Kiel in 1926. For a time he worked in industry on polymers and synthetic rubbers, then in 1940 joined the faculty of the University of Cologne where he remained until his untimely death after a brief illness, shortly before his 56th birthday. He co-authored 173papers which included, besides the stereochemistryand mechanism of the DA reaction, studies on the retro-DAreaction, the "ene" reaction, and applicationsto natural products synthesis. In 1955 Alder signed, with 17 other Nobel laureates, a declaration calling on nations to renounce war as a means of settling disputes.

Kurt Alder 1902-1958

1,3-Butadine Maleic anhydride adduct(100%)(马来酐 )

Maleic anhydride is used to identify conjugate Dienes.Characteristic of the Diels-Alder Reaction:

1. The reaction is reversible.2. Dienophile: with electron-withdrawing group

CH2

HC

HCCH2

HC

HC

C

C

O

O

O

benzene CHCH

C

C

O

O

O

+100 C°

H2C CH C

O

H H2C CH C

O

OCH2CH3 H2C CH C N

HC C COOCH3

3. Diene: with electron-releasing groups and s-cis conformation

3-Dimethyl-1,3-butadiene Adduct (100%)

not

4. Stereospecific: Syn-addition

cis- cis-

H3C

H3CH

OH3C

H3C

H

O

+30 C°

O

+H

H

COCH3

OCOCH3

O

COCH3

OCOCH3

H

H

CH3OOC

H+

CH3

H

Problem:

?

5. Application:a. Identification to conjugate dienes. b. Cyclization by the formation of C-C bond.

Problems to Chapter 5P1464.374.464.47

Additional problems:1.Electrophilic addition of Br2 to isoprene (异戊二烯 )yields the following product mixture:

H2C CCH CH2

CH3Br2 H2C CCHBrCH2Br +

CH3

BrH2C CBrCH CH2

CH3

(3%) (21%)

+ BrCH2C CHCH2Br

CH3

76%)

Of the 1,2-additionproducts, explain why3,4-dibromo-3-methyl-1-butene(21%) predominatesover 3,4-dibromo-1-butene(3%).

2. Draw the possible products resulting from addition of 1 equivalent of HCl to 1-phenyl- 1,3-butadiene. Which would you expect to predominate, and why?

3. How would you use Diels-alder reactions to prepare the following products? Show the starting diene, dienophilie in each case and the reactions.(1) (2)

CN

H

O

O