CH 20 - Dienes, Conjugated Systems, and Pericylic Reactions

there are three types of compounds that contain two C=C

unconjugated, conjugates, cumulated

unconjugated dienes

two alkenes separated by 2 or more single bonds

conjugated dienes -

two alkenes separated by one single bond

cumulated dienes

two alkenes separated by 0 bonds

reactions of dienes - electrophilic addition of HBr

gives a mixture of two constitutional isomers

• different amounts of each with different temperatures

thermodynamic control of HBr addition

forming the more stable product

• moves slower, higher temperature

kinetic control of HBr addition

forms less stable product

• moves faster, lower temperature

kinetic control of HBr justification

proceeding through more stable transition state where + charge is concentrated on 2 allylic position rather than on 1 allylic position

pericyclic reactions occur in

single step

• one transition step, and no intermediates

pericyclic reactions involve

closed loop of orbitals

pericyclic reactions tend to be either

allowed or forbidden

allowed =

low energy transition state

forbidden =

high energy transition state

a pericylic 4+2 cycloaddition (dials-alder reaction) of a diene and dienophile forms

6-membered ring with two new C-C sigma bonds

• reaction is very regio- and stereospecific

diels-alder - diene must be in

s-cis conformation

some dienes can be in cis conformation because of

steric hinderance

• can not undergo reaction

dienophile

EWG on alkene facilitate reaction

Electron releasing/donating groups (EDG)

-CH3, -CH2CH3, CH(CH3)2, -C(CH3)3, - R , -OR, ester

electron withdrawing groups (EWG)

aldehyde, ketone, carboxyl, ester, nitro, cyano

cis/trans substituents of dienophile with

remain cis/trans

diets-alder stereochemistry

end stereochemistry is preferred orientation of bicyclic products

exo =

outside relative to the double bond

endo=

inside relative to the double bond

[3+3] sigmatrophic shifts

reactions with a sigma bond migrates across the face of pi bonds

claisen rearrangement contains

oxygène

cope rearrangment contains

only carbons