Reactions of Conjugated Dienes:
(Same alkene reactions as those in Ch. 7)
Difference: 1,2- versus 1,4-addition reactions
Recall the addition reaction of HX to an alkene, and its mechanism:
HX
X
H
H
H X
X
X
H
Now, consider the addition of HX to 1,3-butadiene… Use the mechanism you are familiar
with to determine what will occur in the process:
HX
H
H X
X
X
2o allylic
H
1,2 addition product
H
X
1o allylic
X
H
1,4 addition product
There are two possible products that can form.
1. Which Carbocation forms fastest?
2. Which Product is most stable?
3. Which product predominates?
Data:
HBr
Temp
-78C
0C
40C
Percentages
1,2
80
71
15
Br
Br
H
1,4
20
29
85
+
H
Under cold temps: 1,2-addition is favored. Why?
Forms from fastest and quickest-to-form (lowest energy) Carbocation Intermediate
(fastest and easiest to form = Kinetic)
Under warm temps: 1,4-addition is favored. Why?
Results in the most stable product being formed (most stable product = most
substituted alkene = Thermodynamic)
H
Energy
H
Br
H
Br
H
Reaction Coordinate
Diels-Alder Reaction (1928, 1950 Nobel Prize)
O
+
O
O
Δ
O
O
100% Yield
O
Cycloaddition Reaction – Ring Formation as 2 molecules become 1
Pericyclic Reaction – takes place w/o intermediates and involves a cyclic redistribution
of bonding eOne-step cyclic transition step (CW or CCW):
Ex.
O
Δ
O
O
1
1
3
4
Ex.
Δ
6
2
6
2
3
5
5
4
CO2Et
Δ
1. The Diene: the 4 pi e- system (“e- rich species”)
Only one requirement – the diene must be able to adopt the “s-cis” conformation
s-cis
s-trans
If the molecule must rotate to become s-cis, it needs to do so before the reaction can
occur:
Any steric hindrance in the rotation, and the reaction slows down!
keep it trans....
no sterics!
keep it cis...
sterics!
Cyclic diene systems – locked in s-cis conformation – fastest to react:
Cyclopentadiene is so reactive, it reacts with itself in a Diels-Alder reaction to form
dicyclopentadiene, at room temperature.
Draw the Product:
+
2
1
3
4
6
5
Δ
160oC
2
3
1
6
H
H
5
4
this is called a
"bridging atom"
watch where this
one winds up
You cannot buy cyclopentadiene to use in a reaction. You must buy dicyclopentadiene,
heat it to 160ºC to do a RETRO (reverse) Diels-Alder reaction, catch the
cyclopentadiene in a flask cooled in an ice-water bath and use IMMEDIATELY.
So - s-cis conformation is required for the diene of this reaction. If you cannot get
into an s-cis conformation, you’ll get no reaction! The diene in the following molecule has
a diene system that is locked in two different rings. It is “s-trans” and this diene
cannot rotate. No Diels-Alder reaction can occur. (I suppose no reaction is as slow as it
gets…)
2. The Dienophile: the 2 π e- system (“e- poor species”)
• Almost always found with a pi bond in conjugation with a carbonyl functional group
(EWG)
• EWG increases reactivity with diene system – the more electron-poor, the more
attractive the dienophile will be to a diene.
Ex.
O
O
O
O
H
OEt
OEt
C N
EWG
OEt
O
Stereochemistry of Diels-Alder Reactions:
A stereospecific reaction is one that favors formation of a specific stereoisomer (recall
that hydrogenation was always a SYN addition, never ANTI while the addition of Br2
always added ANTI, never SYN?).
The Diels-Alder reaction will form racemic mixtures (50:50 mixture of enantiomers –
non-superimposable mirror images).
1.
The stereochemistry of the reacting dienophile must be maintained due to
the Syn Addition
Cis dienophiles à Syn stereochemistry of groups
Ex.
O
O
w/o stereochem
Δ
O
O
now: with steroechemistry
(either enantiomer is fine)
Trans dienophiles à Anti stereochemistry
Ex.
O
O
O
Δ
(either enantiomer is fine)
2. Endo versus Exo Approach:
Two Possibilities:
Endo Approach:
O
O
O
Exo Approach:
O
O
O
Endo is always favored (perhaps due to stability of π e- of diene interacting with π esystem of EWG), resulting in the bridging atom facing in the opposite direction (ANTI)
of the EWG.
Note: Can only “see” the endo stereochemistry when diene is in a cyclic system and you
can “see” the relationship between the bridging atom and the EWG of the dienophile.
Ex.
O
Δ
Ex.
O
Δ
Now – both issues at the same time:
Ex.
O
OEt
Ex.
Δ
O
O
Δ
O
What about this one:
CO2Et
Δ