ch. 30 - orbitals and organic chemistry: pericyclic...

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ORGANIC - MCMURRY 9E

CH. 30 - ORBITALS AND ORGANIC CHEMISTRY: PERICYCLIC REACTIONS

CONCEPT: BASICS OF MOLECULAR ORBITAL THEORY ● As previously discussed, non-bonding orbitals have the unique ability to conjugate with adjacent non-bonding orbitals.

□ Bonding/non-bonding takes place in the outermost shell. Let’s review atomic orbitals of valence electrons:

● When adjacent non-bonded atomic orbitals overlap, they create more favorable molecular orbitals.

□ We can use a linear combination of atomic orbitals (LCAO) to visualize the resultant molecular orbitals

EXAMPLE: Simplified LCAO Model of Ethene.



CONCEPT: DRAWING ATOMIC ORBITALS

Transforming a conjugated molecule into atomic orbitals requires two rules:

EXAMPLE: Provide the correct atomic orbitals for the following conjugated molecules.

a.

b.

c.



CONCEPT: DRAWING MOLECULAR ORBITALS

● Rules for drawing conjugated molecular orbitals: 1. # molecular orbitals = # atomic orbitals 2. One orbital must never change phases (1st is preferred) 3. Last orbital must always change phases 4. Number of nodes must begin = 0 and increase by 1 with each increasing energy level 5. Nodes must be symmetrical as possible. If in doubt, draw sin wave from fake atom [0] to [n + 1]. 6. If a node passes through an orbital, delete that orbital. 7. Fill molecular orbitals according to rules of electron configuration (Aufbau, Pauli, Hund’s)

EXAMPLE: Provide the molecular orbitals of 1,3-butadiene.



PRACTICE: Propose reasonable molecular orbitals for the following conjugated atomic orbitals.



CONCEPT: FRONTIER MOLECULAR ORBITAL THEORY – FINDING HOMO/LUMO

● Frontier orbital interactions are the driving force behind many reactions in organic chemistry

● FMOT is based on being able to identify/understand HOMO and LUMO

□ HOMO = Highest Occupied Molecular Orbital

□ LUMO = Lowest Unoccupied Molecular Orbital

EXAMPLE: Frontier Orbitals of Ethene

PRACTICE: Consider the Molecular Orbitals (MO’s) of the allyl anion. Which are the HOMO and LUMO?

1) HOMO = B, LUMO = C

2) HOMO = B, LUMO = A

3) HOMO = C, LUMO = A

4) HOMO = A, LUMO = C

5) HOMO = C, LUMO = B



CONCEPT: ORBITAL DIAGRAMS: 3-ATOM ALLYLIC IONS

● Allyl positions are famous for their unique ability to resonate, reacting in multiple locations.

□ Regardless to the identity of the ion, this reactivity can be explained through allylic molecular orbitals.

EXAMPLE: Simplified LCAO Model of Propenyl Ions

EXAMPLE: Use both resonance theory and MO theory to predict the reactive sites of the following radical.



PRACTICE: Predict the molecular orbitals and identify the HOMO and LUMO orbitals of 1-propenyl cation (allyl cation).



CONCEPT: ORBITAL DIAGRAMS: 4-ATOM 1,3-BUTADIENE

● Conjugated polyenes are famous for their unique ability to resonate, reacting in multiple locations.

□ They can participate in many types of reactions due to the symmetry of their molecular orbitals.

EXAMPLE: Predict the LCAO Model of 1,3-butadiene. Identify the HOMO and LUMO Orbitals.

Note: You may see these orbitals generated through the addition and subtraction of π-orbitals. Which orbitals would we need to sum to produce the above pattern?



CONCEPT: ORBITAL DIAGRAMS: 5-ATOM ALLYLIC IONS

● Like propenyl ions, 5-atom allylic systems have the ability to resonate, reacting in multiple locations.

□ Regardless to the identity of the ion, this reactivity can be explained through allylic molecular orbitals.

EXAMPLE: Predict the LCAO Model of 5-carbon allylic system. Identify bonding, non-bonding and antibonding orbitals.



PRACTICE: Predict the molecular orbitals and identify the HOMO and LUMO orbitals of the following cation.



CONCEPT: ORBITAL DIAGRAMS: 6-ATOM 1,3,5-HEXATRIENE

● Conjugated polyenes are famous for their unique ability to resonate, reacting in multiple locations.

□ They can participate in many types of reactions due to the symmetry of their molecular orbitals.

EXAMPLE: Predict the LCAO Model of 6-carbon 1,3,5-hexatriene. Identify bonding, non-bonding and antibonding orbitals. Determine the HOMO and LUMO orbitals.



CONCEPT: ORBITAL DIAGRAMS: EXCITED STATES

● Conjugated polyenes have the ability to absorb light energy and kick electrons up to a higher energy state.

□ When this happens, the identity of HOMO/LUMO orbitals change, impacting their reactivity (more later).

EXAMPLE: 1,3-butadiene is irradiated with photons, exciting an electron up to a higher energy molecular orbital. Predict the identity of the HOMO and LUMO orbitals after irradiation.

PRACTICE: 4-Methylbenzylidene camphor (4-MBC) is used by the cosmetic industry for its ability to protect the skin against UV-B radiation. Circle the part of the molecule that you theorize is responsible for its effects on UV light.



CONCEPT: INTRO TO PERICYCLIC REACTIONS

● Conjugated polyenes have the ability to react in non-ionic, concerted, cyclization reactions called pericyclic reactions.

● All pericyclic reactions share the following properties, regardless of the type:

□ Non-ionic. Solvents have no effect on them since there are _____ partial charges.

□ Concerted. All bonds are created and destroyed simultaneously. There are no intermediates.

□ Cyclizations. Mechanisms involve a ring of electrons around a closed loop with ___________ transition states.

□ Reversible. Also known as the “principle of microscopic reversibility”.

□ All can occur either thermally or photochemically.

● Pericyclic reactions can be easily categorized by the number of _________ that are destroyed after a cyclic mechanism.

Cycloadditions: Pericyclic reactions in which ____ π-bonds are destroyed after a cyclic mechanism.

Electrocyclic Reactions: Pericyclic reactions in which ____ π-bonds are destroyed after a cyclic mechanism.

Sigmatropic Shifts: Pericyclic reactions in which ____ π-bonds are destroyed after a cyclic mechanism.



PRACTICE: Determine if the following reactions are cycloadditions, electrocyclic reactions or sigmatropic shifts.

a.

b.

c.



CONCEPT: THERMAL ELECTROCYCLIC REACTIONS

● Pericyclic reactions in which ______ π-bond is destroyed after a _________-activated cyclic mechanism

□ Always intramolecular

● All conjugated polyenes are capable of intramolecular electrocyclic reactions, however stereochemistry is variable.

□ The HOMO orbital is capable of cyclizing in either a ___________________ or ___________________ fashion

● When substituents are involved in cyclization, stereochemistry is dependent on rotation type.

EXAMPLE: Predict the product in the following electrocyclic reaction. Label the reaction as either conrotatory or disrotatory.



CONCEPT: PHOTOCHEMICAL ELECTROCYCLIC REACTIONS

● Intramolecular pericyclic reactions in which ______ π-bond is destroyed after a __________-activated cyclic mechanism

● All conjugated polyenes are capable of intramolecular electrocyclic reactions, however stereochemistry is variable.

□ Light excites ground-state electrons to a ____________ energy state (ψ à ψ*). HOMO / LUMO orbitals change.

● When substituents are involved in cyclization, stereochemistry is dependent on rotation type.

EXAMPLE: Predict the product in the following electrocyclic reaction. Label the reaction as either conrotatory or disrotatory.



CONCEPT: CUMULATIVE ELECTROCYCLIC REACTIONS

Step 1: Determine ROTATION (conrotatory vs. disrotatory)

a. Obtain HOMO through combination of drawing molecular orbitals + activation type —OR—

b. Use Electrocyclic Rotation Summary Chart:

Step 2: Determine STEREOCHEMISTRY

a. Obtain final structure by drawing 3D-representation + ROTATION —OR—

b. Use Electrocyclic Stereochemistry Summary Chart



PRACTICE: Use the summary charts to predict the product of the following reactions. If there is more than one isomer possible, draw them.

a.

b.

PRACTICE: Electrocyclic reactions are not limited to neutral conjugated polyenes, but are also applicable to ionic conjugated systems. Propose a mechanism and product for the following reaction.



CONCEPT: THERMAL CYCLOADDITION REACTIONS

● Pericyclic reactions in which ______ π-bonds are destroyed after ________-activated cyclic mechanism

□ The Diels-Alder reaction is an example of thermal cycloaddition

● In cycloaddition, HOMOA must fill LUMOB.

□ According to FMOT, bonding interaction is strongest when orbital symmetry and energy __________ closely.

□ 1. Reaction must be symmetry-allowed vs. symmetry-disallowed 2. Reaction must minimize HOMO-LUMO Gap

EXAMPLE: Predict the favorability of a bonding interaction between HOMOB and LUMOA



PRACTICE: Use FMOT to predict the mechanism and products for the following cycloadditions. If no product is favored, write “symmetry-disallowed” in place of the product.

a. 2π + 2π cycloaddition

b. 4π + 4π cycloaddition



CONCEPT: PHOTOCHEMICAL CYCLOADDITION REACTIONS

● Pericyclic reactions in which _____ π -bonds are destroyed after a _________-activated cyclic mechanism

● In cycloaddition, HOMOA must fill LUMOB.

□ According to FMOT, bonding interaction is strongest when orbital symmetry and energy match closely.

□ Light excites ground-state electrons to a ____________ energy state (ψ à ψ*). HOMO / LUMO orbitals change.

Cycloadditions Summary:

● Assuming only suprafacial interactions (antrafacial not possible on small rings):



PRACTICE:

a. Use FMOT to predict the mechanism and products for the following cycloaddition. If no product is favored, write “symmetry-disallowed” in place of the product.

2π + 2π cycloaddition (thymine dimerization)

b. Use the cycloaddition summary rules to verify that you have come to the correct conclusion.



CONCEPT: INTRODUCTION TO SIGMATROPIC SHIFTS ● Intramolecular pericyclic reactions in which _______ π-bonds are destroyed after a cyclic mechanism

□ Involve the _______________ of 1 σ–bond and the _____________ of 1 σ–bond

□ Take the form of numerous rearrangements. Products are typically constitutional isomers of the reactant

□ Common examples are the Cope and Claisen Rearrangements

Naming Convention:

● Always described as [x,y]-sigmatropic shifts.

□ σ–bond broken = Atom 1

□ σ–bond created = Atoms [x,y]

EXAMPLE: Provide the correct names and mechanisms for the following sigmatropic shifts



CONCEPT: COPE REARRANGEMENT

● A _________-activated [3,3]-sigmatropic shift that involves only ___________________.

□ Can be differentiated from other pericyclic reactions due to lack of conjugation

□ Molecule may require rotation to visualize the 3,3-location

EXAMPLE: Provide the mechanism and final product for the following reaction.

PRACTICE: Provide the mechanism and final product for the following reaction.



CONCEPT: CLAISEN REARRANGEMENT

● A ________-activated [3,3]-sigmatropic shift that involves an _________ ether

□ Can be differentiated from other pericyclic reactions due to lack of conjugation

□ Molecule may require rotation to visualize the 3,3-location

● A final tautomerization step is required for molecules in which the enol-form is favored.

EXAMPLE: Circle the more favored tautomer of the following Claisen Rearrangement products

EXAMPLE: Provide the mechanism and final product for the following reaction. You may skip the tautomerization

mechanism if one is required.



ch. 30 - orbitals and organic chemistry: pericyclic...

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