Unlocking acyclic π–bond rich structure space with tetraethynylethylene–tetravinylethylene hybrids

Article abstract of DOI:10.1021/jacs.9b08885

Literature reports describe tetraethynylethylene (TEE) as unstable but tetravinylethylene (TVE) as stable. The stabilities of these two known compounds are reinvestigated, along with those of five unprecedented TEE-TVE hybrid compounds. The five new C₁₀ hydrocarbons possess a core, tetrasubstituted C=C bond carrying all possible combinations of vinyl and ethynyl groups. A unified strategy is described for their synthesis, whereupon cross-conjugated ketones are dibromo-olefinated then cross-coupled. Due to an incorrect but nonetheless widely held belief that acyclic π-bond rich hydrocarbons are inherently unstable, a standardized set of robustness tests is introduced. Whereas only TVE survives storage in neat form, all seven hydrocarbons are remarkably robust in dilute solution, generally surviving exposure to moderate heat, light, air, and acid. The first X-ray crystal structure of TEE is reported. Subgroups of hybrids based upon conformational preferences are identified through electronic absorption spectra and associated computational studies. These new acyclic π-bond rich systems have extensive, untapped potential for the production of stable, conjugated carbon-rich materials.

Full text of DOI:10.1021/jacs.9b08885

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Article
Unlocking Acyclic #-Bond Rich Structure Space With
Tetraethynylethylene–Tetravinylethylene Hybrids
Kelsey L. Horvath, Nicholas L. Magann, Madison J. Sowden, Michael G. Gardiner, and Michael S. Sherburn
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 20 Nov 2019
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Unlocking Acyclic π-Bond Rich Structure Space
With Tetraethynylethylene–Tetravinylethylene Hybrids
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Kelsey L. Horvath,‡ Nicholas L. Magann,‡ Madison J. Sowden,‡ Michael G. Gardiner and Michael S. Sherburn*
Research School of Chemistry, Australian National University, Canberra, ACT 2601 (Australia)
KEYWORDS hydrocarbons • polyenes • cross-coupling • carbon-rich materials
ABSTRACT: Literature reports describe tetraethynylethylene (TEE) as unstable but tetravinylethylene (TVE) as stable. The stabilities of
these two known compounds are reinvestigated, along with those of five unprecedented TEE-TVE hybrid compounds. The five new C10 hy-
drocarbons possess a core, tetrasubstituted C=C bond carrying all possible combinations of vinyl and ethynyl groups. A unified strategy is
described for their synthesis, whereupon cross-conjugated ketones are dibromo-olefinated then cross-coupled. Due to an incorrect but none-
theless widely-held belief that acyclic π-bond rich hydrocarbons are inherently unstable, a standardized set of robustness tests is introduced.
Whereas only TVE survives storage in neat form, all seven hydrocarbons are remarkably robust in dilute solution, generally surviving exposure
to moderate heat, light, air and acid. The first X-ray crystal structure of TEE is reported. Sub-groups of hybrids based upon conformational
preferences are identified through electronic absorption spectra and associated computational studies. These new acyclic π-bond rich systems
have extensive, untapped potential for the production of stable, conjugated carbon-rich materials.
INTRODUCTION
Tetraethynylethylene (TEE, 1) and tetravinylethylene (TVE, 2),
and their five hybrid structures 3-7 are amongst the smallest com-
pounds that contain both through-conjugated and cross-
conjugated segments (Figure 1). The Diederich group disclosed
1
2,3
the first synthesis of TEE 1 in 1991. Separate studies subse-
quently reported TEE 1 as unstable as a solid at 25 °C–even in the
absence of O –and caution has been expressed regarding the poten-
2
4
tial for explosive decomposition of TEE. The first synthesis of
5
TVE 2 was reported by Skattebøl and co-workers in the 1960s. In
2
014 we reported a one-step, multigram scale synthesis of TVE 2
6
involving the fourfold Stille coupling of tetrachloroethylene. At the
time, we noted the surprising stability of TVE 2, which is a liquid
that survives storage on the bench at ambient temperature and
pressure.
TEE molecules with substituents at the four terminal alkyne po-
Figure 1. TEE 1 and TVE 2, and the five possible hybrid structures 3-7
that define the structural possibilities between them. Key: TEVE =
triethynylvinylethylene; DVDEE = divinyldiethynylethylene; TVEE =
trivinylethynylethylene.
7
sitions are stable compounds. In fact, the tetraphenyl analog was
8
the first TEE compound to be prepared in 1967. Many substituted
analogues of TEE 1 have subsequently been prepared and studied,
along with oligomeric TEE-based structures, expanded and cyclic
systems, with much interest focused on their enormous potential in
In contrast to the large, established body of important work fo-
cused on molecules carrying a TEE core, TVE 2 chemistry is in its
infancy, with the first steps toward general synthetic approaches
being published in 2014 and 2019. These studies demonstrate
that a compound with a tetravinylethylene unit serves as a hub for a
sequence of up to four pericyclic reactions, each of which creates
greater structural complexity. With a tetracycle and ten stereocen-
ters generated through this process, TVE compounds have great
potential in step- and atom-economic target synthesis.
9
optical and electrical properties. Recent reports describe Glaser-
type couplings of TEE 1 to prepare β-graphdiyne, a two dimension-
1
3
14
al carbon allotrope with predicted applications in energy storage
1
0
and molecular electronics.
Interest in designed carbon allotropes and associated (generally
2
hydrocarbon-based) model molecular materials comprising sp and
1
1
sp carbons is rapidly expanding. These efforts are focused on sys-
tems that rival the conducting, no band gap, “wonder material”
1
2
The reported difference in stability between TEE 1 and TVE 2 is
stark, and their projected applications are in divergent fields, name-
ly functional materials and target synthesis. These facts prompted
us to consider the structures that span the chasm of structural space
graphene, or exhibit specific band gaps required for devices such
as semiconductors.
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Page 2 of 8
between TEE 1 and TVE 2: structures which define the most fun-
vide new insights into selective cross-coupling reactions. This ap-
proach raised several pertinent questions. Firstly, was mono-
coupling of dibromide 13 feasible? If so, would the bromine cis- to
the alkenyl-substituent or the one cis- to the alkynyl-substituent be
replaced by a new C–C bond? Finally, if selective mono-coupling
could be achieved, would it be possible to steer the outcome of the
second coupling to obtain each geometrical isomer, 20 and 21?
With the main strategy defined and potential issues identified,
one final tactical decision was made. We identified the TIPS- or
TMS-protected terminal alkyne derivative of the C10 hydrocarbon
as the primary target, and perform a final stage desilylation to ob-
tain the hybrid molecule 3-7. We adopted this approach since it
was presumed that the trialkylsilylated derivatives would be both
less volatile and more stable than the corresponding hydrocarbon
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
damental combinations of sp and sp carbons, hence serve as the
foundations of applications in this field. Figure 1 depicts the five
possible hybrids of TEE 1 and TVE 2, which feature both vinyl and
ethynyl units attached to a tetrasubstituted central ethylene core.
Structure 3 carries three ethynyl groups and one vinyl group, 7
carries three vinyl groups and one ethynyl group, and 4, 5 and 6 are
the three constitutional and geometrical isomers carrying two
ethynyl and two vinyl groups. Surprisingly, none of these five C10
hydrocarbons have been previously prepared. We devised a method
to access compounds 3-7 for two reasons: firstly, in order to pro-
vide critical information about structure-stability/reactivity rela-
tionships of fundamental acyclic conjugated hydrocarbons; and
secondly, to deliver a significantly greater variety of core C10 struc-
tures to encourage applications of TEE and TVE-based structures
in new materials and target synthesis.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
-7. This prediction, which extrapolates information from literature
1,2,3,4
reports of trialkylsilylated TEE derivatives,
rect.
proved to be cor-
Whereas the parent hydrocarbons 3-7 have not been prepared,
we were encouraged by reports of stable, substituted derivatives of:
Scheme 1. The new transformations under scrutiny (shaded)
and the closest precedent to them (unshaded).
1
5
16
(
a) E-DVDEE 5 prepared by Diederich, Nielsen and Yamamo-
17 18 15d,15e,15g,19
to; (b) TEVE 3 by Hauptmann, Diederich
and Bar-
20
luenga; and (c) of cyclic substituted analogs of gem-DVDEE 6
previously from the Diederich laboratory1,2
21
described by Hopf. No substituted versions of Z-DVDEE 4 or
R
R
Br
Br
TVEE 7 have been reported in the literature.
known
R
4
-TEE
RESULTS AND DISCUSSION
R
R
8
R
R
9
CAUTION: TEE 1 is explosive when in neat crystalline form
vide infra). We have not experienced issues with solutions of TEE
but workers should be cautious when handling this substance in
(
1
previously from our laboratory12
R
R
R
Br
Br
known
any form. We recommend the generation of batches of < 5 mg and
that TEE 1 be maintained in solution. Experienced laboratory re-
searchers should wear appropriate PPE when working with this
compound and ensure it remains in a fume hood behind a blast
shield, as a precaution.
4
R -TVE
R
R
1
8
0
11
R
this work
Br
R
R
Br
not known
Samples of TEE 1 and TVE 2 were prepared by literature meth-
4
R -gem-DVDEE
ods, involving one flask, fourfold cross-couplings of tetrachloroeth-
twofold
4,6,22
coupling
ylene with vinyl and ethynyl nucleophiles.
In principle, stepwise
R
R
1
2
R
R
sequential couplings with the same vinyl and ethynyl nucleophiles
would permit the synthesis of the new hydrocarbons 3-7. The req-
uisite selective cross-couplings of tetrahaloethylenes, however,
R
R
R
R
R
Br
Br
not known
1
3
or
continue to prove elusive.
twofold
coupling
The approach that ultimately proved successful (Scheme 1) in-
volved twofold cross-couplings of 1,1-dibromoalkenes. We were
inspired by the pioneering work of the Diederich group, who de-
R
R
14
R -TEVE
15
R -TVEE
1
3
R
R
R
4
4
previously from our laboratory21
1
5,19
veloped this method to prepare substituted TEEs.
We have
R
R‘
R
R
R’
previously used this process for the synthesis of substituted den-
Br
Br
H
2
3
14
known
Br
known
dralenes and TVEs. On face value, therefore, the synthesis ap-
pears to be trivial. Closer inspection, however, reveals that unprec-
edented cross-coupling processes must be pioneered in order to
achieve a synthesis of hybrids 3-7 using this approach. Firstly,
or
H
H
H
R
1
6
R
17
R
R
18
19
2
whereas twofold sp–sp cross-couplings of 1,1-dialkynyl-2,2-
2
2
this work
R
R
dibromoethylenes 8 into TEEs 9, and twofold sp –sp cross-
couplings of 1,1-dialkenyl-2,2-dibromoethylenes 10 into TEEs 11
are known, there are no examples of gem-DVDEE (12) syntheses
R
R
R
Br
Br
not known
or
stepwise
regio- and
stereoselective
couplings
2
4
from 1,1-dialkynyl-2,2-dibromoethylenes 8. Secondly, whereas 1-
R
R
2
5
R
20
21
4
-E-DVDEE
alkynyl-1-alkenyl-2,2-dibromo-ethylenes 13 are known, there are
no examples of cross coupling reactions involving them. Thirdly,
whilst we have recently reported the stereoselective synthesis of
R
R
1
3
R
4
-Z-DVDEE
R
Syntheses of the five new TEE-TVE hybrid hydrocarbons 3-7
are depicted in Scheme 2. gem-DVDEE 6 was prepared by a route
[
3]dendralenes 18/19 from 1,1-dibromo-1,3-butadienes 16 by way
23
of 3-bromotrienes 17, the conversion of 1-alkynyl-1-alkenyl-2,2-
dibromoethylenes 13 into Z- and E-DVDEE 20 and 21 would pro-
involving a twofold Negishi cross-coupling of known dibromide
1
5a
22 with vinylzinc bromide. Dibromides 24 and 25, prepared by
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Journal of the American Chemical Society
Ramirez dibromo-olefination of reported cross-conjugated ketone stereoinvertive Sonogashira coupling of 28 → 30 and the stereore-
2
6
1
2
3
4
5
6
7
8
9
precursors, were key intermediates en route to the four other TEE-
TVE hybrids 3, 4, 5 and 7. TVEE 7 and TEVE 3 were prepared by
the now familiar twofold coupling reaction from TIPS precursor
24, in the former case a Negishi coupling with vinylzinc bromide
and in the latter case a Sonogashira process with TMS-acetylene.
The final two hybrids, Z-DVDEE 4 and E-DVDEE 5 were the
most challenging to prepare and, as detailed above, required the
development of the most interesting chemistry. Dibromide 25
underwent a selective cross-coupling with vinylzinc bromide at the
bromine cis- to the existing vinyl group, generating bromotrienyne
tentive Negishi coupling of 28 → 29 are without precedent. We
propose that these cross-couplings are both catalyst-controlled and
kinetically-controlled events which follow the general principles
2
8
originally introduced by Negishi, since neither 28 nor 29 isomer-
ize on exposure to the conditions used for stereoinvertive So-
nogashira coupling (i.e. 28 → 30) in the absence of TMS acetylene.
Z-bromotrienyne 28 is, however, converted into its E-stereoisomer
2
2
by heating at 120 °C.
Removal of the terminal alkyne TIPS groups from compounds
23, 26 and 27 to give clean samples of the new hydrocarbons 6, 7
and 3 required considerable experimentation, due to challenges
from both product volatility and separation of the hydrocarbon
from non-polar silicon-containing residues. Solid TBAF in diethyl
ether gave the best outcome by minimizing product loss during the
solvent removal phase of the workup. In addition, the use of Kishi’s
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
8. In a subsequent reaction catalyzed by [Pd
2
(dba)
3
]/t-Bu P, this
3
compound underwent Negishi coupling with the organozinc rea-
gent derived from TMS-acetylene to furnish Z-configured triene-
diyne 29, the product of stereoretention. With [PdCl (dppf)] as
2
precatalyst, the same precursor 28 underwent a Sonogashira cou-
pling with TMS-acetylene to give E-isomer 5, the product of stere-
oinversion.
2
9
non-aqueous work up for TBAF reactions was critical for success.
In contrast, removal of the TMS groups from compounds 29 and
3
0 to give clean samples of new hydrocarbons 4 and 5 was relative-
Scheme 2. Synthesis of TEE 1, TVE 2 and hybrids 3-7.
ly straightforward. An alternative Wittig olefination approach to E-
DVDEE 5 that is inspired by the wealth of information provided by
the Diederich group is described in the SI, along with the lessons
twofold Negishi
BrZn
2
Br
Br
[PdCl
THF, 60 °C
2
(dppf)]
2
TBAF•xH O
learned from a related, unsuccessful approach to Z-DVDEE 4.
Et
2
O, 0 °C
Hydrocarbons 1-7 were purified by flash chromatography and
were fully characterized using standard methods. TEE 1 is a crystal-
line solid at room temperature and ambient pressure, with TVEE 3
being semi-solid and the remainder being oils. We have not been
able to obtain crystals of hydrocarbons 2-7 suitable for single crys-
tal X-ray analysis. We are, however, delighted to report the first X-
ray crystal structure of TEE 1, using small needles obtained by
evaporation from pentane solution (Figure 2), which shows the
near planar structure of the TEE 1 molecules (maximum C devia-
tion 0.029(3) Å).
5
7%
41%
6
TIPS
22
TIPS
23
26
TIPS
TIPS
gem-DVDEE
BrZn
PdCl
THF, 66 °C
twofold
Negishi
[
2
(dppf)]
TBAF•xH
Et O, 0 °C
2
2
O
4
2%
8
4%
Br
Br
TIPS
7
twofold
Sonogashira
30
TVEE
TMS
R
[PdCl (dppf)]
2
TMS
TMS
2
TBAF•xH O
R = TIPS 24
CuI
NEt
R = TMS 25
i-Pr
2
BrZn
THF, 23 °C
2
Et O, 0 °C
[
3 4
Pd(PPh ) ]
73%
68%
3
THF, 35 °C
79%
Z-selective
Negishi
TIPS
ZnBr
27
TEVE
TMS
[
Pd
t-Bu
THF, 35 °C
2
dba
3
P
]
TMS
2 3
K CO
MeOH
23 °C
3
Br
Z
Z
Z
3
7%
61%
stereoretentive
Negishi
TMS
29
4
TMS
2
8
Z-DVDEE
TMS
[PdCl
CuI
2
(dppf)]
TMS
2 3
K CO
i-Pr
THF, 23 °C
2
NH
MeOH
23 °C
E
E
3
8%
66%
stereoinvertive TMS
Sonogashira
(a)
(b)
30
5
E-DVDEE
Figure 2. X-ray crystal structure of TEE 1 highlighting associations
in the solid state through face-face π-stacking (3.459(3) Å) along
the b axis (a) that is offset due to tilting (26.21(8)°). The stacks, in
turn, arrange efficiently (b, viewed down the b axis) via two dis-
The conversion of dibromodienyne 25 into bromotrienyne 28 is
unprecedented: there are neither examples of substrates of this type
2
7
in the literature, nor couplings akin to this. We propose that π-
complexation of Pd(0) with the adjacent vinyl group promotes
oxidative insertion into the cis-disposed C–Br bond. Either π-
complexation to the rival C≡C bond of 25 is sterically disfavored
by the presence of the TMS group, or the resulting π-complex posi-
tions the Pd(0) species less optimally for oxidative insertion.
31
tinct types of CH/π interactions (disorder is removed for clarity).
UV-vis absorption spectra of the seven hydrocarbons 1-7 are de-
picted in Figure 3, which shows three groups of molecules, which
3
2
are color-coded accordingly: (1) in red, TVE 2 and compounds 6
and 7 which exhibit broad absorptions at 280-300 nm and 220-230
nm; (2) in blue, compounds 3, 4 and 5, which show two distinct,
sharper absorption maxima between 300-320 nm and fine structure
While a stereoinvertive Negishi coupling of an alkynylzinc bro-
2
8
mide with a 2-bromo-1,3-butadiene has been reported, both the
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Journal of the American Chemical Society
Page 4 of 8
3
2
in their absorptions around 220 nm; and (3) in black, TEE 1,
from planarity) and lies a substantial 19.1 kJ/mol higher in energy
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
which exhibits a uniquely complex set of absorptions. The longer
wavelength absorptions are attributable to the 6π electron chromo-
phores and the shorter wavelength ones (for compounds 2-7) to 4π
than that shown in Figure 4 (see the SI for details).
In the preferred, C symmetric conformation of gem-DVDEE 6,
2
the [3]dendralene (3-methylene-1,4-pentadiene) moiety adopts a
3
3
systems.
skew s-trans–s-trans conformation, with the skew s-trans–gauche
3
4
conformer only 4.1 kJ/mol higher in energy. Interestingly, the
parent [3]dendralene prefers the skew s-trans–gauche conformation
over the skew s-trans–s-trans conformation by 3.7 kJ/mol. To cause
this reversal in conformational preference, either the ethynyl sub-
stituents in gem-DVDEE 6 destabilize the skew s-trans–gauche con-
former through steric buttressing, or perhaps the skew s-trans–s-
trans conformation is stabilized through conjugation.
6
3
2
TVE
gem-DVDEE
TEVE
7
TVEE
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
E-DVDEE
The lowest energy conformation of TVE 2 is non-planar and C
symmetric, with each gem-divinyl group adopting a skew s-trans–
2
1
TEE
1
3
gauche conformation. The four vinyl groups are slanted in the
same general direction relative to the average plane of the central
C=C bond and its four attached carbons, which has the effect of
placing the four methylene groups on the same face of the central
alkene. TVEE 7 prefers a conformation which mimics that of TVE
4
Z-DVDEE
2, but with a gauche-oriented vinyl group replaced by an ethynyl
unit. The second lowest energy conformer of TVEE 7 (see the SI
for details) lies 8.5 kJ/mol higher in energy and has the orientations
of the two vinyl groups of the gem-divinyl portion of the molecule
reversed.
Thus, in summary, more well-defined UV-vis absorption spectra
are generated by the less conformationally flexible molecules 2, 6
and 7, which strongly prefer a single in-plane conformation, where-
as broad absorption bands result from molecules 3, 4 and 5, which
have several conformations close in energy to the global minimum,
none of which are planar.
Figure 3. UV-visible absorption spectra of hydrocarbons 1-7 (ace-
tonitrile solvent, 23 °C).
Computational studies reveal that the group of three compounds
which give rise to broad absorption bands (2, 6 and 7, red in Fig-
ure 3) are distinct from those that give sharper spectra (3, 4 and 5,
blue in Figure 3). The lowest energy conformations of the five new
hybrid structures were located computationally and are depicted in
2
2
Figure 4. The reported, non-planar calculated conformations of
1
3
As mentioned in the introduction, solid samples of TEE 1 are re-
ported to be explosive. Our results confirm these accounts. On one
occasion, a ca. 0.5 mg sample in an aluminum cap exploded while
being carried in a gloved hand, leaving a charcoal-like dusting on
TVE 2, and the essentially in-plane molecular structure from the
X-ray crystal structure analysis of TEE 1 (Figure 2) were replicated
computationally. Three of the five hybrid structures, namely TEVE
3, Z-DVDEE 4 and E-DVDEE 5 prefer a single, in-plane confor-
3
5
the glove. We assume that solid samples of TEE 1 undergo rapid
mation (Figure 4, top). gem-DVDEE 6 and TVEE 7 prefer non-
planar conformations (Figure 4, bottom), with the latter being
reminiscent of the calculated lowest energy conformation of TVE
exothermic intermolecular addition-polymerization, in a similar
36
manner to that proposed for other polyynes and related systems.
TEE 1 stands alone amongst hydrocarbons 1-7 in this regard. We
recommend extreme caution when handling this substance. TEE 1
should only be handled in dilute solution and not in the solid state.
No such issues were encountered with the remaining hydrocarbons
2.
2-7, which presumably is
a manifestation of their higher
2
C(sp ):C(sp) ratio, hence greater stability. Differential scanning
calorimetry traces for neat samples of compounds 2–7 are included
in the Supporting Information. TEVE 3 shows an exotherm peak-
ing at the lowest temperature (62 °C), E-DVDEE 5 has the highest
3
4
5
TEVE
Z-DVDEE
E-DVDEE
skew anti-gauche
skew anti-gauche
skew anti–anti
(166 °C) and the remainder fall in the range 99–110 °C.
There is an incorrect but widely-held assumption that π-bond-
rich acyclic hydrocarbons are inherently unstable. Evidently these
compounds have the potential to react/decompose under a wide
variety of conditions. We consider that exposure to light, heat, oxy-
gen and acid are the most significant ones in a typical laboratory
skew anti-gauche
6
7
2
TVE
gem-DVDEE
TVEE
3
7
setting. Thus, we performed a series of systematic robustness tests
Figure 4. MP2/cc-pVTZ lowest energy conformations of the five
new hybrids and TVE. The three above the line are planar; the
three below are non-planar.
upon these compounds, both as neat samples and as dilute solu-
tions in common solvents, in order to provide quantitative infor-
mation on their survival. It is important to note that these studies
are not focused on identifying the mechanisms and/or kinetics of
specific decomposition processes. In many cases, neither decompo-
sition products nor processes are easily identified. Instead, the pri-
mary purpose is to provide empirical experimental data that estab-
All 1,3-butadiene residues in the three planar TEE-TVE hybrids
, 4 and 5 are s-trans. Their preference for the conformations de-
3
picted in Figure 4 is strong. For example, the second lowest energy
conformation of TEVE 3 has a skew s-cis 1,3-butadiene (twisted 18°
ACS Paragon Plus Environment

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Journal of the American Chemical Society
lishes conditions required for compound survival. The results of
on exposure to TFA for 1 h. Negligible decomposition was seen for
comparable exposures to dilute solutions of the hydrocarbon.
1
2
3
4
5
6
7
8
9
these tests are depicted in Table S1, and are divided into two broad
classes: those performed on samples held in dilute solution and
those performed on neat samples. Substrates that decompose in
dilute solution will generally (but not always) involve processes
that are first order in hydrocarbon, whereas dimeriza-
tion/oligomerization/polymerization processes will be favored for
neat samples.
CONCLUSION
In summary, the first chemical syntheses of five related through-
conjugated/cross-conjugated C10 hydrocarbons has been achieved.
These structures, along with those of TEE and TVE, represent the
2
The most obvious conclusion from Table S1 is that samples of
hydrocarbons 1-7 generally survive better in dilute solution than on
neat storage. Nonetheless, whereas only TVE 2 survives storage as
a neat sample at ambient pressure and room temperature, all seven
compounds 1–7 can be stored in dilute solution with negligible decom-
position. Furthermore, with dilute solutions, only TEE 1 exhibits
decomposition on exposure to air and only TVE 2 decomposes on
exposure to mild acid.
fundamental arrangements of sp and sp carbons in the majority of
carbon-based conductors. As such, this work serves as a foundation
for applications in this field.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The need to synthesize TVE-TEE hybrids has led to the devel-
opment of unprecedented synthetic transformations of wider value
to the community. Specifically, new regio- and stereoselective
cross-coupling variations have been uncovered, which merit de-
tailed analysis, optimization and generalization.
Moderate levels of decomposition are observed upon storage of
neat samples of hybrids 4-7, and the most alkyne-rich structures 1
and 3 show complete decomposition. Storage at –20 °C slows
down the rate of decomposition but generally with insufficient
effectiveness to permit long term storage in this manner.
The stability of each the seven unsubstituted hydrocarbons has
been documented experimentally through the implementation of a
series of robustness tests, which will have broader value for those
interested in preparing and deploying fundamentally new mole-
cules.
Since each of the hydrocarbons 1-7 carries, in a Z-disposition
about its central tetrasubstituted C=C unit: (a) a pair of –C≡CH
The first X-ray crystal structure of TEE 1 has been obtained, and
an explosive decomposition pathway for this molecule has been
uncovered. TVE 2 is the only one of the seven hydrocarbons that
can be stored as a neat sample at room temperature. Importantly,
all seven hydrocarbons can be stored in dilute solution at room
temperature in ambient light without fear of decomposition. Each
of the five new hybrid hydrocarbons 3–7 are more stable than TEE
1 towards air, and more stable than TVE 2 towards acid. These
findings demonstrate the manageable nature of these new sub-
stances.
The conformational behavior of the seven unsubstituted systems
has been related to their UV-visible absorption spectra. Overall, the
structural requirements for in-plane conformations, hence optimal
conjugation, has been verified. This fundamental information is
essential to the use of these structures in carbon-rich conducting
materials.
groups; and/or (b) a pair of –HC=CH
C≡CH group and a –HC=CH group, these systems can undergo
Bergmann cycloaromatization, (triene) 6π-electrocyclization and
2
groups; and/or (c) a
–
2
3
8
Hopf cycloaromatization, respectively. To our knowledge, these
are the first structures to contain the requisite functionality for
competition between these three related reactions. The primary
decomposition pathway of TVE 2, TVEE 7 and Z-DVDEE 4 on
2
2
heating in dilute solution is 6π-electrocyclization. We have previ-
ously noted that the electrocyclization of TVE 2 is more facile than
13
those of substrates lacking additional vinyl groups and this trend
is followed with substrates carrying ethynyl groups (thermal 6π-
39
electrocyclizations generally require temperatures of ≥140 °C ).
We presume that the faster electrocyclization of TVE 2 (complete
conversion after 2 h at 120 °C) than Z-trienes 4 and 7 (ca. 25-30%
conversion under the same conditions) is due, at least in part, to the
presence of two equivalent Z-trienes in TVE 2.
This study represents the foundations upon which new carbon-
rich materials can be designed and constructed, and new acyclic π-
bond rich hydrocarbons can be exploited.
Thermal Hopf cycloaromatizations of acyclic Z-dienynes usually
40
need temperatures well in excess of 200 °C, hence the stability of
E-DVDEE 5 and gem-DVDEE 6 in dilute solution at 120 °C is ex-
ASSOCIATED CONTENT
Supporting Information
4
1
pected. Bergman cycloaromatization of acyclic Z-enediynes pro-
ceed at temperatures similar to Hopf electrocyclizations. Since the
substrate with an internal competition between Bergman cy-
cloaromatization and 6π-electrocyclization, namely Z-DVDEE 4,
shows no sign of cycloaromatization, we assume that the mecha-
nism of thermal decomposition of other molecules containing Z-
enediynes, specifically TEE 1 and TEVE 3, does not involve this
process.
The Supporting Information is available free of charge on the ACS
Publications website at DOI: XXXXXXX.
1
13
Full experimental details, characterization data, H and C NMR spec-
tra, X-ray crystallographic and computational studies (PDF).
X-ray crystal structure of substituted DVDEE S16 (CCDC: 1942588)
(CIF)
X-ray crystal structure of TEE, 1 (CCDC: 1942589) (CIF)
The aforementioned 6π-electrocyclization products are the only
ones identified, with intractable mixtures of (presumably) polymer-
ic products being formed in all other cases. We strongly suspect
that these other decomposition pathways are initiated by electro-
philic and radical additions to the hydrocarbons, and that these
undesired pathways are promoted when samples are more concen-
trated. Evidence for this proposal comes in the form of exposure of
neat samples of TVEE 7 to air and TFA. On bubbling air though a
neat sample of TVEE 7, 66% decomposition of the hydrocarbon
was witnessed after 10 minutes, and 90% decomposition was seen
AUTHOR INFORMATION
Corresponding Author
* michael.sherburn@anu.edu.au
ORCID
Kelsey L. Horvath: 0000-0003-0297-6982
Nicholas L. Magann: 0000-0002-7278-6642
Madison J. Sowden: 0000-0002-9016-0889
Michael G. Gardiner: 0000-0001-6373-4253
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Journal of the American Chemical Society
Page 6 of 8
Michael S. Sherburn: 0000-0001-5098-0703
optonics and electronics by tetraethynylethene molecular scaffolding. In
Modern Arene Chemistry, D. Astruc, Ed. Wiley-VCH: Weinheim, Germany,
002; pp 196-216; (d) M. B. Nielsen, F. Diederich, Conjugated oligoenynes
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Author Contributions
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
2
based on the diethynylethene unit, Chem. Rev. 2005, 105, 1837-1867.
10
(a) J. Li, Z. Xie, Y. Xiong, Z. Li, Q. Huang, S. Zhang, J. Zhou, R. Liu, X.
‡
These authors contributed equally.
Gao, C. Chen, L. Tong, J. Zhang, Z. Liu, Architecture of b-graphdiyne-
containing this film using modified Glaser-Hay coupling reaction for en-
Funding Sources
This work was supported by the Australian Research Council
(DP160104322).
hanced photocatalytic property of TiO
Z. Jia, Y. Li, Z. Zuo, H. Liu, D. Li, Y. Li, Carbon Ene-Yne. Adv. Electron. Mater.
017, 3, 1700133; (c) J. Li, Y. Xiong, Z. Xie, X. Gao, J. Zhou, C. Yin, L. Tong,
2
, Adv. Mater. 2017, 29, 1700421; (b)
2
C. Chen, Z. Liu, J. Zhang, Template synthesis of an ultrathin b-graphdiyne-
like film using the Eglinton coupling reaction, ACS Appl. Mater. Interfaces
2019, 11, 2734-2739.
0
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Notes
The authors declare no competing financial interest.
1
1
M. D. Kilde, A. H. Murray, C. L. Andersen, F. E. Storm, K. Schmidt, A.
ACKNOWLEDGMENT
Kadziola, K. V. Mikkelsen, F. Hampel, O. Hammerich, R. R. Tykwinski, M. B.
Nielsen, Synthesis of radiaannulene oligomers to model the elusive carbon
allotrope 6,6,12-graphyne, Nat. Commun. 2019, 10, 3714.
Molecular structures from single crystal X-ray analyses and computed
structures were visualized using either CYLview 1.0b; C. Y. Legault,
Université de Sherbrooke, 2009 (http://www.cylview.org) or X-Seed
12
A. K. Geim, K. S. Novoselov The rise of graphene, Nat. Mat. 2007, 6,
183–191.
(
L. J. Barbour, X-Seed - A software tool for supramolecular crystallog-
1
3
E. J. Lindeboom, A. C. Willis, M. N. Paddon-Row, M. S. Sherburn, Com-
putational and synthetic studies with tetravinylethylenes, J. Org. Chem. 2014,
3, 11496–11507.
raphy, J. Supramol. Chem. 2001, 1, 189-191).
5
1
4
K. L. Horvath, C. G. Newton, K. A. Roper, J. S. Ward, M. S. Sherburn, A
ABBREVIATIONS
broad-spectrum synthesis of tetravinylethylenes, Chem. - Eur. J. 2019, 25,
TEE, tetraethynylethylene; TVE, tetravinylethylene; TEVE, tri-
ethynylvinylethylene; DVDEE, divinyldiethynylethylene; TVEE, trivi-
nylethynylethylene; TIPS, tri-isopropylsilyl; TMS, trimethylsilyl; dba,
dibenzylideneacetone; dppf, 1,1'-bis(diphenylphosphino)ferrocene;
TBAF, tetra-n-butylammonium fluoride; UV-vis, ultraviolet-visible; rt,
temp temperature.
4072-4076.
1
5
(a) A. M. Boldi, J. Anthony, C. B. Knobler, F. Diederich, Novel cross-
conjugated compounds derived from tetraethynylethene, Angew. Chem., Int.
Ed. Engl. 1992, 31, 1240-1242; (b) J. Anthony, C. B. Knobler, F. Diederich,
Stable [12]- and [18]Annulenes derived from tetraethynylethene, Angew.
Chem., Int. Ed. Engl. 1993, 32, 406-409; (c) J. Anthony, C. Boudon, F.
Diederich, J. P. Gisselbrecht, V. Gramlich, M. Gross, M. Hobi, P. Seiler, Stable
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M. Boldi, Y. Rubin, M. Hobi, V. Gramlich, C. B. Knobler, P. Seiler, F.
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Nierengarten, M. Schreiber, F. Diederich, V. Gramlich, Novel extended p-
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32
See the Supporting Information for details.
M. S. Sherburn, J. George, J. S. Ward, A General Synthesis of Den-
These UV-vis spectra are consistent with those previously reported. For
23
TVE 2, see reference 6; for TEE 1, see reference 1.
33
dralenes. Chem. Sci. DOI: 10.1039/c9sc03976g
(a) E- and Z-1,3,5-hexatrienes show three absorptions at 245, 255 and
2
4
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
dibromoethylenes 10 could also be used here. Challenges with the prepara-
tion of the parent 1,1-divinyl-2,2-dibromoethylene caused us to focus on this
alternative approach.
2
5
(
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3
4
(
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26
35
(
a) M. Malacria, M. L. Roumestant, Vinylallenes—VI : Synthese de
CAUTION: TEE 1 is explosive when in neat crystalline form. We strong-
cetones de la serie de la jasmine, Tetrahedron 1977, 33, 2813-2817; (b) H.-S.
Yeom, Y. Lee, J. Jeong, E. So, S. Hwang, J.-E. Lee, S. S. Lee, S. Shin, Stereoselec-
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ly recommend that only small quantities be generated and that the compound
is handled in solution to minimize the hazard of this highly reactive com-
pound. Only ca. 0.5 mg was being handled at the time of the explosion and the
worker was not harmed. We presume that heat from the worker’s hand was
involved in the rapid decomposition process. NMR and MS analysis of the
residue gave no evidence for the formation of fullerenes.
2
010, 49, 1611-1614.
27
Selective single couplings of aldehyde-derived 1,1-haloalkenes have been
36
widely reported, with the bromine cis- to the alkene C–H bond being replaced.
First reports: (a) A. Minato, K. Suzuki, K. Tamao, A remarkable steric effect in
palladium-catalyzed Grignard coupling: regio- and stereoselective monoalkyl-
ation and -arylation of 1,1-dichloro-1-alkenes, J. Am. Chem. Soc. 1987, 109,
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some subsequent transformations of tetraethynylmethane J. Am. Chem. Soc.
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1
257-1258; (b) B. M. Trost, R. Walchli, A novel palladium catalyzed reductive
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37
A. Hammoud, G. Linstrumelle, Selective palladium-catalysed substitution of
For precedent showing stability testing, see Bryce and co-workers: K.
1
,1-dichloroethylene, Tetrahedron Lett. 1987, 28, 1649-1650; (d) W. R.
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2
8
Stereo-retentive and stereo-invertive Negishi couplings of 2-bromo-1,3-
butadienes are known: (a) X. Zeng, Q. Hu, M. Qian, E. Negishi, Clean inver-
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dienes J. Am. Chem. Soc. 2003, 125, 13636-13637; (b) X. Zeng, M. Qian, Q.
Hu, E. Negishi, Highly stereoselective synthesis of (1E)‐2‐methyl‐1,3‐dienes
38
For an excellent, general review of these and related reactions, see R. K.
Mohamed, P. W. Peterson, I. V. Alabugin, , Concerted reactions that produce
diradicals and zwitterions: electronic, steric, conformational, and kinetic con-
trol of cycloaromatization processes, Chem. Rev. 2013, 113, 7089-7129.
by
palladium‐catalyzed
trans‐selective
cross‐coupling
of
1,1‐dibromo‐1‐alkenes with alkenylzinc reagents Angew. Chem. Int. Ed. 2004,
3
9
4
3, 2259-2263; (c) reference 23.
Review: M. Shindo, T. Yoshikawa, K. Yaji, Synthesis of 1,3-dienes by cy-
29
Y. Kaburagi, Y. Kishi, Operationally simple and efficient workup proce-
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Org. Lett. 2007, 9, 723-726.
pp. 353-400.
40
(a) H. Hopf, H. Musso, Preparation of benzene by pyrolysis of cis‐ and
3
0
The crystals examined decompose over minutes at ambient temperature.
trans‐1,3‐hexadien‐5‐yne, Angew. Chem. Int. Ed. Engl. 1969, 8, 680; (b) review:
G. Zimmermann, Open and masked 1,3‐hexadien‐5‐ynes − mechanistic and
synthetic aspects, Eur. J. Org. Chem. 2001, 457-471.
They were brought to the microscope cold and selection was done within a
minute or so at ambient temperature with partial coloration of the crystals.
After ca. 5 mins at ambient temperature they no longer diffracted. A number
of crystals were mounted, screened and stored in liquid nitrogen between
screening attempts. This allowed the best crystal to be chosen. The needle-
shaped crystals each showed evidence of twinning/not being single which
could not be handled with twinning routines. Nevertheless, the diffraction
data of the main component yielded good quality metrics, albeit the small
crystals diffracted only to moderate resolution with long collection times. See
4
1
(a) R. R. Jones, R. G. Bergman, p-Benzyne. Generation as an intermedi-
ate in a thermal isomerization reaction and trapping evidence for the 1,4-
benzenediyl structure, J. Am. Chem. Soc. 1972, 94, 660- 661; (b) review: M.
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es, Chem. Rev. 2007, 107, 2861-2890.
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