DIELS ALDER REACTION

1
DIELS- ALDER REACTION
2
The Diels-Alder Reaction
concerted
ring-forming reaction
three pairs of electrons move at one time

• diene dienophile
  cyclohexene

gain of bond order
NOTE
loss of bond order
3
THE DIENE MUST BE ABLE TO ADOPT THE S-CIS
CONFORMATION
rotate

reacts
s-trans conformation
s-cis conformation
does not react
reacts normally
4
DIELS-ALDER REACTION
HOMO
(diene)
The reaction is concerted - all of the
orbitals are aligned in a 6-ring.
2
The HOMO of the diene donates elec- trons into
the LUMO of the dienophile.
1
LUMO
(dienophile)
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BUTADIENE
ETHYLENE
p4
The HOMO of the diene donates electrons into the
LUMO of the dienophile.
p3
LUMO
HOMO
p2
p
p
p1
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EXAMPLE - WITH ELECTRONIC FACTORS
pull
push
D
Diene
Dienophile
A Cyclohexene
Works best if the dienophile has
electron-withdrawing groups, and the diene
has electron-donating groups.
The HOMO of the diene donates PUSHES electrons
into the LUMO of the dieno- phile PULLS.
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FORMATION OF A BICYCLIC COMPOUND
D
8
THE REACTION IS USUALLY STEREOSELECTIVE
endo
exo
The endo product is usually preferred.
It has been suggested that the pi systems like to
establish maximum overlap during the reaction.
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MORE DIELS-ALDER REACTIONS
D
D
10
Retrosynthesis
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ACETYLENES (ALKYNES)
If the dienophile is an alkyne a cyclohexadiene
is formed.
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CARBENES - CYCLOPROPANES
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a-ELIMINATIONS
REVIEW
( also covered in Section 9.13 )
Alkyl halides which have no b-hydrogens will
undergo a-elimination to give carbenes.
b-hydrogen
no b-hydrogens in these groups
a
a
b
a-hydrogen
E1 or E2
carbene
-
alkene
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MECHANISM
Some halides without a-hydrogen atoms
Step 1
fast
-
Step 2
-
..
slow

..
-
carbene
is a strong base such as NaOH, NaOEt, KOH, NaNH2,
or nBuLi
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CARBENES ARE ELECTRON-DEFICIENT
Carbenes are neutral, no charge, but have an
incomplete octet.
They react with alkenes (electron donors) to form
cyclopropanes.
carbene
R
R
syn addition
cis
cis
R
R
a cyclopropane
The reaction is thought to be concerted.
STEPWISE ANALYSIS OF THE CONCERTED REACTION
..

-

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SOME REACTIONS OF DICHLOROCARBENE







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SYNTHESIS OF NORCARANE
phenyl sodium
-




-
strong base


ylide
-
-
a -elimination
tetramethylammonium chloride




carbene
norcarane
(bicyclo 4.1.0 heptane
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explosive if not handled carefully
DIAZOMETHANE
(gas)
UNSTABLE
easily decomposed by light or heat
-
..
..
-




light heat
photolysis
pyrolysis

hn D



nitrogen is a very thermodynamically stable
molecule, and forms readily
nitrogen
methylene
( the specific name for this carbene )
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DIAZOMETHANE REACTIONS
pyrolysis
D

trans
trans
D

cis
cis
photolysis
hn
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CARBENOID REACTIONS
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SIMMONS-SMITH REACTION
a carbenoid reaction
Zn-Cu

Zn-Cu is a zinc-copper couple which is more
reactive than either would be alone. Copper
activates the zinc.
R
R

cis
R
R
cis
concerted
stereospecific
carbenoid reactions give a result as if a carbene
is involved, but it is not.
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WHY USE THE SIMMONS-SMITH ?
1) not hazardous like diazomethane
2) milder reaction conditions
3) doesnt require strong base like a-elimination
4) fewer side products
5) higher yields
.. what else could you want ?