Möbius Aromaticity

Oct. 18, 2011
M1 Kumiko Yamamoto

Contents
Möbius Strip
Criteria of Aromaticity
1964 Heilbronner
Implications of Möbius Aromaticity
1998 Schleyer et al.
A Cationic Möbius Aromatic System (calculation)
2003 Herges et al.
Synthesis and Properties of a Möbius Cycloalkene
2007 Grażyński et al.
Expanded Porphyrinwith a Hückel-Möbius Topology Switch
2008 Osuka et al.
Expanded Porphyrins with a Möbius Aromaticity
Summary and Outlook

Möbius Strip
A nonorientable (one-sided) surface obtained by cutting a closed band or cylinder
into a single strip, giving one of the two ends thus produced a one-half (180˚) twist,
and then reattaching the two ends.

August Ferdinand Möbius
(1790-1866)

Johann Benedict Listing(1808-1882)

http://mathworld.wolfram.com/MoebiusStrip.html

Criteria of Aromaticity (1)Chemical behavior

Energetic

electrophilic aromatic substitution

enhanced stability(large resonance energy)

Aromaticity
Structural

Magnetic

bond length equalization
due to cyclic delocalization

ring current effects

P. v. R. Schleyer, H. Jiao, Pure Appl. Chem. 1996, 68, 209.

Criteria of Aromaticity (2)Magnetic 1H NMR chemical shiftsJ. F. M. Oth, E. P. Woo, F. Sondheimer, J. Am. Chem. Soc. 1973, 95, 7337.

Criteria of Aromaticity (3)Magnetic diamagnetic susceptibility exaltation (Ʌ)
𝜦 = 𝝌𝜧 − 𝝌𝚳′𝜦 < 𝟎: aromatic compounds
𝜦 ~ 𝟎: nonaromatic compounds𝝌𝜧 : the experimentally determined molar susceptibility of a compound𝝌𝜧′ : the susceptibility estimated for a cyclopolyene of that structure
Compound

𝜒Μ𝜒Μ′

Λ

－68.1

－68.1

0.0

－57.5

－58.3

0.8

－48.6

－49.3

0.7

－54.8

－41.1

－13.7

Compound𝜒Μ𝜒Μ′Λ--－17.2**－44.5－38.0－6.5(－2.4**)--32.6***All values of 𝜒Μ , 𝜒Μ′ , and Λ are given in units of 10-6 cm3 mol-1(ppm・cgs). **The calculated magnetic susceptibility exaltations.

H. J. Dauben, Jr., J. D. Wilson, J. L. Laity, J. Am. Chem. Soc. 1968, 90, 811.
H. J. Dauben, Jr., J. D. Wilson, J. L. Laity, J. Am. Chem. Soc. 1969, 91, 1991.

Criteria of Aromaticity (4)Magnetic NICS (nucleus-independent chemical shift)absolute magnetic shieldings, computed at ring centers
NICS < 0 : aromaticityNICS > 0 : antiaromaticity
CompoundΛ

NICSΛ

NICS

0.0－2.2－17.2**

－14.3

－13.7

－9.7

－6.5
(－2.4**)－3.2

32.6**

54.2

Compound*All values of Λ are given in units of 10-6 cm3 mol-1 (ppm・cgs) andNICS are given in units of ppm. **The calculated magnetic
susceptibility exaltations.

P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. van E. Hommes,
J. Am. Chem. Soc. 1996, 118, 6317.

Implications of Möbius Aromaticity
HückelMöbius

𝟒𝒏 + 𝟐 𝝅 : aromatic𝟒𝒏𝝅: antiaromatic

𝟒𝒏𝝅: aromatic
𝟒𝒏 + 𝟐 𝝅 : antiaromatic

𝜀𝑘 = 𝛼 + 2𝛽 cos 2𝑘𝜋𝑁𝜀𝑘 = 𝛼 + 2𝛽′ cos (2𝑘+1)𝜋𝑁E. Heilbronner, Tetrahedron Lett. 1964, 29, 1923.

A Cationic Möbius Aromatic System (1)?

A Cationic Möbius Aromatic System (2)C2

species

C2v(planar geometry)
𝐸𝑟𝑒𝑙 (kcal/mol)∆𝑅𝐶−𝐶 (Å)Λ (ppm•cgs)

NICS (ppm)

C2

0.0

0.043

–18.8

–13.4

C2v

+26.3

0.131

+112.2+42.0

Eight-electron monocyclic cation (CH)9+ actually does prefer C2-symmetricalstructure and is Möbius aromatic rather than nonaromatic.

M. Mauksch, V. Gogonea, H. Jiao, P. v. R. Schleyer, Angew. Chem. Int. Ed. 1998, 37, 2395.

Synthesis and Properties of a Möbius Cycloalkene (1)Strategy

D. Ajami, O. Oeckler, A. Simon, R. Herges, Nature, 2003, 426, 819.

Synthesis and Properties of a Möbius Cycloalkene (2)Synthesis

D. Ajami, O. Oeckler, A. Simon, R. Herges, Nature, 2003, 426, 819.

Synthesis and Properties of a Möbius Cycloalkene (3)Properties

CS Hückel

C2 Möbius
speciescolorHOMA(polyene bridge)

HOMA(all bonds)∆𝑅𝐶−𝐶 (Å)(polyene bridge)

ISEIIuncorr*(kcal/mol)

C2 Möbius

red

0.50

0.35

0.095

4.04

CS Hückel

colorless

0.05

0.17

0.143

–2.22

*An aromatic stabilization energy, computed by the indene/isoindene approach.

“Taken together, the observed trends in bond-length equalization andstabilization energy indicate that the Möbius structure is moderatelyaromatic, whereas the Hückel structure is nonaromatic.”
D. Ajami, O. Oeckler, A. Simon, R. Herges, Nature, 2003, 426, 819.

Synthesis and Properties of a Möbius Cycloalkene (4)Discussiondihedral angle–131.3˚–107.7˚

C2 Möbiusdata source

HOMA∆𝑅𝐶−𝐶 (Å)

Herges et al.
Nature0.35(entire molecule)

0.095(polyene bridge)

Castro et al.
J. Am. Chem. Soc.

–0.02([16]annulene core)

0.135([16]annulene core)“we conclude that the [16]annulene
core of 1 is nonaromatic and that any
aromatic character of 1 is confined tothe benzene rings.” (1 = C2 Möbius)

ISEII(kcal/mol)

NICS(ppm)

𝛬(ppm•cgs)

4.04(uncorrected)--

0.6(corrected)–3.4–30.0 (uncorrected)+19.6 (corrected)

C. Castro, Z. Chen, C. S. Wannere, H. Jial, W. L. Karney, M, Mauksch,R. Puchta, N. J. R. v. E. Hommes, P. v. R. Schleyer, J. Am. Chem. Soc. 2005, 127, 2425.

Expanded Porphyrin with a Hückel-Möbius Topology Switch (1)Synthesis

M. Stępień, L. Latos-Grażyński, N. Sprutta, P. Chwalisz, L. Szterenberg,Angew. Chem. Int. Ed. 2007, 46, 7869.

Expanded Porphyrin with a Hückel-Möbius Topology Switch (2)Properties

X-ray crystal structure

X-ray crystal structure of A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1).

M. Stępień, L. Latos-Grażyński, N. Sprutta, P. Chwalisz, L. Szterenberg,Angew. Chem. Int. Ed. 2007, 46, 7869.

Expanded Porphyrin with a Hückel-Möbius Topology Switch (3)Properties spectra

HückelMöbius

CDCl3, 293 K

M. Stępień, L. Latos-Grażyński, N. Sprutta, P. Chwalisz, L. Szterenberg,Angew. Chem. Int. Ed. 2007, 46, 7869.

Expanded Porphyrin with a Hückel-Möbius Topology Switch (4)Properties electron-absorption spectra and DFT calculationsElectronic-absorption spectra ofA,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1).

DFT-optimized geometries of the Hückel and Möbius
conformers of A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1).

M. Stępień, L. Latos-Grażyński, N. Sprutta, P. Chwalisz, L. Szterenberg,
Angew. Chem. Int. Ed. 2007, 46, 7869.

Expanded Porphyrin with a Hückel-Möbius Topology Switch (5)Conclusion
solvent
temperature

Möbius topology

Hückel topology

A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) is the first example of dynamic switchingbetween Hückel and Möbius topologies in a conjugated molecule.

Expanded Porphyrins with a Möbius Aromaticity (1)Expanded Porphyrins
porphyrin

expanded porphyrins
Macrocycles that contain“pyrrole, furan, thiophene, orother heterocyclic subunits
linked together either directly or
through one or more spacer
atoms in such a manner that theinternal ring pathway contains aminimum of 17 atoms”.
J. L. Sessler, D. Seidel, Angew. Chem. Int. Ed. 2003, 42, 5134.

Expanded Porphyrins with a Möbius Aromaticity (2)Expanded Porphyrins

synthesis

Expanded Porphyrins with a Möbius Aromaticity (3)Expanded Porphyrins

planar Hückel

relationship between topology and aromaticity

figure-eight Hückel

𝟒𝒏 + 𝟐 𝝅 : aromaticity
𝟒𝒏𝝅: antiaromaticity

Möbius𝟒𝒏𝝅: aromaticity
𝟒𝒏 + 𝟐 𝝅 : antiaromaticity

S. Saito, A. Osuka, Angew. Chem. Int. Ed. 2011, 50, 4342.

Expanded Porphyrins with a Möbius Aromaticity (4)Expanded Porphyrins

aromaticity of expanded porphyrins*A normal blue line indicates the signals of the outer pyrrolic β protons
and a bold red line indicates those of inner protons.

S. Saito, A. Osuka, Angew. Chem. Int. Ed. 2011, 50, 4342.

Expanded Porphyrins with a Möbius Aromaticity (5)Bis-PdII [36]octaphyrin

synthesis

Y. Tanaka, S. Saito, S. Mori, N. Aratani, H. Shinokubo, N. Shibata, Y. Higuchi, Z. S. Yoon,K. S. Kim, S. B. Noh, J. K. Park, D. Kim, A. Osuka, Angew. Chem. Int. Ed. 2008, 47, 681.

Expanded Porphyrins with a Möbius Aromaticity (6)Bis-PdII [36]octaphyrin

properties∆𝑅𝐶−𝐶 (Å)

NICS (ppm)

HOMA

0.113

–14.6

0.67

“metal complex is stable, conformationally
locked Möbius aromatic molecule that
displays distinct aromaticity.”
Y. Tanaka, S. Saito, S. Mori, N. Aratani, H. Shinokubo, N. Shibata, Y. Higuchi, Z. S. Yoon,
K. S. Kim, S. B. Noh, J. K. Park, D. Kim, A. Osuka, Angew. Chem. Int. Ed. 2008, 47, 681.

Expanded Porphyrins with a Möbius Aromaticity (7)Conformational Equilibrium synthesis

M. G. P. M. S. Neves, R. M. Martins, A. C. Tomé, A. J. D. Silvestre, A. M. S. Silva, V. Félix,M. G. B. Drew, J. A. S. Cavaleiro, Chem. Commun. 1999, 385.
J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon,J.-Y. Shin, J. M. Lim, Y. Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka,
J. Am. Chem. Soc. 2008, 130, 13568.

Expanded Porphyrins with a Möbius Aromaticity (8)Conformational Equilibrium 1H NMR spectrum[28]hexaphyrin is planar
rectangular conformationand exhibits a moderate
diatropic ring current???

THF-d8, 25 ˚C*Peaks marked with * are due to residual solvents and impurities.

J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon,
J.-Y. Shin, J. M. Lim, Y. Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka,
J. Am. Chem. Soc. 2008, 130, 13568.

Expanded Porphyrins with a Möbius Aromaticity (9)Conformational Equilibrium plausible conformational changes

twisted Möbius topology

planar Hückel topology

J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon,
J.-Y. Shin, J. M. Lim, Y. Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka,
J. Am. Chem. Soc. 2008, 130, 13568.

Expanded Porphyrins with a Möbius Aromaticity (10)Conformational Equilibrium NMR spectra

1H NMR spectra of [28]hexaphyrin in THF-d . Peaks
8

marked with * are due to residual solvents and impurities.

13C NMR spectra of [28]hexaphyrin in THF-d .
8

J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon,
J.-Y. Shin, J. M. Lim, Y. Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka,
J. Am. Chem. Soc. 2008, 130, 13568.

Expanded Porphyrins with a Möbius Aromaticity (11)Conformational Equilibrium absorption spectra

UV/vis spectra in CH2Cl2.

Variable-temperature UV/vis spectra in THF.

J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon,
J.-Y. Shin, J. M. Lim, Y. Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka,J. Am. Chem. Soc. 2008, 130, 13568.

Expanded Porphyrins with a Möbius Aromaticity (12)Conformational Equilibrium X-ray structure and theoretical calculations

X-ray structure of [28]hexaphyrin from CHCl3/n-heptane:
top view and side view.

∆𝑅𝐶−𝐶 (Å)

NICS (ppm)

HOMA

28π Möbius*

0.073-0.85128π Hückel*

0.122-0.48528π Möbius (cal.)**

0.079

–15.2

0.713

X-ray structure of [28]hexaphyrin from 1,2dichloroethane/ethanol: top view and side view
including hydrogen-bonded ethanol molecules.

*[28]hexaphyrin (Ar = 2,6-F2-phenyl). **All of the aromatic substituents werereplaced by hydrogens.

J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon,
J.-Y. Shin, J. M. Lim, Y. Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka,J. Am. Chem. Soc. 2008, 130, 13568.

Expanded Porphyrins with a Möbius Aromaticity (13)Conformational Equilibrium conclusion

twisted Möbius topology

planar Hückel topology“meso-aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been shown to exist
in solution at 25 ˚C largely as an equilibrium of several rapidly interconverting
twisted Möbius conformations with distinct aromaticities, with a small
contribution from a rectangular conformation with antiaromatic character.”J. Sankar, S. Mori, S. Saito, H. Rath, M. Suzuki, Y. Inokuma, H. Shinokubo, K. S. Kim, Z. S. Yoon,J.-Y. Shin, J. M. Lim, Y. Matsuzaki, O. Matsushita, A. Muranaka, N. Kobayashi, D. Kim, A. Osuka,
J. Am. Chem. Soc. 2008, 130, 13568.

Expanded Porphyrins with a Möbius Aromaticity (14)Other Topics
protonation/neutralization switching

intramolecular fusion reactions

S. Saito, J.-Y. Shin, J. M. Lim, K. S. Kim, D. Kim, A. Osuka,Angew. Chem. Int. Ed. 2008, 47, 9657.

S. Tokuji, J.-Y. Shin, K. S. Kim, J. M. Lim, K. Youfu, S. Saito, D. Kim, A. Osuka,J. Am. Chem. Soc. 2009, 131, 7240.

Summary and Outlook (1)Summary
Criteria of Aromaticity
1. Chemical behavior–electrophilic aromatic substitution.
2. Structural–bond length equalization due to cyclic delocalization.
3. Energetic–enhanced stability (large resonance energy).
4. Magnetic–ring current effects.
1H NMR chemical shifts
diamagnetic susceptibility exaltation (ʌ)
NICS

1964 Heilbronner
Implications of Möbius Aromaticity
HückelMöbius4𝑛 + 2 𝜋 : aromatic
4𝑛𝜋: antiaromatic

4𝑛𝜋: aromatic
4𝑛 + 2 𝜋 : antiaromaticSummary and Outlook (2)1998 Schleyer et al.
A Cationic Möbius Aromatic System (calculation)
Eight-electron monocyclic cation (CH)9+ actually does prefer C2-symmetrical structure
and is Möbius aromatic rather than nonaromatic.

2003 Herges et al.
Synthesis and Properties of a Möbius Cycloalkene
Though Herges says that the C2 Möbius structure is moderately aromatic,[16]annulene core of C2 Möbius is nonaromatic.

C2 Möbius

Summary and Outlook (3)2007 Grażyński et al.
Expanded Porphyrin with a Hückel-Möbius Topology Switch
A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) is the first example of dynamic switching
between Hückel and Möbius topologies in a conjugated molecule.

Möbius topology

Hückel topology

2008 Osuka et al.
Expanded Porphyrins with a Möbius Aromaticity
Some expanded porphyrins were the first examples of the molecules which havedistinct Möbius aromaticity.

Summary and Outlook (4)Outlook
Chirality

New Properties+=

αproperties of 4𝑛𝜋
properties of 4𝑛 + 2 𝜋
aromatic compounds aromatic compounds

Möbius Antiaromaticity

Novel Evaluation Criteria
for Aromaticity and Topology
solid-state structural methods

4𝑛 + 2 𝜋 antiaromaticity

Aromaticity
Topology

theoretical methods

novelevaluation
criteria

spectroscopic dataReferences
H. S. Rzepa, Chem. Rev. 2005, 105, 3697.
R. Herges, Chem. Rev. 2006, 106, 4820.
Z. S. Yoon, A. Osuka, D. Kim, Nat. Chem. 2009, 1, 113.
S. Saito, A. Osuka, 化学, 2010, 65, 23.
S. Saito, A. Osuka, Angew. Chem. Int. Ed. 2011, 50, 4342.
A. Osuka, Abstructs of Papers, 21st Banyu Fukuoka Symposium, 2011.