1 Organic Tutorials 3 rd Year MT02 Pericyclic Reactions: There are many approaches which can rationalise the outcome of pericyclic reactions. We will look at two of these: i) the Woodward-Hofmann approach, ii) the FMO (Frontier Molecular Orbital) approach. You need to be acquainted with the Woodward-Hoffmann rules for thermal and photochemical reactions, as well as the following terms: HOMO, LUMO, suprafacial, antarafacial, conrotatory, disrotatory, correlation diagram, the endo-rule etc. There are four classes of pericyclic reactions and you need to know about all of them: i) Cycloadditions (including cheleotropic reactions), the most important of these is the Diels-Alder reaction (a [4+2] cycloaddition) ii) Electrocyclicreactions:4π,6π,8πetc. iii) Sigmatropic rearrangements and how to number them iv) Group transfer reactions (e.g. ene reaction and diimide reduction) You need to able to analyse all of the above classes of reactions using both the Woodward Hoffmann rule and the FMO method. CLEAR diagrams are a must. The best book for this is the OUP Primer by Ian Fleming, Clayden, Greeves Warren and Wothers also has good chapters on this topic, plus all the general texts.
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Organic Tutorials 3rd Year MT02 Pericyclic Reactions: There are many approaches which can rationalise the outcome of pericyclic reactions. We will look at two of these: i) the Woodward-Hofmann approach, ii) the FMO (Frontier Molecular Orbital) approach. You need to be acquainted with the Woodward-Hoffmann rules for thermal and photochemical reactions, as well as the following terms: HOMO, LUMO, suprafacial, antarafacial, conrotatory, disrotatory, correlation diagram, the endo-rule etc. There are four classes of pericyclic reactions and you need to know about all of them: i) Cycloadditions (including cheleotropic reactions), the most important of these is the Diels-Alder reaction (a [4+2] cycloaddition) ii) Electrocyclicreactions:4�,6�,8�etc. iii) Sigmatropic rearrangements and how to number them iv) Group transfer reactions (e.g. ene reaction and diimide reduction) You need to able to analyse all of the above classes of reactions using both the Woodward Hoffmann rule and the FMO method. CLEAR diagrams are a must. The best book for this is the OUP Primer by Ian Fleming, Clayden, Greeves Warren and Wothers also has good chapters on this topic, plus all the general texts.
DCHA 2704 -4-
Gen '05 3. Answer all Parts A, B, C and D.
Part A
Explain what is meant by the term pericyclic reaction.[2]
Part B
When racemic A is heated, only one of the diastereomers of B is formed.
Me
Me
A B
MeCH=CH-CH=CH-CH=CHMeheat
(i) What class of pericyclic reaction is involved in the transformation of A to B?(ii) Explain what is meant by the terms conrotatory and disrotatory.(iii) Clearly indicate which diastereomer of B will be formed and give reasons for your choice.
[6]
Part C
When racemic C is heated, only one of the diastereomers of D is formed.
Me
Me
C D
MeCH=CHCH2CH2CH=CHMeheat
(i) What class of pericyclic reaction is involved in the transformation of C to D?(ii) Explain what is meant by the terms suprafacial and antarafacial.(iii) Clearly indicate which diastereomer of D will be formed and give reasons for your choice.
[6]Part D
When E is heated in an inert solvent, F is formed.
S O
O N
S O
Oheat
E F
(i) Draw a mechanism for the formation of F.(ii) What pericyclic reactions are involved in the formation of F?
[6]
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Give mechanisms for three of the following reactions, classifying any pericyclicreactions involved. [3 x 3 ]
Part C.
Explain the following reactions, commenting on the difference between them. [5]
DCHA 2703 TURN OVER
OCN
OCN
OOEt
OO
O
O
O
OH
CO2MeCO2Me
(b)(i)
(ii) H3O+
(c) LDA, -78oC then 25oC
(d)+ heat
heat(a)
, heat
O O
O O
(a)heat
(b)
heat
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