Conjugated Pi Systems and Pericyclic Reactions

Conjugated Pi Systems and Pericyclic Reactions
Organic Chemistry Second Edition David Klein Chapter 17 Conjugated Pi Systems and Pericyclic Reactions Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.1 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.1 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.1 Classes of Dienes This chapter focuses on conjugated systems
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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 Conjugated Dienes A sterically hindered base can be used to form dienes while avoiding the competing substitution reaction Practice with conceptual checkpoint 17.2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 Conjugated Dienes Single bonds that are part of a conjugated pi system are shorter than typical single bonds What is a pm? The hybridization of a carbon affects its bond length Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 Conjugated Dienes The more s-character a carbon has, the shorter its bonds will be. WHY? Practice with conceptual checkpoint 17.3 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 Conjugated Dienes HOW do heats of hydrogenation provide information about stability? WHY is energy released upon hydrogenation? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 Conjugated Dienes HOW does conjugation affect stability?
Practice conceptual checkpoints 17.4 and 17.5 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 Conjugated Dienes The s-cis and s-trans both allow for full pi system overlap Other possible conformations will be higher in energy WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.2 Conjugated Dienes Why is s-trans lower in energy?

17.4 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The resonance stabilized carbocation can be attacked by the halide at either site that is sharing the (+) charge How is 1,2-addition different from 1,4-addition? Practice with SkillBuilder 17.1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The addition of bromine to a diene also gives both 1,2 and 1,4 addition Predict the MAJOR products for the reaction below. Pay close attention to stereochemistry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.5 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.6 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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

There are three main types of pericyclic reactions Electrocyclic reactions Sigmatropic rearrangements What is the difference between an addition and a rearrangement? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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. WHY is that significant? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.7 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The arrows could be drawn in a clockwise or counterclockwise direction Can you draw a reasonable transition state? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 probably be favored or disfavored by entropy? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 is required) However, high temps do not favor the products thermodynamically Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

When an electron withdrawing group is attached to the dienophile, the reaction is generally spontaneous Show how the groups highlighted in red are electron withdrawing using resonance and induction where appropriate Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Diels-Alder reactions are stereospecific depending on whether the (E) or (Z) dienophile is used Which alkene is (E) and which is (Z)? We will investigate the stereochemical outcome later in this chapter Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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. Show the reaction and products 4 1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Draw four potential bicyclic Diels-Alder products for the reaction below Two of the potential stereoisomers are impossible. WHICH ones and WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The electron withdrawing groups attached to dieneophiles tend to occupy the ENDO position Major Product Minor Product Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The Diels-Alder transition state that produces the ENDO product benefits from favorable pi system interactions Is this a kinetic or thermodynamic argument? Draw an appropriate energy diagram Practice with conceptual checkpoint 17.19 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.9 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Often the energy present at room temperature is sufficient to promote thermal electrocyclic reactions Note that with the use heat, the configuration of the reactant determines the product formed Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The symmetry of the HOMO determines the outcome The terminal carbons rotate as they become sp3 hybridized and overlap lobes that are in phase Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Use MO theory to explain the products for the reactions below Will disrotatory or conrotatory rotation be necessary? Practice with conceptual checkpoint 17.21 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Under photochemical conditions, light energy excites an electron from the HOMO to the LUMO What was the LUMO becomes the new HOMO Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Make the same MO analysis to predict the symmetry-allowed product for the reaction below. Pay close attention to stereochemistry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The disrotatory nature of the photochemical [2+2] electrocyclic ring-closing is difficult to observe directly, because the thermodynamically favors ring opening The ring-opening reaction gives products that result from disrotatory rotation. Predict products below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The Woodward-Hoffmann rules for thermal and photochemical electrocyclic reactions are found in table 17.2 Practice with SkillBuilder 17.4 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.10 Sigmatropic Rearrangements
Sigmatropic Rearrangements – a pericyclic reaction in which one sigma bond is replaced with another Note that the pi bonds move their location Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The Claisen rearrangement is a [3,3] sigmatropic reaction in which one of the 6 atoms in the cyclic transition state is an OXYGEN In general, what factors affect the spontaneity of the reaction? Practice with conceptual checkpoints and 17.28 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.12 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.12 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 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

17.12 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? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Additional Practice Problems
Explain the relationship between heat of hydrogenation and stability of a pi system. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Explain why the instability of electrons in a pi MO directly relates to the number of nodes the orbital posses. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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