Download Chapter 7: Dienes

Dienes & Polyenes: An overview and two key reactions (Ch. 14.1-14.5)
Polyenes contain more than one double bond and are very common in natural
products (ex: carotene). Diene chemistry applies to trienes, tetraenes, etc.
The chemistry of polyenes depends on the relative positions of the C=C bonds:
CH3 – HC = CH – HC = CH – CH3
H2C = CH – CH2 – CH2 – HC = CH2
conjugated dienes have specific reactions
- cycloadditions (e.g. Diels-Alder)
- 1,4-addition
isolated dienes react just
like regular alkenes
Electrophilic additions to isolated dienes resemble those of regular alkenes.
-- Addition of H – X produces alkyl halides with Markovnikov orientation
-- Acid-catalyzed addition of water and oxymercuration produce alcohols with
Markovnikov orientation, alkoxymercuration produces ethers
-- Hydroboration produces alcohols with “anti-Markovnikov” orientation
-- Hydrogenation produces at least some saturated bonds
Stability: Based on heats of hydrogenation, the trend in relative stabilities is:
conjugated dienes
>
isolated dienes
>
cumulated dienes (allenes)
R2C=C=CR2
Why? Hybridization and orbital overlap:
-- All C = C double bonds have sp2 – sp2 overlap
-- C – C single bonds in between puts sp3 hybrids in the overlap
-- The greater the “s” character in each hybrid, the better the overlap
-- Result is greater stability in the conjugated dienes, less in the isolated.
Resonance stabilization and electron delocalization contributes to
stability of the conjugated diene, compared to non-conjugated
Nomenclature of polyenes (review):
1. Chains and rings are numbered and named in the usual way, starting the
numbering at the end closest to a C = C bond, with an ending of “diene” or
“triene”, etc.
2. Substituents are numbered accordingly.
Common names:
H2C = C = CH2
“allene”
“isoprene”
(E/Z) isomerism in dienes:
• For any dienes except those with terminal C = C bonds or identical groups
on a single C, there will be E/Z isomerism at both C = C bonds.
• The E or Z configuration at each double bond, preceded by its bond position
number, appears at the beginning of the name
Draw and name all
the possible
configurations of:
H 3C
H2
C
CH3
C
C
H
C
H
C
H
CH3
Reactions of Dienes: Two Key Types of Reaction
I. Reaction Type:
Reactivity:
Cycloaddition (The Diels-Alder Reaction)
Diene acts as e- donor and acceptor
When a conjugated diene meets a dienophile, the attraction is so strong …
O
CH
+
…that the pi-bonding electrons rearrange themselves into new bonds, joining the
diene and the dienophile together in a pericyclic, concerted reaction!
The Diels-Alder cycloaddition
 results in 2 new σ bonds, with one π bond moving to a new position
 occurs when a diene meets an alkene or alkyne, particularly one with an
electron-withdrawing group attached to the C = C
 results in formation of a new 6-membered ring of C atoms
 requires the diene to have both bonds in an s-cis configuration
 occurs spontaneously with the right dienophile
Electron withdrawing groups?
Certain groups of atoms can pull electron density toward themselves through σ bonds by the
inductive effect. Those groups having a C or N with multiple bonds to an electronegative atom
withdraw e- from dienes through resonance:
O
O
C
R
O
C
H
aldehyde
R
acid
O
C
OH
R
R
ketone
R
C
N
nitrile
N
O
nitro
This makes the reaction more favorable than with unsubstituted dienophiles
If the diene and dienophile are asymmetrical, two structural isomers form.
Example: 1-chlorobutadiene + propenal:
The Diels-Alder reaction is stereospecific: Any geometric isomers (e.g. cis/trans
or E/Z-alkenes) maintain the relative positions of substituents in the product:
Note that in the product, one additional ring is formed. If you start with a cyclic
compound:
cyclic diene + dienophile = bridged bicyclic compound
When the diene approaches the dienophile, proper overlap of pi-bonding orbitals
may favor specific stereoisomers. For example, the reaction you will perform in
the lab could produce "endo" or "exo" orientation:
But the predominant product is the endo isomer.
II. Reaction type:
Electrophilic addition to conjugated dienes
Reactivity:
When conjugated double bonds are present, the reactivity
of one C=C affects the other
Pi bonding electrons in a conjugated arrangement “resonate” between
carbons so the single bonds have some double-bond character:
..
+
H2C– CH = CH – CH2
..
H2C – HC = CH – CH2
+
H2C = CH – HC = CH2
equals “resonance hybrid”
H2C
C
H
C
H
CH2
With conjugated dienes, if excess electrophile is present it reacts with both C=C,
BUT when the electrophile is present in limited supply, it will add preferentially to
the more reactive bond (the one leading to the more stable carbocation, or
surrounded by more R groups)
Electrophilic additions to conjugated dienes form mixtures of products due to the
resonance of the allylic carbocations
Ex: CH2=CH-CH=CH2 + HBr
1,3-butadiene
H2C=CH-CH-CH3 + BrCH2-HC=CH-CH3
Br
1,2 addition
1,4 addition