1. Chemistry 125: Lecture 55 February 24, 2010 (4n+2) Aromaticity Cycloaddition Electrocyclic Reactions This For copyright notice see final page of this file

2. Furan 0 anti-bonding nodes 2 ABNs 4 ABNs

3. N SHMo2 (Simple H ü ckel Molecular Orbital Program) (Simple H ü ckel Molecular Orbital Program) N BenzenePyridine N lower energy node on N identical shape energy larger on N lower energy high N density Crude  calculation shows heterocycle analogy.

4. Cyclodecapentaene  Naphthalene (SHMo2)  Naphthalene same as cyclodecapentaene & butadiene same as ethylene same as butadiene

5. Another Criterion of Aromaticity is the PMR Chemical Shift (coming soon, Chapter 15) The Cyclodecapentaene SystemAngewandte Chemie International Edition in EnglishVolume ﾊ 3, Issue ﾊ 3, Date: ﾊ March 1964, Pages: ﾊ 228- 229E. Vogel, H. D. Roth

6. Generalized Aromaticity pK a 15 vs. 16 for H 2 O H H HH H H Sec. 13.6 pp. 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 Sec. 13.6 p. 591 Same for cyclo-C 7 H 8 cyclo-C 7 H 7 + (cycloheptatrienyl “tropylium”) 6  electrons (4n+2)

7. Electrocyclic Reactions Pericyclic Reactions (in which transition states are “aromatic”) Cycloadditions: Diels-Alder (Sec. 12.12, 14.3)

8. Cycloadditions: Diels-Alder (Sec. 15.3) 4 + 2  electrons Ring 4  + 2  electrons ene diene ene diene

9. Cycloadditions: Diels-Alder Stereochemistry (ene) (Sec. 12.12, p. 549) Ene just “sits down” on Diene

10. Cycloadditions: Diels-Alder Stereochemistry (diene) Diene just “sits down” on Ene

11. Cycloadditions: Diels-Alder (Sec. 14.3, pp. 628-630)

12.   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

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

14. ? HOMO (  ) orthogonal to LUMO (  *) h Shift electron from HOMO to LUMO p. 1046

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

16. 11 33 conrotationdisrotation Möbius 22 Hückel connectivity    requires twist in 1 of 2 ways Hückel Transition State Motion  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  top touches top (even # of nodes) top touches bottom (odd # of nodes) ! 22 11 33 44 55 66

17. Sec. 27.2 pp. 1343-1346 (shown backwards in our text; same favored “aromatic” conrotatory transition state) CON 4e CON for 4n CON 8e DIS 6e DIS for 4n+2

19. 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

20. End of Lecture 55 Feb. 24, 2010 Copyright © J. M. McBride 2010. 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