ch. 20 - dienes, conjugated systems, and...

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ORGANIC - BROWN 8E

CH. 20 - DIENES, CONJUGATED SYSTEMS, AND PERICYCLIC REACTIONS

CONCEPT: INTRODUCTION TO CONJUGATION

Conjugation exists when three or more atoms with the ability to resonate are adjacent to each other (overlapping).

● Conjugation provides an electron “highway” to ______________________ from one side of the molecule to the other.

● Conjugated molecules display unique chemical reactivity. The higher the conjugation, the ________ the UV wavelength.

EXAMPLE: Which of the following molecules exists in a conjugated state?

● Allylic carbocations, carbanions, and radicals are unusually stable due to ________________________

ORGANIC - BROWN 8E


CONCEPT: STABILITY OF CONJUGATED INTERMEDIATES

Regardless of the type of reactive intermediate, conjugation increases the stability.

Carbocations

Radicals

Due to the stability of the allylic position, radical and carbocation intermediated allylic reactions are common.

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CONCEPT: CONJUGATED HYDROHALOGENATION

Recall the addition of a strong halohydric acid on a double bond. This reaction is called hydrohalogenation.

● Carbocation rearrangements are possible

Conjugated hydrohalogenation, also known as hydrohalogenation of dienes, or 1,2 vs. 1,4 addition to dienes, is the same

reaction, except with a possibility of multiple products due to the presence of a conjugated intermediate.

This reaction undergoes kinetic vs. thermodynamic control.

● Temperatures above 40° C favor the __________________, also called the thermodynamic product.

● Temperatures below 0° C favor the ___________________, also called the kinetic product.

EXAMPLE: Products of conjugated hydrohalogenation at different temperatures.

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CONCEPT: CONJUGATED HYDROHALOGENATION – KINETIC VS THERMODYNAMIC CONTROL

Conjugated hydrohalogenation is one of the reactions that undergoes kinetic vs. thermodynamic control.

● Hot reaction conditions favor the thermodynamic product __________________

● Cold reaction conditions favor the kinetic product ___________________

EXAMPLE: Simplified Conjugated Hydrohalogenation Energy Diagram

Summarizing Temperature Control:

The kinetic pathway has a more stable intermediate _________________ but less stable product ___________________

The thermo pathway has a less stable intermediate _________________ but more stable product ___________________

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CONCEPT: DIELS-ALDER REACTION– GENERAL FEATURES

The Diels-Alder reaction is a heat-catalyzed, reversible pericyclic reaction between a conjugated 1,3-diene and dienophile.

● Diels-Alder reactions always yield 6-membered rings as products.

The stereochemistry of all substituents must be ____________________

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CONCEPT: DIELS-ALDER – BRIDGED PRODUCTS

Bicyclic bridged products are obtained when s-cis-1,3-diene is ________________.

EXAMPLE: Cyclopentadiene Dimerization

Exo/Endo Stereochemistry:

When a bridged product is made, substituents must face in the _________________ direction, away from the bridge.

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CONCEPT: DIELS-ALDER – RETROSYNTHESIS

You may be given an end product and asked to provide the original diene and dienophile that were required to cyclize.

EXAMPLE: Which diene and dienophile would you choose to synthesize the following compound?

1. Find the 2. Cross out the new 3. Isolate the

Answer:

EXAMPLE: Which diene and dienophile would you choose to synthesize the following compounds?

a. b.

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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.

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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.

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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.

ORGANIC - BROWN 8E


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

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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

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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.

ORGANIC - BROWN 8E


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

ORGANIC - BROWN 8E


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?

ORGANIC - BROWN 8E


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.

ORGANIC - BROWN 8E


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

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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.

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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.

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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.

ORGANIC - BROWN 8E


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

a.

b.

c.

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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

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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

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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):

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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.

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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

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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.

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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.

ORGANIC - BROWN 8E


ch. 20 - dienes, conjugated systems, and...

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