Conjugated Pi Systems and Pericyclic Reactions

Conjugated Pi Systems and Pericyclic Reactions
Chapter 17 Conjugated Pi Systems and Pericyclic Reactions Suggested Problems –

Classes of Dienes There are three categories for dienes
Cumulated – pi bonds are adjacent Conjugated – pi bonds are separated by exactly ONE single bond Isolated – pi bonds are separated by any distance greater than ONE single bond

Classes of Dienes There are three categories for dienes
Cumulated – pi bonds are perpendicular Conjugated – pi bond overlap extends over the entire system Isolated – pi bonds are separated by too great a distance to experience extra overlap

Classes of Dienes Heteroatoms may be involved in a conjugated system
Draw a picture of the molecule shown to the right indicating where the pi bonds are and how they overlap Practice with conceptual checkpoint 17.1

Conjugated Dienes A sterically hindered base can be used to form dienes while avoiding the competing substitution reaction Practice with conceptual checkpoint 17.2

Conjugated Dienes Single bonds that are part of a conjugated pi system are shorter than typical single bonds The hybridization of a carbon affects its bond length

Conjugated Dienes The more s-character a carbon has, the shorter its bonds will be. WHY? Practice with conceptual checkpoint 17.3

Conjugated Dienes HOW does conjugation affect stability?
Practice conceptual checkpoints 17.4 and 17.5

Conjugated Dienes Rank the following compounds in order of increasing stability Based on degree of unsaturation, conjugation, and degree to which the alkenes are substituted with carbons, the order should be from least to most stable A < C < B < D

Conjugated Dienes In general, single bonds will freely rotate
The two most stable rotational conformations for butadiene are the s-cis and s-trans What does the “s” of s-cis and s-trans stand for? Are there any other rotational conformations that you think might be possible? S stands for single

Conjugated Dienes The s-cis and s-trans both allow for full pi system overlap Other possible conformations will be higher in energy

Conjugated Dienes Why is s-trans lower in energy?
About 98% of the molecules are in the s-trans form

Molecular Orbital Theory
Let’s review Molecular Orbital (MO) theory A MO forms when atomic orbitals overlap A MO extends over the entire molecule Recall the pi bonding and antibonding MOs for ethylene Which is more stable? WHY? What is a node?

The number of MOs must be equal to the number of AOs combined Note how the shorthand drawing matches the actual MOs

The 4 pi electrons in butadiene will occupy the lowest energy MOs. HOW will that affect stability?

MO theory also explains why the central C-C single bond is shorter and stronger than a typical C-C single bond

1,3,5-Hexatriene should have 6 pi MOs What are HOMO and LUMO? The HOMO and LUMO are the frontier MOs

Reactions that molecules undergo can often be explained by studying their frontier orbitals Light can be used to excite an electron from the HOMO to the LUMO End Lecture

Electrophilic Addition
Recall the Markovnikov addition of H-X to a C=C double bond from section 9.3 With a conjugated diene as the substrate, two products are observed

The pi electrons attack the acid to give the most stable carbocation What intermediate would result if the H were added to any of the other carbons in the molecule?

The resonance stabilized carbocation can be attacked by the halide at either site that is sharing the (+) charge Practice with SkillBuilder 17.1 How is 1,2-addition different from 1,4-addition?

The addition of bromine to a diene also gives both 1,2 and 1,4 addition Predict the MAJOR products for the reaction below.

Thermodynamic Control vs. Kinetic Control
The ratio of 1,2 vs. 1,4 addition is often temperature dependant The energy diagram must be examined to see WHY temperature affects the product distribution

Why are the products unequal in free energy?

δ+ δ- Draw a structure for each of the Nuc: attack transition states δ+ δ-

The 1,2 addition should occur more often regardless of temperature. WHY? The H and Br are closer in the 1,2 adduct

Explain how high temps yield one product while low temps yield the other

Predict the MAJOR product for the following reactions

Intro to Pericyclic Reactions
Pericyclic reactions occur without ionic or free radical intermediates There are three main types of pericyclic reactions Cycloaddition reactions How is it an addition reaction?

There are three main types of pericyclic reactions Electrocyclic reactions Sigmatropic rearrangements What is the difference between an addition and a rearrangement?

Pericyclic reactions have 4 general features The reaction mechanism is concerted. It proceeds without any intermediates The mechanism involves a ring of electrons moving around a closed loop The transition state is cyclic The polarity of the solvent generally has no effect on the reaction rate.

Changes in the number of pi and sigma bonds distinguish the pericyclic reactions from one another

Diels-Alder Reactions
Diels-Alder reactions can be very useful They allow a synthetic chemist to quickly build molecular complexity What is meant by [4+2] cycloaddition? Like all pericyclic reactions, the mechanism is concerted

Why do the products generally have lower free energy? Two pi bonds are broken and two sigma bonds are formed. Sigma bonds are lower in energy.

Most Diels-Alder reactions are thermodynamically favored at low and moderate temperatures At temperatures above 200 C, the retroDiels-Alder can predominate Will the reaction be favored or disfavored by entropy? Reaction is disfavored by entropy.

The pi  sigma conversion provides a (-)ΔH

ΔS should be (-) Two molecules combine to form ONE A ring (with limited rotational freedom) forms What will the sign (+/-) be for the –TΔS term?

Given the signs for ΔH and –TΔS, how should temperature affect reaction spontaneity (reactant vs. product favorability)? Are there any disadvantages if the temperature is too low? Think kinetics End Lecture:

In the Diels-Alder reaction, the reactants are generally classified as either the diene or dienophile If a dienophile is not substituted with an electron withdrawing group, it will not be kinetically favored (a lot of activation energy or high temperature will be required) However, high temps do not favor the products thermodynamically

When an electron withdrawing group is attached to the dienophile, the reaction is generally spontaneous The carbonyl group removes electrons inductively

Diels-Alder reactions are stereospecific depending on whether the (E) or (Z) dienophile is used

A CΞC triple bond can also act as a dienophile Practice with SkillBuilder 17.3

Predict products for the following reactions

Predict products for the following reactions The regiochemistry results from the less crowded transition state.

Diels-Alder reactions can also be affected by diene structure Recall that many dienes can exist in an s-cis or an s-trans rotational conformation Which conformation is generally more stable, and WHY? Diels-Alder reactions can ONLY proceed when the diene adopts the s-cis conformation

Dienes that can only exist in an s-trans conformation can not undergo Diels-Alder reactions, because carbons 1 and 4 are too far apart Dienes that are locked into the s-cis conformation undergo Diels-Alder reactions readily Cyclopentadiene is so reactive, that at room temperature, two molecule will react together. 4 1

Draw four potential bicyclic Diels-Alder products for the reaction below Two of the potential stereoisomers are impossible. WHICH ones and WHY?

Draw four potential bicyclic Diels-Alder products for the reaction below The geometry of the concerted mechanism precludes the structures labeled impossible.

When bicyclic systems form, the terms ENDO and EXO are used to describe functional group positioning

The electron withdrawing groups attached to dieneophiles tend to occupy the ENDO position Major Product Minor Product

The Diels-Alder transition state that produces the ENDO product benefits from favorable pi system interactions Practice with conceptual checkpoint 17.19

MO Description of Cycloadditions
In the Diels-Alder, the HOMO of one compound must interact with the LUMO of the other

With an electron withdrawing group, the dieneophile’s LUMO will accept electrons from the diene’s HOMO

The phases of the MOs align to allow for orbital overlap If there is conservation of orbital symmetry, the process is symmetry-allowed Note the carbons that change their hybridization from sp2 to sp3

Similar to a Diels-Alder ([4+2] cycloaddition),the reaction below is a [2+2] cycloaddition Draw a reasonable concerted mechanism Unless the reaction is symmetry-allowed, the process will not occur, so let’s analyze the MOs

The LUMO of one reactant must overlap with the HOMO of the other WHY can’t both of the HOMOs interact together?

The phases of the HOMO and LUMO can not line up to give effective overlap, so the reaction is symmetry-forbidden

[2+2] cycloadditions are only possible when light is used to excite an electron Draw a picture that shows how the reaction is now symmetry allowed Practice with conceptual checkpoint 17.20

[2+2] cycloadditions are only possible when light is used Draw a picture that shows how the reaction is now symmetry allowed

Electrocyclic reactions
Determine how the number of σ and π bonds change for the representative electrocyclic reactions below Explain why the equilibrium favor either products or reactants in the examples above

When substituents are present on the terminal carbons, stereoisomers are possible Note that the use of light versus heat gives different products. See next few slides for explanation

The symmetry of the HOMO determines the outcome The terminal carbons rotate as they become sp3 hybridized and overlap lobes that are in phase 6C

The terminal carbons rotate as they become sp3 hybridized and overlap lobes that are in phase Disrotatory rotation (one rotates clockwise and the other counterclockwise) gives the cis product 6C

The symmetry of the HOMO determines the outcome The terminal carbons rotate as they become sp3 hybridized and overlap lobes that are in phase 4C

The symmetry of the HOMO determines the outcome Conrotatory rotation (one rotates clockwise and the other counterclockwise) gives the trans product 4C

Use MO theory to explain the products for the reactions below Will disrotatory or conrotatory rotation be necessary? Practice with conceptual checkpoint 17.21 conrotatory

Predict the major product for the reaction below. Pay close attention to stereochemistry

Predict the major product for the reaction below. Pay close attention to stereochemistry disrotatory

Under photochemical conditions, light energy excites an electron from the HOMO to the LUMO What was the LUMO becomes the new HOMO This was the old LUMO

Will the new excited HOMO react disrotatory or conrotatory? Draw the expected product conrotatory

Make the same MO analysis to predict the symmetry-allowed product for the reaction below. Pay close attention to stereochemistry

Make the same MO analysis to predict the symmetry-allowed product for the reaction below. Pay close attention to stereochemistry The HOMO undergoes disrotatory motion pushing both ethyl groups up or both down

The ring-opening reaction gives products that result from disrotatory rotation. Predict products below

The Woodward-Hoffmann rules for thermal and photochemical electrocyclic reactions are found in table 17.2 Practice with SkillBuilder 17.4 4n Thermal Conrotatory

Sigmatropic Rearrangements
Sigmatropic Rearrangements – a pericyclic reaction in which one sigma bond is replaced with another Note that the pi bonds move their location

The notation for sigmatropic rearrangements is different from reactions we have seen so far Count the number of atoms on each side of the sigma bonds that are breaking and forming This is a [3,3] sigmatropic rearrangement

The reaction below is a [1,5] sigmatropic rearrangement Practice with conceptual checkpoint 17.25

The Cope rearrangement is a [3,3] sigmatropic reaction in which all 6 atoms in the cyclic transition state are CARBONS In general, what factors affect the spontaneity of the reaction (product favored vs. reactant favored)? Practice with conceptual checkpoint 17.26 Formation of a disubstituted alkene

The Claisen rearrangement is a [3,3] sigmatropic reaction in which one of the 6 atoms in the cyclic transition state is an OXYGEN Practice with conceptual checkpoints and 17.28

Two pericyclic reactions occur in the biosynthesis of Vitamin D End lecture:

UV-Vis Spectroscopy If light with the necessary energy strikes a compound with pi bonds, an electron will be excited from the HOMO to the LUMO Light energy is converted into potential energy

UV-Vis Spectroscopy In general, the necessary energy to excite an electron from π  π* (HOMO to LUMO) is either in the UV or visible region of the spectrum What might happen after the electron is excited?

UV-Vis Spectroscopy UV-Vis spectroscopy gives structural information about molecules A beam of light ( nm) is split in two Half of the beam travels through a cuvette with the analyte in solution The other half of the beam travels through a cuvette with just the solvent (used as a negative control) The intensities of the light that pass through the cuvettes are compared to determine how much light is absorbed by the analyte

UV-Vis Spectroscopy UV-Vis spectroscopy gives structural information about molecules The resulting data is plotted to give a UV-Vis absorption spectrum Compounds require specific wavelengths of energy to excite

UV-Vis Spectroscopy More conjugation gives a smaller π  π* energy gap
The smaller the energy gap, the greater the lambda max 217 258 290 Each additional double bond adds between 30 and 40 nm. Remember Energy is inversely proportional to wavelength.

UV-Vis Spectroscopy The group of atoms responsible for absorbing UV-Vis light is known as the chromophore What you need to know – The greater the conjugation, the smaller the gap between energy levels The less energy needed to promote an electron The longer the wavelength of light absorbed More light emitted in the visible region

UV-Vis Spectroscopy The group of atoms responsible for absorbing UV-Vis light is known as the chromophore Woodward and Fieser developed rules to predict λmax for chromophore starting with butadiene as the base

UV-Vis Spectroscopy Woodward and Fieser developed rules to predict λmax for chromaphore starting with butadiene as the base The Woodward-Fieser rules are a guide to estimate λmax

Color The visible region of the spectrum ( nm) is lower energy than UV radiation Lycopene is responsible for the red color of tomatoes Β-carotene is responsible for the orange color of carrots As conjugation increases, the lambda max increases to above 400 nM. Such a compound will absorb visible light.

Color The color observed by your eyes will be the opposite of what is required to cause the π  π* excitation. WHY? Practice with conceptual checkpoint 17.31 If you see orange light it is because the molecule is absorbing all of the colors of the electromagnetic spectrum but it is reflecting the orange wavelength.

Color Bleaching agents generally work by breaking up conjugation through an addition reaction Destroying long range conjugation destroys the ability to absorb colored light. WHY? Does bleach actually remove stains? No, it breaks the extended conjugation so the stain is no longer colored

Chemistry of Vision Rods and Cones are photosensitive cells
Rods are the dominant receptor in dim lighting. Owls have only rods allowing them to see well at night Cones are responsible for detection of color. Pigeons have only cones providing sensitive daytime vision Rhodopsin is the light-sensitive compound in rods

Chemistry of Vision Sources of 11-cis-retinal include Vitamin A and β-carotene

Chemistry of Vision When rhodopsin is excited photochemically, a change in shape occurs that causes a release of Ca2+ ions The Ca2+ ions block channels through which billions of Na+ ions generally travel each second (double bond goes from cis to trans) The decrease of Na+ ion flow culminates in a nerve impulse to the brain Our eyes are extremely sensitive Just a few photons can cause a nerve impulse The shape change is attributed to the cis double bond being converted into a trans double bond.

Additional Practice Problems
Give a complete mechanism and predict the major product for the addition reaction below.

Additional Practice Problems
Predict the products and give necessary conditions for the compound below to undergo a retro Diels-Alder

Conjugated Pi Systems and Pericyclic Reactions

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