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

Furan 0 anti-bonding nodes 2 ABNs 4 ABNs

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.

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

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 E. Vogel, H. D. Roth

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

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

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

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

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

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

  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

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

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

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

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

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

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

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