Chem 125 Lecture 67 4/13/08 Projected material This material is for the exclusive use of Chem 125 students at Yale and may not be copied or distributed further. It is not readily understood without reference to notes from the lecture.

Bringing the ends of a conjugated chain together to form a ring gives a lowest  MO with an additional bonding interaction. In a conjugated rings peripheral nodes must come in even numbers. e.g. cyclopropenyl 0 nodes2 nodes Lowest MO will have energy = -N/N = -1 2 nodes E = -1E = +1/2

Energy Shifts on “Ring Formation” : : MO Energy (units of 2  ) : : End to End Interaction favorable unfavorable Shifts Alternate (because of increasing number of nodes).

Hückel’s Rule: 4n+2  electrons is unusually favorable in a conjugated ring. On bringing the ends of a chain together, odd-numbered  MOs (1, 3, 5, etc.) decrease in energy (favorable terminal overlap for 0,2,4… nodes), while even-numbered  MOs (2, 4, 6, etc.) increase in energy (unfavorable terminal overlap for 1,3,5… nodes). Thus having an odd number of occupied orbitals (more odd-numbered than even-numbered) insures overall  stabilization of ring (compared to chain). [though there may be strain in the  bonds] an odd number of e-pairs (where n in an integer)

: : : :.. There is always an MO at -1. Circle Mnemonic for  MO Energy in Conjugated Rings. Inscribe regular polygon with point down MO Energy (units of 2  ) Same radius as for open chain Read MO energies on vertical scale. : 4 cyclobutadiene 6 benzene 3 cyclopropenyl reactive SOMOs ! Cation strongly stabilized (vs. allyl + ) : : : 4n “Antiaromatic”! slightly destabilized (vs. butadiene) Stabilized (vs. hexatriene) : : : Radical less stabilized (vs. allyl).. Anion destabilized - open-chain  energies from semicircle mnemonic

Generalization of Aromaticity: 4n+2 Stability NMR Spectroscopy Transition State “Aromaticity”

: H X-X- Y : H. H : N Pyridine HH H H O Furan H HH H H N Pyrrole H H H N N Imidazole Heteroaromatic Compounds (pp. 725, ) : : : :. Y-Y- H X :. Relay for long-range proton transfer by enzymes N.B. Single. denotes contribution of 1 e to  system (redundant with double bond). (occurs in amino acid histidine)

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.

Generalized Aromaticity pK a 15 vs. 16 for H 2 O H H HH H H pp 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

Aromaticity: PMR Chemical Shift Criterion H Cl H H + SbCl 5 SbCl 6 - H H H +  10.4 Downfield! (diamagnetic anisotropy and loss of e-density)

Aromaticity: PMR Chemical Shift Criterion HCCl 3 TMS   electrons (4  3 + 2) DIAMAGNETIC ANISOTROPY! ? DIAMAGNETIC ANISOTROPY

Pericyclic Reactions (in which transition states are “aromatic”) Cycloadditions: Diels-Alder (Ch. 15) Electrocyclic Reactions (Ch. 27)

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

Cycloadditions: Diels-Alder (Sec. 15.3) Stereochemistry (ene) Ene just “sits down” on Diene

Cycloadditions: Diels-Alder (Sec. 15.3) Stereochemistry (diene) Diene just “sits down” on Ene

Cycloadditions: Diels-Alder (Sec. 15.3)

  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

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

End of Lecture 67 April 13, 2009