Methylcyanobutadiyne: Synthesis, x-ray structure and photochemistry; Towards an explanation of its formation in the interstellar medium

Article abstract of DOI:10.1002/chem.201303377

Out of this world: A new convenient synthesis of methylcyanobutadiyne (see figure), an interstellar compound, is described. It was characterized by various techniques including X-ray structure determination. Photolysis experiments were carried out on binary gaseous mixtures of interstellar relevant compounds to understand how methylcyanobutadiyne could be formed in the interstellar medium. Copyright

Full text of DOI:10.1002/chem.201303377

COMMUNICATION
DOI: 10.1002/chem.201303377
Methylcyanobutadiyne: Synthesis, X-ray Structure and Photochemistry;
Towards an Explanation of Its Formation in the Interstellar Medium
Nicolas Kerisit,[a, b] Loꢀc Toupet,[b, c] Yann Trolez,*[a, b] and Jean-Claude Guillemin*[a, b]
À À
(CꢀC)n CN,
Cyanopolyynes, the formula of which is H
N
1978.[12] It involves the reaction of stannylated compound 5
on aluminum chloride and cyanogen chloride (Figure 1).
Given the high toxicity and difficult availability of the
former, we investigated another pathway.
have been detected in the interstellar medium (ISM) with
n=2 to 5.[1] Cyanoacetylene (HC3N, 1), which has been
found in diverse environments from dark molecular clouds
to the envelop of dying carbon stars,[2] has also been detect-
ed on Titan,[3] the largest moon of Saturn, and in comae,[4]
and has been extensively studied during recent decades.[5]
This compound could have played an important role in the
prebiotic synthesis of amino acids and purines or pyrimi-
dines,[6] and consequently, for the emergence of life on
Earth. Cyanobutadiyne HC5N (2), the second member of
this family, has been predicted to also be present on Titan
by some models[7] but has not been detected there so far.
Each cyanopolyyne seems to be formed by starting from
acetylene or maybe polyynes and the corresponding smaller
cyanopolyyne.[8]
Figure 1. Original synthesis of compound 4 by Kroto and co-workers.
Our approach starts with the synthesis of 1,3-pentadiyne
(6) from commercially available 1,4-dichlorobut-2-yne and
methyl iodide according to an adapted procedure described
by Verkruijsse and Brandsma.[13] Compound 6 was then
treated with nBuLi and cannulated to a solution of methyl-
chloroformate to afford ester 7 in 86% yield. Aqueous am-
monia was then added to the latter compound 7 to give the
corresponding amide 8 in 89% yield. This amide was finally
heated between 110 and 1508C under vacuum in a mixture
of sand and P4O10 to give the target methylcyanobutadiyne 4
in 36% yield. During this last step the desired compound
was distilled off from the reaction mixture as soon as it was
formed and trapped in a cell cooled at À508C (Figure 2).
Such an approach involving the dehydration of an amide
was used by Moureu and Bongrand[17] at the beginning of
the 20th century for the preparation of several alkynyl-
Methylcyanopolyynes, the formula of which is
À
À
CH3 (CꢀC)n CN, are the methylated derivatives of cyano-
polyynes. Methylcyanoacetylene (MeC3N, 3) and methylcya-
nobutadiyne (MeC5N, 4), the first two members of this
family, have been detected in the ISM. Even though the first
compound (3) was detected long ago,[9] it has only been re-
cently that 4 was observed in the ISM.[10] The superior coun-
terparts (n>2) have never been detected. The very low
number of studies on methylcyanobutadiyne reported in the
literature[11] prompted us to investigate this compound in
the context of space sciences. We report here a new and
convenient synthesis of methylcyanobutadiyne and studies
on its formation by photolysis in the gas phase for a better
understanding of the ISM. Extension of the synthetic ap-
proach to the kinetically very unstable cyanobutadiyne is
also reported.
AHCTUNGTRENNUNG
tained and confirmed the structure of both molecules
(Figure 3). Crystals of the amide 8 were obtained by slow
diffusion of cyclohexane into an acetone solution. The pack-
ing revealed an important hydrogen bonding pattern as ex-
pected for such an amide, which involves all oxygen atoms
and all hydrogen atoms borne by the nitrogen atoms. p-
Stacking is also involved between the diacetylenic parts.
Crystals of methylcyanobutadiyne (4) were obtained by slow
evaporation of n-pentane. Contrary to compound 8, no hy-
drogen bond is involved, but the title compound exists as
dimer in the unit cell. The two molecules are parallel to
each other in a head-to-tail manner, probably for minimiz-
ing the global dipole moment. According to the distance be-
tween the two molecules (ca. 3.7 ꢀ), p-stacking is likely to
be the most important interaction at stake. Bond lengths of
cyanopolyynes and methylcyanopolyynes have been calcu-
lated in the past in order to deduce the physical properties
To the best of our knowledge, the first and only synthesis
of compound 4 was described by Kroto and co-workers in
[a] N. Kerisit, Dr. Y. Trolez, Dr. J.-C. Guillemin
Ecole Nationale Supꢁrieure de Chimie de Rennes, CNRS, UMR 6226
11 allꢁe de Beaulieu, CS 50837, 35708 Rennes Cedex 7 (France)
E-mail: yann.trolez@ensc-rennes.fr
jean-claude.guillemin@ensc-rennes.fr
[b] N. Kerisit, Dr. L. Toupet, Dr. Y. Trolez, Dr. J.-C. Guillemin
Universitꢁ Europꢁenne de Bretagne (France)
[c] Dr. L. Toupet
Institut de Physique de Rennes, CNRS, UMR 6251
Universitꢁ de Rennes 1, Rennes (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303377.
Chem. Eur. J. 2013, 19, 17683 – 17686
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
17683

and P4O10 to give cyanobuta-
diyne in 14% yield by trapping
it in a cell cooled at À1008C
(Figure 4).
This last strategy does not
really improve the one we re-
ported earlier[15] if one consid-
ers the yield of the last step.
However, it allows
a more
stable amide precursor that
can be easily purified by
column chromatography.
Figure 2. New synthesis of methylcyanobutadiyne (4).
Figure 4. Synthesis of cyanobutadiyne.
Particularly noteworthy is the difference of kinetic stabili-
ty between cyanobutadiyne (2) and methylcyanobutadiyne
(4), which only differ from each other by a methyl group.
Whereas compound 2 decomposes in the solid state from
À408C, 4 melts at 928C without any apparent decomposi-
tion. This huge difference probably originates from the pos-
sibility for HC5N to polymerize because of the terminal
acetylenic function.
By mixing different gases detected in the atmosphere of
Titan and/or in the ISM, we demonstrated the formation of
cyanobutadiyne by photolysis of cyanoacetylene or dicya-
noacetylene with acetylene, or dicyanoacetylene with buta-
diyne at 185 nm.[15] In all cases, the addition of the C3N radi-
cal on acetylene or of the CN radical on butadiyne was pro-
posed for the reaction pathway. Such a hypothesis was sup-
ported by the formation of other photoproducts, such as cya-
noacetylene or a compound tentatively assigned to be
cyanohexatriyne (HC7N). The extension of this finding to
the formation of the methylated cyanopolyynes by the use
of propyne and 1,3-pentadiyne instead of acetylene and bu-
tadiyne would reinforce the proposed mechanism.
For this purpose, a lamp irradiating at 185, 193 and
254 nm was used. Moreover, two different cells were used:
one in ordinary quartz, which absorbs most of the 185 nm
radiation, and one with a Suprasilꢃ window, which allows
185, 193 and 254 nm wavelengths to pass through. After
some hours of irradiation, the volatile compounds were ana-
lyzed by 1H NMR spectroscopy by trapping them in an
NMR tube at low temperature. The presence of MeC5N
could be confirmed or disconfirmed by comparison of spec-
Figure 3. X-ray structure and crystal packing of: a) the amide 8, and
b) methylcyanobutadiyne (4), and c) bond lengths [ꢀ] of methylcyanobu-
tadiyne (4) according to the X-ray structure. The dotted lines represent
H bonds.
of these compounds. These lengths (Figure 3c) are in good
agreement with what has recently been published.[14] How-
ever, these values might serve as standards for future evalu-
ations of the properties of this product and could be ex-
trapolated to cyanopolyynes and derivatives.
Several attempts for the synthesis of cyanobutadiyne (2)
have been reported in the literature before the first isolation
of this compound by our group,[15] but the amide approach
has never been described. Our approach was limited on the
preparative scale by a yield of only 15% for the last step
and was performed starting from 1,3-butadiynetributylstan-
nane, a compound that can only be kept at À208C.[16] Its
synthesis could be simplified by having the corresponding
amide precursor in hand. Starting from commercially avail-
able 1,4-bis(trimethylsilyl)butadiyne, the corresponding
ester 9 was obtained in 55% yield by a first addition of
MeLi·LiBr to form the corresponding monolithiated acety-
lide, which was then treated with an excess of methylchloro-
formate. The resulting compound 9 was turned into the
amide 10 by addition of aqueous ammonia in 53% yield. It
is important to note that the TMS function does not survive
these conditions. The latter compound was then heated
from 120 up to 1808C under vacuum in a mixture of sand
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Chem. Eur. J. 2013, 19, 17683 – 17686

Methylcyanobutadiyne Formation in the Interstellar Medium
COMMUNICATION
tra before and after subsequent addition of an authentic
sample in the tube.
window. The same reason as mentioned above can be in-
voked. However, when 1,3-pentadiyne was used, MeC5N
was formed with and without Suprasil. Nevertheless, particu-
larly noteworthy is that the formation seems to be less fa-
vorable with a Suprasil window, according to the intensity of
The photolysis of HC3N (1)[17] and propyne (11) gave
trace amounts of MeC5N (4). However, particularly note-
worthy is that the procedure did not give rise to MeC5N (4)
with the Suprasil cell but with the quartz cell. This unexpect-
ed effect can be explained by the important coefficient ab-
sorption of 185 nm radiation by propyne (11; about
110 amagatÀ1 cmÀ1 at 185 nm at 293 K).[18] Moreover, some
studies have shown that the major product formed from
photolysis of propyne at 193 nm is the propargyl radical,[19]
which should not result to the formation of MeC5N at this
1
the H NMR peak in both experiments. As for the propyne
case, 185 nm radiation should involves the formation of
more undesired radicals that might inhibit the formation of
MeC5N.
The same pathway of formation can be suggested from
the breaking of C4N2 (14) into C3N and CN radicals
(Scheme 2). The fact that both propyne (11) and 1,3-penta-
diyne (6) lead to the formation of MeC5N (4) with C4N2
wavelength. However, at this wavelength, the homolytic
[20]
À
break of the C H bond of HC3N also occurs, but the ab-
sorption coefficient is approximately the same as that of
propyne (about 10 amagatÀ1 cmÀ1 at 293 K). Therefore, we
conclude that the formation of the two radicals is in compe-
À
tition at 193 nm: the propargyl radical CH2 CCH and
À
CC CN. At 185 nm, the formation of the propargyl radical
is probably predominant because the absorption coefficient
of propyne is ten-times higher than that of HC3N, and this is
the reason why the amount of MeC5N formed during this
1
experiment is probably too low to be detected by H NMR
spectroscopy.
Scheme 2. Proposed mechanism of formation of MeC5N (4) from C4N2
and: a) 1,3-pentadiyne (6), or b) propyne (11).
This result gave us some information about the most
likely mechanism of formation of MeC5N. It suggests that
HC3N (1) partially absorbs 193 nm radiation and is thus
broken into two radicals H and C3N. Radical C3N may then
add on propyne, to give MeC5N and H (Scheme 1). This
result is perfectly consistent with our previous study on
HC5N (2), which could be formed from HC3N and acetylene
(12) with this type of irradiation.[15]
(14) is consistent with the initial formation of C3N and CN
radicals from C4N2 (14), which then react with propyne or
1,3-pentadiyne, respectively, to give MeC5N (4).
In conclusion, a new and convenient synthesis of MeC5N
(4), which has also been applied to the synthesis of HC5N
(2), is proposed. Photolysis experiments in the gas phase
were carried out to investigate the possibility of MeC5N (4)
formation in the ISM from relevant precursors. Indeed,
binary mixtures of HC3N (1) or C4N2 (14), and propyne or
1,3-pentadiyne led to the detection of trace amounts of
MeC5N (4) under certain irradiation conditions. However, it
is likely that other cyanopolyynes were formed during these
experiments. In particular, methylcyanohexatriyne (MeC7N;
13) could be formed in some cases. However, we currently
do not have an authentic sample of this compound that
could allow us to be sure of its formation. Therefore, experi-
ments are currently underway in our laboratory to find
a convenient synthesis of MeC7N (13).
Scheme 1. Proposed mechanism of formation of MeC5N (4) from HC3N
(1) and propyne (11).
To reinforce our hypothesis, a mixture of HC3N (1) and
1,3-pentadiyne (6) was also studied. If these products react
in the same way as the HC3N/propyne mixture, MeC5N (4)
should not be obtained, but rather the superior counterpart
MeC7N (13; methylcyanohexatriyne). Indeed, under the
same conditions, no MeC5N could be detected. However,
1
a peak around d=2.05 ppm in the H NMR spectrum was
Experimental Section
observed that might be assignable to MeC7N (13), but no
1H NMR spectrum of this latter compound is currently
available. Studies are currently in progress in our laboratory
on this compound to confirm or disconfirm its formation.
Two other sets of experiments were carried out by substi-
tuting HC3N (1) by dicyanoacetylene (C4N2; 14).[21] The
same trend was observed: trace amounts of MeC5N were
observed when mixtures of C4N2 and propyne were illumi-
nated in an ordinary quartz cell, but not with a Suprasil
Experimental details can be found in the Supporting Information.
CCDC-935292 (4) and 884433 (8) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Chem. Eur. J. 2013, 19, 17683 – 17686
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Y. Trolez, J.-C. Guillemin et al.
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Acknowledgements
We thank the French Ministry of Higher Education for a PhD fellowship
to N.K. We also acknowledge the PCMI program (INSU-CNRS) and the
CNES (Centre National d’Etudes Spatiales) for their financial support.
Keywords: cyanopolyynes
·
interstellar chemistry
·
methylcyanobutadiyne · synthetic methods · photochemistry
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Received: August 28, 2013
Published online: November 15, 2013
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Chem. Eur. J. 2013, 19, 17683 – 17686