aromaticity: beyond hückel’s rule · 2014-10-19 · aromaticity • no single, concise...
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Aromaticity:Beyond Hückel’s Rule
E. Adrian HenleGroup Meeting
2014.06.25
Aromaticity
• No single, concise definition of what makes an “aromatic” molecule
• Origin of term unclear (Hofmann 1855?)• Defined via inference, on basis of
observables:– Thermodynamics– Geometry– Ring current
Hofmann Proc. R. Soc Lond. 1856 8 1-3
Generalized Definition
Aromatic molecules:– Have empty or singly-occupied atomic orbitals
aligned in phase– Demonstrate electron delocalization– Are diamagnetically anisotropic
Quantification
• Empirical:– 3He encapsulation NMR– 1H, 13C shielding/deshielding– Aromatic Stabilization Energy (ASE)
• Semi-Empirical:– Magnetic susceptibility exaltation (Λ)– Harmonic Oscillator Model (HOMA)– Nucleus-Independent Chemical Shift (NICS)
1H, 13C, 3He Chemical Shift
• Direct measurement of magnetic environment “inside/outside” molecule
• Limitation: target nucleus must be present and resolvable
• Susceptible to convolution by induction, nearby heavy atoms
Gomes & Mallion Chem. Rev. 2001 101 1349-1384Rabinovitz, et al. J. Am. Chem. Soc. 2002 124 8786-8787
NMR Shift Examples
Aromatic Stabilization Energy
• Compare heats of hydrogenation– Full hydrogenation of
• Requires synthesis of partially-hydrogenated derivatives
• Neglects steric contribution
Example: 1,2-Azaborine
Liu, et al. J. Am. Chem. Soc. 2010 132 18048-18050
NBH
tBuNBH
tBu
NBH
tBuNBH
tBu
NBH
tBu
H2
NBH
tBu
-30.0 kcal/mol
-22.7 kcal/mol
-23.9 kcal/mol
H2
H2
ASE = 16.6 kcal/mol
Magnetic Susceptibility Exaltation
• Compares measured diamagnetism to value calculated from increment table
• Pro: all values calculated from same increment table fairly comparable
• Con: increment tables make assumptions/approximations
• Con: difficult to perform measurements
Pascal Ann. Chim. Phys. 1910 19 5
Harmonic Oscillator Model
• Quantifies aromaticity by deviation from “ideal” C-C bond length in ring (1.397Å)
• 퐻푂푀퐴 = 1 − . ∑ (푥 − 1.397)
• 퐻푂푀퐴 = 0 for Kekulé benzene퐻푂푀퐴 = 1 for true benzene structure
• 푛 ≝ number of C-C bonds푥 ≝ length of rth C-C bond
Kruszewski & Krygowski Tetrahedron Lett. 1972 36 3839-3842
NICS
• Either semi-empirical or ab initio• Quantum mechanical calculation of
diamagnetic susceptibility at fixed point• Pro: can calculate aromaticity in difficult or
inaccessible systems• Pro: generally agrees with other methods• Con: “In silico coniecto”
Hückel’s Rule
• Doering 1951: defined a rule for predicting aromatic behavior in ring systems
• Derived from Hückel’s 1931 work• Planar, fully conjugated systems• Aromatic: 4n+2 π-electrons• Anti-aromatic: 4n π-electrons
Doering & Detert J. Am. Chem. Soc. 1951 73 876-877Hückel Z. Phys. 1931 70 204-286; 72 310-337
Benzene
• 6π e- (n=2), planar, fully conjugated• Aromatic, as expected
Cyclooctatetraene (COT)
• 8π e- (n=2), fully conjugated• Anti-aromatic; distorts to break conjugation
Homotropylium
• 6π e- (n=2), but not fully conjugated• VSEPR predicts a planar structure• Distorts to give a “7-membered” arene!• Homoaromaticity (first described for tris-
homocyclopropenium)
Winstein, et al. J. Am. Chem. Soc. 1959 81 6523-6524
Pyrene
• 16π e- (n=4); Hückel’s rule predicts non-planarity to break conjugation
• Aromatic! 4n+2 rule fails in polycycles
Corannulene
• 20π e- (n=5)• Non-planar: surely cannot be aromatic• Aromatic! Hückel’s rule does not apply
Clar’s Rule
• The aromaticity of fused rings is determined by the number of π sextets
• The resonance form with the most sextets contributes the most character
Clar Polycyclic Hydrocarbons Academic Press: New York, 1964
3D Aromaticity
• Fullerene: aromatic for 2(n+1)2 π e-
– Ih symmetric fullerenes spherically symmetric– C60 non-aromatic; C50, 72, 98, etc. aromatic
Hirsch, et al. Angew. Chem. Int. Ed. 2000 39 3915-3917
Möbius Aromaticity
• Hückel’s rule is based on circular topology• Möbius topology allows for 4n πe-
aromatic systems• First Möbius arene isolated in 2003
Herges, et al. Nature 2003 426 819-821
Möbius Arene Synthesis
Hückel and Möbius Isomers
Summary
• Hückel’s rule is a gross oversimplification!• Aromaticity is both hard to define and
difficult to quantify• Methods for quantification are hit-or-miss• There is a huge diversity of non-classically
aromatic molecules (and they are cool!)
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