Synthesis, structural analysis, and properties of [8]circulenes

Article abstract of DOI:10.1002/anie.201303875

Polygons: [8]Circulenes were easily prepared by Pd-catalyzed annulations of tetraiodotetraphenylenes with alkynes. Their saddle-shaped structure with an [8]radialene character was identified by X-ray crystallography. Similar to 1,3,5,7-cyclooctatetraene, they have a tub-shaped eight-membered ring, but all of the bond lengths and bond angles are almost equal. Variable-temperature NMR investigations showed interesting dynamic behavior. Copyright

Full text of DOI:10.1002/anie.201303875

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[8]Circulenes
Polygons: [8]Circulenes were easily pre-
pared by Pd-catalyzed annulations of
tetraiodotetraphenylenes with alkynes.
Their saddle-shaped structure with an
[8]radialene character was identified by X-
ray crystallography. Similar to 1,3,5,7-
cyclooctatetraene, they have a tub-shaped
eight-membered ring, but all of the bond
lengths and bond angles are almost
equal. Variable-temperature NMR inves-
tigations showed interesting dynamic
behavior.
C.-N. Feng, M.-Y. Kuo,
Y.-T. Wu*
&&&& — &&&&
Synthesis, Structural Analysis, and
Properties of [8]Circulenes
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DOI: 10.1002/anie.201303875
[8]Circulenes
Synthesis, Structural Analysis, and Properties of [8]Circulenes**
Chieh-Ning Feng, Ming-Yu Kuo, and Yao-Ting Wu*
[n]Circulene is a macrocyclic arene in which a central n-sided
polygon is completely surrounded and fused by benzenoids.[1]
Only three members of this family, [5]circulene (or corannu-
lene),[2] [6]circulene (or coronene),[3] and [7]circulene (or
pleiadannulene)[4] have been prepared to date. The shapes of
these circulenes change from a bowl ([5]circulene) through
a plane ([6]circulene) to a saddle ([7]circulene). Although
investigations of [4]circulene remain theoretical, tetraben-
zo[4]circulene has recently been synthesized and determined
to be a bowl-shaped molecule.[5] [8]Circulene (1) has not yet
been synthesized in the laboratory, presumably because of its
highly strained structure and instability. In contrast, its
various planar analogues, such as tetraoxo[8]circulene,[6]
octathio[8]circulene,[7] and tetracyclopenta[def,jkl,pqr,vwx]-
tetraphenylene (TCT),[8] have been successfully generated.
[8]Circulene has been predicted to have a saddle-shaped
structure[9] with a higher strain than [7]circulene.[10] Moreover,
a harmonic oscillator model of aromaticity (HOMA)[11]
analysis showed that the central, eight-membered ring and
the peripheral benzenoids of 1 all have significant aromatic
character, and the thus generated concentric aromatic current
causes 1 to be unstable.[12] Despite the interesting geometry
and unusual aromaticity of [8]circulene derivatives, this study
is the first to elucidate their synthesis, structural analysis, and
properties.[13]
Attempts to obtain [8]circulene through the oxidative
photochemical cyclization of [2.2](3,6)phenanthrenophane-
diene have been unsuccessful.[14] This domino reaction was
supposed to form the central eight-membered ring and two
peripheral benzenoids from a planar starting material in one
synthetic step, but it is unsuitable for the synthesis of a highly
strained molecule. In light of this information, the synthetic
strategy presented herein is to first construct the central eight-
membered ring, and then to generate the peripheral benze-
noids. Tetraphenylene should be an ideal starting material,
because the target molecule could be accessed by annulation
of each “bay region” with a C2 building block, thus forming
a phenanthrenyl moiety from a biphenyl moiety. We pre-
viously summarized synthetic methods for the preparation of
phenanthrene,[15] and the most effective protocol is the
palladium-catalyzed annulation of a 2-iodobiphenyl with an
alkyne.[16] Scheme 1 shows the synthetic method toward
substituted [8]circulenes. The key precursors, tetraiodo-sub-
stituted tetraphenylenes
regioisomers, can be easily prepared by the CuII-mediated
cyclodimerization of 2,2’-diiodo-4,4’,5,5’-tetramethylbi-
2
and their iodo-substituted
phenyl[17] and subsequent iodination (see the Supporting
Information). Although the latter reaction is not regioselec-
tive, each “bay region” contains only one iodo substituent and
the regioisomeric mixture was directly used in the following
step. The palladium-catalyzed annulations of tetraiodo-sub-
stituted tetraphenylenes with symmetric diarylethynes gave
peri-substituted [8]circulenes 4 and 5 in moderate yields on
a scale of several hundred milligrams. Although the stability
of both products was not closely examined, they were stable
for a few days at room temperature in aerobic solutions.
Octamethoxy-substituted [8]circulene
6
was similarly
obtained, but its degassed solution was relatively unstable
and decomposed within 24 hours.
The NMR spectra of [8]circulenes 4–6 are very simple,
each showing one distinct structure (see below). For example,
1
the H NMR spectrum of a solution of 4 in CDCl3 at room
[*] C.-N. Feng, Prof. Y.-T. Wu
Department of Chemistry, National Cheng Kung University
No. 1 Ta-Hsueh Rd., 70101 Tainan (Taiwan)
E-mail: ytwuchem@mail.ncku.edu.tw
Prof. M.-Y. Kuo
Department of Applied Chemistry, National Chi Nan University
No. 1 University Rd., 54561 Puli, Nantou (Taiwan)
[**] Metal-Catalyzed Reactions of Alkynes. Part XIV. This work was
supported by the National Science Council of Taiwan (NSC 101-
2628M-006-002-MY3 and NSC 99-2113M-260-007-MY3). We also
thank Prof. S.-L. Wang and Ms. P.-L. Chen (National Tsing Hua
University, Taiwan) for the X-ray structure analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303875.
Scheme 1. Preparation of [8]circulenes. DMF=N,N’-dimethylforma-
mide.
2
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temperature shows four doublets with equal integrations in
the aromatic region and two sharp singlets, which correspond
to methyl groups. The 13C NMR spectra and DEPT (dis-
tortions enhancement by polarization transfer) experiments
show two signals associated with the carbon atoms of the
methyl groups, and six signals of quaternary carbon atoms
(Cquat) and four signals of CH groups in the aromatic region.
The information suggests that the [8]circulenyl core contains
only four types of carbon atoms. p-Tolyl substituents have
more signals than would be expected for a planar environ-
ment, thus showing that [8]circulene is nonplanar at room
temperature on the NMR timescale. Notably, the methyl and
methoxy groups that are directly attached to the backbones of
4/5 and 6, respectively, cause a high-field shift in the 1H NMR
spectra. This shielding effect is probably caused by the ring
current of the pendant aryl substituents. For instance, the
methyl protons in 4 resonate at 1.45 ppm, whereas the
chemical shift of the benzylic hydrogen atoms in toluene is
2.36 ppm in CDCl3.
X-ray crystallography indicated that 4 is a saddle-shaped
molecule (Figure 1 and Table 1).[18] The structure of 4 slightly
deviates from the ideal point symmetry D2d. The peripheral
benzenoids are classified in two types: the curve-up/curve-
down A ring and the “bridged” B ring, which are present in an
alternating ABAB order. Only one conformer, which con-
tains dimethyl-substituted A rings and ditolyl-substituted B
rings, was observed. Similar to 1,3,5,7-cyclooctatetraene
(COT), the central eight-membered ring is tub-shaped, but
all bond lengths (approximately 1.45 ꢀ) and bond angles
(around 1258) are almost identical. The eight-membered ring
is more puckered than that of COT,[19] as determined by
comparing the bending angles (a; 478 vs. 438) and the torsion
angles for C1-C2-C3-C4 (628 vs. 568; Table 1). The dihedral
angle between two adjacent benzenoids A/B has a mean value
of 338, which exceeds the maximum value of 238 in
[7]circulene.[4c] Although no significant distortion of bond
angles in peripheral benzenoids (within 48) is observed, the
twisted conformations of these benzenoids with a torsion
Figure 1. a) Crystallographic structure of 4. Only carbon atoms are
shown for clarity, the thermal ellipsoids are set at 30% probability.
b) Calculated geometry of 4’.
angle of about 308 reflect the strain. The mean length of the
spoke bonds is approximately 1.41 ꢀ. The substituents and
their steric repulsions influence the lengths of the flank and
rim bonds. The A ring has longer flank bonds than the B ring
(fA: 1.452(1) vs. fB: 1.431(1) ꢀ), but shorter rim bonds (rA:
1.352(1) vs. rB: 1.378(1) ꢀ). According to the structural
Table 1: Selected structural data and HOMA analysis of 1 and 4.[a]
1 (R1 =R2 =H)
4 (R1 =Me, R2 =4-tol)
Calcd.
X-ray
Calcd.
Bond length [ꢀ]
hub hA; hB 1.442; 1.470
1.450(1); 1.447(1)
1.414(1)
1.452; 1.447
1.407
spoke s
1.417
flank fA; fB 1.417; 1.432
1.452(1); 1.431(1)
1.352(1); 1.378(1)
1.451; 1.432
1.362; 1.380
rim rA; rB
1.357; 1.349
Bond angle [8]
C1-C2-C3
C2-C3-C4
125.9
125.9
124.7(1)
124.3(1)
124.1
124.1
Torsion angle C1-C2-C3-C4 [8]
Torsion angle C1-C2-C5-C6 [8]
Dihedral angle A/B [8]
58.4
0
31.1
43.7
61.9
0
32.9
46.8
62.7
0
33.3
47.4
Bent angle a [8]
A
B
C
0.689 [À8.9]
0.407 [À4.5]
À0.242 [9.8]
0.369 [À3.4]
0.629 [À3.9]
0.056 [12.9]
0.418 [À4.0]
0.650 [À4.1]
0.007 [12.2]
HOMA index [NICS(0) (ppm)]
[a] The structural data are taken as the averages of symmetrical relevance. The geometric calculations were performed with density functional theory
using Gaussian09 at the wB97X-D/6-31G** level. The NICS(0) values were calculated using the GIAO/HF/6-31+G* level.
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analysis, 4 is more similar to an [8]radialene,[20] in which
double bonds emanate from all vertices of the cyclooctane,
rather than a COT-based arene.
spectroscopy (see below), but the H NMR spectra did not
show 4’. The results demonstrate that either these two
conformers cannot be distinguished in the H NMR spectra
1
The structure of 4 was calculated with density functional
theory using Gaussian09[21] at the wB97X-D/6-31G** level,[22]
and the results correlate well with the experimental bond
lengths and bond angles (Table 1). The maximum deviation
was observed for the bond length of the rim bond rA (1.362 vs.
1.352(1) ꢀ). Notably, this long-range corrected functional
provides more accurate results than that at the B3LYP/6-
31G** level (see the Supporting Information). The calculated
geometries of 1 and 4 are similar, but the former has
significant bond alternation in the eight-membered ring, and
shorter flank and rim bonds. The parent [8]circulene also
possesses an [8]radialene character.
The aromaticities of 1 and 4 were analyzed with the
HOMA index and the nucleus-independent chemical shift
(NICS).[23] Table 1 shows the results of these calculations.
HOMA values of 1 and 0 specify aromaticity and non-
aromaticity, respectively. With respect to 4, the eight-mem-
bered ring is non-aromatic and all six-membered rings have
moderate to weak aromatic characteristics in the order B > A.
The HOMA analysis also supports the fact that 4 is an
[8]radialene. The HOMA indices in 1 and 4 are very similar,
but the former has slight antiaromaticity in the C ring and the
reverse aromatic sequence for peripheral six-membered rings
(A > B). Notably, Salcedao et al. have suggested that the
HOMA values for A/B and C in 1 are 0.803 and 0.953,
respectively.[12] Their results differ significantly from those
obtained herein because they utilized “uncommon” param-
eters.[24]
The NICS(0) indices of 1 and 4 were calculated at the
GIAO/HF/6-31 + G* level. Similar to the results of the
HOMA analysis, the NICS(0) values of peripheral rings B
in 1 and both rings A and B in 4 indicate a moderate to weak
aromatic character (around À4.0 ppm), whereas the aroma-
ticity of rings A in 1 is remarkably high (À8.9 ppm vs.
À9.7 ppm for benzene). The central ring C in both 1 (9.8 ppm)
and 4 (12.2 ppm) cannot be clearly identified as either COT
(1.9 ppm) or [8]radialene (À1.2 ppm). Tetrathiophene (7) is
a saddle-shaped [8]radialene with structural data close to 1.[25]
The NICS(0) value of the central eight-membered ring in 7
was calculated to be 3.3 ppm. NICS(1) indicates the aroma-
ticity at the position 1 ꢀ above the center of the ring, in order
to reduce interference from s electrons and from other rings.
However, NICS(1) was not investigated herein, because the
result can be influenced by the two rings A rings that are
concave. The distance between the test point for NICS(1) and
the center of ring A is around 2.5 ꢀ. Therefore, NICS cannot
provide enough information to distinguish between an
[8]radialene or a COT in [8]circulene.
or the conformer 4’ is thermodynamically much less stable.
The inversion dynamics of saddle-type circulenes are
almost completely unknown. The barrier of the saddle-to-
saddle inversion of [7]circulene was calculated to be approx-
imately 8.5 kcalmolÀ1 via a planar transition structure,
identified in theoretical studies,[26] and 12.2 kcalmolÀ1 from
measurements of 1,2-dihydro[7]circulene.[4b] Owing to its
higher strain, the inversion barrier of [8]circulene (1) should
be higher than that of [7]circulene if the saddle-to-saddle
inversion of the former also proceeds via a planar transition
structure. Our preliminary investigations of the saddle-to-
saddle inversion of 4 and interconversion between 4 and 4’
showed that the inversion mechanism and the transition
structure are very complex, presumably because the molecule
is in a sterically congested environment. Although computa-
tional work on the details of these dynamics is ongoing, some
experimental results of variable-temperature NMR studies
are presented here.
Variable-temperature 1H NMR experiments were per-
1
formed with a 500 MHz instrument. H NMR spectra of 4 in
CDCl3 at 215 and 298 K are nearly identical, showing that the
dynamic process is slower than the NMR timescale or does
not occur in this temperature range. A solution of 4 in [D4]-o-
dichlorobenzene could not be used for measurements because
the signals in the aromatic region overlap at room temper-
ature. Fortunately, a solution of 5 in [D4]-o-dichlorobenzene
clearly gives well-resolved signals, each of which is a sharp
singlet at 303 K (see the Supporting Information). When
a solution of 5 in degassed [D4]-o-dichlorobenzene was heated
to 433 K, the two pairs of xylyl signals, which resonate at 1.89/
2.39 and 6.16/6.86 ppm at 303 K, merged into two broad
bands. In contrast, signals from methyl groups directly
attached to the [8]circulenyl backbone and from 4-H at
xylyl groups were slightly broadened without any significant
change in chemical shift. Although the temperature specifi-
cations of the NMR instrument limited the determination of
the dynamics, the coalescence temperature can be assumed to
be slightly higher than 443 K, giving barrier values of 20.4 and
20.7 kcalmolÀ1, based on aryl and methyl signals, respectively
(see the Supporting Information). The coalescence of xylyl
signals may arise from a structure that incorporates a planar
central eight-membered ring and contains both C2 rotation
axes and mirror planes. Conformations 8 and 9 are two
examples thereof: they could be transition structures for the
saddle-to-saddle inversion, and accordingly, the inversion
barrier for 5 is around 21 kcalmolÀ1. When a solution of 5 in
[8]Circulenes 4–6 should have two saddle-shaped con-
formations, depending on the positions of the substituents.
However, both NMR analysis and X-ray crystallography
showed a single conformer. In contrast to 4, the other
conformer (4’) contains ditolyl-substituted A rings and
dimethyl-substituted B rings (Figure 1). Attempts were
made to study the interconversion between the two con-
formers 4 and 4’ in the temperature range 215–463 K by NMR
4
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Kitaura, Angew. Chem. 1996, 108, 69; Angew. Chem. Int. Ed.
[D4]-o-dichlorobenzene was heated to 463 K, and then cooled
to room temperature, its H NMR spectrum was identical to
1
Engl. 1996, 35, 69; c) K. Yamamoto, T. Harada, Y. Okamoto, H.
Chikamatsu, M. Nakazaki, Y. Kai, T. Nakao, M. Tanaka, S.
Harada, N. Kasai, J. Am. Chem. Soc. 1988, 110, 3578..
that of an unheated sample. This information may suggest the
conformer 5’ could not be observed at temperatures above
that required to induce the coalescence of xylyl signals.
In conclusion, a simple method for the preparation of
[8]circulenes 4–6, which exhibit a unique saddle-shaped
structure with an [8]radialenene character, is presented.
Further investigations of their physical properties and chem-
ical reactivity are under way. The synthetic approach is
currently used to prepare higher circulenes.
´
[5] B. R. Bharat, T. Bally, A. Valente, M. K. Cyranski, L. Dobrzycki,
S. M. Spain, P. Rempała, M. R. Chin, B. T. King, Angew. Chem.
2010, 122, 409; Angew. Chem. Int. Ed. 2010, 49, 399.
[6] H. Erdtman, H.-E. Hçgberg, Tetrahedron Lett. 1970, 11, 3389.
[7] K. Y. Chernichenko, V. V. Sumerin, R. V. Shpanchenko, E. S.
Balenkova, V. G. Nenajdenko, Angew. Chem. 2006, 118, 7527;
Angew. Chem. Int. Ed. 2006, 45, 7367.
[8] S. Nobusue, A. Shimizu, Y. Tobe, The Twelfth International
Kyoto Conference on New Aspects of Organic Chemistry,
November 2012, poster abstract PA-131.
[9] T. Liljefors, O. Wennerstrçm, Tetrahedron 1977, 33, 2999.
[10] H. Christoph, J. Grunenberg, H. Hopf, I. Dix, P. G. Jones, M.
Scholtissek, G. Maier, Chem. Eur. J. 2008, 14, 5604.
[11] a) J. Kruszewski, T. M. Krygowski, Tetrahedron Lett. 1972, 13,
3839; b) T. M. Krygowski, J. Chem. Inf. Comput. Sci. 1993, 33,
70; for a review about HOMA, see: c) T. M. Krygowski, M. K.
Experimental Section
Procedure for the preparation of 1,2,5,6,9,10,13,14-octamethyl-
3,4,7,8,11,12,15,16-octa(4-tolyl)[8]circulene (4): A mixture of tet-
raiodo-tetraphenylenes (2 and its regioisomers; 276 mg, 0.30 mmol),
1,2-di(4-tolyl)ethyne (309 mg, 1.50 mmol), Pd(OAc)2 (33.7 mg,
0.15 mmol), NaOAc (49.2 mg, 0.60 mmol), nBu4NCl (249 mg,
0.90 mmol), and DMF (2 mL) in a thick-walled Pyrex tube was
purged with nitrogen for 5 min. The sealed tube was kept in an oil
bath at 1108C for 36 h. After cooling to room temperature, the
suspension was diluted with hydrochloric acid (4N, 20 mL). The
organic layer was separated and the aqueous layer was extracted with
CH2Cl2 (3 ꢁ 20 mL). The combined organic layers were dried over
anhydrous MgSO4. The solvents of the filtrate were removed under
reduced pressure. The residue was subjected to chromatography on
silica gel, eluting with hexane/CH2Cl2 (10:1), to afford 4 (222 mg,
60%) as a dark red solid, m.p. > 3008C. Single crystals of 4 suitable
for X-ray crystallographic analysis were grown by diffusion of MeOH
into a solution of 4 in CH2Cl2 at room temperature. 1H NMR
(300 MHz, CDCl3): d = 1.45 (s, 24H), 2.28 (s, 24H), 6.53 (d, 3J =
7.7 Hz, 8H), 6.73 (d, 3J = 7.7 Hz, 8H), 6.86 (d, 3J = 7.7 Hz, 8H),
7.08 ppm (d, 3J = 7.7 Hz, 8H). 13C NMR (100 MHz, CDCl3, plus
DEPT): d = 17.8 (CH3), 21.2 (CH3), 127.3 (Cquat), 127.7 (CH), 128.1
(CH), 131.0 (CH), 131.6 (CH), 133.6 (Cquat), 134.9 (Cquat), 136.7 (Cquat),
138.8 ppm (Cquat). One quaternary carbon atom cannot be observed
because of overlapping signals. HRMS (FAB) calcd for C96H80:
1232.6260; found: 1232.6281.
´
Cyranski, Chem. Rev. 2001, 101, 1385.
[12] R. Salcedoa, L. E. Sansoresa, A. Picazoa, L. Sansꢂn, J. Mol.
Struct. (Theochem) 2004, 678, 211.
[13] For attempts toward the synthesis of octa-peri-benzo[8]circu-
lene, see: R. Bhola, B. T. King, Abstracts of Papers, 240th ACS
National Meeting, August, 2010, ORGN-172.
[14] a) B. Thulin, O. Wennerstrçm, Acta Chem. Scand. Ser. B 1976,
30, 369; b) P. J. Jessup, J. A. Reiss, Aust. J. Chem. 1977, 30, 851.
[15] Y.-D. Lin, C.-L. Cho, C.-W. Ko, A. Pulte, Y.-T. Wu, J. Org. Chem.
2012, 77, 9979.
[16] R. C. Larock, M. J. Doty, Q. Tian, J. M. Zenner, J. Org. Chem.
1997, 62, 7536.
[17] S. M. H. Kabir, M. Hasegawa, Y. Kuwatani, M. Yoshida, H.
Matsuyama, M. Iyoda, J. Chem. Soc. Perkin Trans. 1 2001, 159.
[18] Single-crystal X-ray diffraction was performed on a Bruker
APEX DUO [Cu-Ka, (l = 1.54178 ꢀ)] at 100(2) K. Data were
collected and processed by using APEX II 4K CCD detector.
Crystal data for 4: C96H80, orthorhombic, space group Pnma, unit
cell
dimensions:
a = 22.4352(7),
b = 25.3289(8),
c =
17.4665(6) ꢀ, a = 90, b = 90, g = 908. V= 9925.5(6) ꢀ3, Z = 4.
R1 = 0.0484, wR2 = 0.1413. CCDC 938414 contains the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] a) M. Trætteberg, Acta Chem. Scand. 1966, 20, 1724; b) J.
Bordner, R. G. Parker, R. H. Stanford, Acta Crystallogr. Sect.
B 1972, 28, 1069..
Received: May 6, 2013
Published online: && &&, &&&&
Keywords: [8]circulenes · alkynes · annulation · palladium ·
.
tetraphenylenes
[20] a) H. Hopf, G. Maas, Angew. Chem. 1992, 104, 953; Angew.
Chem. Int. Ed. Engl. 1992, 31, 931; b) G. Maas, H. Hopf in The
Chemistry of Functional Groups, Vol. I (Eds.: S. Patai, Z.
Rappoport), Wiley, Chichester, 1997, pp. 927.
[21] Gaussian09, RevisionA.1, M. J. Frisch et al., Gaussian, Inc.,
Wallingford, CT, 2009.
[22] J.-D. Chai, M. Head-Gordon, Phys. Chem. Chem. Phys. 2008, 10,
6615.
[23] Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta, P. v. R.
Schleyer, Chem. Rev. 2005, 105, 3842.
[1] For the definition of circulenes by Wynberg, see: a) J. H.
Dopper, H. Wynberg, Tetrahedron Lett. 1972, 13, 763; for
reviews about circulenes, see: b) J. S. Siegel, T. J. Seiders, Chem.
Br. 1995, 31, 313; c) C.-S. Chang, Y.-T. Wu in Science of Synthesis,
Vol. 45b (Eds.: J. Siegel, Y. Tobe), Georg Thieme, Stuttgart,
2009, pp. 955.
[2] a) W. E. Barth, R. G. Lawton, J. Am. Chem. Soc. 1966, 88, 380;
for reviews about corannulene-based buckybowls, see: b) Frag-
ments of Fullerenes and Carbon Nanotubes: Designed Synthesis
Unusual Reactions, and Coordination Chemistry (Eds.: M. A.
Petrukhina, L. T. Scott), Wiley, Hoboken, 2012; c) A. Sygula,
Eur. J. Org. Chem. 2011, 1611; d) Y.-T. Wu, J. S. Siegel, Chem.
Rev. 2006, 106, 4843.
[3] R. Scholl, K. Meyer, Ber. Dtsch. Chem. Ges. A 1932, 65, 902.
[4] a) K. Yamamoto, T. Harada, M. Nakazaki, T. Naka, Y. Kai, S.
Harada, N. Kasai, J. Am. Chem. Soc. 1983, 105, 7171; b) K.
Yamamoto, H. Sonobe, H. Matsubara, M. Sato, S. Okamoto, K.
À
[24] For the HOMA calculation of C C bonds in an aromatic system,
the normalization constant a and the optimal value Ropt were set
to 257.7 and 1.338 ꢀ, respectively. For details, see: T. M.
´
´
Krygowski, K. Ejsmont, B. T. Stepien, M. K. Cyranski, J.
Poater, M. Solꢃ, J. Org. Chem. 2004, 69, 6634.
[25] T. Fujimoto, R. Suizu, H. Yoshikawa, K. Awaga, Chem. Eur. J.
2008, 14, 6053.
[26] M. Shen, I. S. Ignatyev, Y. Xie, H. F. Schaefer III, J. Phys. Chem.
1993, 97, 3212.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!