Ethynylhydroxycarbene (H-CC-C¨-OH)

Article abstract of DOI:10.1021/jacs.1c00897

The species on the C3H2O potential energy surface have long been known to play a vital role in extraterrestrial chemistry. Here we report on the hitherto uncharacterized isomer ethynylhydroxycarbene (H-CC-C¨-OH, 1) generated by high-vacuum flash pyrolysis of ethynylglyoxylic acid ethyl ester and trapped in solid argon matrices at 3 and 20 K. Upon irradiation at 436 nm trans-1 rearranges to its higher lying cis-conformer. Prolonged irradiation leads to the formation of propynal. When the matrix is kept in the dark, 1 reacts within a half-life of ca. 70 h to propynal in a conformer-specific [1,2]H-tunneling process. Our results are fully consistent with computations at the CCSD(T)/cc-pVTZ and the B3LYP/def2-QZVPP levels of theory.

Full text of DOI:10.1021/jacs.1c00897

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Ethynylhydroxycarbene (H−CC−C−OH)
Bastian Bernhardt, Marcel Ruth, André K. Eckhardt, and Peter R. Schreiner*
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ABSTRACT: The species on the C3H2O potential energy surface have long been known to play a vital role in extraterrestrial
̈
chemistry. Here we report on the hitherto uncharacterized isomer ethynylhydroxycarbene (H−CC−C−OH, 1) generated by
high-vacuum flash pyrolysis of ethynylglyoxylic acid ethyl ester and trapped in solid argon matrices at 3 and 20 K. Upon irradiation at
436 nm trans-1 rearranges to its higher lying cis-conformer. Prolonged irradiation leads to the formation of propynal. When the
matrix is kept in the dark, 1 reacts within a half-life of ca. 70 h to propynal in a conformer-specific [1,2]H-tunneling process. Our
results are fully consistent with computations at the CCSD(T)/cc-pVTZ and the B3LYP/def2-QZVPP levels of theory.
Scheme 1. Proposed Mechanism18,19 of the Observed
eactions of complexes of small carbon clusters with water
Photoinduced Reactions of C3·H2O to C3O, 2, and
Rresemble conceivable entrance channels for the formation
a
Acetylene + CO17
of small oxygenated organic molecules in interstellar media.1−3
It has been shown that carbon atoms in their triplet ground
state are unreactive toward water,4 and only for Cn·H2O with
n ≥ 3 these complexes exist and show photoreactivity under
laboratory conditions.5,6 Hence, the simplest conceivable
addition products all bear the formula C3H2O. Among these,
readily prepared propynal (2, propiolaldehyde)7,8 and cyclo-
propenone (3)9−11 have been also detected in space, while
propadienone (4),12,13 which lies comparably low in energy
(Scheme 1),14 has only been detected under cryogenic
laboratory conditions.15 Herein, we characterize a hitherto
unreported species on the C3H2O potential energy surface
̈
(PES), namely ethynylhydroxycarbene (H−CC−C−OH,
1).
The C3H2O PES has been subject to computational and
experimental research during the past four decades.14 In 1990
Ortman et al. conducted photolysis of matrix-isolated carbon
clusters complexed with water and found the reaction
C3·H2O → 2 + C3O.5 A reactive intermediate detected by
IR spectroscopy was assigned to the title compound 1. Only
two years later, however, Liu et al. suggested that the observed
reactive intermediate is actually 3-hydroxypropadienylidene
(5) according to computed IR spectral data.16 Eventually,
Szczepanski et al. repeated the experiments of Ortman et al.
and confirmed this computational conclusion.17 The con-
troversial IR bands were assigned to cis-3-hydroxy-
propadienylidene (5c) and its trans-conformer (5t). Addition-
ally, the 5c → 5t interconversion in the absence of external
stimuli was observed, which was interpreted on the basis of
quantum mechanical tunneling (QMT). While the reaction
sequence shown in Scheme 1 was later proposed on the basis
of experimental and computational results,18,19 1 remains
elusive.
a
Ethynylhydroxycarbene (1) has not been detected; 2 and 3 are
commodity chemicals. All compounds that have been found in space
have as well been characterized under cryogenic laboratory
conditions. Relative energies in kcal mol−1 of singlet geometries
were computed at the CCSD(T)/cc-pVTZ level of theory. The
relative energy of 6 is corrected for loss of H·: a) refers to C3O + H2
and b) refers to C3O + 2 H·.
b
Received: January 24, 2021
Published: March 5, 2021
In recent years we isolated and characterized several
members of the hydroxycarbene family using high-vacuum
flash pyrolysis (HVFP) of α-keto acids and esters in
conjunction with matrix isolation techniques.20−22 Most
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hydroxycarbenes display QMT to the corresponding aldehyde.
The tunneling half-life depends on the substituent R, which in
the case of R = OH, OMe, and NH2 inhibits QMT.
band vanishes almost completely along with bands at 3545.0
(theor. 3563.1), 3306.9 (3333.6), 2036.2 (2069.9), 1327.1
(1335.2), 797.7 (806.3), and 539.0 (533.2) cm−1 all of which
can be assigned to 1t. Concomitantly, bands that can be
assigned to 2 in accordance with literature data5,17 at 2868.0
(theor. 2833.4), 2107.5 (2160.5), 1688.5 (1720.0), and 940.1
(940.9) cm−1 increase in intensity.
Shorter irradiation times at 436 nm led to an increase of
three bands that cannot be assigned to propynal (2). These
bands at 2029.1, 1263.0, and 792.1 cm−1 are barely visible after
pyrolysis, reach a maximum after 10 min of irradiation at
436 nm, and decrease before they finally vanish completely
after prolonged irradiation times (40 min). We assign these
bands to cis-ethynylhydroxycarbene (1c) based on computed
bands at 2064.3, 1265.9, and 796.8 cm−1 (Figure 1B). This
result is in analogy to previous studies on trans-trifluoro-
In general, ethynylcarbenes are relevant in astrochemical
processes23,24 and can even be used synthetically.25−29 The
electronic structure of parent ground-state triplet propynyli-
dene (HCCCH) has been explored in numerous experimental
and computational studies with the result that both terminal
carbons are equivalent in terms of their reactivity.30−32 The
photoreaction of propynylidene with triplet dioxygen provides
another entrance channel to the C3H2O PES as shown in a
study by Wierlacher et al., who observed 2 besides the
corresponding dioxirane and carbonyl oxide.33 Other triplet
ground-state derivatives of propynylidene have been inves-
tigated spectroscopically and they display similarly delocalized
electronic structures.34−36 In contrast, singlet ethynylcarbenes
(such as 1) can best be represented by a localized carbene
center in α-position to a localized CC bond.37
̈
methylhydroxycarbene (F3C−C−OH, 12) and trans-cyano-
̈
hydroxycarbene (NC−C−OH, 13), which undergo photo-
induced rotamerizations to their corresponding cis-conform-
ers.43,44 Both cis-hydroxycarbenes also vanish after prolonged
irradiation.
Herein we answer the following questions: (1) Can 1 be
generated via pyrolysis of ethynylglyoxylic acid ethyl ester (10,
Scheme 2) and (2) if so, does 1 undergo [1,2]H-tunneling to
2? We support our experiments with computations at the
CCSD(T)/cc-pVTZ and B3LYP/def2-QZVPP levels of theory
and discuss our findings in the context of previous studies.
In order to complement our photochemical experiments we
checked for QMT reactivity by keeping the as-deposited matrix
in the dark and measuring IR spectra in intervals of 1 h. We
performed three such experiments: (1) at 3 K, (2) at 20 K, and
(3) at 3 K and a 4.5 μm cutoff filter installed between the
spectrometer’s light source and the matrix window. In all three
experiments we observed a slow decay of the bands of 1t and a
simultaneous increase of those of 2 (Figure 1C). Additionally,
another such set of bands is apparent, which we assign to trans-
ethynylglyoxylic acid (9t) and its more stable cis-conformer
(9c; Figure 1D), both of which are present due to incomplete
pyrolysis of 10. Both 9c and 9t can be assigned in a spectrum
measured directly after pyrolysis as minor components (see the
Supporting Information for complete band assignments).
We evaluated the kinetics of both QMT reactions by
integrating over the strongest IR bands of all four species (1t,
2, 9t, and 9c) and following their increase (decrease) over
time. The obtained curves were fitted to monoexponential
functions (first-order rate law) from which tunneling half-lives
were calculated (Figure 2); see the Supporting Information for
details, error estimates, and the data for 9t → 9c. The kinetic
profile of the reaction 1t → 2 is not temperature dependent,
and the spectrometer beam does not seem to have a significant
impact on the associated tunneling half-life of ca. 70 h taking
the estimated relative error of ca. 12% into account. This result
clearly points toward a QMT reaction from 1t to 2, which is
further supported by CVT/SCT computations (vide infra).
We computed the potential energy surface around 1 at the
CCSD(T)/cc-pVTZ level of theory (Figure 3). Similar to
other hydroxycarbenes the 1t → 2 reaction is associated with a
substantial barrier of 31.8 kcal mol−1 and, hence, cannot be
initiated solely by excitation from the IR spectrometer in a
single photon process. IR spectra were recorded up to
7000 cm−1 corresponding to a maximum energy output of
only 20.0 kcal mol−1.
Scheme 2. High-Vacuum Flash Pyrolysis (HVFP) of 10
a
Yields 1 via 9
a
1t undergoes QMT to 2. There is a second tunneling reaction from
9t to 9c, both of which are present after pyrolysis. Irradiation of the
matrix at 436 nm leads to an increase of the concentrations of 1c and
2 while 1t vanishes nearly completely over the course of 40 min.
Carbene 1c is not photostable and vanishes after prolonged
irradiation to form 2.
We synthesized ethynylglyoxylic acid ethyl ester (10) via
oxidation of 2-hydroxybut-3-ynoic acid ethyl ester (11) with
Dess-Martin periodinane (see the Supporting Information for
details). Precursor 10 was obtained as a yellow to brown liquid,
which was stable for several weeks in a Schlenk tube connected
to the matrix apparatus under reduced pressure (ca. 3 ×
10−6 mbar). Gaseous 10 was pyrolyzed at 900 °C, and all
pyrolysis products were co-condensed with an excess of argon
onto the cold matrix window for IR measurements.
Mid-IR measurements of the resulting matrix revealed the
formation of CO2 and ethylene38 during the pyrolysis.
Additionally, we detected 2, acetylene,39 CO, and unreacted
10. Bands from hitherto unreported species are discussed
below. Conceivable species that are literature known (Scheme
1) like 3,40,41 4,15 and C3O42 were not observed based on
comparison with published data.
Tunneling computations using canonical variational theory
in conjunction with small curvature tunneling (CVT/SCT) at
the B3LYP/def2-QZVPP level of theory yield a QMT half-life
of 62.1 h for 1t → 2 in excellent agreement with our
experiment. The QMT half-life of 1t nicely fits in the series of
hydroxycarbenes (Figure 4). Ethynyl (sp hybridized) as a
moderate σ acceptor (−I)45,46 is expected to captodatively
After pyrolysis, a strong band at 1249.4 cm−1 coincides well
with the strongest band of 1t computed at 1238.4 cm−1
(determined anharmonically at B3LYP/def2-QZVPP). When
irradiating the matrix at 436 nm for 40 min (Figure 1A) this
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Figure 1. Experimental matrix IR difference spectra (black) and anharmonic spectra computed at the B3LYP/def2-QZVPP level of theory (blue).
(A) After 40 min of irradiation at 436 nm bands of 1t decrease while those of 2 increase in intensity. (B) Three bands that increase after 10 min
irradiation at 436 nm and decrease after prolonged irradiation (red dashed lines) are assigned to 1c. (C and D) By keeping the matrix in the dark
for 409 h at 3 K using a 4.5 μm cutoff filter the reactions 1t → 2 (C) and 9t → 9c (D) were observed. Note that C and D display the same
experimental difference spectrum. Bands that are assigned in C are marked with an asterisk in D and vice versa.
stabilize the carbene center (sp2) in 1, resulting in its longer
half-life compared to parent hydroxymethylene (16).47 As
expected, the effect of the substituent in 1 is smaller than in
trifluoromethylhydroxycarbene (12)43 and cyanohydroxy-
carbene (13),44 both of which also resemble cases of
captodatively stabilized hydroxycarbenes. However, the effect
is stronger than in cyclopropylhydroxycarbene (18),48 in which
the carbene center is stabilized by π donation (+M). The
natural bond orbitals (NBO; Figure 3) of 1 support its carbene
character (HOMO) and show the stabilizing effect of the
adjacent oxygen lone pairs (HOMO−3). The NBOs do not
provide hints for the ethynyl’s π system to interact with the
carbene center (see the SI for details).
Carbene 1 is the only hydroxycarbene without further
heteroatom substituents for which the higher lying cis-
conformer (1c) could be observed, which is another sign of
its increased stability. According to CCSD(T)/cc-pVTZ
computations 1c lies 2.3 kcal mol−1 above 1t. Recently, we
Figure 2. Evaluation of the tunneling half-life of the reaction from 1t
(blue data points, red fit) to 2 (green, gray) at 3 K using a 4.5 μm
reported the spontaneous rotamerization of cis-cis-dihydroxy-
cutoff filter.
carbene to its cis-trans-conformer, resembling the first
tunneling rotamerization in a hydroxycarbene.49 However,
here we could not observe the analogous reaction 1c → 1t.
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We isolated, characterized, and investigated the reactivity of
1 in argon matrices at 3 and 20 K. This complements and
expands previous studies on the C3H2O potential energy
surface and ethynylcarbenes in general. Carbene 1 is a viable
intermediate in astrochemistry (Scheme 1). Upon irradiation
at 436 nm 1t reacts to afford 2; 1c forms as well, but vanishes
upon prolonged irradiation. The 1t → 2 reaction also occurs
spontaneously when keeping the matrix in the dark via QMT
with a tunneling half-life of ca. 70 h in accord with theory and
the general trend of hydroxycarbene QMT half-lives.
Exploiting substituent effects to influence QMT reactivity
and half-lives in a predictable fashion is therefore possible.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c00897.
■
sı
Synthetic details, matrix IR and UV/Vis spectra, matrix
IR band assignments, details on the kinetic analyses, and
computational details (PDF)
Figure 3. Intrinsic reaction coordinates (IRC) of 1t → 2 and 1c → 1t.
The IRC curve was determined at B3LYP/def2-QZVPP, and the
relative energies of the stationary points are given at the CCSD(T)/
cc-pVTZ level of theory. Inset: Highest occupied molecular orbital
(HOMO) and HOMO−3 of 1t obtained at B3LYP/def2-QZVPP.
The HOMO−1 and HOMO−2 (Supporting Information) resemble
the π-system at the alkyne moiety. Color code: Carbon, black;
hydrogen, white; oxygen, red.
AUTHOR INFORMATION
Corresponding Author
■
Peter R. Schreiner − Institute of Organic Chemistry, Justus
Liebig University Giessen, 35392 Giessen, Germany;
orcid.org/0000-0002-3608-5515; Email: prs@uni-
giessen.de
Authors
Bastian Bernhardt − Institute of Organic Chemistry, Justus
Liebig University Giessen, 35392 Giessen, Germany;
orcid.org/0000-0002-7734-6427
Marcel Ruth − Institute of Organic Chemistry, Justus Liebig
University Giessen, 35392 Giessen, Germany
André K. Eckhardt − Institute of Organic Chemistry, Justus
Liebig University Giessen, 35392 Giessen, Germany;
orcid.org/0000-0003-1029-9272
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c00897
Figure 4. Comparison of experimental QMT half-lives of matrix
isolated hydroxycarbenes.44
Funding
B.B. and A.K.E. thank the Fonds der Chemischen Industrie for
doctoral scholarships.
Notes
This is in line with the computed tunneling half-life of 60.8 d
(CVT/SCT//B3LYP/def2-QZVPP) for this reaction. Note
that the 1c → 1t activation barrier amounts to 21.8 kcal mol−1
at the CCSD(T)/cc-pVTZ level of theory (Figure 3) while the
corresponding barrier of cis-cis-dihydroxycarbene is only 9.4
kcal mol−1. As only 1t undergoes QMT to 2 while 1c is
persistent under cryogenic conditions, the Curtin−Hammett
principle is not applicable here. Similar cases of conformer-
specific QMT have been reported for 1243 and 13.44
The feasibility of 1t → 2 supports the mechanism of the
astrochemical formation of small organic molecules from
C3·H2O in Scheme 1.18 Additionally, the intermediacy of 1
might open a channel for the generation of more complex
sugar-like molecules via carbonyl ene reactions with carbonyls,
as has been demonstrated for other hydroxycarbenes.50 As
noted before,17 QMT is a crucial element on the C3H2O PES
and astrochemical reactions in general, as they usually occur at
diluted and low temperature (matrix isolation-like) conditions.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank our colleagues Fumito Saito, Dennis Gerbig, Artur
Mardyukov, Henrik Quanz, Felix Keul, Jan M. Schu
■
̈
mann, and
Friedemann Dressler for fruitful discussions.
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