Unexpected formation of a [4]radialene and dendralenes by addition of tetracyanoethylene to a tetraaryl[5]cumulene

Article abstract of DOI:10.1002/anie.201309355

The use of cumulenes in synthetic transformations offers the possibility to form structurally interesting and potentially useful conjugated molecules. The cycloaddition reaction of a tetraaryl[5]cumulene with the electron-deficient olefin tetracyanoethylene affords unusual products, including functionalized dendralenes and alkylidene cyclobutanes, as well as a symmetric [4]radialene that shows unique solvatochromism, with λ_max values approaching the near-IR region. These carbon-rich products have been investigated spectroscopically and by X-ray crystallographic analysis (five structures). The cycloaddition reaction sequence has also been explored by mechanistic and theoretical studies. The obtained results clearly demonstrate the potential of [5]cumulenes to serve as precursors for unprecedented conjugated structures. Unusual structures and interesting electronic properties characterize the cyclic products from the reaction of TCNE with tetraaryl[5]cumulene. Mechanistic investigations, aided by DFT calculations, outline a likely pathway to the observed products. The addition of MeOH, EtOH, and Br₂ to various intermediates leads to the interesting dendralene compounds.

Full text of DOI:10.1002/anie.201309355

Angewandte
Chemie
DOI: 10.1002/anie.201309355
Reactions of [5]Cumulenes
Unexpected Formation of a [4]Radialene and Dendralenes by Addition
of Tetracyanoethylene to a Tetraaryl[5]cumulene**
Johanna A. Januszewski, Frank Hampel, Christian Neiss, Andreas Gçrling, and
Rik R. Tykwinski*
Dedicated to Professor Kendall (Ken) Houk on the occasion of his 70th birthday
Abstract: The use of cumulenes in synthetic transformations
offers the possibility to form structurally interesting and
potentially useful conjugated molecules. The cycloaddition
reaction of a tetraaryl[5]cumulene with the electron-deficient
olefin tetracyanoethylene affords unusual products, including
functionalized dendralenes and alkylidene cyclobutanes, as
well as a symmetric [4]radialene that shows unique solvato-
chromism, with lmax values approaching the near-IR region.
These carbon-rich products have been investigated spectro-
scopically and by X-ray crystallographic analysis (five struc-
tures). The cycloaddition reaction sequence has also been
explored by mechanistic and theoretical studies. The obtained
results clearly demonstrate the potential of [5]cumulenes to
serve as precursors for unprecedented conjugated structures.
Cumulenes typically undergo thermal or photochemical
cycloaddition reactions. To date, no general guidelines in
regard to these reactions have been reported, but the general
reactivity pattern is summarized in Scheme 1. [5]Cumulenes
can undergo a thermal dimerization reaction at the central g-
bond to give radialenes 1 with either alkyl[7,8] or aryl[9]
substituents. Furthermore, Iyoda and co-workers have
shown that [5]cumulenes also undergo dimerization reactions
at the b-bond under Ni0 catalysis to afford the less-sym-
metrical radialenes 2 (R = aryl) or 3 (R = alkyl).[8,10] The
reaction of [5]cumulenes with electron-deficient acetylenes
seems to occur preferentially at the central g-bond to give the
[2+2] products 4.[7,11] A similar reaction between tetrafluoro-
ethene and tetra(tert-butyl)[5]cumulene gives 5.[7]
These results suggest that reaction at the more electron-
rich b-bond of the cumulene skeleton is not observed because
of unfavorable steric interactions between the end groups.
The reaction of tetraferrocenyl[5]cumulene with C60 and
tetracyanoethylene (TCNE),[11] however, readily occurs at the
b-bond to give 6, which suggests that the reaction outcome is
probably not governed solely by steric factors.
T
he [n]cumulenes (n = number of cumulated double bonds
in a chain of n + 1 carbon atoms) constitute a class of
compounds with interesting physical properties.[1] For exam-
ple, the one-dimensional framework constructed of cumu-
lated sp-hybridized carbon atoms can serve as building blocks
for molecular wires or as linkers in unprecedented carbon
nanostructures.[2] The chemistry and reactivity of lower
[n]cumulenes, namely, n = 2 (allenes)[3] and n = 3 (buta-
trienes),[4,5] has been explored in recent years, but longer
[n]cumulenes (n ꢀ 5) have received only sporadic attention
over the past four decades.[1,6]
Cumulenes are a class of molecules based on sp-hybrid-
ized carbon atoms, and polyynes represent another. The [2+2]
reaction of polyynes with TCNE has been established by
Diederich and co-workers,[12,13] and others,[14] to be a very
efficient “click reaction” that forms new chromophores with
outstanding stability and unique optical properties. Unfortu-
nately, the reported studies offer little guidance as to whether
the regiochemistry of a cycloaddition reaction of a [5]cumu-
lene with TCNE would be governed by steric or electronic
factors. Thus, we performed density-functional (DFT) calcu-
lations to offer insight. These calculations show that the two
central-most carbon atoms bear a slight negative charge,
while the neighboring carbon atoms are slightly positively
charged (see the Supporting Information for details). This
charge distribution may guide the highly electrophilic TCNE
towards the g-bond. Furthermore, the bulky aryl groups at the
terminus are expected to hinder the approach of the TCNE to
the b-bond. Considering the reaction of TCNE and 7 shown in
Scheme 2 and examining the frontier molecular orbitals (see
the Supporting Information), it is clear that a concerted [2+2]
addition of TCNE at the g-bond is forbidden by orbital
symmetry, as also expected on the basis of the Woodward–
Hoffmann rules for a four-electron system. We hypothesized
that the addition of TCNE to cumulene 7 could occur through
a stepwise mechanism that is sterically directed toward the g-
bond of the cumulene (Scheme 2). A subsequent retro [2+2]
[*] J. A. Januszewski, Dr. F. Hampel, Prof. Dr. R. R. Tykwinski
Department fꢀr Chemie und Pharmazie & Interdisciplinary Center
for Molecular Materials (ICMM)
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg (FAU)
Henkestrasse 42, 91054 Erlangen (Germany)
E-mail: rik.tykwinski@fau.de
Homepage: http://www.chemie.uni-erlangen.de/tykwinski
Dr. C. Neiss, Prof. Dr. A. Gçrling
Lehrstuhl fꢀr Theoretische Chemie & Interdisciplinary Center for
Molecular Materials (ICMM)
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg (FAU)
Egerlandstrasse 3, 91058 Erlangen (Germany)
[**] Funding is gratefully acknowledged from the Friedrich-Alexander-
Universitꢁt Erlangen-Nꢀrnberg, the Deutsche Forschungsgemein-
schaft (SFB 953, “Synthetic Carbon Allotropes”), and “Solar
Technologies go Hybrid”—an initiative of the Bavarian State
Ministry for Science, Research, and Art. We thank Dr. Alexandra
Griffin (Agilent Technologies) for collecting the X-ray structure data
of 10a.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309355.
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formed (Scheme 2). Purification by
chromatography allowed the isola-
tion of pure products A and C,
although it was quickly clear that
these products were neither 8 nor 9.
The identity of product A was
tackled first.
A signal at m/
z 979.6557 in the high-resolution
ESI mass spectrum (positive
mode) is consistent with the for-
mula
[7 + TCNE + Na]+
(C68H84N4Na). The aryl region of
1
the H NMR spectrum shows trip-
lets at 7.61, 7.53, and 7.49 ppm
(integration ratio 1:1:2) and dou-
blets at 7.29, 7.19, and 7.12 ppm
(integration ratio 4:2:2). These sig-
nals are consistent with four di-tert-
butylphenyl groups, but also con-
firm a loss of symmetry during the
course of the reaction. There is
a
noteworthy
resonance
at
203.4 ppm in the 13C NMR spec-
trum, while the IR spectrum reveals
a weak signal at 1920 cmÀ1; both
observations suggest an allene
moiety. Finally, overlaying a solu-
tion of A with either MeOH or
EtOH afforded crystals, and struc-
Scheme 1. Examples of cycloaddition reactions reported for [5]cumulenes; Fc: ferrocenyl.
Scheme 2. Hypothesized synthesis of 9, by reaction of 7 with TCNE.
Inset: schematic depiction of the TLC analysis of the reaction after 2 h;
SiO2, CH2Cl2/hexanes=1:1. The identity of products A–C is described
in the text.
reaction from 8 might then give 9. This overall sequence
would be a useful metathesis reaction to form polarized
cumulene products such as 9.[4]
Herein, we report the reaction of [5]cumulene 7 with
TCNE. Although the desired product 9 is not isolated from
this reaction, the resulting synthesis offers a number of
equally intriguing and unprecedented cross-conjugated[15]
structures, including two acyclic [4]dendralenes (after further
bromination),[15,16] a cyclic [3]dendralene, and an electron-
deficient [4]radialene.[15,17] The products and unusual reaction
mechanisms are discussed with the help of five X-ray crystal
structures and DFT calculations.
The reaction of 7 with TCNE was carried out in CH2Cl2 at
room temperature (Scheme 2). After 2 h, a “rainbow” of at
least six products was apparent by TLC analysis. After several
days, however, three predominant products (A, B, and C) had
Figure 1. ORTEP representations (20% probability level) of com-
pounds 10a and 10b.
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tural analysis (Figure 1) showed that the resulting structures
correspond to the ethyl and methyl enol ethers 10a and 10b,
respectively. Thus, product A can be confidently assigned as
cyclic [3]dendralene 11 [Eq. (1)].[18]
The identification of 11 and 12 (products A and C,
respectively) is, in principle, consistent with intermediate 9
being formed during the reaction of 7 with TCNE. As the
study progressed, it became clear that product B (Scheme 2)
is not stable, and transforms to products A and C in a ratio of
about 9:1 over time, on warming, or concentration of the
reaction mixture. Thus, our attention turned to identification
of product B. Viehe and co-workers[23] had shown that
polarized [3]cumulenes could be efficiently trapped by
reaction with bromine. Thus, product B was isolated at low
temperature and treated with bromine at 08C in a solution of
CHCl3 [Eq. (2)]. The product of this reaction is, however, the
The most notable characteristic of product C is its color,
which changes from orange when adsorbed on silica gel to
green in the solid state.[19] UV/Vis spectroscopy indicates that
product C also shows solvatochromism. Specifically, the UV/
Vis spectrum shows a broad low-energy absorption with
lmax > 700 nm (see the Supporting Information) that ranges
from lmax = 720 nm (cyclohexane) to lmax = 771 nm (CHCl3),
characteristic of an intramolecular charge-transfer absorp-
tion.[20] The ESI MS signal at m/z 979.6576 ([M + Na]+) is
analogous to that observed for product A (i.e. compound 11).
The aryl region of the 1H NMR spectrum for product C,
however, shows two sets of signals, namely, only two unique
aryl groups and a structure with twofold symmetry. Further-
more, no resonance that is consistent with an allenic sp-
hybridized carbon atom is found in the 13C NMR spectrum.
Ultimately, X-ray crystallography identified the structure of
product C as radialene 12 (Figure 2). The rectangular
structure of 12[21] suggests a donor–acceptor (push-pull)
interaction between the dicyanovinyl acceptor and the
electron-rich bis(dialkylaryl)vinyl groups, with shortened
symmetrical and stable [4]dendralene 14, as established by X-
ray crystallography (Figure 3). Compound 14 is not, unfortu-
nately, a product that is easily linked to the presence of
[3]cumulene 9 during the reaction of 7 with TCNE.
À
À
C1 C2 bond lengths of 1.469(2) ꢀ and lengthened C1 C1’
À
and C2 C2’ bonds (1.494(3) and 1.504(3) ꢀ, respectively).
This is concurrent with elongation of the alkylidene bonds
À
À
C1 C4 and C2 C3 (1.36–1.37 ꢀ) relative to those in other
[4]radialenes.[22] The rigid structure of 12 is helical in the solid
state, and perhaps configurationally stable in solution because
of restricted rotation of the aryl groups.
Figure 3. ORTEP representation of 14 (20% probability level).
It was hypothesized that dendralene 14 could be
formed from the reaction of Br2 with radialene 12,
which might be produced in situ from product B.
Therefore, a test reaction was carried out in which
radialene 12 was treated directly with excess
bromine (Scheme 3). This reaction does not give
the dibromoadduct 14, but rather [4]dendralene 15,
an isomeric analogue of 14 (bromination of one aryl
ring is also observed). The structure of 15 was
confirmed by X-ray crystallography. The use of less
bromine gave a single product, which was identified
tentatively as [4]dendralene 16.[24]
With the identity of product B not yet con-
firmed, direct isolation and characterization was
attempted. The reaction of 7 and TCNE was
conducted over 24 h at À258C. The desired product
Figure 2. Molecular structure and ORTEP representation (20% probability level) of
radialene 12 (product C). Selected bond lengths [ꢂ]: C1-C1’ 1.494(3), C1-C2
1.469(2), C2-C2’ 1.504(3), C2-C3 1.362(2), C1-C4 1.370(2).
B was purified by column chromatography using
CDCl3, with the column temperature maintained
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a signal at m/z 979.6575 ([M + Na]+), these data support that
product B can be assigned as compound 8 (Scheme 2).[25]
A proposed mechanism for the conversion of 8 into 11 and
12 is shown in Scheme 4.[26] Homolytic cleavage of the central
À
(NC)2C C(CN)2 bond gives intermediate 17, which is stabi-
lized by allylic delocalization of the unpaired electrons.
Cyclization of rotamer 17’ then affords [3]dendralene 11.
Radialene 12 can also be formed from 8 in a stepwise
mechanism, via intermediate 17. However, neither a con-
certed reaction that converts 8 directly into 12 nor a two-step
four-electron electrocyclic ring closing/opening can be ruled
out.[27]
DFT calculations show that a reaction of 7 with TCNE to
give 8 (product B), followed by rearrangement to 11 and 12
(products A and C), is in excellent agreement with predictions
based on stability (Figure 4), namely, compounds 11 and 12
are clearly more stable than 8. Interestingly, the inclusion of
van der Waals corrections in the density-functional calcula-
tions gives enhanced stabilization of products 8, 11, and 12
compared to calculations without this correction. This obser-
vation is easily rationalized by the fact that the products are
stabilized by intramolecular dispersion interactions between
the aryl groups. The computed energies also explain why 9 is
not observed, given that a metathesis reaction from 8 to 9 is
significantly endothermic when dispersive interactions are
included. In the absence of dispersion corrections, the
formation of 11 and 12 is still preferred over 9, although the
differences are less pronounced.
Scheme 3. Top: Bromination of radialene 12 (product C); bottom:
ORTEP representation of 15 (20% probability level); Ar=3,5-di-tert-
butylphenyl.
In summary, we have presented the reaction of TCNE and
a tetraaryl[5]cumulene (7) that affords mainly the novel
vinylidene cyclobutane 8, which contains two exocyclic allene
groups. Compound 8 can be isolated at low temperature and
characterized spectroscopically, although in solution at room
temperature it converts into cyclobutane 11 and the electron-
deficient radialene 12. [4]Radialene 12 has an interesting
electronic absorption approaching the near-IR region that
appears to originate from a charge-transfer transition. The
between À20 and 08C (by using a jacketed column). The
fractions containing product B were stored at low temper-
ature (i.e. on dry ice) to prevent conversion into A and C. The
combined CDCl3 fractions (ca. 250 mL) were reduced to less
than 1 mL under vacuum, and NMR spectra were acquired.
The 1H NMR spectrum shows only one set of signals
representative of the 3,5-di-tert-butylphenyl group, namely
a broad singlet (suggesting a triplet) at 7.39 ppm and a doublet
at 7.16 ppm. These signals indicate a highly symmetrical
structure for product B. More significantly, the 13C NMR
spectrum shows the signal corresponding to an allene at
203.6 ppm, as well as that of an sp3-hybridized carbon atom at
42.9 ppm. Combined with ESI MS analysis, which shows
Figure 4. Computed energies (kcalmolÀ1) of the products 8 (product
B), 11 (product A), 12 (product C), and the hypothesized product 9
relative to that of the reactants TCNE+7. Calculations based on DFT
including (black) and without (light gray) dispersion interaction
corrections (see the Supporting Information for computational
details).
Scheme 4. Proposed mechanism for the conversion of 8 into 11 and 12
(products B, A, and C, respectively); Ar=3,5-di-tert-butylphenyl.
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Received: October 26, 2013
Published online: February 26, 2014
Keywords: cumulenes · cycloaddition · dendralenes ·
.
radialenes · TCNE
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