Download Frontier Molecular Orbitals and Sigmatropic reactions.

CHE 538 Lecture 25
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Frontier Molecular Orbitals and Sigmatropic reactions.
1. Mentally cut through the clutter in this outline and hang on to the thought that all the
reactions presented have some things fundamental in common!
1.1. They are all single elemental steps.
1.1.1. No intermediates between reactant and product.
1.2. All these reactions are dependent on orbital symmetry.
2. The ring closure elemental step in the mechanism below is an electrocyclic ring closure.
2.1.
2.1.1. Do you think you can explain what elemental changes in structure are driving this
reaction?
2.1.2. Do you think you could explain with sufficient detail how this ring closure is
similar to the Cope rearrangement?
Cope rearrangement.
2.1.2.1.
2.1.3. Or how the first two examples have so much in common with the Diels-Alder
reaction?
CN
CN
CN
40%
60%
2.1.3.1.
2.1.4. Electrocyclic ring closures are disrotatory when the transition state possesses an
aromatic number of electrons (4N+2)=2, 6, 10 etc. and conrotatory when the
transition state possesses an anti-aromatic number of electrons. =4, 8, 12 etc.
not
Ph
Ph
Ph
Ph
Ph
2.1.5.
2.1.5.1. six-electron transition state - - > disrotatory
Ph
Ph
Ph
CHE 538 Lecture 25
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Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2.1.6.
2.1.6.1. four electron transition state - - > conrotatory
2.1.7. By analogy the Nazarov cyclization should proceed in a conrotatory fashion.
OH
C
O
O H
Ph
Ph
Ph
Ph
Ph
H
Ph
2.1.8.
2.1.8.1. This fact has no synthetic significance for the molecule above because the
zwitterion in the figure above quickly looses a proton in the next step, thus
erasing any record of the diastereoselectivity of the cyclization.
O
2.1.9.
R
H
O
O
254 nm
benzene
R
H
H3PO4, HOAc
R
R
R
R
2.1.9.1. However the reaction above demonstrates that the initial rotation of the ring
closure under thermal conditions is conrotatory. Whereas in the excited state
the initial rotation of the ring closure under thermal conditions is disrotatory.
2.1.10. In any case the mechanistic examples follow the Woodward, Hoffman rules of
orbital overlap and symmetry conservation. At least a rudimentary knowledge of
this stuff is necessary for an Organic Chemist.
3. The reactions discussed above also have a lot in common with closed-shell 1, 5-hydrogen
atom transfer reactions. Shown below.
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3.1. Nominally 6 electrons are involved with this transposition.
3.2. The reaction transfers hydrogen atom across the same face of the molecule. Why is this
so?
H
H
H
H
H
Ph
3.3.
4. Woodward and Hoffmann formulated the MO-based hypothesis in terms of complimentary
overlap in transition states constructed from the HOMOs of the acyclic fragments.
Ph
2 nodes HOMO
interaction of
Frontier HOMOS
H
C
Ph
1 node
H
Ph
HOMO
five pi
electrons
Ph
4.1.
+
0 nodes
best bonding
on one side
of pi-system
CHE 538 Lecture 25
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4.2.
4.2.1. For stereochemical insights, you don’t have to know the value of the coefficients;
you simply have to know the relative signs.
4.3. Construction of the MOs of polyene fragments (crude).
4.3.1. The HOMOs of neutral polyene fragments with odd numbers of carbon atoms
have open shells with nodes that bisect atoms.
4.3.1.1. These are odd alternate hydrocarbon polyenes and as such they have a node
at every other atom in the HOMO and the HOMO must be non-bonding at zero
beta.
4.3.1.2. From here we can construct the HOMOs straight away.
C
C
C
C
C
C
C
C
C
4.3.1.3.
4.3.2. The HOMOs of neutral polyenes with even numbers of carbon atoms have closed
shells with nodes that bisect bonds.
4.3.2.1. These are even alternate hydrocarbon polyenes and as such they have a node
at every other bond in the HOMO; the HOMO must have some bonding
character.
4.3.2.2.
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5. Allowed versus forbidden is a function of energy.
H
5.1.
H
400 C
5.2. 200 C should have done it for a symmetry-allowed reaction.
6. How would we expect the reaction below to occur?
6.1. Suprafacial or antarafacial hydrogen atom transfer?
O
O
H
H
+ ?
Heat!
6.2.
6.2.1. How would we tell?
6.2.2. What would drive the reaction?