CHEM267 Chapter Notes - Chapter 17: Electrocyclic Reaction, Cyclopentadiene, Pericyclic Reaction

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Chem 267 Notes Chapter 17 Notes Winter 2012
17.6 An Introduction to Pericyclic Reactions
3 major groups of pericyclic reactions: cycloaddition, electrocyclic, and sigmatrophic
Cycloaddition: +2 σ bonds, -2 π bonds
Electrocyclic: +1 σ bonds, -1 π bond
Sigmatrophic: no change in bond number
These are all concerted reactions (1 step); therefore, there are no intermediates!
17.7 Diels-Alder Reactions
In this process, a ring is formed (therefore, it is a cycloaddition reaction)
Called a [4+2] cycloaddition (2 different π systems are involved: one with 4 atoms, one with 2)
NOTE: The product of a Diels-Alder reaction is always a substituted cyclohexane!
Moderate temperatures favour the formation of products in a Diels-Alder reaction
High temperatures (>200 °C) favour the reverse Diels-Alder reaction: retro Diels-Alder reaction
NOTE: Entropy (-TΔS) will always be positive for a Diels-Alder reaction!
the starting materials for a Diels-Alder reaction are a diene and a dienophile
o when dienophile has no substituents (X), the reaction has a large Ea and is very slow
high temp. must be used (>200 °C) for reaction to proceed (yield will still be low)
o reaction will proceed faster and have a higher yield when the dienophile has an
electron-withdrawing substituent (ex. halogen, carbonyl group)
o when the dienophile is disubstituted, the reaction/product has stereospecificity
RULE: cis alkene produces cis disubstituted ring; trans alkene produces trans disubstituted ring
NOTE: the Diels-Alder reaction only occurs when the diene is in an s-cis conformation!
o Dienes that are in an s-trans conformation will result in no reaction
when cyclopentadiene is used as the starting diene, a bridged bicyclic compound is formed
2 possible cycloadducts: endo and exo
o Endo position = syn to the larger bridge
o Exo position = anti to the larger bridge
RULE: endo is favoured over exo!
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Document Summary

3 major groups of pericyclic reactions: cycloaddition, electrocyclic, and sigmatrophic. Cycloaddition: +2 bonds, -2 bonds. Electrocyclic: +1 bonds, -1 bond. These are all concerted reactions (1 step); therefore, there are no intermediates! In this process, a ring is formed (therefore, it is a cycloaddition reaction) Called a [4+2] cycloaddition (2 different systems are involved: one with 4 atoms, one with 2) Note: the product of a diels-alder reaction is always a substituted cyclohexane! Moderate temperatures favour the formation of products in a diels-alder reaction. High temperatures (>200 c) favour the reverse diels-alder reaction: retro diels-alder reaction. Rule: cis alkene produces cis disubstituted ring; trans alkene produces trans disubstituted ring. Note: the diels-alder reaction only occurs when the diene is in an s-cis conformation: dienes that are in an s-trans conformation will result in no reaction. When cyclopentadiene is used as the starting diene, a bridged bicyclic compound is formed.

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