Ring Enlargement of Three-Membered Heterocycles by Treatment with In Situ Formed Tricyanomethane

Article abstract of DOI:10.1002/chem.202000089

Although the chemistry of elusive tricyanomethane (cyanoform) has been studied during a period of more than 150 years, this compound has very rarely been utilized in the synthesis or modification of heterocycles. Three-membered heterocycles, such as epoxides, thiirane, aziridines, or 2H-azirines, are now treated with tricyanomethane, which is generated in situ by heating azidomethylidene-malonodinitrile in tetrahydrofuran at 45 °C or by adding sulfuric acid to potassium tricyanomethanide. This leads to ring expansion with formation of 2-(dicyanomethylidene)oxazolidine derivatives or creation of the corresponding thiazolidine, imidazolidine, or imidazoline compounds and opens up a new access to these push–pull-substituted olefinic products. The regio- and stereochemistry of the ring-enlargement processes are discussed, and the proposed reaction mechanisms were confirmed by using ¹⁵N-labeled substrates. It turns out that different mechanisms are operating; however, tricyanomethanide is always acting as a nitrogen-centered nucleophile, which is quite unusual if compared to other reactions of this species.

Full text of DOI:10.1002/chem.202000089

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Title: Ring Enlargement of Three-Membered Heterocycles by
Treatment with In Situ Formed Tricyanomethane
Authors: Klaus Banert, Madhu Chityala, and Marcus Korb
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Ring Enlargement of Three-Membered Heterocycles by Treatment
with In Situ Formed Tricyanomethane✱✱
Klaus Banert,✱[a] Madhu Chityala, and Marcus Korb
[a]
[b]
Dedicated to Professor Akhila Kumar Sahoo, University of Hyderabad
Abstract: Although the chemistry of elusive tricyanomethane
ketenimine 3.[5] In 1963, Trofimenko used the salt 1b to repeat
the synthesis of 2 and called this compound “aquoethereal
cyanoform”.[6] He successfully liberated 2 from the solvents by
rapid evaporation and vacuum sublimation to receive unstable
white crystals, to which he erroneously assigned the structure of
3. When Dunitz et al. performed the rapid evaporation of 2
without sublimation, they were able to isolate single crystals of
hydronium tricyanomethanide (4), and that facilitated the
(cyanoform) has been studied during a period of more than 150 years,
this compound has very rarely been utilized in the synthesis or
modification of heterocycles. Three-membered heterocycles, such as
epoxides, thiirane, aziridines, or 2H-azirines, are now treated with
tricyanomethane, which is generated in situ by heating
azidomethylidene-malonodinitrile in tetrahydrofuran at 45 °C or by
adding sulfuric acid to potassium tricyanomethanide. This leads to
ring expansion with formation of 2-(dicyanomethylidene)oxazolidine
derivatives or creation of the corresponding thiazolidine,
imidazolidine, or imidazoline compounds and opens up a new access
to these push–pull substituted olefinic products. The regio- and
stereochemistry of the ring-enlargement processes are discussed,
[
7]
structure verification by X-ray diffraction studies. In 2017, it was
shown that Trofimenko’s experiment did not produce 3 because
15
and the proposed reaction mechanisms were confirmed by using N-
labeled substrates. It turns out that different mechanisms are
operating; however, tricyanomethanide is always acting as
a
nitrogen-centered nucleophile, which is quite unusual if compared to
other reactions of this species.
Introduction
The history of tricyanomethane (5) dates back to 1864 when the
first isolation of so-called cyanoform was reported.[1] But this and
a
second synthesis[2] could not be reproduced by other
chemists,[
2,3]
and later it was found that 5, even if generated,
would not have been able to survive the described drastic
reaction conditions. In 1896, Schmidtmann treated sodium
tricyanomethanide (1a) with dilute sulfuric acid and then with
diethyl ether (Scheme 1).[4] He obtained a three-phase system,
which comprised a greenish yellow middle layer 2 that was
claimed to include cyanoform (5), although first experiments to
remove the solvents did not lead to characterizable substances.
Three years later, Hantzsch and Osswald confirmed these
phenomena and stated that tautomerism of 5 is likely to form
[
[
a]
b]
Prof. Dr. K. Banert, Dr. M. Chityala
Organic Chemistry, Chemnitz University of Technology
Strasse der Nationen 62, 09111 Chemnitz (Germany)
E-mail: klaus.banert@chemie.tu-chemnitz.de
Dr. M. Korb
The University of Western Australia, Faculty of Science, School of
Molecular Scienes
35 Stirling Highway, Crawley, Perth, Western Australia 6009
(
Australia)
[✱✱
]
This work is Part 39 in the series “Reactions of Unsaturated Azides”;
for Part 38, see reference [8].
Scheme 1. Previous syntheses of tricyanomethane (5).

Supporting information, including full experimental details and
compound characterization, and the ORCID identification number(s)
for the author(s) of this article can be found under …
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rapid evaporation and sublimation of 2 led to the isolation of a
mixture of 4 and 5.[8] Under special conditions of vacuum
sublimation, single crystals of tricyanomethane (5) were
accessible, which allowed structure confirmation by X-ray
diffraction. If moisture was excluded, such single crystals could
be handled at ambient temperature for a short time. But
cyanoform (5) could not be analyzed in solution by NMR
spectroscopy at room temperature due to rapid decomposition.
Moreover, 5 was easily converted into 4 even if only trace
amounts of water were present. Especially, mixtures of 4 and 5
possibly react to 12a by nucleophilic addition of the hydroxy
group at a cyano unit. The former assumption was seemingly
supported
by
the
well-known[15]
C-alkylation
of
tricyanomethanide salts such as 1b with the help of alternative
electrophiles,[16–18] for example, primary alkyl halides. But instead
of 10a or 12a, we obtained the isomeric product 13a after
treatment of 9a with 7. Obviously, protonated 9a was attacked by
a nitrogen atom, and after ring opening, the resulting ketenimine
11a was transformed into the final product 13a, which includes a
push–pull substituted C=C unit. Whereas both rings are cis-fused
in the starting compound 9a, the 1H NMR data of the ring-
expanded product 13a indicated a large vicinal coupling of the
1
tended to dynamic processes, which led to extremely broad H
1
3
and C NMR signals at temperatures between –50 and 0 °C.
Thus, even lower temperatures, for example, –70 °C, were
necessary to detect sharp 13C NMR signals for pure 5 in
3
two axial bridgehead protons with J = 12.0 Hz. Consequently,
both rings in 13a are trans-fused.
[
D
8
]THF.[8]
Cyanoform (5) was prepared not only from 1a or 1b via 2, but
also by reacting 1c with hydrogen sulfide,[9] or alternatively by
treatment of 1d with an excess of anhydrous hydrogen
fluoride,[ and finally by short-time thermolysis or photolysis of
vinyl azide 7.[8] The products 5 and 6 could not be separated in
the case of precursor 1d; however, convincing spectroscopic
10]
identification of
5
was nevertheless possible.[10] Whereas
thermolysis of 7 produced only 5 and small amounts of 4,
irradiation of 7 in solution led to 5 and the 2H-azirine 8.[8] The
photolysis of 7 isolated in an argon matrix did not generate 5
since ketenimine 3 and the heterocycle 8 were formed instead.
During a period of more than 150 years, the chemistry of
cyanoform (5) has been studied thoroughly.[11] Its gas-phase
structure was investigated by photoelectron spectroscopy[12] and
microwave spectroscopy.[9] The relative stabilities, spectroscopic
4 3
data, and isomerization reactions of 3, 5, and other C HN
species were analyzed by utilizing quantum-chemical
methods.[ In particular, the acidic properties of 5 as well as its
13]
tautomer
3 5 is
were discussed intensively.[5,6,14] Hence,
presented in textbooks of organic chemistry as one of the
strongest carbon acids.
As a Brønsted acid, tricyanomethane (5) should be able to
catalyze the ring opening of three-membered heterocycles, for
example, epoxides. If no other reactive species such as
competing nucleophiles are present, interesting novel types of
open-chain or ring-expanded products may result. Herein, we
report on surprising transformations which were induced by
treatment of oxiranes, thiirane, aziridines, and 2H-azirines with in
situ formed cyanoform (5).
Scheme 2. Ring enlargement of 9a by treatment with 7 and in situ generation
of tricyanomethane (5).
Besides 9a, we similarly treated also the epoxides 9b–h with
vinyl azide 7 to obtain the ring-enlargement products 13 (Table
1). While the parent oxirane (9b) exclusively led to the single
product 13b,[19] the reaction of propylene oxide (9c) resulted in
the formation of two regioisomeric oxazole derivatives, which
were substituted with a methyl group in the 5- or in the 4-position.
Both isomers were easily separated by chromatography, and
assignment by H and 13C NMR spectroscopy was simple since
the adjacent oxygen atom always induced significantly stronger
deshielding properties than the amine unit. In the case of styrene
oxide (9d), treatment with 7 regioselectively led to 13d as the
sole product, and the analogous reaction of the unsymmetrical
epoxide 9e similarly resulted in the exclusive formation of the
oxazole derivative 13e, which was isolated in 79% yield. Thus,
the well-known[20] regiochemistry of acid-catalyzed epoxide ring
1
Results and Discussion
At first, we utilized the commercially available cyclohexene oxide
(
9a) as a substrate that was exposed to the precursor 7 in
anhydrous THF at 45 °C (Scheme 2). We expected that 5
resulting from 7 would O-protonate 9a, and the nucleophilic
attack of the central carbon atom of the tricyanomethanide
counterion would lead to the ring-opened product 10a, which can
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opening was also observed for the conversion of 9 into 13
although the involved reagents did not include an acid on the first
view. However, slight warming of vinyl azide 7 generated the
Brønsted acid cyanoform (5), which initiated the desired
transformation by O-protonation of 9.
Table 1. Treatment of epoxides 9 with azide 7 to prepare the ring-enlargement
products 13.
Figure 1. ORTEP (30% ellipsoid probability) of the molecular structure of 13g.
A second molecule in the asymmetric unit has been omitted for clarity. Selected
bond properties (Å/°): C1–O1 1.45(3), C1–N1 1.31(3), C1–C4 1.37(3), O1–C2
Substrate 9
Product 13
1.45(3), C2–C3 1.60(3), C3–N1 1.43(2), C4–C5 1.40(3), C5–N3 1.15(3), C4–
Yield
[
C6 1.43(3), C6–N2 1.21(3), N1–C1–O1 111(2), C1–N1–C3 114(2), C5–C4–C6
120(2), C7–C2–C3–C13 128(2).
R1
R2
R3
R1
R2
R3
%]]
a]
a
b
c
H
H
H
(CH
2
)
4
H
H
(CH
2
)
4
56
72
The ring-enlargement reaction with the help of vinyl azide 7
was also transferred from oxiranes 9 to thiirane (14) and
aziridines 16 and 18 to obtain the five-membered heterocycles
15,[21] 17, and 19, respectively (Scheme 3). The 13C NMR
H
H
H
H
H
Me
H
Me
H
H
Me
H
25
22
6
spectrum of imidazole derivative 17, measured in [D ]DMSO at
room temperature, showed only a broad signal for both cyano
groups, whereas the corresponding spectrum of a solution in
[D ]acetone exhibited two signals at ambient temperature. When
6
d
e
f
Ph
Ph
Ph
H
H
Ph
Ph
H
H
H
58
79
Ph
H
H
Ph
the temperature was raised to 45 °C in the latter case,
coalescence of the two signals was observed. Rapid exchange
of both cyano groups was obviously initiated by rotation about
the push–pull substituted C=C bond. The π system of this bond
is weakened owing to a dipolar resonance structure, which
includes a positive charge, stabilized by the ring nitrogen atoms,
and a negative charge delocalized by the cyano groups. The
molecular structures of the heterocyclic products 15, 17, and 19
were confirmed by the usual spectroscopic characterization and
also by single crystal X-ray diffraction analysis of 17 (Figure 2).
Bz
Ph
H
H
Ph
Bz
Bz
27
26
g
h
H
Ph
H
Ph
Ph
H
H
Ph
Ph
Ph
Ph
46]b]
47]b]
Ph
[
[
a] Isolated yields; after separation of two products in the case of 13c and 13f.
b] 1,2-Diphenylethanone and diphenylacetaldehyde were additionally formed
and isolated with a total yield of 13–14%.
When the epoxide 9f, which includes a phenyl group and an
electron-withdrawing benzoyl substituent, was subjected to 7, the
product 13f was formed with perfect regioselectivity. However,
13f was composed of two stereoisomers isolated with nearly
equal yields; the structural assignment of cis-13f and trans-13f
was based on 1H NMR spectroscopy utilizing vicinal coupling
constants and NOE experiments. In the case of trans-4,5-
diphenyloxazole derivative 13g/h, the synthesis was successful
starting with oxirane 9g or alternatively with the stereoisomer 9h.
Thus, both reactions with 7 are stereoselective, but not
stereospecific since the same product (13g = 13h) was formed.
The transformations of 9g and 9h were accompanied by the
generation of 1,2-diphenylethanone and diphenylacetaldehyde
(
total isolated yield: 13–14%). These side products obviously
resulted from ring opening of the O-protonated epoxides followed
by hydride shift or phenyl migration of the substituted benzyl
cation. Similar carbocations were involved in the reactions of 9d
or 9e with 7 which explains the regioselectivity in the formation of
13d and 13e, respectively. The structure of 13g was confirmed
not only by the usual spectroscopic characterization, but also by
single crystal X-ray diffraction analysis (Figure 1).
Scheme 3. Ring enlargement of 14, 16, and 18.
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Figure 2. ORTEP (50% ellipsoid probability) of the molecular structure of 17.
A second molecule in the asymmetric unit has been omitted for clarity. Selected
bond properties (Å/°): S1–N1 1.6929(13), N1–C8 1.4979(18), C8–C9 1.525(2),
C9–N2 1.463(2), N2–C10 1.3239(18), C10–N1 1.4121(18), C10–C11 1.388(2),
C11–C12 1.418(2), C12–N3 1.147(2), C11–C13 1.421(2), C13–N4 1.1508(19),
C8–N1–C10 105.08(11), C9–N2–C10 112.78(13), N1–C10–N2 110.05(13),
C12–C11–C13 117.06(13), N1–C8–C9–N2 26.89(14).
Scheme 4. Reaction of different 2H-azirines with cyanoform (5); for the key of
1
2
R and R , see Table 2.
Finally, we treated the 2H-azirines 21 with the azide 7 to
induce the reaction of in situ generated cyanoform (5) with these
highly strained three-membered heterocycles (Scheme 4). Since
the addition of 5 at the C=N bond of azirine 8, which was
performed by irradiating of 7 at low temperatures and warming
the resulting mixture of the photoproducts 5 and 8 to ≥ –30 °C,
was reported to lead to the unstable aziridine 20,[8] we expected
analogous final products from substrates 7 and 21. But the latter
starting compounds afforded novel ring-extended imidazole
derivatives 22 instead of aziridines. The products 22a–d were
formed with 56–62% yield; and in the case of the very unstable[22]
Table 2. Treatment of 21a–d with 7 to produce 22a–d.
2
1/22
R1
Ph
H
R2
H
Yield of 22 [%][a]
a
58
61
56
62
b
c
d
Ph
Ph
Me
Ph
Ph
[
a] Isolated yields.
2H-azirine 21b, which does not possess a substituent in the 3-
Whereas the formation of 20 is connected with an addition
position, the yield was even slightly higher than that for the
transformation of the robust[23] substrate 21a, although the
corresponding products 22a and 22b are identical (Table 2). It
turned out that acetonitrile was a better solvent for this ring-
enlargement reaction than tetrahydrofuran since more
convenient workup was possible and pure products were
obtained easily. The new heterocycles 22 were characterized not
only by the usual spectroscopic methods, but also by 15N NMR
spectra of 22a and 22c. After assignment of the two NH proton
at the C=N bond of N-protonated 8 and obviously induced by
tricyanomethanide, acting as a carbon nucleophile, the genesis
of 22 can only be explained by interaction of the protonated 2H-
azirines 21 with tricyanomethanide, which reacts as a nitrogen
nucleophile. Since at least two mechanisms may rationalize the
21 → 22 ring enlargement, we utilized the 15N-labeled substrate
15N-21a,[25] which was prepared by thermolysis of the
corresponding α-azidostyrene, and included 49% of the nitrogen
label (Scheme 5). After treatment of 15N-21a with 7, we obtained,
besides unlabeled 22a, the products 15N-22a and 15N-22a' in a
ratio of 11:1. The quantitative analysis of 15N-22a was facilitated
15
1
signals of 22a by homonuclear NOE experiments, 2D- N, H shift
correlation (HSQC) also facilitated the allocation of the 15N NMR
signals. In the 13C and 15N NMR spectra of the unsymmetrical
heterocyclic compound 22a, both cyano groups led to a single
signal because of a rapid rotation about the exocyclic C=C bond.
1
15
by the H NMR signal of NH-1 at δ = 12.50, which indicated
1
15
1
1
1
J( N, H) = 97.6 Hz with H, H triplet splitting (J = 2.2 Hz),
whereas N-22a' showed a H NMR signal of 15NH-3 at δ = 12.68
with J( N, H) = 96.0 Hz and additional H, H triplet splitting (J =
2.2 Hz). These results were supported by the 15N NMR spectrum,
1
5
1
]DMF), the 13C
1
15
1
1
1
Even at low temperature (–60 °C, 100 MHz, [D
7
NMR signal of the two cyano groups was not split into two lines.
Apparently, the dipolar resonance structure of 22a,[24] in which
the positive charge is delocalized in an aromatic imidazolium ring,
is more pronounced than in the case of non-aromatic heterocycle
15
15
which included the NH-1 signal of N-22a at δ = –233.6 with
1
15
1
J( N, H) = 97.4 Hz and additional long-range coupling (dd, J =
4.6 and 3.7 Hz) as well as the 15NH-3 signal of 15N-22a' at δ = –
9. This assumption might be supported by the fact that 13C NMR
chemical shifts of the exocyclic C=C moiety in 22a (δ = 24.5 and
49.3, Δδ = 124.8) show a greater difference than those of 19
Δδ = 109.6).
1
15
1
3
1
238.9 with J( N, H) = 96.4 Hz and triplet splitting with J = 3.4
1
15
Hz. The assignment of all H and N NMR signals were based
not only on the previous allocation of the corresponding NMR
signals of 22a (see above), but also on additional NOE
experiments with 15N-22a/ N-22a'.
1
(
15
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Scheme 5. Ring enlargement of 15N-21a.
We assume that formation of tricyanomethane (5) from 7 and
subsequent N-protonation of 15N-21a with generation of the ion
pair 23 are always the first steps in the ring enlargement reaction
of 15N-21a (Scheme 6). Thereafter, attack of the nitrogen
nucleophile tricyanomethanide at the highly activated C=N unit
of 23 leads to the addition product 24, which undergoes a ring
expansion by a 1,3-migration process. The resulting imidazole
derivative 25 tautomerizes to yield the main product 15N-22a.
However, a minor pathway with nitrogen-centered nucleophilic
3
attack of tricyanomethanide at the sp -hybridized carbon atom
achieves ring opening of 23 with formation of 26; and
successional cyclization of this ketenimine intermediate, followed
by tautomerism of 25', generates 15N-22a'. The sequence N-
15
21a → 23 → 26 → 25' is similar to the mechanisms, which we
suggest for the corresponding ring-enlargement reactions of
epoxides 9, thiirane 14, and aziridines 16 and 18.
In order to confirm the proposed reaction mechanisms of the
transformation 7 + 21a → 22a, we utilized also the 15N-labeled
azide
chloromethylidene)malonodinitrile and sodium azide that
included 98% of the isotope label at one of the terminal positions
Scheme 7). Since we used a smaller amount of 15N-7 and
because of the label distribution on the three cyano positions of
15N-7,
which
was
prepared
from
Scheme 6. Reaction mechanisms which explain the formation of 15N-22a and
N-22a' from 7 and N-21a.
(
15
15
(
1
5
the resulting cyanoform, the experiment with
N-7 was
15
significantly less sensitive than that with N-21a. As expected,
we detected just a small NH signal of 15N-22a besides strong
signals of 15N-22a' and NH-unlabeled 22a and 15N-22a'' in the H
1
15
NMR spectrum, which was measured after treating N-7 with
1
5
15
2
3
1a. The corresponding N NMR spectrum indicated only NH-
1
3
15
15
(δ = –238.6, dt, J = 96.4 Hz, J = 3.4 Hz) of N-22a' and C≡ N
δ = –113.3, s) of 15N-22a''. Thus, the transformation of 15N-7
(
proved to be a complementary confirmation of the experiment
with 15N-21a.
Scheme 7. Ring enlargement of 21a with the help of 15N-7.
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Ring-expansion reactions of 9, 14, 16, 18, and 21 were
performed with the aid of cyanoform precursor 7, which was used
under aprotic starting conditions and without competing
nucleophiles. Hence, the question arose whether the
corresponding ring-enlargement products can also be prepared
if aquoethereal cyanoform (2) or similar reagents are utilized. In
order to find an answer to this question, we treated epoxide 9a
with compound 2 and obtained the desired product 13a in 75%
yield (Scheme 8). However, 13a was accompanied by a small
amount of trans-cyclohexane-1,2-diol, which was simultaneously
generated from 9a owing to the presence of water. Thus, the
separation and purification of 13a was quite tedious if compared
to the workup after the transformation 7 + 9a → 13a and similar
reactions. Consequently, we tried anhydrous acidification of 1b
and added concentrated sulfuric acid to a solution of 1b and 9a
in 1,2-dimethoxyethane (DME). After optimization of the
reactions conditions, we isolated 13a by simple washing of the
crude product with a limited amount of dichloromethane in 79%
yield. When we exposed the azirine 21a to the reagent 2, we
obtained the wanted product 22a, but the yield was disappointing
low (10%). By using the procedure with 1b and concentrated
sulfuric acid in DME, we did not get the desired compound 22a
from 21a at all. Hence, ring enlargement of three-membered
heterocycles with the help of the alternative cyanoform precursor
show in near future that tricyanomethane (5), also generated in
situ by warming of 7 or by acidification of 1b, operates as a
carbon nucleophile in Michael addition reactions.
Products of ring expansion, such as 13, 15, 17, 19, and 22,
include an exocyclic push–pull substituted C=C unit, and similar
compounds were previously investigated intensively because of
their structures, electronic properties, and rotational barriers.[
Moreover, such substances were utilized for organic synthesis,
in particular, as precursors of (other) heterocyclic skeletal
structures,[27] and in some cases, the biological properties were
24,26]
[
28]
tested, for example, as plant growth regulators or for histamine
H
3
receptor-binding affinities.[ Several methods are known to
29]
prepare these push–pull substituted olefinic compounds. In most
cases, however, the access required multi-step
a
synthesis.[19,21,30] Thus, the simple cyanoform-induced
transformation of three-membered heterocycles into ring-
enlargement products, such as 13, 15, 17, 19, and 22, offers a
new synthetic approach for the desired push–pull substituted
alkenes.
Experimental Section
1H, 13C, 15N NMR spectra, crystal structure data and
Experimental details,
1b is possible, but there are limitations in some cases.
refinement details are given in the Supporting Information.
Acknowledgements
M. C. is grateful to DAAD (Deutscher Akademischer
Austauschdienst) for a Ph.D. fellowship. M. K. thanks the Fonds
der Chemischen Industrie for a Ph.D. fellowship and the Forrest
Research Foundation for a Postdoctorial Fellowship. We are
grateful to Dr. M. Hagedorn for his assistance with special NMR
measurements and Dr. A. Ihle for his help in connection with the
manuscript. We thank Inorganic Chemistry at Chemnitz University
of Technology and especially Prof. H. Lang for support by making
available X-ray equipment.
Keywords: cyanoform • nitrogen heterocycles • reaction
mechanisms • reactive intermediates • ring expansion
Scheme 8. Ring enlargement of 9a and 21a with cyanoform generated from
tricyanomethanide salts.
[
[
1]
2]
T. Fairly, Ann. Chem. Pharm. 1864/1865, Supl. 3, 371–374.
a) F. Pfankuch, J. Prakt. Chem. 1871, 4, 35–39; b) F. Pfankuch, J. Prakt.
Chem. 1873, 6, 97–116.
Conclusions
In summary, we have demonstrated that tricyanomethane (5), in
situ generated by thermal decay of vinyl azide 7 or by
acidification of tricyanomethanide salt 1b, can successfully be
used to perform ring expansion reactions with three-membered
heterocycles. The resulting products are formed via varying
reactions mechanisms; however, tricyanomethanide always acts
as nitrogen nucleophile. This is quite different to the known
simple alkylation of this nucleophilic species in the presence of
alkyl halides, which leads to 1,1,1-tricyanoalkanes by attack of a
carbon nucleophile.[15] It can be argued that different reaction
conditions of ring enlargement and alkylation are the reason for
distinct nucleophilic properties of tricyanomethanide. But we will
[
3]
a) A. Claus, Ber. Dtsch. Chem. Ges. 1876, 9, 223–228; b) A. Claus,
Justus Liebigs Ann. Chem. 1878, 191, 33–93.
[
[
[
[
4]
5]
6]
7]
H. Schmidtmann, Ber. Dtsch. Chem. Ges. 1896, 29, 1168−1175.
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Hidden acid: Cyanoform, which
is generated in situ from
azidomethylidene-malonodinitrile
or other precursors, induces ring-
expansion reactions of epoxides,
thiirane, aziridines, or 2H-
azirines. The transformations are
✱
Klaus Banert, Madhu Chityala, and Marcus Korb
Page No. – Page No.
Ring Enlargement of Three-Membered Heterocycles by
Treatment with In Situ Formed Tricyanomethane
always
initiated
through
protonation of the heteroatom X
by cyanoform, but lead to the
five-membered heterocycles via
different
mechanisms
(see
scheme).
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