Upload
others
View
47
Download
0
Embed Size (px)
Citation preview
! 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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
Page 7
PRACTICE: Propose reasonable molecular orbitals for the following conjugated atomic orbitals.
ORGANIC - BRUICE 8E
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
Page 10
PRACTICE: Predict the molecular orbitals and identify the HOMO and LUMO orbitals of 1-propenyl cation (allyl cation).
ORGANIC - BRUICE 8E
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
Page 13
PRACTICE: Predict the molecular orbitals and identify the HOMO and LUMO orbitals of the following cation.
ORGANIC - BRUICE 8E
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
Page 17
PRACTICE: Determine if the following reactions are cycloadditions, electrocyclic reactions or sigmatropic shifts.
a.
b.
c.
ORGANIC - BRUICE 8E
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
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
CH. 28 - PERICYCLIC REACTIONS
Page 29