Sulfur Ylides

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  • Topic: Organic reaction, Chemical reaction, Rearrangement reactions
  • Pages : 24 (2417 words )
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  • Published : April 2, 2013
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SUPERVISOR
Professor R.H Duncan Lyngdoh
DEPARTMENT OF CHEMISTRY

Kamminlen S.Haokip
Roll no.: Chem-08-06
Regd. No ; 7523 of 2005-2006

CONTENTS
 INTRODUCTION
 METHODOLOGY

 RESULTS AND DISCUSSION
 CONCLUSION

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INTRODUCTION
 Pericyclic reaction
A pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions. The major classes of pericyclic reactions are: 1. Electrocyclic ring closing and ring opening reaction

2. Cycloaddition and Cycloreversion reaction
3. Sigmatropic Rearrangements
4. Chelotropic Reaction
5. Group transfer Reaction
Sigmatropic rearrangement
Sigmatropic rearrangements is a concerted intramolecular shift of an atom or a group of

atoms, where a sigma-bond is broken in the reactant and a new sigma-bond is formed in the product. Sigmatropic rearrangements

uses the terms [m + n] to indicate the atoms involved in each

component.
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[2,3] Sigmatropic Rearrangement in Sulfur Ylides
Sulfur ylides bearing an allylic group are converted on heating to unsaturated sulphides.

new bond
R1

R2

R3

C

R4

R2

R3

R1

R4
S

S

R

R
R6

R5

R5

R6

migrating bond
When the migrating group is allylic, two pathways are possible, viz; (1) Radical-pair or ion-pair mechanism, and (2) The concerted pericyclic [2, 3]-sigmatropic rearrangement. Further they can be easily differentiated as the [2,3] sigmatropic rearrangement always involves an allylic shift.

Woodward- Hoffman Rules
These predict whether a pericyclic reaction under thermal and photochemical conditions will be allowed or not and predicts the stereochemistry followed by the allowed pathway.
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Allowedness of the reaction
System
(no of electrons)

Mode of reaction

4n

Thermal

Photochemical

conrotatory

Allowed

Forbidden

4n

disrotatory

Forbidden

Allowed

4n+2

conrotatory

Forbidden

Allowed

4n+2

disrotatory

Allowed

Forbidden

AIM OF THE PROJECT
The aim of this project is to show that, of the many mechanistic pathways studied here, it is the concerted pericyclic pathway which has the lowest activation barrier and the reaction occurs in accordance with the Woodward-Hoffman’s rule.

The rearrangement reaction studied here is a thermal reaction involving 4n+2 electrons and is thus expected to undergo disrotatory motion for the groups involved if it is allowed by the Woodward-Hoffman rules.

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Reaction studied
The reaction studied is a [2,3]-sigmatropic rearrangement of a simple sulfur ylide shown by the given reaction scheme.
H

H
1

H

S
C

C
H

H

H

3

H

1C

C

2

H

H

C4

C

H
H

H

H

H

C4

S
C

C
H

H

H

Reactant

H

3

H
C2
H

Product

The reaction is a [2,3]-sigmatropic rearrangement where simultaneous bond breaking and bond making reaction takes place. The transition state shows both the processes of bond breaking and bond making and has the following general structure:

HB HA

HA
1C

H
H

S
C
H

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HB
C4

2

C
3

C
HA

new bond forming between
C1 and C4 with the simultaneous breaking of S-C2
bond.
H

HB

Transition state

6

Eight different mechanistic pathways are described below, for 8 different hydrogens on the three CH2 groups present in eight different way, capable of moving up, down or sideways in order to form the product and are given as Cases 1 to 8. (FIGURE 1). The three CH2 groups present in the reactant include :

(a) the two hydrogens on the C=C alkene terminus of the reactant, (b) the two hydrogens on the methylene group adjoining the sulfur atom of the reactant (c) the two hydrogens on the terminal carbanionic carbon atom bonded to sulfur

Figure 1 The eight rotatory...
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