ORGANIC - BRUICE 8E CH. 28 - PERICYCLIC …lightcat-files.s3.amazonaws.com/packets/admin_organic-2...ORGANIC - BRUICE 8E CH. 28 - PERICYCLIC REACTIONS Page 2 CONCEPT: DIELS-ALDER –

! www.clutchprep.com

!

ORGANIC - BRUICE 8E

CH. 28 - PERICYCLIC REACTIONS

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 ____________________

ORGANIC - BRUICE 8E


Page 2

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.

ORGANIC - BRUICE 8E


Page 3

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.

ORGANIC - BRUICE 8E


Page 4

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.

ORGANIC - BRUICE 8E


Page 5

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.

ORGANIC - BRUICE 8E


Page 6

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 - BRUICE 8E


Page 7

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

ORGANIC - BRUICE 8E


Page 8

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

ORGANIC - BRUICE 8E


Page 9

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 - BRUICE 8E


Page 10

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

ORGANIC - BRUICE 8E


Page 11

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 - BRUICE 8E


Page 12

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 - BRUICE 8E


Page 13

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

ORGANIC - BRUICE 8E


Page 14

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.

ORGANIC - BRUICE 8E


Page 15

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.

ORGANIC - BRUICE 8E


Page 16

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 - BRUICE 8E


Page 17

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

a.

b.

c.

ORGANIC - BRUICE 8E


Page 18

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.

ORGANIC - BRUICE 8E


Page 19

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.

ORGANIC - BRUICE 8E


Page 20

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

ORGANIC - BRUICE 8E


Page 21

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.

ORGANIC - BRUICE 8E


Page 22

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

ORGANIC - BRUICE 8E


Page 23

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

ORGANIC - BRUICE 8E


Page 24

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):

ORGANIC - BRUICE 8E


Page 25

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.

ORGANIC - BRUICE 8E


Page 26

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

ORGANIC - BRUICE 8E


Page 27

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.

ORGANIC - BRUICE 8E


Page 28

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 - BRUICE 8E


Page 29

Documents

ORGANIC - BRUICE 8E CH. 28 - PERICYCLIC …lightcat-files.s3.amazonaws.com/packets/admin_organic-2...ORGANIC - BRUICE 8E CH. 28 - PERICYCLIC REACTIONS Page 2 CONCEPT: DIELS-ALDER –