N2/CO Exchange at a Vinylidene Carbon Center: Stable Alkylidene Ketenes and Alkylidene Thioketenes from 1,2,3-Triazole Derived Diazoalkenes

Article abstract of DOI:10.1021/jacs.1c06906

We present a new class of room-temperature stable diazoalkenes featuring a 1,2,3-triazole backbone. Dinitrogen of the diazoalkene moiety can be thermally displaced by an isocyanide and carbon monoxide. The latter alkylidene ketenes are typically considered as highly reactive compounds, traditionally only accessible by flash vacuum pyrolysis. We present a new and mild synthetic approach to the first structurally characterized alkylidene ketenes by a substitution reaction. Density functional theory calculations suggest the substitution with isocyanides to take place via a stepwise addition/elimination mechanism. In the case of carbon monoxide, the reaction proceeds through an unusual concerted exchange at a vinylidene carbon center. The vinylidene ketenes react with carbon disulfide via a four-membered thiete intermediate to give vinylidene thioketenes under release of COS. We present spectroscopic as well as structural data for the complete isoelectronic series (R2C═C═X; X = N2, CO, CNR, CS) including 1J(13C-13C) data. As N2, CO, and isocyanides belong to the archetypical ligands in transition-metal chemistry, this process can be interpreted in analogy to coordination chemistry as a ligand exchange reaction at a vinylidene carbon center.

Full text of DOI:10.1021/jacs.1c06906

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N2/CO Exchange at a Vinylidene Carbon Center: Stable Alkylidene
Ketenes and Alkylidene Thioketenes from 1,2,3-Triazole Derived
Diazoalkenes
Patrick W. Antoni, Justus Reitz, and Max M. Hansmann*
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ABSTRACT: We present a new class of room-temperature stable
diazoalkenes featuring a 1,2,3-triazole backbone. Dinitrogen of the
diazoalkene moiety can be thermally displaced by an isocyanide and
carbon monoxide. The latter alkylidene ketenes are typically
considered as highly reactive compounds, traditionally only accessible
by flash vacuum pyrolysis. We present a new and mild synthetic
approach to the first structurally characterized alkylidene ketenes by a
substitution reaction. Density functional theory calculations suggest
the substitution with isocyanides to take place via a stepwise addition/
elimination mechanism. In the case of carbon monoxide, the reaction
proceeds through an unusual concerted exchange at a vinylidene carbon center. The vinylidene ketenes react with carbon disulfide
via a four-membered thiete intermediate to give vinylidene thioketenes under release of COS. We present spectroscopic as well as
structural data for the complete isoelectronic series (R2CCX; X = N2, CO, CNR, CS) including 1J(13C−13C) data. As N2, CO,
and isocyanides belong to the archetypical ligands in transition-metal chemistry, this process can be interpreted in analogy to
coordination chemistry as a ligand exchange reaction at a vinylidene carbon center.
Scheme 1. (A) Carbon Suboxide (I) and
INTRODUCTION
■
Phosphoranylideneketene (II) are Rare Examples of Well-
Characterized Cumulenones Containing the XCCO
Fragment and (B) Synthesis of Alkylidene Ketenes via FVP
The activation of carbon monoxide with main group elements
is a vibrant area of research, exemplified by the recent
characterization of carbonyl complexes based on silicon,1,2
boron,3−6 phosphorus,7,8 or alkaline earth metals.9,10 Carbonyl
complexes of carbon, ketenes (R2CCO) are well explored
in organic chemistry and can even be generated by the
combination of electrophilic carbenes with CO.11−14 In
contrast to ketenes, the higher unsaturated homologues,
alkylidene ketenes or more specifically vinylidene ketenes
(R2CCCO) are much less investigated, presumably a
result of their challenging synthetic access and high
reactivity.15,16 Carbon suboxide17 (I; Scheme 1A) is one of
the few relatively stable compounds, which has received great
interest and was suggested by Frenking and co-workers to be
described as a carbon atom flanked by two neutral CO donor
ligands (L → C ← L; carbone).18 Following this analogy, the
[CCO] fragment can be stabilized by a phosphine ligand
leading to phosphoranylideneketenes (II).19−22 Besides
phosphacumulene ylides, which are the only structurally
characterized compounds containing the XCCO frag-
ment, alkylidene ketenes or cumulenones (X = CR2) are
exceedingly rare. Typically, alkylidene ketenes are generated in
the gas phase by flash vacuum pyrolysis (FVP) (T > 500 °C),
trapped, and analyzed in matrixes at cryogenic temperatures
(Scheme 1B).23 Common precursors are Meldrum’s acid
derivates (III), which upon FVP eliminate acetone and CO2 to
Received: July 3, 2021
© XXXX The Authors. Published by
American Chemical Society
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afford alkylidene ketenes (IV);24−27 the latter are prone to
dimerize or subsequently decarbonylate to form highly reactive
vinylidenes (V).28−30 Note, in the reverse sense, alkylidene
ketenes can be considered as carbonylation products of elusive
unsaturated carbenes (R2CC:), a rare reactivity observed for
matrix isolated F2CC:.30,31 Interestingly, Wentrup and co-
workers prepared alkylidene ketene VI via FVP (450 °C, 10−4
Torr) followed by trapping at low temperatures.32 As far as we
know, VI is the only relatively stable alkylidene ketene, besides
iminopropadienones (VII),33 allowing even its room-temper-
ature NMR characterization.
Similar observations of ketenes (R2CCO) and alkylidene
ketenes (R2CCCO) can be applied to diazoalkanes
(R2CN2) and diazoalkenes (R2CCN2). While the first
class is well-studied,34,35 diazoalkenes are typically high-energy
intermediates proposed in a series of reaction mechanisms
such as the Seyferth−Gilbert homologation or the Colvin
reaction.36,37 As a result of their tendency to rapidly eliminate
dinitrogen, they are challenging to characterize, and their
detection was so far limited to matrix isolation studies.30,31
However, by tuning the electronics, we recently were able to
describe the synthesis of the first room temperature stable
diazoalkene based on an imidazole heterocycle, enabling a new
entry into vinylidene chemistry.38 In this work, we present a
new class of diazoalkenes and demonstrate a new mild
synthetic strategy to access vinylidene ketenes by a N2/CO
exchange reaction.
terminal N atom of nitrous oxide (I), followed by proton
migration to II and dehydration (Scheme 2).40,41
The transfer of N2 from N2O to form diazo species is
typically a challenging process,42 with few literature prece-
dents.43,44 Note, the analogy between the imidazole mNHO39
and the 1,2,3-triazole mNHO-based reactions is not obvious,
considering that the reaction of normal N-heterocyclic olefins
(NHOs) with N2O gives dimeric species of the type R2C
CH(NN)HCCR2 and not the crucial diazoalkenes.45 This
change in selectivity highlights the unique reactivity of
mesoionic N-heterocyclic olefins, since Tolman electronic
parameters indicate the 1,2,3-triazole mNHOs to be only
slightly stronger donors compared to normal N-heterocyclic
olefins.39,46 Some differences between the two mNHO classes
should be pointed out: The reaction rate is slower compared to
the imidazole system. This finding is in line with the reduced
nucleophilicity of the 1,2,3-triazole mNHOs39 and the rate-
determining step being the initial addition. Furthermore, while
the reaction of the imidazole system leads to a 1:1
stoichiometric water hydrolysis side product limiting the
theoretical yield to 50%, in this case, only several minor
hydrolysis products are detected and water can even be
partially trapped by addition of molecular sieves.
The new diazoalkene class can be described through a series
of zwitter-ionic Lewis structures (2I-2III) as well as dative
descriptions such as a carbone (2IV). In the IR (ATR, solid)
the diazo moiety shows up as a sharp signal at v
cm−1 (2a-c), outside of the classical range for organic diazo
compounds (v
∼ 2017−2180 cm−1)34 and at slightly larger
̃
= 1953−1956
RESULTS AND DISCUSSION
̃
■
We started by investigating the reactivity of 1,2,3-triazole
derived mesoionic N-heterocyclic olefin (mNHO) 1a39 with
nitrous oxide (Scheme 2). Upon addition of 2−3 bar N2O to
wavenumbers compared to the only so far reported room-
temperature stable diazoalkene (1944 cm−1).38 While
imidazole derived NHOs are more ylidically polarized,
compared to 1,2,3-triazole mNHOs,39 the polarization trend
is also reflected in the IR of the diazoalkenes. The low IR
absorption supports a CNN fragment disfavoring
diazonium representations. The 13C NMR signal of the labeled
Scheme 2. Synthesis of 1,2,3-Triazole Derived Diazoalkenes
2 from mNHOs 1 with the Corresponding Lewis Structures
13
 CNN species appears at δ = 34.5−35.2 ppm (2a−c),
being strongly high-field shifted, typical for diazo compounds.
Natural abundance 15N NMR of 2a allows to detect all five
nitrogen atoms, with the diazo N atoms being at δ (15N; rel. to
NH3): 288.7 ppm (N2) and 255.0 ppm (N1; Figure 1B).
The assignment is based on the 15N−13C coupling constants
derived from the labeled sample and supported by quantum
chemical predictions (see SI). While N1 is in a usual 15N NMR
shift range for diazo compounds, N2 (288.7 ppm) is unusually
high field shifted (N2 for diazoalkanes typically δ(15N) ∼
350−400 ppm).34,38
In the X-ray solid-state structures (Figure 1A), the C1−C2
bond lengths [2a: 1.401(2) Å; 2c: 1.399(1) Å] are positioned
between a single and double bond similar to the C−C bond in
benzene (1.398 Å).47 The diazoalkenes are not linear but bent
[2a: 121.6(1)°; 2c: 123.51(8)°]. Interestingly, in the solid-
state, the CNN moiety is pointing down in the case of 2a
and up for 2c, suggesting an unusual type of E/Z stereo-
isomers. Note, NMR data only show one isomer, suggesting a
fast exchange on the NMR time scale (vide infra). The CN2
moiety is in plane with the heterocycle [C3−C2−C1−N1; 2a:
−17.1(3)°; 2c: 178.1(1)°], supporting a partial double bond
contribution. The N1−N2 [2a: 1.157(2) Å; 2c: 1.153(1) Å]
bonds are shorter compared to the previously reported
diazoalkene [1.184(7) Å],38 however, still long compared to
regular diazoalkanes [diazomethane: N−N 1.12 Å]48 disfavor-
ing N−N triple bond representations.
1a, the reaction turns over the course of 3−4 h from a
homogeneous purple solution to a dark suspension. The
desired diazoalkene 2a precipitates and can be isolated as an
orange solid in 65% yield. The reaction can also be performed
with the new mesoionic olefins 1b and 1c to afford 2b and 2c,
respectively. 2a is stable in the solid-state and in solution under
inert atmosphere, while differential scanning calorimetry
(DSC) measurements indicate stability up to 180 °C (see
SI). In analogy to the recently described synthesis of
imidazole-based diazoalkenes, the mechanism most likely
involves initial attack of the strong carbon donor onto the
B
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Scheme 3. Ligand Exchange Reactions of Diazoalkenes with
a
(A) an Isocyanide and with (B) CO
a
Xyl: 2,6-Dimethylphenyl.
Figure 1. (A) X-ray solid-state structures of 2a and 2c. Thermal
ellipsoids are shown with 50% probability. Selected bond lengths and
angles in [Å] and [°]: (2a) C1−C2 1.401(2), C1−N1 1.274(2), N1−
N2 1.157(2), C2−C1−N1 121.6(1), C1−N1−N2 169.1(1), C3−
C2−C1−N1 −17.1(3); (2c): C1−C2 1.399(1), C1−N1 1.276(1),
N1−N2 1.154(1), C2−C1−N1 123.51(8), C1−N1−N2 168.71(9),
C3−C2−C1−N1 178.1(1). (B) Natural abundance 15N NMR
spectrum (60 MHz, d8-THF, ref against liq. NH3) of 2a.
Next, we investigated the reactivity of the diazoalkenes
toward ambiphilic reagents such as 2,6-dimethylphenyl
isocyanide (Scheme 3A). At room temperature, a clean
substitution of dinitrogen takes place to give vinylidene
ketenimine 3a.
Figure 2. X-ray solid-state structures of 3a (left) and 4a (right).
Thermal ellipsoids are shown with 50% probability. Selected bond
lengths and angles in [Å] and [°]: 3a: C1−C2 1.365(2), C2−C3
1.414(2), C1−C4 1.233(2), C4−N1 1.262(2), C2−C1−C4 176.4(1),
C1−C4−N1 170.8(1), C4−N1−C5 129.9(1); 4a: C1−C2 1.387(2),
C2−C3 1.404(2), C1−C4 1.237(2), C4−O1 1.201(2), C2−C1−C4
146.0(2), C1−C4−O1 171.5(2). In the case of 4a, a solvent molecule
of THF is omitted for clarity.
13C labeling of 3a allows to assign Cα: 48.4 ppm and Cβ:
137.2 ppm (verified by INADEQUATE measurements). In the
solid-state structure the C2−C1−C4 angle [176.4(1)°] is
significantly widened, while the C1−C4 [1.233(2) Å] and
C1−C2 bond lengths [1.365(2) Å] are short (Figure 2),
supporting a resonance hybrid between a cumulenic structure
(3a) and a zwitter-ion with a C−C triple bond (3a′).
Previously, we speculated that such an exchange reaction
most likely proceeds via a (3 + 2) cycloaddition leading to
IntA (Scheme 3A) followed by a cycloreversion with N2
liberation.38 At first sight, a restriction to a cycloaddition
mechanism appears to be a severe limitation for a more general
concept. In order to probe if this approach is limited to
isocyanides, we selected carbon monoxide as an ambiphilic,
isoelectronic species. We were delighted to see that carbon
monoxide (2−3 bar) reacted highly selectively (no side
products) with diazoalkene 2a at room temperature in 3−4
h to give vinylidene ketene 4a as a yellow solid in 79% yield
(Scheme 3B).
a zwitter-ionic ketene (4II), a zwitter-ionic alkoxy alkyne (4III),
and a zwitter-ionic oxonium ion (4IV) as well as by additional
dative descriptions (4V−4VI) (Scheme 3B). In the IR, the 
CCO moiety appears as a strong signal at 2096−2099
cm−1 (4a; 4b) and 2085 cm−1 (6) similar to Wentrup’s stable
ketene (2080 cm−1)32 and matrix isolated methyleneketenes
(typically ∼ 2100 cm−1).49 The IR band is at lower
wavenumbers than in free CO (2143 cm−1), indicating π-
backbonding into the CO fragment. 13C-labeled 4a allows to
unambiguously assign the 13C NMR signals (Cα): 13.9 ppm
(4a) [Cα: 12.7 ppm (6)] and Cβ: 149.2 ppm (4a) [Cβ: 144.5
ppm (6)]. The strong high field shift from 34.5 ppm (Cα: 2a)
to 13.9 ppm (Cα: 4a) suggests a significant negative charge at
Cα. At first sight, this appears rather counterintuitive since CO
should be a superior π-acceptor than N2, but it follows the
same trend observed for diazomethane H2CN2 (23.1 ppm)
and ketene H2CCO (2.5 ppm).50 Note, the 13C chemical
shift is quite different to Wentrup’s alkylidene ketene VI
In order to prove the generality of this transformation, we
also performed the reaction with 2b to give 4b (72%) and with
the recently described diazoalkene 5 to give vinylidene ketene
6 (46%). Vinylidene ketenes 4/6 can be described through an
array of Lewis resonance structures such as a cumulenone (4I),
C
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(Scheme 1B) [76 ppm (Cα) and 161 ppm (Cβ)],32 resembling
more carbon suboxide [13C: −14.6 (Cα) and 129.7 ppm (Cβ)],
which was interpreted to show considerable oxonium
character.51 The strong shielding effect is in agreement with
high electron density at Cα, favoring resonance structure 4aII
rather than an alkyne (4aIII). However, it should be pointed
out that the interpretation of 13C NMR shifts solely based on
the diamagnetic contributions to shielding (electron density
arguments) might be problematic since paramagnetic con-
tributions might play a role. In case of 4a and 4b (see SI), we
were able to obtain single crystals suitable for X-ray diffraction
(Figure 2). Strikingly, the C2−C1−C4 angle [146.0(2)° (4a);
147.5(1)° (4b)] is significantly widened compared to 2a
[121.6(1)°], but still far away from linearity. Interestingly, in
1985, Brown and co-workers proposed by microwave spec-
troscopy the parent propadienone (H2CCCO) to feature
a zigzag structure with two angles of 144.5° (CCC) and
169.4° (CCO),52 but so far, no X-ray data on any
alkylidene ketene were reported. The here described solid-state
structures confirm the zigzag microwave data. Furthermore,
the C1−C2 [1.387(2) Å 4a; 1.378(1) Å 4b] and C1−C4
[1.237(2) Å 4a; 1.236(2) Å 4b] bond lengths are in the range
of cumulenic bond distances. In order to gain more
experimental insight into the electronic structure, we
determined the 1J(13C−13C) coupling constants for the
isoelectronic series 2a, 3a, and 4a based on the 13C(Cα)
Figure 3. (A) Computational comparison (BP86/def2TZVPP) of the
isoelectronic series R2CCX (X = N2 2S; CO 4S; CNMe 3S). (B)
Frontier molecular orbitals (BP86/def2TZVPP) shown with an
isovalue of 60%.
confirm the differences in bending, we performed for the series
2S, 3S, and 4S a relaxed potential energy surface scan for the
bending angle C−C−X (Figure 4; for the full system see the
1
labeled compounds. The J(13C−13C) coupling constants for
Cα−Cγ follow the trend: 60 Hz (2a), 107 Hz (4a), and 120 Hz
(3a), while the 1J(13C−13C) coupling constants for Cα−Cβ are
154 Hz (4a) and 177 Hz (3a). It has been established that the
1
magnitude of the J(13C−13C) coupling is primarily deter-
mined by the hybridization of the two carbon atoms
involved.53 The absolute values for 1J(sp2-sp2) coupling
constants of ca. 60−70 Hz53 agree with 1a and 2a to feature
a central (Cα) sp2 carbon atom. Interestingly, the increase in
1J(13C−13C) for 4a (107 Hz) and 3a (120 Hz) can be
correlated with an increase in s-character in the order CC
N2 < CCO < CCNR, in agreement with the trend in
bending angles. Note, coupling constants for 1J(sp2-sp) such as
in H2CCCH2 are typically around 100 Hz.53 The large
Cα−Cβ coupling constants of 154 Hz (4a) and 177 Hz (3a)
support the high s-orbital contribution (sp hybridized) in a
typical range for alkynes (ca. 170−190 Hz).53 Note, while the
hybridization of the two carbon atoms involved can be
estimated, the bond order is not necessarily a triple bond [for
Figure 4. Relaxed potential energy surface scan (BP86/def2TZVPP)
for R2CCX (X = N2 2S; CO 4S; CNMe 3S) as a function of the
bending angle (CCN, CCC). The energy at the minimum was
referenced to zero to allow comparison.
1
comparison: HCCH J(13C−13C) = 171 Hz; HCC−C
53
1
CH J(13C−13C) = 154 Hz].
1
SI). In agreement with the increasing J(13C−13C) coupling
In order to gain additional insight into the electronic
structures of the isoelectronic series [R2CCX; X = N2,
CO, CNR], we performed quantum chemical calculations on
the simplified systems (for the full systems, see SI) (Figure
3A). The structural data (BP86/def2TZVPP) are in very good
agreement with the data obtained from the solid-state
structures. Natural charges at Cα: −0.29e (2) < −0.55e (4)
agree with the 13C NMR data δ ∼ 35 ppm (2) and ∼ 13 ppm
(4). Interestingly, the frontier molecular orbitals of the three
systems are fairly similar (Figure 3B). The LUMO is centered
on the heterocyclic ring, while HOMO and HOMO−1 are
primarily centered on the CNN, CCO, CCN fragments (three
center 4-electron bond). The HOMO is in plane with the
heterocycle, while the HOMO−1 is perpendicularly oriented
in agreement with theoretical predictions by Frenking and co-
workers on related systems.54 In order to computationally
constants in the series 2a < 4a < 3a, a higher s-orbital
contribution would expect minima shifted toward larger
bending angles (also observed by X-ray diffraction).
Interestingly, bending 2a into a linear CCNN
species proceeds with a barrier of ca. 10 kcal/mol. This
relatively low value renders the isolation of potential E/Z
isomers to be unlikely. In case of vinylidene ketene 4a and
vinylidene ketenimine 3a, the minima are shifted to ca. 150°
[exp. 146.0(2)° (4a)] and ca. 175° [exp. 176.4(1)° (3a)]. In
both cases, the bending potential is very shallow with an
energy difference of <1 kcal/mol between 140° and 180°,
indicating the CCX entity to be flexible in solution. This
observation is reminiscent to the shallow bending potential of
carbon suboxide55 and carbones in general.18
Besides the electronic structure, we were interested in the
reaction mechanism for this unusual substitution reaction at a
D
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sp2-carbon center. Even though singlet11−14 and triplet56,57
carbenes are known to activate CO, a dissociative mechanism
via a free vinylidene (SN1 type) seems highly unlikely.
Computational analysis (BP86/def2TZVPP) revealed the
substitution to be strongly exergonic [ΔG = −50.6 kcal/mol
(CO); ΔG = −55.7 kcal/mol (isocyanide)]. Optimizations of
the full system indicated a single transition state II (ΔG⧧ =
18.9 kcal/mol) which connects 2 and 4 in an intrinsic reaction
coordinate (IRC) calculation via a shoulder III (Figure 5).
Figure 5. IRC calculations conducted on TS-II (BP86/def2TZVPP).
Transition-state II can be rationalized by an attack of a
carbon centered lone pair of 2 (HOMO/HOMO−1) into the
antibonding lowest unoccupied molecular orbital (LUMO) of
carbon monoxide. Note, this polarization is the opposite to a
regular SN2 reaction as well as the opposite to the activation of
CO (HOMO) with carbenes (LUMO).11−14 The activation is
supported by reports of carbanion-mediated carbonylation
reactions such as the addition of organolithium compounds to
CO to give highly reactive acyllithium intermediates.58−61
While typically such acyl anions are trapped by an electrophile,
in this case, the adjacent diazonium moiety allows for
synchronous elimination.62 In order to support the unusual
mechanism, we calculated the potential energy surface for the
substitution reaction based on the small system 2S as a function
of the C−C and C−N bond distances for CO and an
isocyanide (Figure 6). The data agree with the IRC calculation
of the full system with a single transition state (TS-IIS) and no
acyl intermediate being involved. The direct downhill trans-
formation on a temporarily flat potential surface (area around
IIIS) leads to a strongly exergonic dinitrogen liberation (Figure
6A). This reaction mechanism might be classified as two-step
(addition/elimination) no intermediate transformation, even
though no bifurcation scenario is involved.63 Note, the
elementary step involves the concerted bond cleavage of a
CN multiple bond to form a new CC multiple bond. The
conclusions derived from the energy surface of the small
system agree with the optimizations of the full systems.
Figure 6. Relaxed potential energy surfaces (electronic energy) for the
exchange of N2 by CO (A) and N2 by an isocyanide (B) as a function
of the C−N and C−C bond distances at the BP86/def2TZVPP level
of theory. The dotted lines indicate the energetically lowest energy
path (IRC) from starting material to product. Optimized transition
states and intermediates of the isocyanide reaction (C).
product 3S. Optimizations of the full system are in excellent
agreement with the potential energy surface of the small
system, featuring an activation energy of ΔG⧧ = 21.6 kcal/mol
(TS-IV) on the reaction path to the (3 + 2) cycloaddition
product Int-V (ΔG = −5.8 kcal/mol) in an overall strongly
exergonic reaction (ΔG = −55.7 kcal/mol; for the optimized
structures see the SI).
Finally, we were curious if it is possible to transfer the
vinylidene ketene into its sulfur derivative, thereby extending
the isoelectronic series R2CCX to XS. Some cumu-
lenes, such as [PCO]−, are known to give the
corresponding thio derivatives [PCS]− by addition of
CS2 under release of COS.64 In analogy, we attempted the
synthesis of the thio analog of 4 (Scheme 4).
Upon addition of 1 equiv of carbon disulfide to 4a, an
instant color change from dark brown to orange was observed.
Monitoring the reaction by in situ NMR spectroscopy
indicated the immediate formation of a new species, which
rearranged into another compound (see SI). Low-temperature
(−20 °C) NMR spectroscopy allowed the clean character-
ization of the intermediate, the unknown 2-oxo-2H-thiete-4-
thiolate heterocycle 7b. Additionally, we also obtained single
crystals at −25 °C and could establish the solid-state structure
Interestingly, we also compared the potential energy surface
for the reaction with an isocyanide (Figure 6B). While the first
transition state [TS-IVS] is structurally and energetically
similar to (TS-IIS; Figure 6C) and can also be rationalized
by a nucleophilic activation of the isocyanide, it leads to an
initial intermediate, the (3 + 2) cycloaddition product Int-VS.
A second transition state (TS-VIS) connects the cyclic
intermediate Int-VS with the strongly exergonic substitution
E
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Scheme 4. Reaction of Vinylidene Ketenes 4 with CS2 to
Give Vinylidene Thioketenes 8 via a Four-Membered
Intermediate (7)
CONCLUSION
■
In summary, we report a new stable diazoalkene class based on
the dinitrogen transfer from N2O to a 1,2,3-triazole derived
mNHO. As such mNHOs are easily accessible and structurally
variable, this finding opens up a simple synthetic access to a
structurally rich class of diazoalkenes. We show that the
exchange reaction of N2 with ambiphilic compounds is not
limited to isocyanides, but can also be performed at room
temperature with CO. Computations suggest the mechanism
proceeds through a nucleophilic activation of CO via a
concerted mechanism under release of dinitrogen. The new
type of substitution offers a mild and atom-economic synthetic
approach to alkylidene ketenes. We present the first structural
data of vinylidene ketenes, confirming their proposed structure
to feature a zigzag conformation, and give an experimental
comparison between the complete isoelectronic series R2C
of the reactive thiete heterocycle 7b (Figure 7). Interestingly, a
thiete heterocycle was proposed as an unstable intermediate by
CX (X = N2, CO, CNR, CS) including J(13C−13C) data.
1
The vinylidene thioketenes can be accessed by reaction of
alkylidene ketenes with CS2. We verified the mechanism takes
place via a structural characterized four-membered thiete
intermediate. Even though the spectral data of the new
compounds indicate cumulenic character, it remains to be
evaluated if the new substitution approach might also add
additional insights into carbone chemistry. Previously, carbon-
based ylidic compounds have been interpreted as coordination
compounds.18,70 Considering that N2, CO, and isocyanides
belong to the archetypical ligands in transition-metal
chemistry, the here described concept of ligand exchange
might add a new dimension in the understanding of carbon-
based “coordination” complexes, thereby mimicking transition-
metal reactivity. Importantly, the substitution with other small
molecules should be applicable since no restriction to a
cycloaddition mechanism is required. We believe the exchange
of entities at a vinylidene carbon center opens a new atom-
economic synthetic approach to access novel unsaturated
compounds, which we are currently further investigating.
Figure 7. X-ray solid-state structures of 7b and 8b. Thermal ellipsoids
are shown with 50% probability. Selected bond lengths and angles in
[Å] and [°]: 7b: C1−C2 1.437(2), C2−C3 1.398(2), C1−C4
1.443(2), C1−C5 1.411(2), C4−S1 1.875(2), C4−O1 1.198 (2),
C5−S1 1.814(2), C5−S2 1.633(2), N3−C2−C1−C4 36.3; 8b: C1−
C2 1.379(3), C2−C3 1.409(3), C1−C4 1.227(3), C4−S1 1.616(2),
C2−C1−C4 166.4(2), C1−C4−S1 179.4(2). In the case of 7b, two
molecules of CH2Cl2 are omitted for clarity.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c06906.
■
sı
Birum and Matthews in the chemistry of phosphorus ylides,
but so far not characterized.65 In a closed J-Young NMR tube,
7b releases quickly (∼ 1 h) at room-temperature COS (13C
NMR 153.8 ppm; CD2Cl2) until an equilibrium (∼3:1 7b:8b)
is reached, indicating a reversibility in the presence of COS.
Note, it is important to remove excess of CS2 since 8b remains
reactive. Upon removing COS under reduced pressure, the
vinylidene thioketene can be obtained as a single species.
Vinylidene thioketenes are rare compounds which have been
derived from thiophosgene66−68 or by FVP.69 In the ATR-IR, 8
features an intense band at 2035−2036 cm−1 (8a; 8b). 13C
labeling of 8a allows to assign Cα: 51.2 ppm and Cβ: 140.4
Experimental procedures, characterization data, and
computational details (PDF)
Accession Codes
CCDC 2094184−2094190 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
ppm with J(13Cα−13Cβ) = 172 Hz and J(13Cα−13Cγ) = 122
Hz. The NMR parameters are similar to the data obtained for
vinylidene ketenimine 3a, supporting a significant triple-bond
contribution. Additionally, we were able to obtain single
crystals of 8b suitable for X-ray diffraction (Figure 7). Upon
O/S exchange, the C−C−C bond angle widens from
147.5(1)° (O; 4b) to 166.4(2)° (S; 8b), while the C1−C4
distance shortens [1.236(2) Å (O; 4b); 1.227(3) Å (S; 8b)].
These data agree with a lower tendency of sulfur to form CS
double bonds and favor an alkyne representation 8′ similar to
the structural data for vinylidene ketenimine 3a.
1
1
AUTHOR INFORMATION
Corresponding Author
■
Max M. Hansmann − Fakultät fur Chemie und Chemische
̈
Biologie, Technische Universität Dortmund, 44227
Dortmund, Germany; orcid.org/0000-0003-3107-1445;
Email: max.hansmann@tu-dortmund.de
Authors
Patrick W. Antoni − Fakultät fur Chemie und Chemische
̈
Biologie, Technische Universität Dortmund, 44227
Dortmund, Germany
F
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J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
r Chemie und Chemische Biologie,
pubs.acs.org/JACS
Article
with Implications for Synthesis. J. Am. Chem. Soc. 2011, 133, 3557−
3569.
Justus Reitz − Fakultät fu
̈
Technische Universität Dortmund, 44227 Dortmund,
(13) Allen, A. D.; Tidwell, T. T. Ketenes and Other Cumulenes as
Reactive Intermediates. Chem. Rev. 2013, 113, 7287−7342.
(14) Peltier, J. L.; Tomás-Mendivil, E.; Tolentino, D. R.; Hansmann,
M. M.; Jazzar, R.; Bertrand, G. Realizing Metal-Free Carbene-
Catalyzed Carbonylation Reactions with CO. J. Am. Chem. Soc. 2020,
142, 18336−18340.
Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c06906
Author Contributions
All authors performed the experimental work. M.M.H.
performed the calculations and wrote the manuscript. All
authors have given approval to the final version of the
manuscript.
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function. New J. Chem. 2007, 31, 1853−1873.
Notes
̈
(17) Kappe, T.; Ziegler, E. Kohlensuboxid in der praparativen
The authors declare no competing financial interest.
Organischen Chemie. Angew. Chem., Int. Ed. Engl. 1974, 13, 491−504.
(18) Tonner, R.; Frenking, G. C(NHC)2: Divalent Carbon(0)
Compounds with N-Heterocyclic Carbene LigandsTheoretical
Evidence for a Class of Molecules with Promising Chemical
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(19) Matthews, C. N.; Birum, G. H. Triphenylphosphoranylidene-
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ACKNOWLEDGMENTS
■
This work was supported by the Fonds der chemischen
Industrie and BMBF (tenure-track program for M.M.H.).
Computational resources were provided by LiDO3, the high-
performance computing facility at TU Dortmund (DFG
project 271512359). Thomas Seidensticker is thanked for
providing carbon monoxide. Christian Sindlinger is greatly
acknowledged for refining the X-ray solid-state structures. The
members of the chemistry department at TU Dortmund are
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NMR and INADEQUATE measurements.
N
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