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The Diels-Alder Reaction. Synthetic method for preparing compounds containing a cyclohexene ring. In general. +. conjugated diene. alkene (dienophile). cyclohexene. via. transition state. Mechanistic features. concerted mechanism [4+2] cycloaddition pericyclic reaction - PowerPoint PPT Presentation
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The Diels-Alder ReactionThe Diels-Alder Reaction
Synthetic method for preparing Synthetic method for preparing compounds containing a cyclohexene ringcompounds containing a cyclohexene ring
conjugated conjugated dienediene
alkene alkene (dienophile)(dienophile)
cyclohexenecyclohexene
++
In general...In general...
transition statetransition state
viavia
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concerted mechanismconcerted mechanism
[4+2] cycloadditioncycloaddition
pericyclic reactionpericyclic reaction
a concerted reaction that proceeds a concerted reaction that proceeds through a cyclic transition statethrough a cyclic transition state
Mechanistic featuresMechanistic features
conjugated conjugated dienediene
alkene alkene (dienophile)(dienophile)
cyclohexenecyclohexene
++
Recall the general reaction...Recall the general reaction...
The equation as written is somewhat The equation as written is somewhat misleading because ethylene is a relatively misleading because ethylene is a relatively unreactive dienophile.unreactive dienophile.
+
diene dienophile transitionstate
new bond
new bond
What makes a reactive dienophile?What makes a reactive dienophile?
The most reactive dienophiles have an The most reactive dienophiles have an electron-withdrawing group (electron-withdrawing group (EWGEWG) directly ) directly attached to the double bond.attached to the double bond.
Typical EWGs Typical EWGs
CC OO
CC NN
CC CC
EWGEWG
http://www.brunel.ac.uk/depts/chem/ch241s/re_view/barry/diels2.htm
++
benzenebenzene 100°C100°C
(100%)(100%)
HH22CC CHCHCHCH CHCH22 HH22CC CHCH CHCH
OO
CHCH
OO
ExampleExample
++
benzenebenzene 100°C100°C
(100%)(100%)
HH22CC CHCHCHCH CHCH22 HH22CC CHCH CHCH
OO
CHCH
OO
ExampleExample
CHCH
OOvia:via:
++
benzenebenzene 100°C100°C
(100%)(100%)
OO
OO
OO
ExampleExample
HH22CC CHCCHC CHCH22
CHCH33
HH33CCOO
OO
OO
++
benzenebenzene 100°C100°C
(100%)(100%)
OO
OO
OO
ExampleExample
HH22CC CHCCHC CHCH22
CHCH33
HH33CCOO
OO
OO
via:via:
HH33CCOO
OO
OO
++
benzenebenzene 100°C100°C
(98%)(98%)
HH22CC CHCHCHCH CHCH22
Acetylenic DienophileAcetylenic Dienophile
OO
CCOCHCCOCH22CHCH33CHCH33CHCH22OCCOCC
OO
COCHCOCH22CHCH33
COCHCOCH22CHCH33
OO
OO
syn addition to alkenesyn addition to alkene
cis-trans relationship of substituents on alkene cis-trans relationship of substituents on alkene retained in cyclohexene productretained in cyclohexene product
Diels-Alder Reaction is Stereospecific*Diels-Alder Reaction is Stereospecific*
*A stereospecific reaction is one in which *A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric starting materials give stereoisomeric products; characterized by stereoisomeric products; characterized by terms like syn addition, anti elimination, terms like syn addition, anti elimination, inversion of configuration, etc.inversion of configuration, etc.
Stereospecific, concerted, syn addition:
Predict the reaction products:
1. Consider the alignment of the reactants
2. Consider the charge distribution in each of the reactants
Only product.Only product.But, is it enantiomeric?But, is it enantiomeric?
++HH22CC CHCHCHCH CHCH22
ExampleExample
CC CC
CC66HH55 COHCOH
HH HH
OO
HH
CC66HH55
HH
COHCOH
OO
Only the s-cis conformation of the diene can participate in a
Diels–Alder reaction
only productonly product
++HH22CC CHCHCHCH CHCH22
ExampleExample
CC CC
CC66HH55
COHCOHHH
HH
OO
HH
CC66HH55
HH
COHCOH
OO
Cyclic dienes yield bridged bicyclicCyclic dienes yield bridged bicyclicDiels-Alder adducts.Diels-Alder adducts.
Since only cis dienes can participate in Diels–Alder reactions: 5- & 6- membered rings are ideal
++
CC CC
COCHCOCH33HH
HH
OO
CHCH33OCOC
OO HH
HH
COCHCOCH33
OO
COCHCOCH33
OO
is theis thesame assame as
HH
HH
COCHCOCH33
OO
COCHCOCH33
OO
HH
HH
COCHCOCH33
OO
COCHCOCH33
OO
The The Molecular Orbitals Molecular Orbitals
ofof
Ethylene and 1,3-ButadieneEthylene and 1,3-Butadiene
Reactants’ HOMO and LUMO
Orbitals and Chemical ReactionsOrbitals and Chemical Reactions
A deeper understanding of chemical reactivity A deeper understanding of chemical reactivity can be gained by focusing on the can be gained by focusing on the frontier frontier orbitals orbitals of the reactants.of the reactants.
Electrons flow from the highest occupied Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital the lowest unoccupied molecular orbital (LUMO) of the other.(LUMO) of the other.
We can illustrate HOMO-LUMO interactions by We can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between way of the Diels-Alder reaction between ethylene and 1,3-butadiene.ethylene and 1,3-butadiene.
We need only consider only the We need only consider only the electrons of electrons of ethylene and 1,3-butadiene. We can ignore ethylene and 1,3-butadiene. We can ignore the framework of the framework of bonds in each molecule. bonds in each molecule.
Orbitals and Chemical ReactionsOrbitals and Chemical Reactions
The The MOs of Ethylene MOs of Ethylene
red and blue colors red and blue colors distinguish sign of wave distinguish sign of wave functionfunction
bonding bonding MO is MO is antisymmetric with antisymmetric with respect to plane of respect to plane of moleculemolecule
Bonding Bonding orbital of ethylene; orbital of ethylene;two electrons in this orbitaltwo electrons in this orbital
The The MOs of Ethylene MOs of Ethylene
Bonding Bonding orbital of ethylene; orbital of ethylene;two electrons in this orbitaltwo electrons in this orbital
Antibonding Antibonding orbital of ethylene; orbital of ethylene;no electrons in this orbitalno electrons in this orbital
LUMOLUMO
HOMOHOMO
FourFour pp orbitals contribute to the orbitals contribute to the system of 1,3- system of 1,3-butadiene; therefore, there are butadiene; therefore, there are fourfour molecular orbitals.molecular orbitals.
Two of these orbitals are bonding; two are Two of these orbitals are bonding; two are antibonding.antibonding.
The The MOs of 1,3-Butadiene MOs of 1,3-Butadiene
The Two Bonding The Two Bonding MOs of 1,3-Butadiene MOs of 1,3-Butadiene
Lowest energy orbitalLowest energy orbital
4 4 electrons; 2 in electrons; 2 ineach orbitaleach orbital
HOMOHOMO
The Two Antibonding The Two Antibonding MOs of 1,3-Butadiene MOs of 1,3-Butadiene
Highest energy orbitalHighest energy orbital
Both antibondingBoth antibondingorbitals are vacantorbitals are vacant
LUMOLUMO
Two Possible Configurations of Bridged Bicyclic Compounds
Secondary orbital overlap favors the endo productformation
A A Molecular Orbital Analysis Molecular Orbital Analysis
of theof the
Diels-Alder ReactionDiels-Alder Reaction
MO Analysis of Diels-Alder ReactionMO Analysis of Diels-Alder Reaction
Inasmuch as electron-withdrawing groups Inasmuch as electron-withdrawing groups increase the reactivity of a dienophile, we increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the assume electrons flow from the HOMO of the diene to the LUMO of the dienophile.diene to the LUMO of the dienophile.
LUMO of ethylene (dienophile)LUMO of ethylene (dienophile)
HOMO of 1,3-butadieneHOMO of 1,3-butadiene
HOMO of 1,3-butadiene HOMO of 1,3-butadiene and LUMO of ethylene and LUMO of ethylene are in phase with one are in phase with one anotheranother
allows allows bond formation bond formation between the alkene and between the alkene and the dienethe diene
MO Analysis of Diels-Alder ReactionMO Analysis of Diels-Alder Reaction
LUMO of ethylene (dienophile)LUMO of ethylene (dienophile)
HOMO of 1,3-butadieneHOMO of 1,3-butadiene
MO Analysis of Diels-Alder ReactionMO Analysis of Diels-Alder Reaction
A "forbidden" reactionA "forbidden" reaction
The dimerization of ethylene to give cyclobutane The dimerization of ethylene to give cyclobutane does not occur under conditions of typical does not occur under conditions of typical Diels-Alder reactions. Why not?Diels-Alder reactions. Why not?
HH22CC CHCH22
HH22CC CHCH22
++
A "forbidden" reactionA "forbidden" reaction
HH22CC CHCH22
HH22CC CHCH22
++
HOMO of HOMO of one ethyleneone ethylenemoleculemolecule
LUMO of LUMO of other ethyleneother ethylenemoleculemolecule
HOMO-LUMOHOMO-LUMOmismatch of twomismatch of twoethylene moleculesethylene moleculesprecludes single-stepprecludes single-stepformation of two newformation of two newbondsbonds
