third year organic chemistry course chm3a2 frontier molecular orbitals and pericyclic reactions part...
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Third Year Organic Chemistry CourseCHM3A2
Frontier Molecular Orbitals and Pericyclic Reactions
Part 4:
Advanced Cycloaddition Reactions
Facial SelectivitiesTandem ReactionIonic Reactions
1,3-Dipolar CycloadditionsSecondary Orbital Interactions
O
O
O
KINETIC CONTROL
THERMODYNAMIC CONTROL
LOW TSHORT t
HIGH TLONG t
O
O
O
H
H
ENDO ADDUCT
O
O
H
O
H
EXO ADDUCT
– Learning Objectives Part 4 –
Advanced Cycloaddition Reactions
CHM3A2– Frontier Molecular Orbitals and Pericyclic Reactions –
After completing PART 4 of this course you should have an understanding of, and be able to demonstrate, the following
terms, ideas and methods.
(vi) Cycloaddition reactions can be regioselective. The regioselectivity cannot be predicted from the simple
treatment given to frontier molecular orbitals in this course. However, generalisations can be made from
looking at classes of substituents (C, Z, X) which are in conjugation with the -systems, which allow us to
predict the regioselectivity in an empirical manner (CHM2I3).
(vii) Tandem cycloaddition reactions are useful synthetic reactions for the construction of fused cyclic systems,
however, one has to consider the implications of kinetic and thermodynamic control, in many cases.
(viii) 1,3-Dipolar cycloadditions are an important class of cycloaddition reactions, as they are a versatile route to
highly functionalised heterocycles. They involve the reaction of a 1,3-dipole with a dipolarophile.
(viii) Cycloaddition reactions can be initiated by photoexcitation. This allows many reactions that are not
thermally allowed to occur. However, it is not clear in some cases whether the mechanism is truly concerted
in nature, or has some component of radical generation and a step-wise reaction.
Facial
Selectivities
anti- and syn-Diels-Alder Adducts
MeMe
MeMe
Me
XO
O
O
X Me
O
O
O
Me X
O
O
O
Exo-Diels-Alder Adducts (Thermodynamic Product)
anti-Adduct syn-Adduct
X % anti-Adduct % syn-Adduct
H 20 80SMe 90 10S(O)Me >95 <5
S(O)2ME >95 <5
iPr >95 <5Et >95 <5CHO >95 <5
CH2OH >95 <5
OH <5 >95
Sterically Favoured
Me O
OO
O
H
?
[4s + 2s]‡
Other Dienophiles and -Facial Diastereoselectivity:
OO
Singlet
O
O
OO
27%
73%
[4s + 2s]‡
N
O
O
Ph
N
O
O
Ph
NO
O
Ph>90%
<10%
[4s + 2s]‡
Tandem
Diels-Alder
Reactions
Tandem Diels-Alder Reactions
O
O
Z
Z
Z
Z
Z
Z
Z
ZLow T High T
KINETIC PRODUCT
THERMODYNAMIC PRODUCT
LUMO of Dienophile Closest to Butadiene
HOMO
Z groups least sterically hindered
O
O[4s + 2s]‡Retro
[4s + 2s]‡
NN
CF3
CF3
N
N
CF3
CF3
N
N
CF3
CF3
N NCF3
CF3
25°C
100%
N NCF3
CF3
100°C
100%
KINETIC PRODUCT
THERMODYNAMICPRODUCT
100°C 100%Thermal [4s + 2s]‡
Ionic
Cyclo-additions
Cationic Cycloadditions (Allyl Cation)
R2 electron system
R' R'
R
LUMO 2
R' R'
HOMO 2
R
I
Thermal [2s + 4s]‡
R
R' R'
R
R' R'
Ph
Ph Ph
Anionic Cycloadditions (Allyl Anion)
Ph
Ph Ph
Ph
HOMO 2
Ph Ph
LUMO 2
4e -system
Ph
H
B:H
Ph
Ph PhThermal [4s + 2s]‡
1,3-Dipolar
Cyclo-additions
Sub
Sub
1,3-Dipolar Cycloadditions
YX Z
X Y Z ZY
X
Sub
ZY
X
Sub
ZY
X
Sub
ZY
X
Sub
REGIOISOMERS
4s
+
2s
4s
+
2s
[4s + 2s]‡
[4s + 2s]‡
Linear 1,3-Dipoles
X Y Z X Y ZGeneral
HC N CH2 H2C N CH2Nitrile ylids
NHNHCNHNHCNitrile Imines
HC N O HC N ONitrile oxides
CH2NNCH2NNDiazoalkanes
NHNNNHNNAzide
ONNONNNitrous oxide
General YX Z
YX Z
Azomethine ylids
NH2C CH2
NH2C CH2
Azomethineimines
NH2C NH
NH2C NH
Nitrones NH2C O
NH2C O
Carbonyl ylids
OH2C CH2
OH2C CH2
Carbonylimines
OH2C NH
OH2C NH
CarbonylOxides
OH2C O
OH2C O
Bent 1,3-Dipoles
H2CC
CH2 H2CN
NH
H2CC
CH2 H2C
NNH
Heteroatoms desymmetrise the
coefficients.This has effect on regiochemistry of
cycloaddition reaction
K.N. Houk, J. Sims, R.E. Duke, R.W. Strozier, J.K. GeorgeJ. Am. Chem. Soc., 1973, 95, 7287
Desymmetrising Coefficients and Regiochemistry
2 HOMO 2 HOMO
Driving force: -bond conjugation
1,3-Dipolar Cycloaddition Reaction Examples
NHN
C-OMeOproton
transfer
N
N
1,3-Dipole
C-OMeO
Dipolarophile
NN
C-OMe
H
O
NN
C-OMeO
H
Regioisomers
[4s + 2s]‡
Major Product
Minor Product
NN
C-RO
H
NN
C-ROH
NN
C-R
Oproton transfer
H
NHN
C-R
O
Most Acidic: -ve Charge Delocalised into C=O group
as well as into aromatic ring
NHN
C-RO
MinorProduct
MajorProduct
NN
O
1,3-Dipole Dipolarophile
R
[4s + 2s]‡
Secondary Orbital
Interactions
Secondary Orbital Interactions. 1
ENDO ADDUCT
KINETIC PRODUCT
EXO ADDUCT
THERMODYNAMIC PRODUCT
LUMO
HOMO
1
2
3
4
1
LUMO
2
3
4
5
6
HOMO
O
OO
H
HLow T
Short t
High T
Long t
O
O
H
O
H
O
O
O
Thermal [4s + 2s]‡
Secondary Orbital Interaction Control
O
HOMO of Cyclopenatadiene
LUMO of Maleic Anhydride
Primary Orbital
Interactions
Secondary Orbital
Interactions
Note that all of the -conjugated system of maleic anhdride needs to be considered, and not just the monoene unit, in order that the secondary orbital interactions can be taken into account
The ENDO Transition State
[4s + 2s]‡
The EXO Transition State
This orientation of reactants in the transition state does not facilitate any secondary orbital interactions.
Thermal [4s + 2s]‡
OOO
Energy
Reaction Coordinate
Endo Isomer:KineticProduct
G2‡< G1
‡
O
O
OO
OO
H
H
O
O
H
O
H
G1°
G2‡
G1‡
G2°
Exo Isomer:Thermodynamic
ProductG1
°>G2°
Kinetic & Thermodynamic Product
O
[4s + 2s]‡
OOO
Secondary Orbital Interactions. 2
O
O
O
O
O
Antibonding Secondary Orbital Interactions
Thermal [4s + 6s]‡
Thermal 10 (4n+2, n=2) e D-A
– Summary Sheet Part 4 –
Apied Cycloaddition Reactions
CHM3A2–Frontier Molecular Orbitals and Pericyclic Reactions –
Attack of a monoene on appropriately substituted dienes can take place in a suprafacial/suprafacial process on either face of the diene, leading two diastereoisomers. It is often found that there is some p-facial diastereoselectivity, which can usually be explained by considering the ease of accessibility of the monoene to the two different faces.
In many cycloaddition reactions with the correct substitution on the two partners, the two components are able to react to afford regioisomers. In practice it is found that one regioisomer is formed in preference to the other. The reason for this is attributed to the matching of the coefficients of the HOMO and LUMO in the transition state. Calculation predicts that the smallest coefficient of one partner (HOMO) will interact with the smallest coefficient of the second partner (LUMO), and vice versa for the large coefficients, because this results in the smallest energy gap and the lowest energy transition state. In many, instances the use of a Bronsted (H+) or a Lewis acid, increases the propensity for one regioisomer. The reason for this is that the coefficients at the termini of p-system are made increasingly larger and smaller, respectively, and as a result, the overlap with the second reacting partner becomes even more efficient.
Cycloaddition reactions can also be utilised to construct multiple ring systems, via what are referred to as tandem cycloaddition reactions. These are important reactions in synthetic chemistry as complex fused ring systems with high degrees of stereo and regio control can be formed in one-pot reactions. The nature of tandem cycloaddition reactions generally means that the second cycloaddition has two choices, one of which is controlled by thermodynamics (the product stability) and the other by kinetics (the ease of attainment of the transition state). Thus, it is important to bear this in mind in experimental design: should the reaction be carried out a low temperatures (favouring the kinetic product) or high temperatures (favouring the thermodynamic product), for example.
Another extremely important class of cycloaddition reactions are the 1,3-dipolar cycloadditions. This class of reactions allows the construction of heterocycles with high degrees of stereo and regioselectivity, and is thus very valuable in organic synthesis. The reactions involve a 1,3-dipole (X—Y+=Z, a 4 p-electron system) and a dipolarophile (generally an alkene, a 2 p -electron system), reacting in a concerted fashion via an ss transition state.
Cycloaddition reactions which were not permitted by thermal routes because the phase overlap of the FMOs was not appropriate, can be accomplished under photochemical conditions. For example, monoenes can be photodimerised. However, the product outcomes sometimes suggest that the reaction is not concerted (a stepwise radical mechanism may be involved). Furthermore, the issue is complicated by the fact that photoexcited singlet and triplet states can be formed, which results in different product outcomes.
In appropriate cases, the Diels-Alder reaction proceeds kinetically with endo selectivity. The so-called endo rule can be rationalised in terms of favourable secondary orbital interactions. Additionally, these so-called secondary orbital interaction can also explain the formation of only one diastereoisomer, by observing antibonding secondary orbital interactions (a point we shall return to when examining sigmatropic rearrangements).
Remember the following very wise phrase:
The man who says he can
and
The man who says he can’t
Are both right!Ali G!!
Good Luck with your exams…
…but remember they are nothing more than a stepping stone in life.
J.D. Walker
Tandem Diels-Alder Cycloadditions in Organic Synthesis
Chem. Rev., 1996, 96, 167.
W. Adams, M. Prein
-Facial Diastereoselectivity in the [4+2] Cycloaddition of Singlet Oxygen as a Mechanistic Probe
Acc. Chem. Res., 1996, 29, 275.
P.J. Parsons, C.S. Penkett, A.J. Shell
Tandem Reactions in Organic Synthesis: Novel Strategies for Natural Product Elaboration and the Development of New Synthetic Methodology
Chem. Rev., 1996, 96, 195.
Further Reading on Cycloaddition Reactions
Exercise 1: 4n+2 CycloadditionsIdentify the starting materials and propose arrow pushing mechanisms for the formation of the following products
OO
O
O
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
O O
80C
Benzene
80C
Benzene
64%
23%
45%
Answer 1: 4n+2 CycloadditionsIdentify the starting materials and propose arrow pushing mechanisms for the formation of the following products
80C
Benzene45%
OO
23%
Cl
Cl
Cl
Cl
O O
OO
O
O
Cl
Cl
Cl
Cl
O
O
Cl
Cl
Cl
Cl
80C
Benzene
64%O
O
Cl
Cl
Cl
Cl
Exercise 2: 4n+2 CycloadditionsIdentify the starting materials and propose arrow pushing mechanisms for the formation of the following products
O
CN
CN
20C
Ether92%
N
O20C
Ether66%
OO
BrBr80C
Benzene
Answer 2: 4n+2 CycloadditionsIdentify the starting materials and propose arrow pushing mechanisms for the formation of the following products
O
CN
CN
20C
Ether92%
O
NC CN
N
O20C
Ether66%
OO
BrBr80C
Benzene
O
N
O
Br
O
Br
Exercise 3: 4n+2 Cycloadditions and a bit more!
Rationalise the following reaction scheme utilising frontier molecular orbitals and identify reagent A.
CN
NC
CO2Me
CO2Me
NC CO2Me
CO2Me
Reagent A
Answer 3: 4n+2 Cycloadditions and a bit more!
Rationalise the following reaction scheme utilising frontier molecular orbitals and identify reagent A.
CN
NC
CO2Me
CO2Me
NC CO2Me
CO2Me
Reagent A
NC
CN
HOMO 3
H H
H
Attacking from least hindered side
4s +2s
DIS
CN
H
H
CN
H
H
NC
E
E
E
E
Exercise 4: 4n+2 CycloadditionsRationalise the following reaction scheme utilising secondary orbital interactions.
NN
N
Low Temp
HighTemp
Answer 4: 4n+2 CycloadditionsRationalise the following reaction scheme utilising secondary orbital interactions.
N
N
Low Temp
N
HighTemp
ThermodynamicProduct
KineticProduct
N
2 HOMO
3 LUMO
N
Primary Orbital Interactions
Secondary Orbital Interactions