diels alder reaction.power point
DESCRIPTION
Organic chemistryTRANSCRIPT
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Diels-Alder Reaction
• Method for synthesis of 6-membered ring• One-step, concerted reaction• Termed [4+2] cycloaddition reaction where 4 and 2 electrons react.
+
Otto Diels Kurt Alder
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Diels-Alder Reaction
• Discovered by O. Diels and K. Alder in 1928.• Occur between a conjugated diene and substituted
alkene (dienophile) to form cyclohexene ring system.
• Concerted reaction (single step), can be accelerated by heating or using some catalysts.
• [ 4+2 ] cycloaddition reaction.
• In retro Diels-Alder reaction, the six membered ring is break down to regenerate the diene and dienophile using high temperature usually.
• Stereoselective reaction ( mainly one product formed).
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Diels-Alder Reaction
• Stereospecific reaction ( reactants can keep their stereochemistry).
• No transition states or charged intermediates.• All electrons moving in same time to form two new σ
bonds.• 100 % economic ( No. of reactants atoms = No. of
products atoms).• If one or more of diene or dienophile atoms are not
carbon ,the reaction is hetero-Diels-Alder reaction.
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The dienes• Can be cyclic , acyclic carring many kinds of substituents.
• Must have s-cis conformation.
• +I substituted dienes are more reactive than others.
R= Me, OMe, CH(Me)OMe
R
CO2Na
OMe
OTMS
Danishefsky diene
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Cyclopentadiene
• Cyclopentadiene which is well know as a standard diene can undergoes self D.A at R.T .
• This dimer can be cracked by distillation.
25oC
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The dienophiles
• Wide range of dienophiles can be used including cyclic, acyclic and hetero compounds.
• Perfect dienophiles are alkenes conjugated to electron withdrawing groups such as carbonyl, nitro, cyano, halogens…etc
• -I groups increase the rate of D.A reaction.
• D.A reaction between alkene and diene without any substituents can take place but with low yield.
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The dienophiles
O
O
O
O
O
N
NN Ph
O
O
S
SiMe3
O2NMe
O
NMe
O
O
Br
F3C CF3
Ph Ph
O O
CNNC
NC CN
P
O
OEtOEt
N NO O
OMeMeO
O
PhO
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How to know its D.A reaction?
• Since it s {4+2} cycloaddition, the product is, six membered ring, double bond inside the ring , conjugate group outside the ring opposite to double bond.
O O
O
O
O
O
+
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How it works?
• Concerted reaction via aromatic transition state.
• There are two approches, the first depends on the interaction between HOMO of The diene and LUMO of the dienophile.
• The second, depends on +I, -I groups affect, since they form negative and positive cherges on the diene and dienophile.
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Stereochemistry
• The stereochemistry of substituents can be retained in the product.
• Cyclic dienes must be in s-cis are highly reactive .
+
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
MeO2C
CO2Me
CO2Me
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Endo rule• D.A between cyclic dienes and dienophiles can lead two
diasteremers, endo and exo.• Endo product is kinetically controlled product.• Exo product is thermomdynamically product.
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Regioselectivity
• Diels-Alder reaction can lead to different structural isomers.• Electronic and steric effects of the substituents.• Position of these groups is the main factor.• Usually its ortho and para directiong.
Z
Z
X
X X
Z
X
Z
Z= electron-withdrawing gruopx= electron-donating group
ortho
para
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Regioselectivity
Me
Me
O
Me
+
Me
O
CHOOHC OHC
+
ortho meta
+
CO2H
HO2CHO2C
ortho
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Catalysed Diels-Alder reaction
• To increase the rate of the reaction.
• To reduce high temperarture and long reaction times.
• To improve the regioselectivity.
• As a source of enetioselectivity.
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Catalysed Diels-Alder reaction
• Using Lewis acids catalysts (including chiral members).
• AlCl3 and its derivatives, M(OTf), BF3 .OEt…..
• Using organic catalysts (organic molecules can be used individually or combined with Lewis Acids).
• Imidazolidinone derivatives, thiourea derivatives, BINOL and its derivatives…..
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Lewis Acid catalysis
• L.A binding with EWD group then make the dienophile more electron deficient.
• Then, decrease LUMO energy, strong interaction with diene`s HOMO.
OOO OSnCl4+-
+ +
71 29
93 7
:
:
without catalystwith SnCl4
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Asymmetric Diels-Alder reaction
• Preparation of chiral cyclohexene derivatives .• Using chiral dienes or dienophiles• Using chiral Lewis Acids.
+
O
N NO
CO2Bn CO2Bn
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Chiral dienophiles
S
N
OO
Me
O
OR
O
OMeN
OO
R'
RN R
O
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Chiral dienes
• Using chiral dienes is less developing as the use of chiral dienophiles.
NN OOMe
O
EtO
O
O
OTMS
R
OAc
OAc
OAc
OAc
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Chiral Lewis Acids
OAlCl2+
72%Me CHO
MeCHO
endo:exo98:2
+
O
ON
O
OH
Me
OHMe
endo : exo93 : 07
OO
OO
OTMS
TMSO
+ TiCl4
+
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References
• Organic chemistry; Clayden, J.; Greeves, N.; Warren, S.;
Wothers, P. Oxford University Press, Oxford, 2006. • Lewis Acids and Selectivity in Organic Synthesis;
Santelli, M.; Pons, J. CRS Press, USA, 1995.• Wikipedia.co.uk