Spectroscopic study of reaction of propargyl bromide with pyridine

Article abstract of DOI:10.1023/A:1019572605480

Pyridine was reacted with propargyl bromide to obtain the dibromide [C ₅H₅NCH=CHCH=CHCH=CHC₅H₅N] ²⁺ 2Br^-. The effect of the vinyl group on its UV spectrum is qualitatively interpreted using the results of nonempirical quantum-chemical calculations (basis HF/6-31G*).

Full text of DOI:10.1023/A:1019572605480

Russian Journal of General Chemistry, Vol. 72, No. 5, 2002, pp. 778 784. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 5, 2002,
pp. 832 838.
Original Russian Text Copyright
2002 by Kanitskaya, Elokhina, Fedorov, Shulunova, Nakhmanovich, Turchaninov, Lopyrev.
Spectroscopic Study of Reaction
of Propargyl Bromide with Pyridine
L. V. Kanitskaya, V. N. Elokhina, S. V. Fedorov, A. M. Shulunova,
A. S. Nakhmanovich, V. K. Turchaninov, and V. A. Lopyrev
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, Russia
Received July 11, 2000
Abstract Pyridine was reacted with propargyl bromide to obtain the dibromide [C H NCH=CHCH=
5
5
2
+
CHCH=CHC H N] 2Br . The effect of the vinyl group on its UV spectrum is qualitatively interpreted using
5
5
*
the results of nonempirical quantum-chemical calculations (basis HF/6-31G ).
It is known [1] that N,N-dimethylhydrazones react
the most characteristic feature of the IR spectrum of
+
with methyl iodide in anhydrous ethanol to give
[C H NCH CH ] Br (I) is the high-frequency shift
5 5 2 3
+
N,N,N-trimethylhydrazonium iodides Me N N=CRR
of the strong stretching vibration bands of the pyri-
3
1
I (R = R = Ph, PhCH , i-Bu; R = Me, R = Ph; R =
dinium ring from 1430, 1530, and 1600 cm (pyri-
2
1
Me, R = PhCH2).
dine) to 1480 and 1620 cm . The (C H) band of the
CH N group of this cation is located in a region
We earlier found [2] that reactions of N,N-dimethyl-
hydrazones with propargyl bromide and 1,3-dibromo-
propyne (in diethyl ether, methanol, or acetonitrile at
2
characteristic of salts and has a maximum at
1
3
010 cm . The bending vibration bands of the five
adjacent hydrogen atoms of the heteroring, located in
2
0 C) involve alkylation by the tertiary nitrogen atom
1
+
the spectrum of pyridine at 700 and 740 cm , are
and yields bromides RCH=NN (Me) (CH C CR ) Br
2
2
shifted in the spectrum of the cation to 680 and 780
(R = Me, Ph, 2-thienyl; R = N, Br).
1
cm . Along with these bands, the high-frequency
With 4(3)-formylpyridine hydrazones, however,
region of the IR spectrum of the reaction product
the reaction pathways changes considerably. This is
primarily evidenced by the appearance of an intense
color and the lack of (C C) and ( C H) bands in
the IR spectrum of one of the isolated reaction pro-
ducts. These data led us to propose that the hetero-
cyclic fragment competes with amine in the reactions
in question, and at their initial stages propargyl
bromide adds to the pyridine nitrogen atom to give
a cation.
1
1
shows (C C) (2110 cm ) and ( C H) (3120 cm )
bands. Thus, the IR spectrum provides unambiguous
evidence showing that the reaction product formed
+
with excess is the salt [C H NCH C CH] Br (II).
5
5
2
Since the unsaturated fragments of the cation
+
[
C H NCH C CH] are intervened by a methylene
5
5
2
unit, we might expect that the UV spectrum of a
solution of salt II would be similar to the spectra of
solutions of salts of N-alkylpyridinium cations. The
UV spectrum of the N-ethylpyridinium cation, we
obtained in the present work, is identical to those
reported in the literature [3 5] and contains a fine-
+
Me NN=CH
2
N CH C CH
2
1
structured band at 35000 41000 cm . The salt of
this cation is readily soluble in aprotic solvents, and
its absorption maximum, for instance, in acetonitrile
To verify this hypothesis, we studied reaction of
pyridine with propargyl bromide. It was found that
the reaction time and the structure of the reaction
products are strongly dependent on the reactant ratio
and the medium.
1
is at 38600 cm . The vibronic splitting here is
00 cm 1 (Fig. 1). We found that the spectra of the
6
two cations in polar media are not only similar, but
also fully identical to each other, both in shape and in
position (Figs. 1a and 1b). The spectrum in the low-
The IR spectrum of the reaction product formed
with excess BrCH C CH in the reaction mixture
polarity CHCl contains, along with the band of the
2
3
slightly differs from the spectrum of N-ethylpyridi-
ion pair, a Br
cation charge-transfer band (Fig. 1c).
1
nium cation in the 1700 400 cm range. Note that
Such band is also characteristic of pyridinium salts [6].
1
070-3632/02/7205-0778$27.00 2002 MAIK Nauka/Interperiodica

SPECTROSCOPIC STUDY OF REACTION OF PROPARGYL BROMIDE WITH PYRIDINE
779
The reaction of propargyl bromide with excess
pyridine gave a complex intractable mixture of
(a)
products. Therefore, its composition was studied by
spectral methods.
D
The IR spectrum of the product formed with excess
pyridine or with equimolar reactant ratio characteris-
tically lacks C C and C H absorption bands but
shows enhanced absorption in the region of stretching
vibrations of the pyridinium C=C and C=N bonds
1
(1630 cm ).
(b)
By measuring the IR spectra at certain intervals
under conditions when the reaction is sufficiently
slow, i.e. at equimolar reactant ratio in acetonitrile,
we could trace the overall reaction dynamics. It
proved that the initial stage involves formation of a
new cation, a precursor of the final reaction product.
One might suggest that it is an N-allylpyridinium
cation incorporated as a structural unit in the final re-
action product. This suggestion follows from a com-
parison of the IR spectrum of the final reaction
product with the spectrum of N-allylpyridinium
bromide obtained from pyridine and allyl bromide
(c)
(
Figs. 2a and 2b). However, this hypothesis conflicts
44
40
36
32
28
0 , cm
3
1
with the intense color of the final product, in view of
the fact that the N-allylpyridinium cation has no
visible absorption bands. Therefore, in further studies
we used UV spectroscopy.
1
+
Fig. 1. UV spectra of (a) [C H NCH CH ] in MeCN,
(b) [C H NCH C CH] in MeCN, and (c) ion pair
5
5
2
3
+
5
5
2
+
[
C H NCH C CH] Br in CHCl . (Dashed lines) Bands
5 5 2 3
According to UV data, if the propargyl bromide:
pyridine ratio is slightly lower than 2:1, one more salt
is accumulated in the reaction mixture. The absorp-
tiom maximum of the cation of this salt is strongly
recorded for highly concentrated solutions.
1
(
23400 cm ), thus imparting color to the sample, and
1
is registered even in such a polar solvent as aceto-
nitrile. This fact points to an unusually high resistance
of the ion pair to dissociation into free ions.
shifted (by 8500 cm ) to low frequencies compared
with those of N-alkylpyridinium cations (Fig. 3a). The
absorption band of the new species, unlike that of N-
alkylpyridinium cations, is modulated by the vibration
1
At equimolar reactant ratio, a compound is formed,
whose UV spectrum exhibits a well-defined third band
with fine structure. This band is similar in shape to
frequency 1000 1200, rather than 600 cm , and has
the strongest 0 0 component. This vibration range
is characteristic of an exocyclic double bond
weakened in the electronically excited state. In its
that of the pyridinium cation, but its
is
max
1
27500 cm 1 (Fig. 3b). The charge-transfer band over-
laps with a weak band of different nature, presumably
belonging to some structurally more complex cation.
Evidence for this assignment comes from the fact that
the latter band is present in the spectrum of aqueous
silutions (Fig. 3c) of the reaction product obtained
with excess pyridine. The spectra all contain an N-
alkylpyridinium absorption band. Consequently, this
fragment enters into the composition of the reaction
products. Inspection of data in Table 1 shows that the
effect of the pyridine nitrogen atom on the absorption
position ( max 30000 cm ) and fine structure the
new band is close to the absorption bands of hetero-
cyclic cations like A having two double bonds at the
pyridinium nitrogen atom (Table 1).
+
N
A
Importantly, the counterion couterion charge-trans-
fer band here is shifted as strongly as the intrinsic
absorption band of the cation. The charge-transfer by only 700 cm than the increment of a double bond
band is already in the visible spectral range elongating the conjugation chain in homoannular
maximum of the cation is roughly additive (on
1
average,
is 3800 cm ). This value is smaller
max
1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 5 2002

780
KANITSKAYA et al.
(a)
(b)
30
25
20
15
10
0 2, cm 1
1
+
Fig. 2. IR spectra of (a) [C H NCH CH=CH ] Br and (b) the final reaction product of pyridine with BrCH C CH.
5
5
2
2
2
1
The 13C NMR spectrum of the reaction products of
pyridine with propargyl bromide is given in Fig. 4.
As seen, we deal with a complex mixture of struc-
turally similar compounds, and, therefore, traditional
C, not to mention H NMR spectroscopies are fully
unsuitable here.
dienes (4500 cm ) [10]. As known, increasing
number of double bonds diminish the red shift of the
absorption band of a polyene. For instance, in the
H(CH=CH) H series, in going from n = 3 to n = 4,
is 4400 cm and gets smaller for further pairs:
000 (n = 4 5), 2400 (n = 5 6) and 1900 (n =
n
13
1
1
max
3
6
7) cm 1 [10].
This mixture of products was examined in terms
of quantitative approaches of NMR spectroscopy of
multicomponent organic mixtures, as well as using
multipulse experiments to obtain the subspectra of
primary and tertiary and secondary and quaternary
carbon atoms [11].
The smallest shift of the UV spectrum with respect
to that of the N-ethylpyridinium cation [8400 cm 1
(
Table 1)] is characteristic of a cation in which the
pyridine nitrogen atom is attached to a polyene chain
containing no more than three C=C bonds. The same
analysis is readily performed for the other cations
observed in the UV spectrum.
The lack in the 13C NMR spectrum (Fig. 4,
Table 2) of C C carbon signals suggests complete
conversion of propargyl bromide. Calculations
Thus, the initial stage of cation formation with
excess pyridine can be represented by Scheme 1.
showed that the carbon atoms giving signals at
118 150 ppm comprise 89% of the total number of
C
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 5 2002

SPECTROSCOPIC STUDY OF REACTION OF PROPARGYL BROMIDE WITH PYRIDINE
781
Scheme 1.
C H N
+
II
5
5
+
Br + C H NH
+
+
5
5
Br
N
N
N
CH=C=C
CH=C=CH
CH=C CH2
CH C CH
2
H
II
III
N
+
+
Br
C H NH
5
5
+
Br
N
IV
CH=C=CH
CH=CH CH2
N
+
+
Br
Br
N
VI
CH=C CH=CH CH=CH
+
N
+
C H NH
+
5 5
Br
Br
N
CH=CH CH=CH CH=CH
V
N
+
Br
VII
(a)
D
(b)
(c)
4
8 44 40 36 32 28 24 20 16
0 , cm 1
3
1
2
00 180 160 140 120 100 80 60 40 20
ppm
,
C
Fig. 3. UV spectra of the reaction products of pyridine
with BrCH C CH at various molar ratios. Molar ratio:
2
(
a) 1:1.5 1.8 (EtOH), (b) 1:1 (EtOH), and (c) excess
1
3
pyridine (H O). (Dashed lines) Bands recorded for
Fig. 4. C NMR spectrum of the reaction product of
2
highly concentrated solutions.
pyridine with propargyl bromide (with excess pyridine).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 5 2002

782
KANITSKAYA et al.
Table 1. Long-wave absorption bands of certain hetero-
cyclic cations containing a pyridine fragment
sions concerning the chemical structure of the reaction
products of pyridine with propargyl bromide. The
major product is compound VII represented by
several rotamers, which explains the observation in
the spectrum of a great number of signals with close
Cation
N Et
(H O)
2
max
max
3
8500
0
values. The signals of equal intensity at 199.9
C
+
(=C=) and 61.3 (CH N) ppm are likely to belong to
2
compound VI. According to calculations, the fractions
of compounds VII and V are 86 and 10 wt %, res-
pectively, and 4 wt% fall at unidentified compounds.
The theoretical elemental compositions of compounds
VI and VII are consistent with experiment (Table 3).
35100a
35000a
3400a
+
N
CH3
Thus, the proposed structure of cation VII is con-
sistent with all the experimental evidence. The shift
of its UV absorption maximum with respect to those
characteristic of N-alkyl cations is readily interpreted
+
0a
N
H
*
with the aid of HF/6-31G nonempirical quantum-
chemical calculations. Analysis of the structure of
+
-
MOs (HOMO and LUMO) of the model N-vinyl-
N
3
0900b
4100b
8400c
pyridinium cation points to strong mixing of the vinyl
and pyridine fragment orbitals (Scheme 2). This result
explains the low-frequency shift of the long-wave
electronic transition of the vinylpyridinium cation
(Figs. 1, 3).
C H N:BrCH C CH
30100
5
5
2
(
1: 2)
C H N:BrCH C CH
30100
27500
2
8400c
11000c
15500c
Scheme 2.
5
5
2
(
1: 1)
3000
+
a
b
c
N
Data of [7]. Data of [8, 9]. Relative to the N-ethylpyridi-
nium cation.
carbon atoms. The signals at
56 68 ppm, assig-
C
nable to >CHN and CH N carbons [13], comprise 5%.
2
The spectrum contains a signal at
assignable to the allenic =C= carbon [13].
199.9 ppm,
C
HOMO
4.19 eV
LUMO
3.28 eV
1
In view of the data in Table 2, the assigned
quaternary, tertiary, secondary, and primary carbon
signals, as well as the results of qualitative and
quantitative analysis of the C NMR specrum of the
sample (Table 3), we came to the following conclu-
EXPERIMENTAL
1
3
The 13C NMR spectrum of the sample with proton
decoupling and the subspectra of primary and tertiary
Table 2. Chemical shifts in the 13C NMR spectra of pyridine and pyridinium cations
C, ppm
Compound
C2,6
C3,5
C4
C
C
C
a
C H N
150.2
142.4
144.4
144.7
123.9
129.0
127.9
128.1
135.9
148.3
145.3
146.4
5
5
+
a
[
[
[
C H N] H
C H NCH CH ] Br
C H NCH C CH] Br
5 5
+
55.9
49.6
16.2
81.3
5
5
2
3
+
75.2
5
5
2
a
Data of [12].
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SPECTROSCOPIC STUDY OF REACTION OF PROPARGYL BROMIDE WITH PYRIDINE
783
Table 3. Relative contents of carbon atoms in the reaction
c. A solution of 2.38 g of propargyl bromide in
product of excess pyridine with propargyl bromide (in
5 ml of absolute acetonitirile was added dropwise
with stirring to 3.16 g of pyridine. The mixture
warmed up to 50 55 C and got brown. It was stirred
for 2 h at 20 C, and the solvent was removed in a
vacuum to leave a glassy material (mp 149 151 C)
which was a mixture of dimers VI and VII of N-(2-
propyn-1-yl)pyridinum bromide (II). Yield 3.72 g
a
fractions q ) and its elemental composition (%) (parenthe-
x
b
sized are calculated values)
qx
C, ppm
0
0
0
0
0
0
0
.006 199.9 (CH=C=CH)
4
.120 146.0 148.0 (C in pyridinium cations)
(94% per propargyl bromide), soluble in water and
2
3
,6
,5
.211 142.0 146.0 (C
.221 127.0 129.0 (C
in pyridinium cations)
in pyridinium cations)
ethanol. Found, %: C 48.72; H 4.13; Br 40.02; N 7.40.
C H Br N . Calculated, %: C 48.48; H 4.04; Br
1
6
16
2
2
.306 132.0 139.0, 124.0 125.5 [CH=CH]
4
0.40; N 7.07.
.006 61.3 (CH N)
2
.130 118.7, 1.7 68.0 (unidentified compounds)
REFERENCES
a
13
Relative to the total integral intensity in the C NMR spec-
b
1
. Smith, P.A.S. and Most, E.E., J. Org. Chem., 1957,
vol. 22, no. 4, p. 358.
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(
40.40); N 7.40 (7.07).
and secondary and quaternary carbon atoms, obtained
by the spin-echo technique with multiplet dephasing,
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[
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0 ) in DMSO-d . The relaxant was chromium tris-
3
. Brown, H.C. and Cahr, A., J. Am. Chem. Soc., 1955,
9
6
vol. 77, no. 7, p. 1715.
acetylacetonate (c 0.02 M). Structural assignment
from the 13C NMR spectra was based on calculations,
4. Kosower, E.M. and Skorcz, J.A., J. Am. Chem. Soc.,
2
,6
3,4
assuming the sum of the fractions of the C , C ,
1960, vol. 82, no. 9, p. 2195.
4
and C atoms of the pyridinium cations equal to five
5
6
. Kosower, E.M., J. Am. Chem. Soc., 1958, vol. 80,
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carbon atoms. Calculation of the fraction of each
1
3
compound in the mixture (wt%) from the C NMR
spectrum was performed with the elemental composi-
tions obtained by chemical methods. The fractions of
compounds VI and VII were determined with a
relative error of no higher than 7%. Nonempirical
calculations of the geometry of cation VI in the
. Burger, K., Solvation, Ionic and Complex Formation
Reactions in Non-Aqueous Solvents, Amsterdam:
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ionnye reaktsii i kompleksoobrazovanie v nevodnykh
sredakh, Moscow: Mir, 1984, p. 256.
*
HF/6-31G basis and of its UV spectra were perfomed
7
. Armagero, L.F., J. Chem. Soc., 1964, no. 11, p. 4226.
using GAUSSIAN 94 [14].
Rection of pyridine with propargyl bromide.
a. Propargyl bromide, 2.38 g, was slowly added with
stirring to a solution of 1.58 g of pyridine in 5 ml of
absolute acetonitrile. Heat release was observed, and
the reaction mixture got dark brown. The solvent was
removed in a vacuum to leave a tarry noncrystallizing
material, yield 3.96 g (100%).
8. Holland, D.O. and Nayler, J.H.C., J. Chem. Soc.,
1955, no. 6, p. 1657.
9
. Boekelheide, V. and Gall, W.G., J. Am. Chem. Soc.,
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1
10. Gillam and Stern’s Introduction to Electronic Absorp-
tion Spectroscopy in Organic Chemistry, Stern, E.S.
and Timmons, C.J., Eds., London: Arnold, 1970,
b. Propargyl bromide, 4.76 g, was slowly added
with stirring to a solution of 1.58 g of pyridine in 5
ml of absolute acetonitrile, and the resulting mixture
was allowed to stir at 20 C for 4 h. The precipitate
that formed was filtered off, washed on the filter with
cold acetonitrile, and dried in a vacuum to obtain
N-(2-propyn-1-yl)pyridinium bromide as a single
product, yield 3.15 g (80% per pyridine), mp 122
3
rd ed. Translated under the title Elektronnaya absor-
btsionnaya spektroskopiya v organicheskoi khimii,
Moscow: Mir, 1974, p. 296.
1
1. Polonov, V.M., Kalabin, G.A., Kushnarev, D.F., and
Latyshev, V.P., Khim. Tverd. Topl. (Moscow), 1984,
no. 4, p. 9.
1
24 C. Found, %: C 48.40; H 3.96; Br 40.38; N 6.90.
12. Levy, G.C. and Nelson, G.L., Carbon-13 Nuclear
Magnetic Resonance for Organic Chemists, New
York: Wiley, 1972.
C H BrN. Calculated, %: C 48.48; H 4.04; Br 40.40;
N 7.07.
8
8
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 5 2002

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