Interaction of acetonitrile with trifluoromethanesulfonic acid: Unexpected formation of a wide variety of structures

Article abstract of DOI:10.1039/c2ob06841a

Interaction of acetonitrile with trifluoromethanesulfonic acid has been studied by multinuclear NMR and ESI-MS. It has been found that the interaction results in formation of a great variety of different cations and neutral compounds which is controlled by the ratio of CH₃CN to TfOH. In the presence of an excess of the acid (molar ratio 1:8-14) diprotonated N-acetylacetamidine 1 is formed as the major product, which eventually transforms into protonated acetamidine 3 and acetic acid 4. At molar ratio of (1:1-2) diprotonated 2,4-dimethyl-6-methylidene-3H-1,3,5-triazine 12, tautomer of the diprotonated trimethyl-s-triazine 11, becomes the main product at an early stage of the reaction and diprotonated 1-(dimethyl-1,3,5-triazin-2-yl) prop-1-en-2-ol 15 at a later stage. In the case of a large excess of acetonitrile (4-20:1) trication 17 is formed as a result of the interaction between 11 and 12 along with some oligomers [(CH₃CN) ₃]_n (n = 4-12). The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob06841a

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PAPER
Interaction of acetonitrile with trifluoromethanesulfonic acid: unexpected
formation of a wide variety of structures†
George E. Salnikov, Alexander M. Genaev,* Vladimir G. Vasiliev and Vyacheslav G. Shubin
Received 2nd November 2011, Accepted 5th January 2012
DOI: 10.1039/c2ob06841a
Interaction of acetonitrile with trifluoromethanesulfonic acid has been studied by multinuclear NMR and
ESI-MS. It has been found that the interaction results in formation of a great variety of different cations
and neutral compounds which is controlled by the ratio of CH3CN to TfOH. In the presence of an excess
of the acid (molar ratio 1 : 8–14) diprotonated N-acetylacetamidine 1 is formed as the major product,
which eventually transforms into protonated acetamidine 3 and acetic acid 4. At molar ratio of (1 : 1–2)
diprotonated 2,4-dimethyl-6-methylidene-3H-1,3,5-triazine 12, tautomer of the diprotonated trimethyl-s-
triazine 11, becomes the main product at an early stage of the reaction and diprotonated 1-(dimethyl-
1,3,5-triazin-2-yl)prop-1-en-2-ol 15 at a later stage. In the case of a large excess of acetonitrile (4–20 : 1)
trication 17 is formed as a result of the interaction between 11 and 12 along with some oligomers
[(CH3CN)3]n (n = 4–12).
Introduction
Results and discussion
Acetonitrile is a common solvent in organic chemistry and tri-
fluoromethanesulfonic acid (TfOH) is often used to acidify
acetonitrile solutions. However, although the chemical behaviour
of nitriles in acids has been studied for many years,1 data on the
interaction of the simplest nitrile, acetonitrile, with TfOH are
limited. It is generally assumed that these substances mix, in
ambient conditions, without any reaction,2 though on occasion
some unspecified chemical reactions were observed3 (cf. ref. 4
and 5). The authors of the paper6 had suggested that the inter-
action of CH3CN with TfOH resulted in formation of rather
stable acetonitrilium triflate, but later it was found that such
interaction resulted in formation of protonated trimethyl-s-tria-
zine.7,8 We have found that the interaction of CH3CN with TfOH
gives rise to a large number of other products. The purpose of
this paper is to determine the structures of these products and to
discuss the mechanisms of their formation.
We have studied the interaction of acetonitrile with trifluoro-
methanesulfonic acid (TfOH) at different ratios using multinuc-
lear NMR and ESI-MS. Structure elucidation and NMR signal
assignment for the products presented on Schemes 1, 4, and 6
were performed on the basis of 2D NMR correlation spectra
1H–1H (NOESY, ROESY), 1H–13C (HSQC, HMBC), and
1H–15N (HMBC).
All reaction mixtures in this study contain both neutral and pro-
tonated acetonitrile as indicated by the 15N chemical shifts 246
and 144 ppm,9 respectively. For the mixtures under investigation
the 15N chemical shift changes from 240 to 173 ppm when the
initial CH3CN : TfOH ratio varies from 4 : 1 to 1 : 14.
The products formed upon the interaction of CH3CN with
TfOH (molar ratio 1 : 14) are given in Scheme 1. At the early
stages of the reaction the main product is dication 1, cf. ref. 1
and 10. By NOESY data, this dication exists in the s-cis-form,
apparently due to formation of the intramolecular hydrogen
1
bond. The H NMR signal of the OH group of 1 is not observa-
ble obviously due to a rapid proton exchange with TfOH. Along
with 1, cation 2 is also observed (Fig. 1, see also Table 1 in
ESI†), cf. ref. 5 and 11. The 1H NMR spectrum of 2 is consider-
ably different from the spectrum of protonated acetamide [δ
2.62 ppm (CH3), 8.12 ppm (NH), 8.32 ppm (NH), cf. ref. 12]
and similar to that of cation 913 bearing
a
OTf
group (Scheme 2), the 19F chemical shift being equal to that of
cation 9.
In the course of the reaction the content of cations 1 and 2
decreases with time and the other species, 3, 4, and 5, appear
(Fig. 1). According to DOSY data (Fig. 2) hydrodynamic
volumes of structures 3, 4, and 5 are roughly equal, while that of
structure 1 is doubled. The chemical shifts of 3, 4, and 5 are
close to those of acetamidinium cation,14 protonated acetic acid
and acetyl cation,15 respectively. According to 2D NMR spectra
both 4 and 5 have one set of correlations corresponding to the
CH3–C– fragment and no nitrogen atoms. Cross-peaks corre-
sponding to chemical exchange between 4 and 5 are observed in
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Academician
Lavrent’ev Ave., 9, Novosibirsk 630090, Russian Federation. E-mail:
genaev@nioch.nsc.ru; Fax: +7-383-330-97-52
†Electronic supplementary information (ESI) available: Tables and
spectra. See DOI: 10.1039/c2ob06841a
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Scheme 1 NMR data for CH3CN, TfOH (molar ratio 1 : 14) and for the products of their interaction.‡
Scheme 2 Interaction of HCN with TfOH.‡
Fig. 1 Progress of the reaction of CH3CN with TfOH (mol. ratio
1 : 14) with time.
the NOESY spectrum. Such exchange is typical for the equili-
brium between protonated acetic acid and acetyl cation.16 So, we
have concluded that 3 is an acetamidinium cation, 4 is a proto-
nated acetic acid and 5 is an acetyl cation.
Almost from the very beginning of the reaction a signal for
Tf2O appears in the 19F NMR spectra. Its identification has been
made from the observation of a long-range spin–spin coupling
constant 19F–19F (J = 0.85 Hz in the 13C satellites) and was
confirmed by comparison with the 19F NMR spectrum of the
mixture Tf2O–TfOH. The content of Tf2O in solution passes
Fig. 2 2D DOSY spectrum of CH3CN–TfOH mixture (mol. ratio
1 : 14) after 192 h.
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methyl group protons of cation 5 as detected by heteronuclear
19F–1H NOE difference experiments. The longitudinal magneti-
1
zation is transferred from the irradiated 19F to H nuclei in D
where these nuclei are spatially close to each other and trans-
ferred further via chemical exchange to cation 5. An alternative
mechanism of magnetization transfer, due to formation of rather
strong associates of cation 5 with TfOH, is not justified, as diffu-
sion of 5 according to DOSY spectra is not slowed down in
comparison with the other species. Hydration of cation 5 gives
cation 4. TfOH apparently plays the role of water source, being
partially transformed into Tf2O. The latter partially transforms
into CF3OTf. This process is known.18
Species 7 and 8 are obviously formed as a result of the inter-
action of protonated acetic acid (cation 4) with Tf2O. This con-
clusion is confirmed by the following experiments. Thus, cation
4 formed from acetic acid in TfOH (AcOH–TfOH, molar ratio
1 : 7) is stable for 7 h at 110 °C, however, upon heating the
mixture AcOH–TfOH–Tf2O 1 : 7 : 0.5 (7 h at 110 °C) a reaction
proceeds that results in formation of the mixture of 7 and 8
(molar ratio 5 : 1). Neutral precursors of cations 7 and 8, triacetic
acid lactone and dehydroacetic acid, are well known and can
transform into each other in acidic media.19 The neutral precur-
sor of cation 6, 2,4-dimethyl-1,6-dihydro-6-pyrimidone, is also
known and can arise from acetamidine and various derivatives of
acetic acid.20
The same products at similar ratios are formed in the systems
CH3CN–TfOH 1 : 10 and 1 : 8. However, when the ratio of
CH3CN to TfOH increases to 1 : 2 the composition of the pro-
ducts changes sharply (Scheme 4, Fig. 3, Table 2 in ESI†).
According to literature data,7 the main product observed upon
neutralization of such mixture is trimethyl-s-triazine. The authors
of that paper suggested that in the 1 : 2 mixture of CH3CN–
TfOH trimethyl-s-triazine is present in triprotonated form.
According to our data, diprotonated form 11 can be identified in
the solution (as reasoned below). 11 is in slow exchange, on the
NMR time scale, with its tautomer 12, the latter being more
stable. The equilibrium mixture of 11 and 12 remains unchanged
for a long time. The other products, 13, 14, and 15, have also
been detected by NMR spectroscopy. Relative sizes of cations 1,
3, 11, 12, and 13 are in accordance with the DOSY data (Fig. 4):
the ratio of their hydrodynamic volumes estimated from the
respective diffusion coefficients is 2 : 1 : 3 : 3 : 3.
In the ESI-MS spectra (at m/z > 100, see ESI†) we observed
signals corresponding to products 11 (12), 15, and additionally a
weaker signal with m/z 208.107 corresponding to the molecular
formula (CH3CN)3(CH2CO)2+H+. Concentration of the respect-
ive cation in solution is too low to allow its unambiguous
identification by NMR. By analogy to 15, we suggest structure
16 for this cation (Scheme 5).
The suggested mechanism of the reactions proceeding in the
system CH3CN–TfOH 1 : 2 is given in Scheme 5. The first step
consists of the interaction of dication 1, formed according to
Scheme 3, with acetonitrile resulting in formation of intermedi-
ate E. The majority of this intermediate undergoes cyclization
into dication 13, which transforms into dications 12 and 11 as a
result of dehydration. The rest of E undergoes scission into
cations 5 and 14, the former transforming almost quantitatively
into its hydrated form, cation 4. When acetonitrile is exhausted,
formation of intermediate E becomes impossible and dication 1
Scheme 3 Mechanism of the reaction between CH3CN and TfOH
(molar ratio 1 : 14).
through a maximum and then decreases. When the reaction
mixture is kept for a long time an additional species appears, its
structure is tentatively assigned as 6 on the basis of its 2D NMR
data. Further ageing and heating leads to the appearance of two
more species, identified as 7 and 8.
The suggested mechanism is given in Scheme 3. At first
cation 2 is formed (cf. ref. 1). The interaction of this cation with
acetonitrile gives intermediate B, subsequently transforming into
dication 1, with the source of the OH group being TfOH itself
while undergoing dehydration to Tf2O. Thus, acetonitrile for-
mally plays the role of a dehydrating reagent, cf. ref. 17. An ana-
logous process was observed earlier upon the interaction of
HCN with TfOH13 without indication of Tf2O formation, but
now we have found that Tf2O is formed according to Scheme 2.
The transformation of 9 into protonated formamide 10 is very
slow (t1/2 is approx. 30 days at ambient temperature). This is
quite natural as it has to proceed through unstable intermediate
A, bearing an extra positive charge. Analogous conversion of 2
into the protonated acetamide is also very slow, much slower
than its conversion into 1, and we have not managed to detect
this reaction. On the contrary, conversion of intermediate B into
1 is more favourable as it can proceed via intramolecular tauto-
merization into C, bearing no extra charge. Addition of TfOH to
dication 1 followed by β-scission of the N–C bond results in for-
mation of amidinium cation 3 and intermediate D. The latter
transforms into acetyl cation 5. The conclusion of intermediate
D formation was supported by the NOE from 19F nuclei to the
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Scheme 4 Chemical shifts of CH3CN (in a molar ratio of 1 : 2 with TfOH) and of the products of their interaction.‡ The respective signals of 11 are
averaged over the triazine ring due to proton exchange.
Fig. 4 2D DOSY spectrum of CH3CN–TfOH mixture (mol. ratio 1 : 2)
after 316 h.
TfOH protons, in contrast to those of deuterated analogs of 4, 5
Fig. 3 Progress of the reaction of CH3CN with TfOH (mol. ratio 1 : 2)
(cf. ref. 21), 11, 12, 13, and 15. Equilibrium distribution of deu-
with time.
terium is observed at all the positions of cations 11, 12, 13, and
15. For cations 11 and 12 it follows from their tautomerization.
continues to react according to the other mechanism, which is
analogous to Scheme 3, resulting in cation 3 and intermediate D.
The latter adds to exo-double bond of dication 12 producing
dication 15. This dication can add one more fragment of D
giving 16.
Scheme 5 is consistent with the data on deuterium distribution
in products formed in the system CD3CN–TfOH. CD3 groups of
deuterated 1, 3 and 14 are not involved in isotope exchange with
Obviously an analogous process is also possible for dication 15.
All the positions of the deuterated dication 13 are involved in
isotope exchange with TfOH, probably via dication 12.
When the ratio CH3CN to TfOH is 1 : 1 only 11 and 12 are
formed in the beginning of the reaction (see ESI†). In 24 h the
reaction mixture becomes viscous, so that high-resolution NMR
spectra are unavailable. Upon increase of the acetonitrile content
in the mixture with TfOH to 4 : 1 ratio the interaction of these
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Scheme 5 Mechanism of the reaction between CH3CN and TfOH (molar ratio 1 : 2).
Scheme 6 Chemical shifts of CH3CN, TfOH (molar ratio 4 : 1) and of the products of their interaction.‡
components results in formation of cations 11, 12, and 17
(Scheme 6, Fig. 5, Table 3 in the ESI†). The same products are
formed upon increase of the ratio to 20 : 1, see ESI.† Trication
17 obviously results from an electrophilic attack at the exo-
methylene bond of dication 12 by its aromatic tautomeric form
11 (Scheme 7). It is quite natural to suggest that the triazinium
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acetonitrile. The concentration of all the cations larger than 18 is
too low to be detected by NMR. In principle, linear as well as
branched aggregates with n > 3 are probable.
When the ratio CH3CN–TfOH is increased to 200 : 1, the pro-
tonated acetamide is formed as a sole product (cf. ref. 8)
obviously as a result of hydrolysis of acetonitrile due to the pres-
ence of water traces.
On degree of trimethyl-s-triazine protonation
Although the proton signals of the NH groups in 11 are not
observed, consideration of its NMR spectra, in a combination
with the literary data, allows estimation of the degree of tri-
methyl-s-triazine protonation in media with various amounts of
TfOH. 15N chemical shifts strongly depend on the extent of pro-
tonation. A signal of the protonated nitrogen atom is at higher
field than that of the neutral one, e.g. the 15N chemical shift for
pyridine is δ 306 ppm, while that for the pyridinium cation is δ
210 ppm. The 15N chemical shift for neutral trimethyl-s-triazine
is δ 267.2 ppm.22 On the basis of these data it is reasonable to
suggest that one of the nitrogen atoms of the aromatic ring of tri-
cation 17 is protonated (δ 195.5 ppm), the two others are neutral
Fig. 5 Progress of the reaction of CH3CN with TfOH (mol. ratio 4 : 1)
with time.
(δ 269.3 ppm). The average value due to proton exchange of 15
N
chemical shift for monoprotonated trimethyl-s-triazine is
expected to be δ 245 ppm, while that for the diprotonated
species is δ 220 ppm. So, in the 1 : 2 and 1 : 1 mixtures of
CH3CN–TfOH the diprotonated form of trimethyl-s-triazine is
predominantly formed (the 15N chemical shifts are 221.5 and
222.6 ppm, respectively), and in the 4 : 1 mixture of CH3CN–
TfOH a substantial part of trimethyl-s-triazine is monoprotonated
(the 15N chemical shift is 233.5 ppm).
Conclusions
Interaction of the two simple compounds, CH3CN and TfOH,
results in formation of a great variety of complicated structures,
the composition of the reaction products being a function of the
ratio of CH3CN to TfOH.
Scheme 7 Mechanism of the reaction between CH3CN and TfOH
(molar ratio 4 : 1).
fragment of 17 can produce tautomeric forms having exo-
methylene bonds and these forms can add more fragments of 11
by analogy to Scheme 7. As a result, the extent of oligomeriza-
tion can increase. Indeed, the set of NMR signals corresponding
to cation 18 is observed.
Experimental
General methods and materials
Upon keeping the reaction mixture for a longer time, a growth
of broadened signals with chemical shifts very close to that of
18 is observed. This may indicate the formation of further
similar structures with even higher oligomerization degree.
In the ESI-MS spectra (see ESI†) signals corresponding to
products 11, 12 and 17 are observed. There are also strong
signals corresponding to 18, [(CH3CN)3]3. In addition, signals
corresponding to other products with even higher degree of oli-
gomerization are observed: [(CH3CN)3]n (n = 4, 5). There are
also weak signals corresponding to structures with n = 6, 7, and
(under conditions optimized for observation of large ions)
signals of even larger oligomers (n is up to 12). Signals corre-
sponding to oligomers with n > 12 do not appear in the ESI-MS
spectra at all, and the amount of oligomers with n > 4 is rather
low, indicating that there is no significant polymerization of
NMR and ESI-MS spectra were obtained at the Chemical
Service Centre of the Siberian Branch of the Russian Academy
2
of Sciences. Multinuclear NMR spectra (1H, H, 13C, 15N, 19F)
1
were obtained on Bruker AV-600 (resonance frequency for H
600.3 MHz), AV-400 (400.13 MHz), and AV-300 (300.13 MHz)
spectrometers. Chemical shifts for 1H, 2H and 13C measured
1
relative to the internal standard CD2Cl2 (δ 5.33 ppm for H and
2H, 53.6 ppm for 13C), for 15N relative to external NH3, for 19F
relative to external CFCl3. For structure elucidation and NMR
signal assignment 2D correlation spectra 1H–1H (NOESY,
ROESY with mixing times τm = 0.4–0.7 s), 1H–13C (HSQC,
1
1
HMBC), H–15N (HMBC), and heteronuclear (19F → H) NOE
difference spectra were used. For separation of NMR subspectra
corresponding to different cationic species and for measurement
of diffusion coefficients diffusion oriented spectroscopy (DOSY)
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with longitudinal eddy current delay (LED)23 on 1H and 19F
nuclei was used.
Notes and references
‡1H chemical shifts are given in italics, 13C in ordinary font, 15N in
bold face, the chemical shifts of acetonitrile are in fact those of its
mixture with the protonated form, the signals are averaged through rapid
exchange of both forms on the NMR time scale.
The ESI mass spectra were recorded for m/z 100–3000. For
the mixture CH3CN–TfOH 4 : 1 additionally ESI mass spectra
from m/z 200 to 10 000 were recorded. A hybrid QTOF (quadru-
pole-time-of-flight) mass spectrometer with an electrospray inter-
face (micrOTOF-Q, Bruker Daltonics) was utilized. Nitrogen
1 I. D. Gridnev and N. A. Gridneva, Russ. Chem. Rev., 1995, 64, 1021–
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was used as a drying gas at 220 °C and at flow of 4 L min−1
.
2 P. Savu, in Kirk-Othmer Encyclopedia of Chemical Technology, John
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Nebulizer pressure was set to 1.0 bar. The capillary voltage was
set at −4.0 kV in the positive scan mode. Sample solutions were
infused into the ESI source by LC Agilent 1200 at FIA mode
(Flow Injection Analysis, 2–5 μL at a flow rate of CH3CN
0.1 mL min−1).
4 J. Umezawa, O. Takahashi, K. Furuhashi and H. Nohira, Tetrahedron:
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CH3CN Panreac (HPLC-gradient grade, 99.9%), CD2Cl2 was
dried over molecular sieves 4 Å, TfOH reagent grade, 98% was
purchased from Aldrich and used without further purification.
All the mixtures under study contained 20% (v/v) of CD2Cl2
(for field-frequency lock). Special experiments showed that the
usage of freshly distilled TfOH and adding CD2Cl2 to the reac-
tion mixtures did not influence the products composition.
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TfOH (50–670 mg, 0.33–4.5 mmol), CD2Cl2 (0.1 mL), and
CH3CN (13–270 mg, 0.32–6.6 mmol) put into NMR tube in
such portions to obtain the ratio CH3CN–TfOH needed (from
1 : 14 to 20 : 1). The tube closed and the three-layered mixture
shook under cooling with cold water just before the NMR
spectra registration. For ESI-MS determination a drop (approx.
5 mg) of the reaction mixture was solved in 1 mL of acetonitrile.
The reaction mixture CH3CN–TfOH 200 : 1 was boiled under
argon for 5 h (cf. ref. 8).
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We thank the Russian Academy of Sciences (Project No. 5.1.4)
for financial support. Helpful remarks by Dr S. Nagy are greatly
appreciated.
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This journal is © The Royal Society of Chemistry 2012