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Chapter 17

CHEM 267: Chapter 17 Textbook Notes

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CHEM 267
Monica Barra

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 larae E 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 i
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