Read Microsoft Word - PERICYCLIC.doc text version

Engineering Chemistry III

Prof. S. Sankararaman

Pericyclic Reactions

Definition: 1. Concerted reaction that proceed via a cyclic transition state 2. No distinct intermediates in the reaction 3. Bond forming and bond breaking steps are simultaneous but not necessarily synchronous Classification: 1. Electrocyclic ring closing and ring opening reaction 2. Cycloaddition and Cycloreversion reaction 3. Sigmatropic Rearrangements 4. Chelotropic Reaction 5. Group transfer Reaction Methods of Analyzing Pericyclic Reaction 1. Orbital symmetry correlation method (Woodward, Hoffmann, Longuet-Higgins and Abrahamson) 2. The frontier orbital method (Woodward, Hoffmann and Fukui) 3. Transition state aromaticity method (Dewar and Zimmerman) Woodward-Hoffmann Rules: Predicts the allowedness or otherwise of pericyclic reactions under thermal and photo- chemical conditions using the above methods. Therefore a basic understanding of molecular orbitals of conjugated polyene systems and their symmetry properties is essential to apply the above methods.

1

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Constructing MO diagram of polyene systems: 1. Although there are C-C and C-H sigma bonds present in the molecule, the MOs can be constructed independently of them. Although there may be a change in the hybridization of carbon atoms during the course of a pericyclic reaction, the MO levels of the sigma framework are relatively unaffected. 2. For a conjugated polyene system containing n (n = even) electrons, there will be n/2 bonding molecular orbitals that are filled MOs and n/2 antibonding MOs that are empty in the ground state electronic configuration of the molecule. 3. The lowest energy MO has zero nodes, the next higher one has one node and the second higher has two nodes and so on. The nth MO will have (n-1) nodes. 4. The nodal points are found at the most symmetric points in a MO. In other words, no MO can be symmetric as well as antisymmetric at the same time with respect to any existing molecular symmetry element. For example the 2 MO of butadiene has a node at the center of the bond connecting C2 and C3. It is incorrect to assign this node to the center of the bond connecting C1 and C2.

node

node

2 Correct

2 Incorrect

2

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Formation of MOs of butadiene from MOs of ethylene

*

4

3

1

4 ethylene butadiene ethylene

3

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

MOs of ethylene Butadiene and hexatriene

6

4

5

antibonding

4 3

bonding

3 2

2

1 1

4

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Frontier orbital method: Highest occupied MO (HOMO) ­ filled Lowest unoccupied MO (LUMO) ­ empty Analysis based on the interaction of HOMO of one Component and LUMO of the other component. If HOMO-LUMO interaction leads to bonding then the reaction is allowed. If not it is forbidden. HOMO-LUMO gap is important. The closer it is the faster the reaction.

ELECTROCYCLIC REACTIONS 1. Cyclization of an acyclic conjugated polyene system 2. The terminal carbons interact to form a sigma bond 3. Cyclic transition state involving either 4n electrons or 4n+2 electrons.

(1)

(2)

Electrocyclization of butadiene (4n) and hexatriene (4n+2)

5

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Modes of ring closing/ring opening reactions - Stereochemistry

H3C H

cis

CH3 H

conrotatory

H CH3 H

Z, E

CH3

disrotatory H

CH3

(3)

H3C

trans

H

H3C H

trans

H

conrotatory

H CH3 3C H

Z, Z

H

disrotatory H

H

(4)

CH3

H3C

cis

CH3

H H3C

CH3 H

trans

conrotatory

H3C H

E, E

CH3 H

disrotatory H

H CH3

(5)

H3C

cis

6

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Frontier orbital method for electrocyclic reactions

conrotation

2 HOMO of butadiene

bonding interaction in the TS

bonding orbital

disrotation

2 HOMO of butadiene

antibonding interaction in the TS

antibonding orbital

disrotation

3 HOMO of hexatriene

bonding interaction in the TS

bonding orbital

conrotation

3 HOMO of hexatriene

antibonding interaction in the TS

antibonding orbital

7

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

disrotation

HOMO of excited state butadiene

bonding interaction in the TS conrotation

bonding orbital

HOMO of excited state butadiene

antibonding interaction in the TS

antibonding orbital

conrotation

bonding orbital HOMO of excited state hexatriene bonding interaction in the TS

disrotation

antibonding orbital HOMO of excited state hexatriene antibonding interaction in the TS

8

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Woodward ­ Hoffmann rules for electrocyclic reactions

System (no of electrons) 4n 4n 4n+2 4n+2

Mode of reaction

Allowedness of the reaction Thermal Photochemical allowed forbidden forbidden allowed forbidden allowed allowed forbidden

conrotatory disrotatory conrotatory disrotatory

Four-membered Ring Systems: The synthesis of cyclobutene was first reported by Willstätter. The thermal ring opening of cyclobutene occurs readily at 150 oC to give 1,3-butadiene.

150 C

o

Thermal isomerization of cyclobutene to 1,3-butadiene The stereochemistry of the ring opening has been studied systematically in detail by Vogel and Criegee even before the theory of pericyclic reactions and WoodwardHoffmann rules were developed. The electrocyclic ring opening of 3,4-disubstitued cyclobutenes yield products arising from the conrotatory mode of ring opening with high stereospecificity as illusrated below.

9

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Me

Me H H Me

H

150 oC

Me Me

Me H Me

Me

only E, Z isomer

Me

Me H Me H

Me

150 oC

H Me Me H

E, E isomer not formed

Me Me

H H Me

Me Me

Me

only Z, Z isomer

Ph

COOMe Me 70 oC Me COOMe

COOMe Ph Ph Me COOMe Me

Ph

Stereochemistry of thermal electrocyclic ring opening of cyclobutenes. Thermal isomerization of the highly substituted dienes shown below takes place through the formation of the cyclobutene intermediate by a conrotatory pathway. None of the symmetry disallowed disrotatory products were formed even after 51 days at +124 oC which allowed the estimation of a lower limit of 7.3 kcal/mole of energy difference between the conrotatory and disrotatory modes of reaction.

10

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Stereoselective thermal isomerization of 1,3-butadiene derivatives.

CH3 Ph Ph Ph Ph CD3 CH3 Ph Ph CD3 Ph Ph Ph CH3 Ph CD3

Ph

Ph

Ph Ph Ph CH3 or CD3 Ph Ph Ph

CH3 Ph Ph CD3

51 days at 140 C The photochemical ring closing of butadiene and E,E and Z,E-hexa-2,4-diene has been studied by Srinivasan and the reaction follows the disroatory mode as predicted by the Woodward-Hoffmann rules.

h 253 nm

o

h

h

Photochemical electrocyclization of 1,3-butadiene derivatives.

Benzocyclobutene is another well studied 4 electron system and the electrocyclic ring opening gives a very reactive intermediate, namely ortho quinodimethane. 11

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Thermal isomerization of benzocyclobutene to ortho quinodimethane.

Stereoselective thermal isomerization of benzocyclobutene derivatives

Ph

rt CH2Cl2 Ph con meso

Ph Ph

TCNE

H CN CN CN CN Ph H CN CN CN CN

Ph Ph Ph

rt CH2Cl2 Ph con TCNE

H Ph

quantitative

racemic

Ph

Ph

H

quantitative

Examples of thermal and photochemical electrocyclic reaction of cyclohexadienehexatriene system are abundant in the literature.] According to the Woodward-Hoffmann rules this six electron system is predicted to undergo disrotatory cyclization under thermal and conrotatory ring closure under photochemical conditions. octatrienes conform to the above predictions and undergo electrocyclization as shown below. Stereospecific electrocyclic ring closure of isomeric hexa-1,3-5-trienes.

Me H H Me H

o Me 150 C dis H

Isomeric

stereospecific

150 oC dis

Me Me

Me Me

Me

12

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

The photochemical reaction proceeds by a conrotatory ring closure / opening mode and in general a photostationary state is reached consisting of an equilibrium mixture of both the hexatriene and cyclohexadiene (shown below). Stereospecific thermal ring closure of triphenylhexatrienes.

Ph Ph H H Ph Ph H

o Ph 110 C dis H

Ph

80 oC dis 92 %

Ph Ph Ph Ph Ph

110 oC

Ph

Ph

H

> 90 %

dis Ph > 90 %

Ph

H

Photochemical electrocyclic ring opening / closure of cyclohexdiene / hexatriene systems

Me H H Me Ph Me Me Ph Ph Me Me Ph Ph

h h h dis

Me Me Ph Me H H Ph Ph Me H Me H Me

13

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

CYCLOADDITION REACTIONS 1. Reaction of two components to form a cyclic compound 2. Ring forming reactions 3. Pericyclic type ­ both components are systems 4. Intramolecular and intermolecular versions Classification Based on the number of electrons involved in each component The numbers are written within a square bracket e.g. [2 + 2], [2 + 4] etc

EXAMPLES OF CYCLOADDITION REACTIONS

[2 + 2]

+

[4 + 2]

[4 + 4]

[4 + 6]

O

O

[14 + 2]

NC NC

CN CN

H NC

H CN CNCN

14

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Stereochemistry of cycloaddition reactions Suprafacial and antarafacial approaches to a bond

suprafacial approach syn addition

antarafacial approach anti addition

It is necessary to specify with respect to each component whether the approach is suprafacial or antarafacial The cycloaddition of ethylene to form cyclobutane is a [2s + 2s] process. The thermal Diels-Alder reaction is a [4s + 2s] process Frontier Orbital Method: HOMO-LUMO interaction for a [2s+2s] cycloaddition.

LUMO

LUMO

anti bonding HOMO

both bonding HOMO [ethylene]* [2s + 2s] photochemically allowed

[2s + 2s] thermally forbidden

15

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

HOMO-LUMO interaction for a [4s+2s] cycloaddition

LUMO butadiene

HOMO [butadiene]*

HOMO ethylene

anti bonding

LUMO ethylene [4s + 2s]

photochemically forbidden HOMO butadiene

LUMO ethylene

[4s + 2s] thermally allowed

Concerted [2+2] cycloaddition reactions of alkenes

*

1. Convenient way to form cyclobutanes 2. Reaction occurs from singlet excited -* state 3. Triplet sensitizer is required to form T1 state 4. Acyclic alkenes undergo competing cis-trans isomerization

16

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

5. Reaction is generally suprafacial-suprafacial additon and hence highly stereospecific Photochemical [2 + 2] Cycloadditions: The concerted photochemical [2 + 2] cycloaddition reaction is suprafacial on both of the systems. The dimerization of cis- and trans-2-butene have been reported to take place in a highly stereospecific manner. The structure of the four possible isomers are given in Scheme below. The original 2-butene fragment in the product is shown by thick lines. Only two isomers namely the cis-syn-cis (syn) and the cis-anti-cis (anti) isomer are formed when pure cis 2-butene was photolysed in the liquid state. Similarly when pure trans-2-butene was photolysed it gave only trans-anti-trans and cis-anti-cis isomers. The fourth isomer, namely cis-anti-trans, was formed only when a mixture of cis- and trans2-butene was photolysed. This experiment clearly points to the fact that the reaction is highly stereospecific and suprafacial in each of the reacting partners. Photochemical cycloaddition reactions of cis- and trans-2-butene.

h direct

+ anti / syn = 0.8

h direct

+

+

h direct

+ the above products

Chapman has reported an efficient photochemical cross addition of trans-stilbene with tetramethylethylene with high quantum yield ( = 1.0) and high stereospecificity. The inverse dependence of the rate of cycloaddition with temperature provided evidence for an exciplex formation.

17

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Photochemical cross addition of trans-stilbene and tetramethylethylene

Ph Ph

+ hv

Via exciplex, = 1.0

Ph

Ph

[2+2] Photocycloadditions of cyclic alkenes

h

+

h acetone

H H H H H H H H

+ 12 : 88

H H H

h sens = PhCOCH3

H

Synthesis of cage structures by photochemical cycloaddition

H H

h cyclohexane 62 %

H O h H acetone O O O O

O

basketene

MeO

OMe

1. h 2. H+

O

pentaprismane

18

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Diels-Alder Reaction: Thermal cycloaddition between a cisoid conjugated diene and a dienophile, usually a olefin or an acetylene Six membered ring is formed It is a concerted [4s + 2s ] addition

R1 + R2 CHR3 CHR4

* * * *

Diels-Alder Diene

s-cisoid

s-transoid

conjugated transoid dienes

transoid

cisoid

>>

>

>

Order of reactivity of cyclic conjugated dienes

19

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Dienophiles:

CN

CN CN

NC

CN

NC

NC

CN

CN

O O O

COOCH3

COOCH3

COCH3

COOCH3

O

N N N-Ph

O

O

O

The "cis" rule :

COOEt R HH O O O R HH O R COOEt R HH O O O H RH O O O R COOEt R H R COOEt R H COOEt O O R COOEt R H R H COOEt COOEt

R = Me, Ph

R = Me, Ph

20

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Me

Me H

H COOH COOH H

Me Me

COOH COOH COOH COOH

Me H Me H

COOH H COOH H

Me HOOC COOH

COOH

Me H

COOH

Alder's "endo" rule (secondary orbital interactions):

+

rt

H H

O

+

O O

rt

H HO O O

HOMO

LUMO O

O O O

21

Indian Institute of Technology Madras

Engineering Chemistry III

Prof. S. Sankararaman

Regioselectivity in Diels-Alder reaction:

R R'

+

R R'

+

R

R'

"ortho" "meta"

R

+

R'

R

R'

+

R R'

"meta"

"para"

Diels-Alder reactions are highly ortho/para selective. The regioselectivity in Diels-Alder reactions is exemplified below

R X

+

R X

R

X

R Me OAc OMe OMe OMe X COOMe COOMe COOMe CN CHO ortho 89 100 100 100 100 : : : : : meta 11 0 0 0 0

R

+

X

R

+

R X

X

R Me Me OMe OMe

X COOMe CHO COMe CHO

para 80 100 100 100

: : : : :

meta 20 0 0 0

22

Indian Institute of Technology Madras

Information

Microsoft Word - PERICYCLIC.doc

22 pages

Find more like this

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

872425


You might also be interested in

BETA
Microsoft Word - PERICYCLIC.doc