The Impact of Antiaromatic Subunits in [4 n +2] π-Systems: Bispentalenes with [4 n +2] π-Electron Perimeters and Antiaromatic Character

Article abstract of DOI:10.1021/jacs.5b03074

Three series of stable, neutral, π-extended bispentalene derivatives, with two pentalenes fused to a central benzene or naphthalene moiety, have been prepared through a modified double carbopalladation cascade reaction. While these chromophores feature skeletons with [4n+2] π-electron perimeters, the two 8 π-electron pentalene subunits strongly influence bonding and spectral properties. ¹H NMR spectra showed large upfield shifts of the protons in the pentalene moieties, comparable to antiaromatic monobenzopentalenes. Further investigations on magnetic ring currents through NICS-XY-scans suggest a global paratropic current and a local diatropic current at the central benzene ring in two of the series, while the third series, with a central naphthalene ring, showed more localized ring currents, with stronger paratropic ring currents on the pentalene moieties. X-ray diffraction analyses revealed planar bispentalene cores with large double- and single-bond alternation in the pentalene units, characteristic for antiaromaticity, and small alternation in the central aromatic rings. In agreement with TD-DFT calculations, both optical and electrochemical data showed much smaller HOMO-LUMO energy gaps compared to other neutral, acene-like hydrocarbons with the same number of fused rings. Both experimental and computational results suggest that the molecular properties of the presented bispentalenes are dominated by the antiaromatic pentalene-subunits despite the [4n+2] π-electron perimeter of the skeletons. (Chemical Equation Presented).

Full text of DOI:10.1021/jacs.5b03074

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Article
The Impact of Antiaromatic Subunits in [4n+2] #-Systems: Bispentalenes
with [4n+2] #-Electron Perimeters and Antiaromatic Character
Jing Cao, Gabor London, Oliver Dumele, Margarete von Wantoch Rekowski, Nils
Trapp, Laurent Ruhlmann, Corinne Boudon, Amnon Stanger, and François Diederich
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b03074 • Publication Date (Web): 15 May 2015
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Journal of the American Chemical Society
The Impact of Antiaromatic Subunits in [4n+2] π-Systems:
Bispentalenes with [4n+2] π-Electron Perimeters and
Antiaromatic Character
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Jing Cao,†,|| Gábor London,†,⊥,|| Oliver Dumele,†,|| Margarete von Wantoch Rekowski,† Nils Trapp,†
Laurent Ruhlmann,‡ Corinne Boudon,‡ Amnon Stanger,*,§ and François Diederich*,†
†Laboratory of Organic Chemistry, ETH Zurich, VladimirꢀPrelogꢀWeg 3, CHꢀ8093 Zurich, Switzerland
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‡Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, Institut de ChimieꢀUMR 7177, C.N.R.S., Université de
Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France
§Schulich Faculty of Chemistry and the Lise MeitnerꢀMinerva Center for Computational Quantum Chemistry, Technionꢀ
Israel Institute of Technology, Haifa, 3200008, Israel
ABSTRACT: Three series of stable, neutral,
or naphthalene moiety, have been prepared through a modified double carbopalladation cascade reaction. While these
chromophores feature skeletons with [4n+2] ꢀelectron perimeters, the two 8 ꢀelectron pentalene subunits strongly influence
bonding and spectral properties. H NMR spectra showed large upfield shifts of the protons in the pentalene moieties, comparable
to 4n ꢀelectron antiaromatic monobenzopentalenes. Further investigations on magnetic ring currents through NICSꢀXYꢀscans
πꢀextended bispentalene derivatives, with two pentalenes fused to a central benzene
π
π
1
π
suggest a global paratropic current and a local diatropic current at the central benzene ring in two of the series, while the third
series, with a central naphthalene ring, showed more localized ring currents, with stronger paratropic ring currents on the pentalene
moieties. Xꢀray diffraction analyses revealed planar bispentalene cores with large doubleꢀ and singleꢀbond alternation in the
pentalene units, characteristic for antiaromaticity, and small alternation in the central aromatic rings. In agreement with TDꢀDFT
calculations, both optical and electrochemical data showed much smaller HOMO–LUMO energy gaps compared to other neutral,
aceneꢀlike hydrocarbons with the same number of fused rings. Both experimental and computational results suggest that the
molecular properties of the presented bispentalenes are dominated by the antiaromatic 4n πꢀsubunits despite the [4n+2] πꢀelectron
perimeter of the skeletons.
dominant contribution to the overall properties, are of special
interest, although experimentally rare.8
INTRODUCTION
The concepts of aromaticity and antiaromaticity are of
fundamental importance in classifying physical and chemical
properties of planar,
According to Hückel’s rule,2 planar,
systems with [4n+2] ꢀelectrons exhibit conjugative
stabilization in the ground state and are called aromatic
compounds. Later, Frost and Musulin,3 and Breslow4 extended
Hückel’s work and introduced the term antiaromaticity to
Nowadays, polycyclic aromatic hydrocarbons (PAH) are
attracting tremendous interest as advanced materials.9,10
Especially studied are linear acenes, such as pentacene and
derivatives,10 because of their planar conjugated structures and
fascinating optical and electronic properties that have great
potential in organic electronics. However, insufficiently
substituted acenes are prone to oxidation and dimerization
under ambient conditions.10b,11 Thus, research toward
alternative, aceneꢀlike structures earns increasing attention.
π
ꢀconjugated cyclic molecules.1
πꢀconjugated monocyclic
π
describe conjugated cyclic systems whose πꢀelectron energy is
higher than that of a suitable reference compound, which is not
cyclically delocalized.4 A series of intuitively acceptable
criteria for aromaticity and antiaromaticity of neutral, evenꢀ
electron singlet species were thus established by Breslow and
summarized by Krygowski et al.1c,5 These criteria describe
aromaticity and antiaromaticity from six different aspects,
including electronic nature, energy, geometry, magnetic
properties, reactivity, and spectroscopic properties. However,
until now, no unique definition for aromaticity and
antiaromaticity is generally accepted.1,6 Additionally,
theoretical studies7 suggest that the aromaticity (or
Investigations show that the combination of extended
πꢀ
conjugation and proper lowering of aromaticity within the
same scaffold is an effective way to achieve high charge
mobilities in fused ring systems.10b Furthermore, theoretical
predictions indicate the high performance of antiaromatic rings
in electrical conductivity.12 Apart from the widely used
methods of replacing benzene rings in acenes by
heteroaromatic rings13 to lower aromaticity,10b another recently
developed important strategy is the incorporation of nonꢀ
hexagonal hydrocarbon rings into the acene framework.14,15 In
this regard, fully
(e.g., Figure 1a,b) with a 6
possess an overall 4n ꢀelectronic structure have recently
π
ꢀconjugated indenofluorene derivatives15
6 fused ring system that
antiaromaticity) of neutral,
hydrocarbons should be determined by the number of
conjugated ꢀelectrons as well as by the nature of their
πꢀconjugated polycyclic
–
5–
6
–5–
π
π
received particular attention. Within that class, especially
promising candidates are the indeno[1,2ꢀb]fluorene and
indeno[2,1ꢀb]fluorene derivatives (Figure 1a,b), as they exhibit
ambipolar carrier transportation properties both in singleꢀ
crystal organic fieldꢀeffect transistors (OFETs)15b and in thinꢀ
fragmental structures. Therefore, investigations on the
electronic structures, physical properties, and chemical
reactivity of
πꢀconjugated polycyclic systems with [4n+2]
πꢀ
electron perimeter containing 4n
πꢀsubunits, which make a
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film OFETs.15c However, aceneꢀlike
πꢀconjugated polycyclic
synthesized compounds 1–3 were characterized by means of
NMR spectroscopy, nucleusꢀindependent chemical shift
(NICS)ꢀXYꢀscan,24 Xꢀray crystallographic analysis, UV/Vis
spectroscopy, and electrochemistry. The obtained data are
indicative of the antiaromatic nature of the bispentalene core
despite the [4n+2] πꢀelectron perimeter of the molecules.
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hydrocarbons containing more than two cyclopentadiene rings,
have rarely been explored.16
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RESULTS AND DISCUSSION
Synthesis. The key steps toward bispentalene derivatives 1–3
are outlined in Scheme 1. Bis(gemꢀdibromoolefin) derivatives
4, 6, and 7 were prepared by the Ramirez olefination reaction25
starting from the corresponding dialdehydes (for complete
synthetic details, see Section S2, Supporting Information).
When bis(gemꢀdibromoolefin) 4 was first treated under the
standard cascade carbopalladation reaction conditions
established in our initial work,23a 90% of it was recovered and
only traces of the desired product 1a were detected by mass
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Figure 1. Structures of (a) indeno[1,2ꢀb]fluorene, (b) indeno[2,1ꢀ
b]fluorene, (c) pentalene, (d) bisannulated[a,e]pentalene, (e)
monoannulated pentalene.
1
spectrometry and H NMR spectroscopy. Extension of the
Pentalene (Figure 1c) is a fully antiaromatic compound7d
consisting of two fused cyclopentadiene rings with
reaction time as well as addition of up to 1.0 equivalent of
palladium "catalyst" only enhanced the formation of
unidentified byproducts. Furthermore, due to the large excess
of the acetylene reagents and limited solubility of 1a, further
purification was deemed unfeasible.
8
π
electrons.17 It dimerizes above –196 °C without
electronic or steric stabilization of the core.18 Major recent
synthetic advances, with focus on bisannulated[a,e]pentalene
derivatives (Figure 1d), include the transition metalꢀmediated
annulation,16b,19 B(C6F5)3ꢀinduced intramolecular coupling,20
and anionic or radical anionic transꢀannulation.21
Dinaphtho[a,e]pentalene thin films19a,f,g and dibenzopentalene
(DBP)ꢀbased conjugated polymers22 showed high charge
mobilities, being potential candidates for organic/polymeric
thinꢀfilm transistors.
Scheme 1. Key Steps Towards Bispentalene Derivatives 1–
3a
a
Reaction conditions: alkyne 5 (6.0 equiv), [Pd(PPh3)2Cl2]
(0.2 equiv), K2CO3 (4.0 equiv), Zn dust (2.0 equiv),
hydroquinone (4.0 equiv, added after heating 1–2 h at 110 °C),
toluene, 110 °C, total heating time: 25–26 h.
Figure 2. Structures of extended bispentalene derivatives 1–3.
Recently, our group developed a simple and rapid method
toward monoannulated pentalene derivatives (Figure 1e),
based on a cascade carbopalladation reaction between gemꢀ
dibromoolefins and alkynes.23 This methodology provides
access to extended polycyclic hydrocarbons with pentalene
moieties. Herein, we report the synthesis of bispentalene
derivatives 1–3 (Figure 2) with different fusion patterns and
functional groups through a double cascade carbopalladation
reaction between the corresponding bis(gemꢀdibromoolefins)
and alkynes involving the formation of six C–C bonds during
the oneꢀpot reaction. Structurally, bispentalenes 1–2 consist of
four cyclopentadiene subunits replacing four of the benzene
rings in the parent pentacene structure, as compared to two
cyclopentadiene rings in indenofluorene derivatives (Figure
1a,b).14,15 The structure and physicoꢀchemical properties of the
The use of hydroquinone in the reaction turned out to be as
effective as we found in our more recent studies.23b Not only
could bispentalene 1a be obtained in a reasonable yield of 24%
with the formation of six new C–C bond in a oneꢀpot cascade
reaction, but also the amount of acetylene reagent could be
decreased from 40 equivalents to 6 equivalents, which
substantially facilitates the purification process. Prominently,
hydroquinone was found beneficial only when it was added
after the reaction mixture had been stirred at 110 °C for 1–2 h,
but its exact role remains unknown. The reaction conditions
shown in Scheme 1 were used for the preparation of all three
series of compounds. Bispentalene derivative 1a exhibited
limited solubility in organic solvents except for CS2, which
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was subsequently used in both chromatographic purification
and structural characterization. Double cascade
reflections on 1H NMR chemical shifts, magnetic
susceptibility exaltation,27 and nucleusꢀindependent chemical
shifts (NICS),28 besides others.1c
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2
3
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7
carbopalladation reaction of bis(gemꢀdibromoolefin) 4 with
1,2ꢀbis(4ꢀmethoxyphenyl)ethyne 5b was performed to
improve the solubility through introduction of methoxyꢀ
substituents on the peripheral phenyl rings. Compound 1b was
obtained as a black solid that showed good solubility in
common organic solvents such as CH2Cl2 and CHCl3.
In the 1H NMR spectra of 1–3, protons of the fused
bispentalene chromophores appeared significantly upfield
compared to those of the peripheral phenyl substituents. As
shown in Figure 3, the proton signals of H1 in the
cyclopentadiene rings of 1–3 appear in the region between
8
9
The reaction of bis(gemꢀdibromoolefin) 6 with three
different diphenylacetylenes 5a–c provided the three
bispentalene derivatives 2a–c as dark purple solids in
moderate yields (Scheme 1). This series shows excellent
solubility in common organic solvents, such as CH2Cl2 and
CHCl3, and methoxyꢀsubstituted 2b could even dissolve in
apolar hexane. Cyanoꢀsubstituted 2c is the least soluble of the
three compounds.
5.80–6.05
ppm,
whereas
the
same
proton
in
monobenzopentalene S9 bearing the same two 4ꢀ
methoxyphenyl substituents is located at about 6.29 ppm
(Figure S28, Supporting Information). The upfield shift of the
protons in the central benzene ring of 2b (H2, Figure 3b) is
even larger than those of the protons in 6,12ꢀdiaryl substituted
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indeno[1,2ꢀb]fluorenes, 20
πꢀelectron molecules, by about
0.70 ppm.15b There are clearly pronounced ring current effects
in bispentalenes 1–3, however, the contribution of electronic
substituent effects and shielding/deshielding effects from the
peripheral phenyl ring cannot be neglected for specific protons
at the annellated core.
To further extend the πꢀsystem, naphthoꢀfused bispentalene
derivatives 3a–c with 5–5–6–6–5–5 ring patterns were also
prepared by reacting acetylenes 5a–c with bis(gemꢀ
dibromoolefin) 7. While methoxyꢀsubstituted 3b showed the
highest solubility in common organic solvents, the comparably
poor solubility of cyanoꢀsubstituted 3c rendered its
purification difficult, which lead to its low yield of 8%.
The molecular structures of 1a–b and 2a–c with 18
πꢀ
electrons and 3a–c with 22 ꢀelectrons in their perimeters
π
were fully characterized by highꢀresolution mass spectrometry
1
(HRꢀMS), H NMR and 13C NMR spectroscopy, and Xꢀray
crystallography (see below). Their aromatic (or antiaromatic)
properties were investigated through four aspects: magnetic
properties, geometry, spectroscopic properties, and energy.1c,5
Thermal Stability of the Bispentalenes. Compounds 1–3
are deeply colored solids, which show high stability in the
solid state at 298 K, with no obvious degradation even when
exposed to air and light for months. Bispentalene 2a is even
thermally stable up to its melting point (295 °C) under ambient
conditions, while 1a–b, 2c, and 3c completely decompose
when heating up to 250 °C.
1
Figure 3. Partial H NMR spectra (400 MHz, CDCl3, 298 K) of
compound (a) 1b, (b) 2b, and (c) 3b.
The relative stability of 1a–b, 2a–b, and 3a–b in solution
was examined by UV/Vis spectroscopy under reported
conditions,11d by monitoring the strongest absorption in the
Vis region (for details, see Section S4, Supporting
Information). Molecules 3a–b degraded faster than the
corresponding compounds from the other two series, with halfꢀ
life times (t1/2) of 50 h and 20 h for 3a and 3b, respectively.
This is likely due to the more electronically isolated nature of
the antiaromatic pentalene and aromatic naphthalene units
within compounds 3a–b, as discussed below. Furthermore, the
electronꢀdonating methoxy substituents reduced the stability in
solution, in agreement with the comparably high HOMO
energy levels of these compounds (see below). Among the
Comparison of the proton signals in the central sixꢀ
membered rings of 1b, 2b, and 3b sheds light on these
multiple effects. For instance, in compound 3b, the chemical
shifts of the naphthalene protons appear at
6.02 (H3) ppm (Figure 3c) in CDCl3 at 298 K. While in benzoꢀ
δ = 5.93 (H2) and
fused bispentalene 2b, the corresponding signals are located at
δ
= 6.43 (H2) ppm (Figure 3b). One possibility for this
difference might be the different ring current effects. Notably,
all three protons are situated in the shielding regions of the
peripheral phenyl substituents (on C(3), C(8) in 2b; on C(4),
C(10) in 3b, Figure 3). The dihedral angle between the plane
of these phenyl rings and the central core is only relevant if the
hydrogen atoms are in close proximity to the peripheral phenyl
rings: in this case, the more perpendicular the two planes are,
the stronger the shielding effect. The distances between the
protons and peripheral phenyl ring could not be neglected
either. In compound 1b (Figure 3a), the larger upfield shift of
bispentalenes
presented,
2a
showed
the
highest
thermodynamic stability with no obvious degradation
observed even after two weeks. This result indicates that
introduction of bulky substituents is beneficial in both
increasing the solubility and enhancing the stability of the
bispentalene core.
1H NMR Spectroscopy. It is widely accepted that aromatic
systems have the ability to sustain a diatropic circulation of
ring current in the presence of an external magnetic field.1 In
contrast, antiaromatic compounds sustain paratropic ring
H2 (δ = 6.13 ppm), compared to H3 (δ = 6.46 ppm), could be
attributed to the shielding effects from the two proximal
phenyl rings (on C(3) and C(5), Figure 3a). However, although
no shielding effect comes from the phenyl substituents, the
currents.26 As a result, planar, cyclic conjugated
systems exhibit specific magnetic properties that have
π
ꢀelectron
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proton signal of H3 in 1b is still shifted upfield with respect to currents at each fiveꢀmembered ring were obtained. As seen in
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the normal aromatic region.
Figure 4b, the paratropic ring current of 1a at the pentalene
units is reduced (ca. 11 ppm vs 24 ppm in pentalene), while at
the central benzene ring there is a small paratropic current (ca.
3 ppm) – very different from benzene24 (ca. –16 ppm). This
Bispentalene derivatives 2a–c (Figure S40, Supporting
Information) showed no significant effects of the substituents
at the peripheral phenyl rings on the chemical shifts. We
therefore conclude that the major part of the magnetic
shielding effects observed in 1–3 originates from induced ring
currents in the fused bispentalene chromophores.
suggests a global paratropic current for the 18
πꢀelectron
perimeter of 11 ppm and a local diatropic current at the sixꢀ
membered ring of ca. –8 ppm (Figure 5). But we still could
not neglect the semiꢀglobal currents at the 5–5–6 membered
ring subunits. However, even if these currents exist, the
picture of global paratropic currents in the systems remains the
same (for further details, see Section S6.2, Supporting
Information). Thus, after fusion, both the paratropic ring
current of the pentalene moieties and the diatropic ring current
of the benzene unit in 1a are reduced by ca. 50%.
NICS-XY-Scans. Complementary to 1H NMR spectroscopy,
NICSꢀbased methods28 are more convincing and widely used
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for the identification of induced ring currents within
πꢀ
conjugated cyclic systems. Negative NICS values are denoted
to induced diatropic ring currents, while positive values are
indicative of paratropic ring currents.28a
Since bispentalenes 1–3 are
πꢀconjugated polycyclic
hydrocarbons, their aromatic (or antiaromatic) properties
cannot be understood by looking at individual rings. Thus, the
magnetic shielding properties of compounds 1–3 were
investigated by the recently established NICSꢀXYꢀscan, which
can explore the types of ring currents (namely, diatropic,
(a)
(b)
paratropic, global, and local) in
πꢀconjugated polycyclic
(c)
(d)
systems.24 A global ring current refers to the current for the
entire molecule, while a local ring current describes the
induced current circuit at a specific reference point in and near
a molecule.24,29 Existence of a global current only results in a
flat NICSꢀXYꢀscan above the molecule (see, for example
naphthalene24), while the coꢀexistence of global, semiꢀglobal
and local current results in a nonꢀflat curve (see, for example,
anthracene24).
Figure 5. Scheme of induced ring currents from NICSꢀXYꢀ
scan results for (a) bispentalene 1, (b) bispentalene 2, (c) one
possibility for bispentalene 3, and (d) the second possibility
for bispentalene 3. Substituents and the small local paratropic
currents at the fiveꢀmembered rings (Figure S45, Supporting
Information) are omitted for clarity.
Since the substituents on the peripheral phenyl rings in 2a
have a negligible effect on the induced ring currents of the
system (Table S1, Supporting Information), an analogue
compound, 2d, without tertꢀbutyl groups was employed for
calculation. Compound 2d (Figure 4c) shows a very similar
NICSꢀXYꢀscan to bispentalene 1a (Figure 4b vs 4c), both the
shape of the curves and the values are nearly the same.
Moreover, the NICS(1)π,ZZ values, which report the magnetic
environment above each ring (Table S1, Supporting
Information), are almost identical in 1a and 2d. It is thus
concluded that the magnetic properties of 1 and 2 are very
similar, regardless of their different topologies (fused at the
same sites).
The NICSꢀXYꢀscan of bispentalene 3a shows NICSπ,ZZ
values of 17 ppm above the pentalene unit, while above the
naphthalenic moiety the curve is constantly flat with a value of
–6 ppm (Figure 4d). There are two plausible interpretations for
the computed curves and values. One possibility is a 22
πꢀ
electron global diatropic current of –6 ppm and a “semiꢀlocal”
paratropic current at the pentalenes of 23 ppm. The second
possibility is a relatively weak global paratropic current and
“semiꢀlocal” diatropic and paratropic currents at the
naphthalene and pentalene moieties, respectively (Figure 5). In
any event, it looks like the ring currents in naphthaleneꢀ
centered bispentalene 3 are much more localized than in
benzoꢀcentered bispentalenes 1 and 2. Further investigations
on the NICSꢀXYꢀscan of S10, a topologic isomer of 3, and
natural resonance theory (NRT) analysis of these two isomers
Figure 4. NICSꢀXYꢀscans of (a) pentalene, (b) bispentalene
1a, (c) bispentalene 2d, and (d) bispentalene 3a. Black:
NICSZZ. Red: NICSπ,ZZ
.
The NICSꢀXYꢀscan of pentalene serves as a reference: a
global pentalenic paratropic current (Figure 4a) of ca. 24
ppm30 and two local, relatively small (ca. 3 ppm) paratropic
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suggest that 3 is better viewed as separate pentalenic and
Above all, NICSꢀXYꢀscans suggest a global paratropic
current in the 18 electron perimeter and a local diatropic
current at the central sixꢀmembered ring for bispentalenes 1
and 2, characteristic for antiaromaticity, while the 22
electron systems showed more electron localization
compared to 1 and 2, with stronger paratropic ring currents on
the pentalene moieties and diatropic current at the naphthalene
moiety (Figure 5).
1
2
3
4
5
6
7
naphthalenic units (for further details, see Section S6.3,
Supporting Information). The less reduced local paratropic
ring current of the pentalene moieties, arising from the
reduced delocalization with the adjacent naphthalene ring,
might lead to the upfield shifts of proton H1 of 3b compared
to the corresponding resonances of 1b and 2b, which are
observed by 1H NMR spectroscopy (Figure 3).
π
πꢀ
3
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(a)
(c)
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1.350
1.349
1.482
1.470
1.476
1.471
1.462
1.379
1.384
1.367
1.382
1.469
8
1.401
1.427
1.412
1.419
4
1.477
1.358
1.507
3
1.462
1.502
1.456
1.359
1.390
10
1.399
(b)
(d)
48o
81o
10
8
4
3
Figure 6. Xꢀray structures of (a) top view and (b) side view of 2b; (c) top view and (d) side view of 3b. Solvent molecules and hydrogen
atoms are omitted for clarity. Compound 2b and 3b are C2hꢀsymmetric molecules with space group P21/c; only half of the bond lengths are
presented. Thermal ellipsoids are drawn at 50% probability level at 100 K.
X-ray Crystallographic Analyses. Single crystals suitable
for Xꢀray diffraction were grown by slow evaporation of
CHCl3/hexane (2a, for Xꢀray crystal structure, see Section
S7.2, Supporting Information), CDCl3 (2b), and CH2Cl2 (3b).
and 3b (1.421 Å), which are slightly larger than that of
monobenzopentalene S11 (1.415 Å).23a
(a)
Views of the ORTEP drawings of bispentalenes 2b and 3b
as well as the relevant bond lengths of the cores are shown in
Figure 6. The crystal structures of both 2b and 3b show
significant doubleꢀ and singleꢀbond alternation in the
pentalene units, characteristic for antiaromaticity, similar to
that of monobenzopentalene S1123a (Figure S50, Supporting
Information). The degree of bond length alternation in 3b and
S11 is slightly enhanced compared to that in 2b. It is
noteworthy that the length of a C–C bond in the pentalene
moieties of 2b and 3b (around 1.50 Å, Figure 6a, 6c) is even
longer than the expected bond length (1.48 Å) for a C(sp2)–
C(sp2) single bond.31 On the other hand, the alternation of
bond lengths in the central sixꢀmembered rings is less
pronounced pointing to a rather aromatic character.31 These
a
c
d
b
a
d
d
c
e
e
c
d
c
a
b
a
(b)
1
bond length alternations as well as the H NMR chemical
shifts indicate that the electron delocalization in the fused
bispentalene cores of 2 and 3 is significantly decreased,
compared to benzoꢀtype aromatic systems. Although
informative for the understanding of the global molecular
properties, examination of only the fragmentary structures
Figure 7. Xꢀray crystal structure of bispentalene 3b. (a) View of
the nonꢀcovalent interactions (dotted blue lines) between the
adjacent molecules. Intermolecular distances (d(C···
Å; b: 3.418 Å; c: 3.544 Å; d: 3.589 Å; e: 3.416 Å. d(H···
2.737 Å; b: 2.631 Å; c: 2.595 Å; d: 2.752 Å; e: 2.628 Å). (b) View
of the 3D layered structures; hydrogen atoms and solvent
molecules (CH2Cl2) are omitted for clarity.
π
): a: 3.507
π): a:
without considering the
could be misleading. Therefore, average bond lengths of the
πꢀconjugated scaffold as a whole
entire chromophoric skeleton were calculated for 2b (1.418 Å)
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Page 6 of 11
Similar to S11,23a the bispentalene cores of 2b, and 3b are
which are symmetryꢀforbidden as confirmed by timeꢀ
dependent DFT calculations.
1
2
3
4
5
6
7
8
also planar with slight distortion. The aryl substituents are all
twisted with different dihedral angles relative to the
bispentalene cores. The largest dihedral angle is much smaller
for 2a (43°) (Figure S47, Supporting Information) and 2b
(48°, phenyl rings on C(3), C(8), Figure 6b) than for 3b (81°,
phenyl rings on C(4), C(10), Figure 6d). These observations
support the hypothesis for the stronger shielding effects on
protons H2 and H3 in 3b (Figure 3c).
Clear differences between their lowꢀenergy absorptions
could be seen for bispentalene derivatives with different
central ring fusion. The lowestꢀintensity absorption of
bispentalene 2a with a central benzene ring appears at λmax
820 nm (1.51 eV, = 380
–1cm–1), whereas for 3a with the
central naphthalene moiety, λmax is observed around 770 nm
(1.61 eV, = 400
–1cm–1). The differences might be due to
=
ε
M
ε
M
The crystal packing of bispentalene derivative 3b in the
solid state was also investigated. As shown in Figure 7a, each
molecule is connected to adjacent molecules in the same plane
through C–H···π interactions with a distance d(C···π) of 3.507
Å (a). Furthermore, each molecule is tightly interlaced by four
other molecules in two adjacent layers, and the structure is
9
the reduced delocalization between the electronically more
isolated pentalenic and naphthalenic units of 3a, as confirmed
through natural resonance theory analysis (NRT) (for further
details, see Section S6.2, Supporting Information).
Furthermore, the near orthogonal orientation between the
peripheral phenyl rings and the bispentalene backbone in 3a
reduces the conjugation pathlength. No clear lowꢀenergy
absorption maximum was observed for the Uꢀshaped
bispentalene 1a–b. It might be overlapping with the broad
shoulder peak covering the region of 550–700 nm.
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packed together by several C–H···
π interactions (b–e) with
distances d(C··· ) between 3.416 and 3.589 Å. By virtue of
π
these multiple weak nonꢀcovalent interactions, compound 3b
selfꢀassembles into a layered structure (Figure 7b) in the solid
state, with CH2Cl2 located in the interlayer space.
The influence of the functional groups at the peripheral
rings on the optical properties in 1–3 seems negligible.
Comparison of the UV/Vis spectra (Figure S52, Supporting
Information) of unsubstituted 2a, methoxyꢀsubstituted 2b, and
cyanoꢀsubstituted 2c revealed that only small shifts of the lowꢀ
energy absorptions were observed. For compound 2b with
electronꢀdonating methoxyꢀgroups, a slight bathochromic shift
of approximately 65 nm, whereas for 2c with electronꢀ
withdrawing cyano groups, a hypsochromic shift of about 30
Opto-electronic Properties. The optoꢀelectronic properties
of bispentalene derivatives 1–3 were investigated by UV/Vis
absorption spectroscopy, TDꢀDFT calculations, as well as
cyclic voltammetry (CV) and rotating disc voltammetry
(RDV). The results are summarized in Table 1 (for full data
set, see Section S9, Supporting Information).
The new compounds show specific colors in the solution of
CH2Cl2 or CHCl3: compounds in series 1 are purple, those in
series 2 pink, and those in series 3 orange (Figure S51,
Supporting Information). These colors originate from intense
absorptions in the Vis region. The UV/Vis absorption spectra
of bispentalenes 1–3 in CHCl3 at 298 K are depicted in Figure
8.
1
nm was detected. In agreement with the results from H NMR
spectroscopy, this suggests that the molecular properties of
bispentalenes 1–3 are dominated by the conjugated core and
not significantly influenced by the functional groups on the
peripheral rings.
2000"
1600"
7"
1200"
800"
6"
400"
1a"
ε
ε
ε
ε
5"
4"
3"
2"
1"
0"
0"
550"
2a"
3a"
1b"
2b"
3b"
2c"
3c"
650"
750"
850"
950"
1050" 1150" 1250"
λ
/ nm
ε
ε
ε
ε
300"
400"
500"
600"
700"
800"
900"
1000" 1100"
λ / nm
Figure 8. UV/Vis spectra of bispentalene derivatives 1, 2, and 3
recorded at 298 K in CHCl3 solution (4×10–5
). Inset: longꢀ
Figure 9. Calculated (B3LYP/6ꢀ31G(d)) orbital energy
diagram of HOMO–1, HOMO, LUMO, and LUMO+1 for
bispentalenes 1a, 2a, and 3a. Red and blue arrows represent
symmetryꢀforbidden and symmetryꢀallowed transitions,
respectively.
M
wavelength, lowestꢀintensity absorptions corresponding to the
symmetryꢀforbidden HOMO–LUMO transitions.
All compounds exhibited distinct absorptions in the 450–
580 nm region with absorption coefficients (
ε
reaching
2.5×104 –1cm–1. Different from bispentalenes 2 and 3,
M
Calculations of the excitedꢀstate electronic properties of
compounds 1–3 were performed by TDꢀDFT at the CAMꢀ
B3LYP/6ꢀ31G(d) level of theory using the software package
Gaussian 09.32a In all cases, the computed transition energies
are slightly larger than the experimental values with
differences in the range of 0.03–0.12 eV for the lowest energy
transition S0→S1, well within the expected error of recent
compounds 1a–b displayed broad shoulders in the region of
550–700 nm. Most interestingly, all compounds feature weak,
broad lowꢀenergy absorptions, which extend into the nearꢀ
infrared region of the spectra and end beyond 1300 nm (0.95
eV). These absorptions represent HOMO–LUMO transitions,
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benchmarks on common TDꢀDFT methods.32b These
1b correspond to the broad shoulders that were observed in the
absorption spectra at ca. 600nm.
1
2
3
4
5
6
7
8
HOMO→LUMO transitions are symmetryꢀforbidden, as they
would conserve symmetry with respect to an inversion center
of the pentalene unit, which is reflected by the very low
calculated oscillator strength f (typically f < 0.01). The second
excited states all consist of a ~ 1:1 composition of both
HOMO–1→LUMO and HOMO→LUMO+1 transitions,
resulting in a similarly lowꢀintense optical absorption band for
the bispentalenes 2 and 3 (f < 0.001, each), because the
HOMO→LUMO+1 vertical transition is symmetryꢀforbidden
as revealed by TDꢀDFT calculations (Figure 9). Whereas
slightly higher oscillator strengths of 0.07 and 0.08 for 1a and
Trends in the properties in the series of bispentalenes 1–3
can be well predicted by TDꢀDFT calculations. The calculated
energetic sequence of the lowestꢀenergy absorption of
1a ≈ 2a < 3a is in good agreement with the experimental
UV/Vis spectra (Figure 8 and Section S8, Supporting
Information) and electrochemical results. Similarly reproduced
is the trend of the third excitation state in the energetic
sequence 1 ≈ 3 > 2, all showing intense oscillator strength of
f > 0.5, in agreement with the absorptions around 500 nm.
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59
60
Table 1. Computational, Electrochemical, and Optical Data for Bispentalene Derivatives 1–3
Calca
Electrochemicalb
Opticale
Comp.
c
Egap(S0→S1)
E1ox
E1red
EHOMO
ELUMO
Egap
Egap
[eV]
λmax
[nm]
λmax
[nm]
[eV]
[V]
[V]
[eV]
[eV]
[eV]
1.59
1.56
1.59
1.57
1.59
1.63
1.62
1.62
+0.25
–1.23d
–1.32d
–1.25c
–1.25c
–1.07c
–1.35c
–1.33c
–1.10c
–5.05
–4.92
–5.05
–4.95
–5.28
–5.07
–5.00
–5.27
–3.57
1.48
1.44
1.50
1.40
1.55
1.62
1.53
1.57
838
861
827
885
800
765
810
790
–
–
1a
1b
2a
2b
2c
3a
3b
3c
+0.12
+0.25
+0.15
+0.48
+0.27
+0.20
+0.47
–3.48
–3.55
–3.55
–3.73
–3.45
–3.47
–3.70
–
–
820
885
790
770
801
772
1.51
1.40
1.57
1.61
1.55
1.61
aCalculations were performed at the CAMꢀB3LYP/6ꢀ31G(d)//
ωB97XꢀD/6ꢀ31G(d) level of theory with chloroform solvation (PCM).
bElectrochemical data obtained at a scan rate of 0.1 V s–1 in CH2Cl2 containing 0.1
M nBu4NPF6 on a glassy carbon working
electrode. All potentials are given versus the Fc+/Fc couple used as internal standard. HOMO and LUMO energy levels in eV were
approximated from the reversible halfꢀpotential reduction waves or the irreversible first reduction wave using the equation, HOMO
= –(4.80+E1ox), LUMO = –(4.80+E1red),33 Egap = LUMO–HOMO. λmax is the calculated optical gap from redox data (λmax
d
e
=1240/Egap).13d cReversible first reduction or oxidation wave. Irreversible first reduction or oxidation wave. The optical gap, Egap
,
is defined as the energy corresponding to the lowestꢀenergy absorption and estimated from corresponding spectra
(Egap=1240/
λ
max).13d
By cyclic voltammetry, bispentalenes 1–3 commonly
of –5.05 eV. Interestingly, πꢀelectron extension only raises the
exhibit two reversible oxidation waves and two reversible
LUMO energy levels and has no obvious effects on the
HOMO energy levels in these series of polycyclic
hydrocarbons. The higher LUMO energy level of 3a signifies
that the naphthaleneꢀfused bispentalene is less easily reduced
than benzeneꢀfused 1a and 2a, suggesting a stronger coupling
reduction waves in CH2Cl2 at 298 K (vs Fc+/Fc, + 0.1
M
nBu4NPF6; for detailed cyclic voltammogramms, linear sweep
voltammogramms or redox potentials, see Section S9,
Supporting Information). For example in compounds 2a and
3b, the first reversible oxidation occurs at +0.25 V and +0.20
V, while reduction occurs at –1.25 V and –1.33 V,
respectively. Compounds 1b and 3c, however, showed a first
irreversible reduction peak and a second reversible electron
transfer. In contrast, the first reduction for bispentalene 1a was
reversible whereas a second step presented an irreversible
reduction peak. Substitution effects are as expected: methoxy
groups shifts the first oxidation potential to more negative
values, whereas cyano substitution leads to anodic shifts of
first reduction potentials. On the basis of the experimental
values, LUMO energy levels for 1a, 2a, and 3a are estimated
as –3.57 eV, –3.55 eV, and –3.45 eV, respectively, while all
HOMO energy levels are maintained at around the same value
across the
πꢀframework of 1 and 2. Furthermore, the same
HOMO energy level indicates that the extension of the
π
ꢀ
system has almost no effect on the oxidation properties in
these series or the effect is balanced by some other factors.
These energy levels lead to HOMO–LUMO energy gap of
1.48 eV, 1.50 eV, and 1.62 eV for 1a, 2a, and 3a, respectively,
in agreement with the optical gaps calculated from the lowestꢀ
energy in the UV/Vis spectra.
The small effect of the functional groups on the peripheral
phenyl rings is also reflected by the values of HOMO–LUMO
gaps (Table 1). Bispentalene derivative 2b has the lowest
HOMO–LUMO energy gap of 1.40 eV, and this value is lower
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equiv.) was added. The reaction mixture was continued
πꢀconjugated polycyclic hydrocarbons
Page 8 of 11
than in other neutral,
1
2
3
4
5
6
7
8
stirring at 110 °C for another 24 h. After evaporation, the
residue was purified by column chromatography. The excess
of the alkyne could be recovered after the chromatographic
separation by recrystallization.
containing the same number of fused rings. The same value
was obtained for unsubstituted hexacene in a thin film, which
is thermally stable in the solid state in dark, but highly
unstable in solutions or when exposed to light.34 The
electrochemical gaps are all in good agreement with the
energy gaps derived from the absorption spectra, and
calculated by the DFT methods.
1,2,6,7-Tetrakis(4-methoxyphenyl)-3,5-diphenyldicyclo-
penta[a,h]-s-indacene 1b. A mixture of 4 (50 mg, 0.08
mmol), 5b (111 mg, 0.47 mmol), [Pd(PPh3)2Cl2] (11 mg, 0.02
mmol), K2CO3 (43 mg, 0.32 mmol), Zn dust (11 mg, 0.16
mmol), and hydroquinone (34 mg, 0.32 mmol) in toluene (2
mL) was treated according to the general procedure to give 1b
(13 mg, 21%) as a darkꢀpurple solid. Column chromatography
(Alumina N; hexane/CH2Cl2 1:2). Dec. > 250 °C; Rf = 0.30
SUMMARY AND CONCLUSIONS
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Three series of stable, neutral, πꢀextended bispentalene
derivatives with different fusion patterns and functional groups
on the peripheral phenyl groups were synthesized by a
modified double carbopalladation cascade reaction between
acetylenes and bis(gemꢀdibromoolefins) with the formation of
six C–C bonds during the oneꢀpot reaction. Although the
synthesized hydrocarbons have overall [4n+2] πꢀelectrons in
the backbone, their H NMR spectra showed intensive upfield
shifts of the protons in the pentalene subunits, comparable to
1
(Alumina B; cyclohexane/CH2Cl2 1:2); H NMR (400 MHz,
CDCl3)
δ = 3.74 (s, 6H, 2 OCH3), 3.76 (s, 6H, 2 OCH3), 6.05
(s, 2H, H–C(8,10)), 6.13 (s, 1H, H–C(4)), 6.46 (s, 1H, H–
C(9)), 6.63 (d, J = 8.8 Hz, 4H, arom.), 6.71 (d, J = 8.9 Hz, 4H,
arom.), 6.74 (d, J = 8.8 Hz, 4H, arom.), 6.88–6.95 (m, 4H,
arom.), 7.00 (d, J = 8.9 Hz, 4H, arom.), 7.05–7.09 (m, 4H,
arom.), 7.14–7.23 (m, 2H, arom.) ppm; 13C NMR (100 MHz,
1
antiaromatic monobenzopentalene derivatives with 4n
πꢀ
electrons.23a Further investigations on the magnetic shielding
properties were performed through NICSꢀXYꢀscans,
CDCl3)
δ = 55.05, 55.14, 113.14, 113.47, 114.67, 120.23,
127.21, 127.31, 127.80, 127.83, 128.93, 129.50, 130.77,
132.48, 133.07, 134.15, 137.22, 139.92, 145.28, 145.91,
147.54, 153.06, 158.46, 158.49 ppm; UV/Vis (CHCl3): λmax
suggesting for 18
paratropic ring current and a local diatropic ring current at the
central benzene ring. In contrast, ring currents in 22 ꢀelectron
πꢀelectron systems 1 and 2 a global
(lgε) 367 (ca. 4.78), 492 (ca. 4.27), 608 (ca. 4.01) nm; HRꢀ
π
MALDI/ESIꢀMS: m/z (%) 804.3136 (20), 803.3105 (63),
bispentalene systems 3 with a central naphthalene moiety are
found to be more localized compared to 1 and 2. Xꢀray crystal
structures of 2a,b and 3b revealed planar bispentalene cores
with large doubleꢀ and singleꢀbond alternation in the pentalene
subunits while much smaller bond alternation is seen in the
central benzene or naphthalene rings. Compound 3b was
shown to selfꢀassemble into a layered 3D structure in the solid
state. Interestingly, the UV/Vis spectra of the new compounds
all exhibit broad, lowꢀenergy absorptions extending into the
nearꢀinfrared region. In agreement with TDꢀDFT results, both
optical and electrochemical data showed much smaller
HOMO–LUMO energy gaps compared to other stable neutral
aceneꢀlike conjugated hydrocarbons containing the same
number of fused rings. The experimental results in
combination with theoretical calculations are indicative of the
overall antiaromatic character of bispentalenes 1–3 containing
+
802.3074 (100, [M]+, calcd for C58H42O4 : 802.3078).
3,8-Bis(3,5-di-t-butylphenyl)-1,2,6,7-tetrakis(4-methoxy-
phenyl)dicyclo-penta[a,g]-s-indacene 2b. A mixture of 6 (50
mg, 0.06 mmol), 5b (82 mg, 0.36 mmol), [Pd(PPh3)2Cl2] (8
mg, 0.01 mmol), K2CO3 (32 mg, 0.24 mmol), Zn dust(8 mg,
0.12 mmol), and hydroquinone (25 mg, 0.24 mmol) in toluene
(2 mL) was treated according to the general procedure to give
2b (12 mg, 20%) as a black solid. Column chromatography
(Alumina N; hexane/CH2Cl2 5:1). Mp. 263–264 °C; Rf = 0.28
1
(Alumina B; cyclohexane/CH2Cl2 1:1); H NMR (400 MHz,
CDCl3)
δ = 1.17 (s, 36H, 4 CMe3), 3.73 (s, 6H, 2 OCH3), 3.76
(s, 6H, 2 OCH3), 5.92 (s, 2H, H–C(5,10)), 6.43 (s, 2H, H–
C(4,9)), 6.68–6.75 (m, 14H, arom.), 6.85 (s, 4H, arom.), 6.99–
7.02 (m, 4H, arom.) ppm; 13C NMR (100 MHz, CDCl3)
δ =
31.27, 34.76, 55.21, 113.30, 113.50, 117.62, 121.86, 123.94,
127.27, 127.76, 129.50, 130.88, 132.11, 132.24, 132.90,
135.44, 139.06, 146.08, 146.23, 148.86, 149.52, 153.70,
two 8 πꢀelectron pentalene units in spite of their [4n+2] πꢀ
electron perimeters. Due to its low HOMO–LUMO band gap,
environmental stability and excellent solubility, bispentalene
2a is a promising candidate for use in organic electronic
devices.
158.41, 158.54 ppm; UV/Vis (CHCl3): λmax (lgε) 370 (ca.
4.82), 535 (ca. 4.44), 885 (ca. 2.79) nm; HRꢀMALDI/ESIꢀMS:
m/z (%) 1028.5652 (32), 1027.5617 (79), 1026.5580 (100,
+
[M]+, calcd for C74H74O4 : 1026.5582).
The present study not only introduces new bispentaleneꢀ
based chromophores, but also emphasizes the importance of
local structures in global molecular properties. Thus, it is
broadening the design strategies toward molecules with lowꢀ
energy band gaps and higher stability for organic materials
applications.
2,3,8,9-Tetrakis(4-methoxyphenyl)-4,10-diphenyl-
dipentaleno[1,2-a:1',2'-f]-naphthalene 3b. A mixture of 7
(100 mg, 0.14 mmol), 5b (205 mg, 0.84 mmol),
[Pd(PPh3)2Cl2] (20 mg, 0.03 mmol), K2CO3 (80 mg, 0.56
mmol), Zn dust (19 mg, 0.28 mmol), and hydroquinone (63
mg, 0.56 mmol) in toluene (4 mL) was treated according to the
general procedure to give 3b (34 mg, 28%) as a brown solid.
Column chromatography (Alumina N; hexane/CH21Cl2 4:1).
Mp. 291–292 °C; Rf = 0.65 (Alumina B; CH2Cl2); H NMR
EXPERIMENTAL SECTION
General Procedure for the Synthesis of Bispentalene
Derivatives via Carbopalladation Cascade Reaction.
Bis(gemꢀdibromoolefin) (1.0 equiv.), alkyne (6.0 equiv.),
[Pd(PPh3)2Cl2] (0.2 equiv.), zinc dust (2.0 equiv.), and K2CO3
(4.0 equiv.) were placed in a dry Schlenk tube. The tube was
evacuated and refilled with nitrogen five times. Dry toluene
a 0.04 M solution of bis(gemꢀ
dibromoolefin)) was added, the tube was sealed, and the
mixture stirred at 110 °C for 1–2 h, then hydroquinone (4.0
(400 MHz, CDCl3)
δ = 3.71 (s, 6H, 2 OCH3), 3.75 (s, 6H, 2
OCH3), 5.82 (s, 2H, H–C(1,7)), 5.93 (d, J = 8.2 Hz, 2H, H–
C(5,11)), 6.02 (d, J = 8.4 Hz, 2H, H–C(6,12)), 6.57 (d, J = 8.9
Hz, 4H, arom.), 6.68 (2d, J = 8.9 Hz, 8H, arom.), 6.94 (d, J =
8.8 Hz, 4H, arom.), 7.01–7.09 (m, 4H, arom.), 7.17–7.25 (m,
(enough to generate
6H, arom.) ppm; 13C NMR (100 MHz, CDCl3)
55.15, 113.08, 113.43, 122.10, 125.57, 126.64, 127.41,
δ
= 55.03,
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(8) Stowasser, B.; Hafner, K. Angew. Chem., Int. Ed. Engl. 1986, 25,
466–468.
128.05, 129.21, 129.31, 129.48, 130.55, 132.58, 133.48,
134.86, 135.52, 142.79, 146.11, 146.63, 148.71, 155.16,
158.42, 158.55 ppm UV/Vis (CHCl3): λmax (lg ) 374 (ca.
1
2
3
4
(9) For some recent examples, see: (a) Hoheisel, T. N.; Schrettl. S.;
Szilluweit, R.; Frauenrath, H. Angew. Chem., Int. Ed. 2010, 49, 6496–
6515. (b) Pisula, W.; Feng, X.; Müllen, K. Chem. Mater. 2011, 23, 554–
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699–704.
ε
4.68), 507 (ca. 4.29), 801 (ca. 2.59) nm; HRꢀMALDI/ESIꢀMS:
m/z (%) 854.3300 (23), 853.3269 (66), 852.3233 (100, [M]+,
+
calcd for C62H44O4 :852.3234).
5
6
7
8
9
(10) For some recent reviews, see: (a) Wang, C.; Dong, H.; Hu, W.; Liu,
Y.; Zhu, D. Chem. Rev. 2012, 112, 2208–2267. (b) Mei, J.; Diao, Y.;
Appleton, A. L.; Fang, L.; Bao, Z. J. Am. Chem. Soc. 2013, 135, 6724–
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2005, 17, 3001–3005. (b) Anthony, J. E. Chem. Rev. 2006, 106, 5028–
5048. (c) Palayangoda, S. S.; Mondal, R.; Shah, B. K.; Neckers, D. C. J.
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ASSOCIATED CONTENT
Supporting Information.
Materials and methods, experimental details and data for new
compounds as well as copies of H and 13C NMR spectra for
1
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new compounds, Xꢀray crystallographic data and cif files for
2a, 2b, and 3b, full data set of UV/Vis spectroscopy, cyclic
voltammetry (CV) and rotating disc voltammetry (RDV)
results, computational details for 1, 2, and 3, and relative
stabilities in solution. This material is available free of charge
via the Internet at http://pubs.acs.org.
(12) (a) Breslow, R.; Foss, F. W. Jr. J. Phys: Condens. Matter 2008, 20,
374104. (b) Breslow, R.; Schneebeli, S. T. Tetrahedron 2011, 67, 10171–
10178.
AUTHOR INFORMATION
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and eightꢀmembered rings, see: (a) Feng, C.ꢀN.; Kuo, M.ꢀY.; Wu, Y.ꢀT.
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L. N.; Haley, M. M. Angew. Chem., Int. Ed. 2011, 50, 1127–1130. (b)
Chase, D. T.; Fix, A. G.; Kang, S. J.; Rose, B. D.; Weber, C. D.; Zhong,
Y.; Zakharov, L. N.; Lonergan, M. C.; Nuckolls, C.; Haley, M. M. J. Am.
Chem. Soc. 2012, 134, 10349–10352. (c) Nishida, J.; Tsukaguchi, S.;
Yamashita, Y. Chem.—Eur. J. 2012, 18, 8964–8970. (d) Fix, A. G.; Deal,
P. E.; Vonnegut, C. L.; Rose, B. D.; Zakharov, L. N.; Haley, M. M. Org.
Lett. 2013, 15, 1362–1365. (e) Shimizu, A.; Kishi, R.; Nakano, M.;
Shiomi, D.; Sato, K.; Takui, T.; Hisaki, I.; Miyata, M.; Tobe, Y. Angew.
Chem., Int. Ed. 2013, 52, 6076–6079. (f) Tobe, Y. Chem. Rec. 2015, 15,
86–96.
Corresponding Authors
diederich@org.chem.ethz.ch
stanger@tx.technion.ac.il
Present Address
⊥MTAꢀSZTE Stereochemistry Research Group, Dóm tér 8,
Szeged 6720, Hungary.
Author Contributions
||These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by a grant from the Swiss National
Science Foundation (SNF). O.D. acknowledges support by a
Kekulé fellowship (Fonds der Chemischen Industrie) and the
Studienstiftung des Deutschen Volkes. A.S. thanks the Israel
Science Foundation (ISF, grant number 1485/11) for financial
support.
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