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Chemistry 125: Lecture 56 February 25, 2011 Generalized Aromaticity Cycloaddition – Diels-Alder Electrocyclic Stereochemistry Dewar Benzene This For copyright.

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Presentation on theme: "Chemistry 125: Lecture 56 February 25, 2011 Generalized Aromaticity Cycloaddition – Diels-Alder Electrocyclic Stereochemistry Dewar Benzene This For copyright."— Presentation transcript:

1 Chemistry 125: Lecture 56 February 25, 2011 Generalized Aromaticity Cycloaddition – Diels-Alder Electrocyclic Stereochemistry Dewar Benzene This For copyright notice see final page of this file

2 Generalization of Aromaticity: 4n+2 Stability Transition State “Aromaticity” Cycloadditions & Electrocyclic Reactions e.g. J&F Sec. 13.6 pp. 582-595

3 Generalized Aromaticity pK a 15 vs. 16 for H 2 O H H HH H H e.g. J&F Sec. 13.6pp. 587, 592 cyclo-C 7 H 8 cyclo-C 7 H 7 - pK a 39 (despite more resonance structures) 6  electrons (4n+2) 8  electrons (4n, antiaromatic) R H R R + Ph 3 C + 2  electrons (4n+2) H HH H H OH - unusually stable cation (triply benzylic) + Ph 3 CH R R R + even more stable e.g. J&F Sec. 13.6 p. 591 Same for cyclo-C 7 H 8 + Ph 3 C + cyclo-C 7 H 7 + (cycloheptatrienyl or “tropylium”) 6  electrons (4n+2)

4 Electrocyclic Reactions Pericyclic Reactions (in which transition states are “aromatic”) Cycloadditions: Diels-Alder (e.g. J&F Sec. 12.12, 14.3)

5 H H Cycloadditions: Diels-Alder 4  + 2  electrons Ring 4  + 2  electrons enediene LUMO HOMO How does  become  ? Approach parallel to p-orbital axes. folded transition state flattened product H H H H H H cis Z H H H H Z E trans

6 Cycloadditions: Diels-Alder Regiochemistry 9% yield 45% yield 20°C Perhaps an allylic + / enolate - intermediate stabilized by terminal CH 3 or unsymmetrical Transition State? ? Perhaps Steric Hindrance? Note: Diene is over C=O as well as C=C

7 trans alkenetrans cyclohexene cis alkenecis cyclohexene Cycloadditions: Diels-Alder Stereochemistry (Ene) Diene just “sits down” on Ene 68% yield 84% yield e.g. J&F Sec. 12.12, p. 549 150-160°C forming two  -bonds simultaneously from the same face. No rotatable intermediate with only one new  bond

8 H H H H Cycloadditions: Diels-Alder Stereochemistry (Diene) CH 2 OH CH 3 5 min 120°C (2E,4E)-2,4-hexadien-1-ol maleic anhydride all cis 81% yield 15 hr 150°C one trans H CH 3 Prefers s-trans conformation, which is not reactive. CH 3 H (2E,4Z)-2,4-hexadiene

9 Diels-Alder Variety propenal (“acrolein”) e.g. J&F Sec. 14.3, pp. 628-630 160°C 150°C 20°C k ~1 M -1 s -1

10   LUMO   HOMO Diels-Alder Reaction cyclic  electron transition state  HOMO   LUMO Transition State Motion front viewside view Transition State HOMO-1 Transition State HOMO p. 1351

11 Diels-Alder Reaction cyclic   electron transition state Transition State Motion front viewside view

12 ? HOMO (  ) orthogonal to LUMO (  *) h Shift electron from HOMO to LUMO e.g. J&F p. 1046

13 A-T-T-G DNA Double Helix T-A-A-C T-T h (UVB) Thymine photodimerization causes a chain kink that inhibits DNA replication & transcription and is believed to be the main source of mutation / melanomas.

14 Pericyclic Reactions (in which transition states are “aromatic”) Cycloadditions: Diels-Alder Electrocyclic Reactions

15 conrotationdisrotation    requires twist in 1 of 2 ways Hückel Transition State Motion top touches top (even # of nodes) top touches bottom (odd # of nodes) David Benbennick Möbius Preserves Mirror Preserves Axis node

16 11 33 conrotationdisrotation Möbius 22 Aromatic Analogue (Hückel Connectivity) Hückel 22 11 33 44 55 66  11 22 33 44  22 11 33 44 55 66 22 11 33 44 55 66  11 22 33 44  Möbius ! Track the MOs of hexatriene as they transform into those of cyclohexadiene: Preserves Mirror Preserves Axis

17  H -16 kcal/mole  H +11 kcal/mole How to study whether Conrotation is preferred for 4n-electron shift? Disrotation preferred for 6-electron shift CH 3 (forms the less stable isomer) (less stable isomer) (4n+2) The transition state favored in going from A to B, must also be favored in going from B to A. (“Microscopic Reversibility”)

18 4-electron cycloaddition!

19 CON 4e CON for 4n CON 8e DIS 6e DIS for 4n+2 e.g. J&F Sec. 27.2 pp. 1343-1346 H3CH3C CH 3 ~0.005% CH 3 99.9 % 280°C CH 3 (forms the less stable isomer) (less stable isomer) Bias >11 kcal/mole -10°C CH 3

20 22  Transition State HOMO 11  disrotation 6e Hückel bottom touches top (odd # of nodes) top touches top (even # of nodes) conrotation 4e Möbius If you could run it forwards!

21 Opening Dewar Benzene (1866) Strained Stable Really wants to open up

22 Calculated Isomers of Benzene (2004) 84 are calculated to be < 100 kcal above benzene. 6 > 100 kcal above benzene have been prepared. (single bond breaking gives even less stable species) Dewar Benzene (1963) is 74 kcal above benzene but lasts 2 days at room temperature!

23 van Tamelen & Pappas (1963) 4-electron disrotation!

24 CCC angles require disrotatory motion  HOMO  * LUMO t 1/2 = 2 days (room temp) -11 kcal -75 kcal 25 33 conrotatory more strain aromatic  HOMO 66 kcal/mole more exothermic, but only 8 kcal/mole “faster”? good for 4n electrons  * LUMO

25 But shouldn’t “aromatic” 6-  -electron transition state be good for disrotation? It is more fundamental that   LUMO doesn’t overlap  HOMOs (& vice versa).   

26 Spectroscopy for Structure and Dynamics “Sunbeams..passing through a Glass Prism to the opposite Wall, exhibited there a Spectrum of divers colours” Newton (1674) “Specters or straunge Sights, Visions and Apparitions” (1605) O.E.D. Electronic (Visible/UV) e.g. F&J sec. 12.7-12.8 pp. 533 Vibrational (Infrared) e.g. F&J sec. 15.4, pp. 707-713 NMR (Radio) e.g. F&J sec. 15.5-15.9, pp. 713-749

27 End of Lecture 56 February 25, 2011 Copyright © J. M. McBride 2011. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0


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