Comparison of the electronic and steric structures of 1-vinyl-and 1-(prop-1-en-1-yl)pyrroles according to the 1H and 13C NMR data

Article abstract of DOI:10.1134/S1070428007030116

According to the ¹H and ¹³C NMR data, 1-isopropenylpyrroles are characterized by larger dihedral angles between the heteroring and exocyclic double bond planes, as compared to isostructural 1-vinylpyrroles, due to steric effect of th

Full text of DOI:10.1134/S1070428007030116

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 3, pp. 397–405. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © A.V. Afonin, I.A. Ushakov, D.E. Simonenko, O.A. Tarasova, M.A. Maksimova, B.A. Trofimov, 2007, published in Zhurnal
Organicheskoi Khimii, 2007, Vol. 43, No. 3, pp. 398–405.
Comparison of the Electronic and Steric Structures
of 1-Vinyl- and 1-(Prop-1-en-1-yl)pyrroles
according to the 1H and 13C NMR Data
A. V. Afonin, I. A. Ushakov, D. E. Simonenko, O. A. Tarasova,
M. A. Maksimova, and B. A. Trofimov
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: olga@irioch.irk.ru
Received February 16, 2006
1
Abstract—According to the H and 13C NMR data, 1-isopropenylpyrroles are characterized by larger dihedral
angles between the heteroring and exocyclic double bond planes, as compared to isostructural 1-vinylpyrroles,
due to steric effect of the α-methyl group in the propenyl fragment. As a result, p–π conjugation with the
propenyl group is weaker than with the vinyl group. The propenyl group in 1-isopropenylpyrrole having no
other substituents in the heteroring is forced out from the heteroring plane, while the 1-vinylpyrrole molecule is
planar. If substituents are present in positions 2 and 5 of the pyrrole ring, the propenyl group on the nitrogen
atom becomes orthogonal with respect to the pyrrole ring plane, so that no p–π conjugation is possible. The
steric structures of (E)-1-(prop-1-en-1-yl)pyrrole and (Z)-1-(prop-1-en-1-yl)pyrrole are different: the propenyl
group in the former is turned relative to the heteroring plane, while the latter molecule is planar.
DOI: 10.1134/S1070428007030116
The steric and electronic structures of 1-vinylpyr-
para position of the 2-aryl group is transmitted to the
vinyl group through the pyrrole ring [7–9].
1
roles were studied in detail by H and 13C NMR spec-
troscopy [1–6]. It was shown that π systems of the
vinyl group and pyrrole ring efficiently interact with
each other via p–π conjugation mechanism, which
leads to excess shielding of the β-carbon atom of the
vinyl group. The vinyl group and pyrrole ring in
1-vinylpyrrole lie in one plane, so that the maximal
degree of p–π conjugation is attained. The presence of
alkyl groups in the 2-position of the pyrrole ring forces
the vinyl group to go out from the pyrrole ring plane;
as a result, p–π conjugation with the vinyl group
weakens, and the degree of shielding of its β-carbon
atom decreases. The larger the alkyl group in position
2 of pyrrole ring, the greater the deviation of the vinyl
group from the pyrrole ring plane and the weaker the
p–π conjugation. If substituents are present in both
2- and 5-positions of the pyrrole ring, deviation of the
vinyl group from the pyrrole ring plane becomes
especially strong, and p–π conjugation with the vinyl
group is disrupted almost completely [1–6]. The aro-
matic ring in 2-aryl-1-vinylpyrroles is involved in con-
jugation, and electronic effect of a substituent in the
Distortion of coplanarity between the vinyl group
and pyrrole ring is reflected in a number of H and
1
13C NMR parameters, including deshielding of the
β-carbon atom [1–6]. In addition, the geminal coupling
constants between the HA and HB vinyl protons in-
crease [3, 5], while the long-range coupling constants
between the vinyl group protons and protons in the
pyrrole ring [3, 5] and the direct 13C–1H coupling
constants of the Cβ carbon atom change in opposite
directions [9, 10].
We recently synthesized a new series of 1-(prop-1-
en-1-yl)pyrroles [11]. Even at first glance, parameters
1
of the H and 13C NMR spectra of these compounds
revealed a strong dependence of the efficiency of p–π
conjugation with the olefinic fragment on the presence
of substituents in the pyrrole ring. With a view to gain
a deeper insight into the electronic and steric structures
of 1-(prop-1-en-1-yl)pyrroles we performed a detailed
analysis of the 1H and 13C NMR parameters of a wider
series (than that described in [11]) of substrates (com-
pounds I–XV) and compared the obtained data with
397

AFONIN et al.
398
R2
R2
4
3
2
5
R1
R3
HB
R4
R4
R1
R3
HB
N
α
N
N
N
N
Me
Me
Me
HX
β
HA
R4
Me
HA
XVI–XXII
I–VII
VIII–XI
XII, XIV
XIII, XV
I, XVI, R1 = R2 = R3 = H; II, XVII, R1 = R2 = Me, R3 = H; III, XVIII, R1 = R2 = R3 = Me; IV, XIX, R1R2 = (CH2)4, R3 = H;
V, XX, R1R2 = (CH2)4, R3 = Me; VI, XXI, R1 = Ph, R2 = R3 = H; VII, XXII, R1R2 = (CH)4, R3 = H; VIII, R4 = Ph; IX, R4 =
4-t-BuC6H4; X, R4 = 4-MeOC6H4; XI, R4 = MeO; XII, XIII, R4 = H; XIV, XV, R4 = Ph.
those typical of isostructural 1-vinylpyrroles XVI–
XXII.
spectrum unambiguously indicated that these com-
pounds exist as E isomers (structure A). The con-
figuration of pyrroles XII–XV was determined on the
basis of vicinal coupling constants between the olefinic
protons, which were equal to 14.2 and 14.0 Hz for XII
and XIV and 9.1 and 8.6 Hz for XIII and XV, respec-
tively. Therefore, compounds XII and XIV were as-
signed the structure of E isomers, while compounds
XIII and XV, Z isomers.
1
The 13C and H parameters of pyrroles I–XXII are
collected in Tables 1 and 2, and the 13C–1H coupling
constants are given in Table 3. Table 4 contains 13C
chemical shifts of reference compounds [12]: ethylene,
1
propene, styrene, and methoxyethene. The H and 13C
NMR signals were assigned on the basis of the data
1
obtained using H–1H 2M COSY and NOESY and
1
heteronuclear H–13C 2M HSQC and HMBC tech-
1
niques. The H and 13C NMR signals of the pyrrole
H2
N
ring were assigned according to the scheme described
in [13]. Signals from methyl groups at the exocyclic
double bond in the 1H NMR spectra of pyrroles having
other methyl groups were identified by the presence in
their HMBC spectra of a cross peak with the olefinic
carbon atoms through two bonds. Likewise, signals
from protons of the methyl groups in positions 2 and 3
of pyrroles II and XVII were assigned on the basis of
cross peaks with C2 and C3 through two bonds in the
HMBC spectra. The presence of a cross peak with 4-H
in the NOESY spectra of pyrroles III and XVIII
allowed us to distinguish signals from protons in the 3-
and 5-methyl groups from the 2-Me signal. All methyl
group signals in the 13C NMR spectra were identified
by correlation with the corresponding proton signals in
the HSQC spectra. The 4-H and 7-H protons in the
fused cyclohexene ring of tetrahydroindoles IV, V,
XIX, and XX gave rise to off-diagonal peaks with 3-H
and protons of the olefinic fragment (HA, HB, HX, or
CH3), respectively. The 5-H and 6-H protons in the
same ring gave cross peaks through three bonds with
C9 and C8, respectively, in the HMBC spectra. The cor-
responding carbon atoms (C4–C7) of tetrahydroindoles
IV, V, XIX, and XX showed in the HSQC NMR spec-
tra correlations with protons attached thereto.
NOESY
HB
Me
R
A
It is seen (Table 1)that the signal from the β-carbon
atom in the propenyl group of 1-isopropenylpyrrole (I)
is displaced strongly upfield relative to the correspond-
ing propene signal. This means that p–π conjugation
between the pyrrole ring and propenyl group is fairly
effective. However, the difference ΔδC = 17.6 ppm for
1-isopropenylpyrrole (I) is considerably smaller than
that for 1-vinylpyrrole (XVI) (ΔδC = 26.4 ppm). Even
the overall effect including p–π conjugation with the
pyrrole ring and σ–π conjugation with the methyl
group in I induces smaller shift of the β-carbon signal
(ΔδC = 25.4 ppm; Table 1) as compared to 1-vinylpyr-
role (XVI). These findings indicate weakening of p–π
conjugation with the exocyclic double bond in 1-iso-
propenylpyrrole (I) as a result of distortion of coplan-
arity between the double bond and heteroring planes
N
φ
H
Me
Compounds VIII–XI may have either Z- or E-
isomer structure. The presence of a cross peak between
the olefinic proton and 2-H (or 5-H) in the NOESY
H
B
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 3 2007

COMPARISON OF THE ELECTRONIC AND STERIC STRUCTURES
399
(increase of dihedral angle φ, as shown in structure B)
due to steric effect of the α-methyl group in the
propenyl fragment.
Analysis of the two-dimensional NOESY spectra
allowed us to determine preferential orientation of the
propenyl group in 1-isopropenyl-2,3-dimethylpyrrole
(II) and 1-isopropenyl-4,5,6,7-tetrahydroindole (IV).
The NOESY spectra contained cross peaks between
protons of the α-methyl group and 5-H (2-H) and
between HB and 2-CH3 in pyrrole II and between
HB and 7-H in tetrahydroindole IV. These findings
indicate trans orientation of the α-methyl group with
respect to the substituent in the pyrrole ring (structure
C); obviously, mutual arrangement of the above frag-
ments in molecules II and IV is determined by steric
repulsion.
The signal from the β-carbon atom of the propenyl
group in 1-isopropenyl-2,3-dimethylpyrrole (II) is loc-
ated in a considerably weaker field (ΔδC = 10.4 ppm)
relative to the corresponding signal of 1-isopropenyl-
pyrrole (I), whereas no such difference is observed in
going from 1-vinylpyrrole (XVI) to 2,3-dimethyl-1-
vinylpyrrole (XVII) (Table 1). Spatial interaction
between the methyl groups in the α-position of the
propenyl group and in position 2 (or 5) of the pyrrole
ring leads to a strong deviation of the propenyl group
from the heteroring plane and considerably weakens
p–π conjugation between these fragments; therefore,
excess shielding of the propenyl β-carbon atom in
pyrrole II is as small as 7.1 ppm. The position of the
Cβ signal in the 13C NMR spectrum of 1-isopropenyl-
2,3,5-trimethylpyrrole (III) is even more downfield,
so that it approaches the position of the corresponding
carbon signal in the spectrum of propene (Table 1).
The absence of excess shielding of Cβ in III suggests
complete distortion of p–π conjugation in its molecule,
which is possible when the propenyl and heteroring
planes are mutually orthogonal. 2,3,5-Trimethyl-1-
vinylpyrrole (XVIII) retains fairly effective p–π con-
jugation with the vinyl group (excess shielding of
the vinyl β-carbon atom is 19.2 ppm); therefore, the
dihedral angle between the vinyl group plane and the
pyrrole ring is relatively small. Thus, methyl group in
the α-position of the vinyl group exerts a very strong
effect on the steric and electronic structures of 1-prop-
enylpyrroles.
H3C
H3C
HB
H5
N
NOESY
CH3
HA
C
Excess shielding of the β-carbon atom in
1-(1-methyl-2-phenylvinyl)pyrrole (VIII) due to p–π
conjugation with the olefinic fragment is estimated at
20.3 ppm relative to Cα in styrene; moreover, the effect
is transmitted to the phenyl ring, leading to additional
shielding of the para-carbon atom by 2.2 ppm
(Table 1). On the other hand, conjugation between the
phenyl ring and exocyclic double bond in molecule
VIII induces only 4.8-ppm upfield shift of the Cα
signal (relative to the Cα signal of 1-isopropenylpyr-
role), while the difference in the chemical shifts of Cβ
between styrene and ethylene is 10.1 ppm (Tables 1,
4). Thus, strong p–π conjugation between the pyrrole
ring and the exocyclic double bond inhibits π-donor
effect of the phenyl group on the double bond. This is
the reason why the electronic effect of the methoxy
group on the double bond through the benzene ring in
pyrrole X is very weak: the upfield shift of the Cα
signal in going from pyrrole VIII to X is as small as
1 ppm (Table 1).
Shielding of the β-carbon atom in 1-isopropenyl-
4,5,6,7-tetrahydroindole (IV) and 1-isopropenylindole
(VII) is slightly greater, while in 1-isopropenyl-2-
phenylpyrrole (VI) smaller, than in 1-isopropenyl-2,3-
dimethylpyrrole (II). The observed differences may
originate from both electronic effects of substituents in
the pyrrole ring and change of the dihedral angle be-
tween the propenyl group and pyrrole ring planes. To
distinguish between these two factors, analysis of
1
the H NMR spectra is necessary (see below). Excess
Weakening of p–π conjugation with the double
bond is clearly illustrated by the NMR data for
1-(2-methoxy-1-methylvinyl)pyrrole (XI). The upfield
shift of the β-carbon signal due to joint effects of p–π
conjugation with the pyrrole ring and σ–π conjugation
in molecule XI is only 12.9 ppm (relative to the Cα
signal of methoxyethene) against 25.4 ppm for 1-iso-
propenylpyrrole (I); p–π conjugation with the methoxy
group induces an upfield shift of the α-carbon signal
shielding of the β-carbon atom in 1-isopropenyl-2-
methyl-4,5,6,7-tetrahydroindole (V), as well as in
1-isopropenyl-2,3,5-trimethylpyrrole (III), is almost
absent (Table 1). Here, spatial interaction between the
substituents in positions 2 and 5 of the pyrrole ring, on
the one hand, and methyl group at the α-carbon atom,
on the other, leads to orthogonal orientation of the
propenyl group plane with respect to the pyrrole ring.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 3 2007

AFONIN et al.
400
Table 1. 13C NMR parameters of 1-propenyl- and 1-vinylpyrroles I–XXIIa
Chemical shift δC, ppm
Comp.
Cα
Cβ
C2
C3
C4 (C9) C5 (C8)
CH3, CH2
no.
140.69
97.93
118.24 109.56 109.56 118.24 20.39 (α-CH3)
I
(–17.6)b
(–25.4)d
(–0.7)c (–0.7)c
142.19 108.37 124.49 116.17 109.34 118.15 22.70 (α-CH3), 10.88 (2-CH3), 11.49 (3-CH3)
II
(–7.1)b
142.00 115.11 123.12 113.58 107.40 125.94 22.97 (α-CH3), 10.00 (2-CH3), 11.05 (3-CH3), 12.14
III
IV
V
(–0.4)b
(5-CH3)
141.15 105.24 118.27 107.39 119.06 127.78 22.37 (α-CH3), 23.35 (4-CH2), 23.88 (5-CH2), 23.35
(–10.3)b
(6-CH2), 24.22 (7-CH2)
141.33 114.28 126.71 105.36 116.44 126.71 22.88 (α-CH3), 12.25 (2-CH3), 23.02 (4-CH2), 23.85
(–1.2)b
(5-CH2), 23.65 (6-CH2), 22.25 (7-CH2)
143.34 110.14 133.08 110.50 108.34 123.58 22.38 (α-CH3)
VI
VII
VIII
(–5.4)b
140.85 105.73 126.33 103.11 129.58 135.61 22.16 (α-CH3)
(–9.8)b
135.91 116.44 118.70 109.59 109.59 118.70 16.83 (α-CH3)
(–4.8)e (–20.3)f
135.48 116.48 118.74 109.50 109.50 118.74 17.00 (α-CH3), 34.58 (t-Bu, C), 31.38 (t-Bu, CH3)
134.86 116.38 118.75 109.39 109.39 118.75 16.94 (α-CH3), 55.32 (OCH3)
IX
X
119.76 140.66 119.76 108.58 108.58 119.76 14.14 (α-CH3), 60.13 (OCH3)
XI
(–20.9)e (–12.9)g
128.21 109.52 118.53 109.65 109.65 118.53 15.02 (β-CH3)
XII
(–4.9)c (–24.1)b
(–0.6)c (–0.6)c
127.15 113.95 121.11 108.79 108.79 121.11 12.84 (β-CH3)
(–5.9)c (–19.6)b (–1.5)c (–1.5)c
XIII
XIV
XV
127.02 112.94 133.75 109.55 109.43 119.60 15.25 (β-CH3)
(–20.7)b
127.47 123.25 134.33 108.86 108.78 118.53 12.75 (β-CH3)
(–10.3)b
133.14
130.82
96.86
118.77 110.29 110.29 118.77
XVI
XVII
XVIII
XIX
XX
(–26.4)g
96.75
124.77 116.23 111.33 114.63 9.57 (2-CH3), 11.22 (3-CH3)
(–26.5)d
131.22 104.12 123.99 115.01 109.47 126.94 11.13 (2-CH3), 11.06 (3-CH3), 13.63 (5-CH3)
(–19.2)d
130.19
95.73
114.85 109.27 118.89 127.54 23.00 (4-CH2), 23.40 (5-CH2), 23.11 (6-CH2), 21.77
(7-CH2)
(–27.6)d
130.46 101.52 127.40 107.39 118.14 127.06 23.05 (4-CH2), 23.29 (5-CH2), 23.71 (6-CH2), 23.93
(–21.8)d
(7-CH2), 13.37 (2-CH3)
132.08
129.56
98.81
134.39 110.15 110.12 118.35
123.35 104.89 135.50 129.33
XXI
XXII
(–24.5)d
96.25
(–27.0)d
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COMPARISON OF THE ELECTRONIC AND STERIC STRUCTURES
401
Table 1. (Contd.)
a
Chemical shifts of the aromatic carbon atoms, δC, ppm: VI: 133.82 (Ci), 127.53 (Co), 128.30 (Cm), 123.58 (Cp); VII: 121.18 (C4), 122.35
(C5), 120.36 (C6), 112.02 (C7); VIII: 136.50 (Ci), 129.07 (Co), 128.36 (Cm), 126.58 (Cp); IX: 133.58 (Ci), 128.81 (Co), 125.32 (Cm),
149.57 (Cp); X: 128.92 (Ci), 130.22 (Co), 113.83 (Cm), 158.92 (Cp); XIV: 132.98 (Ci), 129.16 (Co), 128.46 (Cm), 126.71 (Cp); XV: 133.19
(Ci), 129.16 (Co), 128.35 (Cm), 126.71 (Cp); XXI: 132.52 (Ci), 128.49 (Co), 128.35 (Cm), 127.28 (Cp); XXII: 121.13 (C4), 122.69(C5),
120.77 (C6), 109.53 (C7).
Relative to propene.
b
c
d
e
f
Relative to 1-vinylpyrrole.
Relative to ethylene.
Relative to 1-(1-methylvinyl)pyrrole (I).
Relative to styrene.
g
Relative to methoxyethene.
by 20.9 ppm relative to the Cα signal of I, whereas the
corresponding difference between methoxyethene and
atom is lesser by 24.1 ppm than the chemical shift of
Cα in propene (due to p–π conjugation). The difference
is almost the same as that found for 1-vinylpyrrole
(XVI) (ΔδC = 26.4 ppm). A slightly weaker p–π
interaction in pyrrole XII compared to XVI may be
ethylene is 37.3 ppm (Table 1).
In the 13C NMR spectrum of (E)-1-(prop-1-en-1-
yl)pyrrole (XII), the chemical shift of the β-carbon
Table 2. 1H NMR parameters of 1-propenyl- and 1-vinylpyrroles I–XXIIa
Chemical shift δ, ppm
2-H 3-H 4-H 5-H
6.95 6.21 6.21 6.95 2.19 (α-CH3)
5.96 6.57 2.09 (α-CH3), 2.14 (2-CH3), 2.02 (3-CH3)
5.68 1.95 (α-CH3), 2.06 (2-CH3), 1.97 (3-CH3), 2.12 (5-CH3)
Comp.
no.
Δδ
HA
HB
HX
CH3, CH2
0.37 4.54 4.91
–0.08– 4.94 4.82
–0.36– 5.28 4.92
–0.02– 4.80 4.78
–0.32– 5.22 4.90
–0.02– 4.97 4.95
0.10 5.06 5.16
6.59
I
II
III
6.64 5.96
5.96
2.12 (α-CH3), 2.52 (4-H), 1.75 (5-H, 6-H), 2.59 (7-H)
1.98 (α-CH3), 2.46 (4-H, 7-H), 1.74 (5-H, 6-H)
2.12 (α-CH3)
IV
V
6.30 6.22 6.78
7.21 6.55
VI
1.98 (α-CH3), 2.16 (2-CH3)
VII
VIII
IX
7.02 6.26 6.26 7.02 2.33 (α-CH3)
6.56
7.02 6.25 6.25 7.02 2.34 (α-CH3), 1.32 (t-Bu)
7.01 6.25 6.25 7.01 2.31 (α-CH3), 3.82 (OCH3)
6.72 6.17 6.17 6.72 2.06 (α-CH3), 3.64 (OCH3)
6.53
X
6.30
XI
5.63 6.61 6.79 6.16 6.16 6.79 1.77 (β-CH3)
6.59 6.80 6.21 6.21 6.80 1.84 (β-CH3)
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
5.21
5.42
5.73 6.66
6.52
6.24 6.24 6.99 1.76 (β-CH3)
6.32 6.27 6.86 1.80 (β-CH3)
0.47 4.54 5.01 6.72 6.81 6.18 6.18 6.81
0.43 4.57 5.00 6.82
6.00 6.87 2.14 (2-CH3), 2.00 (3-CH3)
0.15 4.86 5.01 6.71
5.72
2.16 (2-CH3), 1.96 (3-CH3), 2.23 (5-CH3)
0.45 4.52 4.97 6.74 6.83 5.99
2.46 (4-CH2), 1.70 (5-H), 1.79 (6-H), 2.55 (7-H)
0.22 4.75 4.97 6.72
5.70
2.25 (2-CH3), 2.44 (4-H), 1.75 (5-H2), 1.81 (6-H), 2.61
(7-H)
0.49 4.63 5.12 6.86
6.23 6.26 7.07
XXI
0.43 4.73 5.16 7.20 7.40 6.66
XXII
a
Chemical shifts of aromatic protons, δ, ppm: VI: 7.42 (o-H), 7.32 (m-H), 7.22 (p-H); VII: 7.62 (4-H), 7.21 (5-H), 7.11 (6-H), 7.62
(7-H); VIII: 7.27 (o-H), 7.35 (m-H), 7.23 (p-H); IX: 7.23 (o-H), 7.37 (m-H); X: 7.22 (o-H), 6.90 (m-H); XIV: 7.39 (o-H), 7.35 (m-H),
7.29 (p-H); XV: 7.45 (o-H), 7.42 (m-H), 7.29 (p-H); XXI: 7.34 (o-H), 7.32 (m-H), 7.27 (p-H); XXII: 7.71 (4-H), 7.30 (5-H), 7.27 (6-H),
7.45 (7-H).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 3 2007

AFONIN et al.
402
Table 3. Direct coupling constants 13C–1H (Hz) in the 13C NMR spectra of 1-propenyl- and 1-vinylpyrroles I–XIII
and XVI–XXII
Compound no.
ΔJ
Cβ–HA
Cβ–HB
Cα–HX
C2–H
184.8
C3–H
171.4
C4–H
C5–H
4.9
0.0
161.9
158.8
157.3
158.7
157.6
159.5
159.5
157.0
158.8
159.3
158.7
159.0
159.5
158.9
153.4
152.6
155.4
177.9
151.9
171.4
167.8
165.9
184.8
184.0
I
II
–2.0–
0.0
III
184.2
168.2
166.0
170.4
173.4
171.0
171.2
171.0
170.5
171.0
171.0
171.4
IV
–1.4–
0.0
V
171.4
186.0
VI
0.6
183.3
184.7
185.2
185.0
184.5
184.7
185.2
185.2
VII
VIII
IX
171.0
171.2
171.0
170.5
171.0
171.0
171.4
169.1
167.1
184.7
185.2
185.0
184.5
184.7
185.2
185.2
184.5
X
XI
172.8
176.7
174.2
173.7
172.0
172.1
171.4
176.1
173.0
XII
XIII
XVI
XVII
XVIII
XIX
XX
XXI
XXII
156.8
164.0
163.2
161.5
163.1
161.1
163.5
164.1
7.0
6.5
2.7
6.2
2.3
6.5
7.1
157.0
156.7
158.8
156.9
158.6
157.0
157.0
184.5
168.7
167.4
171.1
171.7
185.7
rationalized in terms of the opposite effect of donor
σ–π interaction with the β-methyl group in the
propenyl fragment. Taking into account the lack of
steric repulsion between the pyrrole ring and fairly
distant trans-methyl group, coplanar arrangement of
the pyrrole ring and propenyl group in (E)-1-(prop-1-
en-1-yl)pyrrole (XII) may be assumed.
pyrrole ring plane due to steric effect of the cis-β-
methyl group. As the p–π conjugation with the prop-
enyl group in molecule XIII weakens, σ–π interaction
with the β-methyl group becomes stronger, and the Cα
signal shifts upfield by 1 ppm relative to the corre-
sponding signal of XII (Table 1).
Some deshielding of the Cβ atom in (E)-2-phenyl-1-
(prop-1-en-1-yl)pyrrole (XIV) compared to (E)-1-
(prop-1-en-1-yl)pyrrole (XII) is similar to that ob-
served in going from 2-phenyl-1-vinylpyrrole (XXI) to
1-vinylpyrrole (XVI). However, the Cβ atom in (Z)-2-
phenyl-1-(prop-1-en-1-yl)pyrrole (XV) is deshielded
much more strongly than in its isomer (XIV) (ΔδC =
10.3 ppm). Steric interaction between the β-methyl
group and 2-phenyl substituent leads to considerable
deviation of the propenyl group from the heteroring
plane.
In going from (E)-1-(prop-1-en-1-yl)pyrrole (XII)
to isomeric (Z)-1-(prop-1-en-1-yl)pyrrole (XIII), the
β-carbon atom becomes less shielded (ΔδC = 4.4 ppm).
The reason is partial rupture of p–π conjugation as
a result of deviation of the propenyl group from the
Table 4. 13C NMR chemical shifts of substituted ethenesa
δC, ppm
Compound
Cα
Cβ
CH2=CH2
123.3
133.6
136.7
153.6
123.3
115.5
113.2
086.0
Change of the orientation of the propenyl group
CH3CH=CH2
PhCH=CH2
CH3OCH=CH2
1
relative to the pyrrole ring is reflected in the H NMR
spectra (Table 2), where different effects of the
magnetically anisotropic pyrrole ring on protons in the
propenyl group are observed. In the 1H NMR spectrum
a
Data of [12].
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COMPARISON OF THE ELECTRONIC AND STERIC STRUCTURES
403
of 1-isopropenylpyrrole (I), which is characterized by
a small dihedral angle (φ) between the propenyl group
and pyrrole ring planes, the chemical shift of protons
in the α-methyl group is δ 2.19 ppm. Increase in the
angle φ in going to pyrroles II, IV, and VI is accom-
panied by upfield shift of the α-methyl proton signal to
δ 2.09–2.12 ppm, and the corresponding signal of III
and V with orthogonal orientation of the propenyl
group and heteroring appears at δ 1.95–1.98 ppm.
We previously showed that direct 13C–1H coupling
constants for the vinyl β-carbon atom are sensitive to
the steric structure and that they depend on the di-
hedral angle (φ) between the vinyl group and hetero-
ring planes [10, 16]. The coupling constant of Cβ with
trans-β-H decreases, while that with cis-β-H increases,
as the angle φ rises; correspondingly, the difference
ΔJ = 1J(Cβ–HA) – 1J(Cβ–HB) decreases [10, 16]. The ΔJ
value for 1-isopropenylpyrrole (I) is 4.9 Hz (Table 3).
In going to 2-substituted pyrroles II and VI, tetra-
hydroindole IV, and indole VII, ΔJ decreases to 0–
0.6 Hz in parallel with considerable increase of the
dihedral angle φ. The difference ΔJ for indole VII is
somewhat larger than the corresponding parameters of
compounds II, IV, and VI, indicating that molecule
VII is more planar. Orthogonal orientation of the prop-
enyl group and heteroring in 2,5-substituted pyrrole III
and 2-substituted tetrahydroindol V is characterized by
negative values of ΔJ. The range of variation of ΔJ
for propenyl-substituted compounds I–VII (ΔΔJ =
6.9 Hz) exceeds that for vinyl-substituted analogs
XVI–XXII (ΔΔJ = 4.8 Hz), in keeping with the wider
range of variation of the dihedral angle between the
planes of the propenyl group and heteroring, as com-
pared to vinyl group.
The chemical shifts of the β-protons in the vinyl or
propenyl group are sensitive to the steric structure.
Distortion of the planar structure and hence rupture of
p–π conjugation induces downfield shift of the trans-β-
proton signal (HA) (as well as of the Cβ signal), while
the cis-β-proton signal (HC) shifts upfield due to aniso-
tropic effect of the pyrrole ring [14]. As the exocyclic
double bond deviates from the heteroring plane, the
difference in the chemical shifts of HB and HA (ΔδAB
=
δHB – δHA) decreases, and it becomes negative at
large dihedral angles φ [15, 16]. The parameter ΔδAB
for 1-isopropenylpyrrole (I) insignificantly differs
from that found for 1-vinylpyrrole (XVI) (0.37 and
0.47 ppm, respectively; Table 2). However, in going to
1-isopropenyl-2,3-dimethylpyrrole (II), the ΔδAB value
decreases so strongly that it becomes negative
(–0.08 ppm). The difference between ΔδAB values for
pyrroles II and I exceeds the whole range of variation
of ΔδAB for 1-vinylpyrroles XVI–XXII, indicating con-
siderable acoplanarity of the propenyl group and pyr-
role ring. It should be noted that ΔδAB for 1-isoprop-
enylindole (VII) is appreciably greater than those for
2-substituted isopropenylpyrroles II, IV, and VI. Pre-
sumably, steric effect of the fused benzene ring on the
propenyl group is much weaker than the effects of
alkyl substituents; therefore, the corresponding devia-
tion of the propenyl group from the heteroring plane is
much smaller. Compounds III and V in which the
propenyl group is orthogonal to the pyrrole ring are
characterized by large negative values of ΔδAB (–0.36
and –0.32 ppm, respectively; Table 2).
EXPERIMENTAL
1
The H and 13C NMR spectra were recorded on
a Bruker DPX-250 spectrometer (250.1 and 62.9 MHz,
respectively) using CDCl3 as solvent (c = 5–10 wt %)
and hexamethyldisiloxane as internal reference. The
1
pulse sequence parameters for recording the H and
13C NMR spectra were reported in [17]. Two-dimen-
sional COSY, NOESY, HSQC, and HMBC spectra
were measured using standard built-in programs
1
n
optimized for JCH = 160 Hz (HSQC) and JCH = 8 Hz
(HMBC). The IR spectra were obtained on a Bruker
IFS 25 spectrometer from samples prepared as thin
films (liquid substances) or KBr pellets (crystalline
substances). The mass spectra of compounds XVIII
and XX (electron impact, 60 eV) were recorded on
an LKB-2091 GC–MS system (38-m SE-54 capillary
column, injector temperature 250°C, oven temperature
programming from 70 to 200°C at 10 deg/min; ion
source temperature 240°C). Compound XI was iso-
lated by preparative gas–liquid chromatography on
a PAKhV-07 instrument equipped with a 5-m×10-mm
column; stationary phase 5% of XE-60 on Chromaton
N-AW-HMDS; thermal conductivity detector; carrier
gas helium; oven temperature 100°C, detector tem-
perature 200°C, injector temperature 230°C.
1
The 7-H signal in the H NMR spectrum of 1-iso-
propenyl-2-methyl-4,5,6,7-tetrahydroindole (V) is dis-
placed upfield by 0.09–0.15 ppm relative to the cor-
responding signals of tetrahydroindoles IV, XIX, and
XX. A probable reason is that the 7-H proton in IV,
XIX, and XX is deshielded due to magnetically aniso-
tropic effect of the vinyl or propenyl fragment; the
propenyl group in V is oriented orthogonally to the
heteroring plane, so that it does not exert deshielding
effect on 7-H.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 3 2007

AFONIN et al.
404
of water. The mixture was heated to 120–127°C, a so-
lution of 5.8 g (50 mmol) of prop-2-yn-1-ylbenzene in
5 ml of DMSO was added dropwise under stirring over
a period of 30 min, the mixture was cooled and diluted
with water, and the precipitate was filtered off,
repeatedly washed with water (until neutral washings),
and dried. We thus isolated 6.75 g of compound VIII.
The aqueous phase was extracted with 4 portions of
diethyl ether, the extracts were combined, washed with
water, and dried over MgSO4, the solvent was re-
moved, and the residue was evacuated at a residual
pressure of 2 mm to isolate an additional portion of
VIII, 1.54 g, as brown crystals. The two portions of
the product were combined and recrystallized from
hexane to obtain 5.77 g of pure compound VIII as
colorless crystals with mp 78°C. IR spectrum, ν, cm–1:
3132 w, 3103 w, 3045 w, 3022 w, 3005 w, 2921 w,
2855 w, 1638 s, 1596, 1569, 1554, 1518, 1477 s, 1444,
1393, 1360 w, 1320, 1308 s, 1283, 1256, 1202 w,
1180 w, 1156 w, 1124, 1090, 1075, 1045 w, 1028 w,
1000 w, 978, 927, 855, 824 w, 758, 723 s, 697, 619,
579, 443 w. Found, %: C 85.18; H 7.16; N 7.78.
C13H13N. Calculated, %: C 85.21; H 7.15; N 7.84. The
mother liquor was evaporated, and the residue was
subjected to column chromatography on Al2O3 (diethyl
ether–hexane, 1:3) to isolate 1.09 g (75%) of light
yellow crystals containing (according to the 1H and 13C
NMR data) 15% of 1-[(Z)-1-methyl-2-phenylvinyl]-
1H-pyrrole and 85% of E isomer VIII.
1-Isopropenylpyrroles I–VI were synthesized by
reactions of the corresponding NH-pyrroles with
propyne and allene according to the procedure de-
scribed in [11]. 1-(Prop-1-en-1-yl)pyrroles XII and
XIII were obtained by isomerization of 1-allylpyrrole
in the system KOH–DMSO [18]. 1-Vinylpyrrole
(XVI) and its derivatives XVIII and XX were pre-
pared by vinylation of the corresponding NH-pyrroles
with acetylene in the system KOH–DMSO under
atmospheric pressure [19]. Compounds XVIII and XX
were not described previously.
2,3,5-Trimethyl-1-vinyl-1H-pyrrole (XVIII) was
obtained from 2,3,5-trimethyl-1H-pyrrole [20] and
acetylene (120–126°C, 2 h). Yield 51%, colorless
liquid, bp 73°C (7 mm), nD24 = 1.5201, purity 95%. IR
spectrum, ν, cm–1: 3122 w, 3088 w, 2972, 2921 s,
2862 s, 2737 w, 1642 s, 1602, 1424 s, 1397 s, 1535,
1376, 1341 s, 1302 s, 1167 w, 1140 w, 1111 w, 1074 w,
1037 w, 1006 w, 969 s, 866 s, 782 s, 717, 694 w, 631,
575 w, 510 w. Mass spectrum, m/z (Irel, %): 135 (100)
[M]+, 134 (100), 120 (46), 118 (13), 108 (13), 106 (15),
93 (15), 91 (24), 79 (43), 77 (30), 67 (48), 65 (33), 53
(20), 51 (17), 42 (65), 39 (63).
2-Methyl-1-vinyl-4,5,6,7-tetrahydro-1H-indole
(XX) was obtained from 2-methyl-4,5,6,7-tetrahydro-
1H-indole [20] and acetylene (115–125°C, 2 h). Yield
49%, light yellow liquid, bp 105°C (6 mm), nD21
=
1.5534, purity 95%. IR spectrum, ν, cm–1: 3123 w,
3087 w, 3049 w, 2972, 2969, 2930, 2843 s, 1642 s,
1600, 1535 s, 1429 s, 1396 s, 1382, 1347, 1328,
1290 s, 1254, 1238, 1205 w, 1154, 1101, 1074 w,
1058 w, 1034 w, 993, 968 s, 938, 858 s, 779 s, 724 w,
693, 639, 604 w, 580. Mass spectrum, m/z (Irel, %): 161
(89) [M]+, 147 (22), 134 (100), 117 (1), 91 (13), 77 (8),
65 (7), 40 (6), 33 (6), 28 (32).
Compounds IX and X were synthesized in a similar
way.
1-[(E)-1-Methyl-2-(4-tert-butylphenyl)vinyl]-1H-
pyrrole (IX). Yield 60%, light yellow crystals,
mp 80°C (from hexane). IR spectrum, ν, cm–1: 3139 w,
3100 w, 3053 w, 3024 w, 3000 w, 2962, 2901, 2866,
1723 w, 1686 w, 1639, 1607 w, 1551, 1517, 1480 s,
1458, 1445, 1412 w, 1395, 1368, 1358, 1313 s, 1296,
1284, 1270 w, 1257 w, 1202 w, 1191, 1119, 1089,
1074, 1042 w, 1025 w, 1013 w, 979, 928, 862, 836,
816, 725 s, 668, 637 w, 563, 516 w, 456 w. Found, %:
C 85.28; H 8.79; N 5.83. C17H21N. Calculated, %:
C 85.30; H 8.84; N 5.85.
Substituted 1-vinylpyrroles XVII, XIX, and XXI
were synthesized by the Trofimov reaction from the
corresponding ketone oximes and acetylene [6, 21].
1-Vinylindole was prepared by vinylation of indole
with acetylene in aqueous dioxane in the presence of
30% KOH under pressure [22]. Previously unknown
compounds VIII–XI were synthesized by reaction of
pyrrole with prop-2-yn-1-ylbenzene, para-substituted
prop-2-yn-1-ylbenzenes, and 3-methoxypropyne in the
system KOH–DMSO [23].
1-[(E)-2-(4-Methoxyphenyl)-1-methylvinyl]-1H-
pyrrole (X). Yield 60%, large light brown crystals,
mp 101–102°C (from hexane). IR spectrum, ν, cm–1:
3138 w, 3113 w, 3044 w, 3015 w, 2955 w, 2933 w,
2913 w, 2837 w, 1645, 1606, 1478, 1461, 1441,
1413 w, 1387, 1359 w, 1312 s, 1290 s, 1247 s, 1201 w,
1177, 1128, 1100, 1091, 1076, 1027, 977, 928, 852,
833, 817, 774 w, 731 s, 704 w, 668 w, 633 w, 616, 551,
1-[(E)-1-Methyl-2-phenylvinyl]-1H-pyrrole
(VIII). Pyrrole, 3.35 g (50 mmol), was added to a sus-
pension of 3.25 g (50 mmol) of powdered KOH·
0.5H2O in 25 ml of DMSO containing less than 0.2%
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 3 2007

COMPARISON OF THE ELECTRONIC AND STERIC STRUCTURES
405
8. Afonin, A.V., Sigalov, M.V., Korostova, S.E., Voro-
nov, V.K., and Aliev, I.A., Izv. Akad. Nauk SSSR, Ser.
Khim., 1988, p. 2765.
9. Afonin, A.V., Sigalov, M.V., Korostova, S.E., Ali-
ev, I.A., Vashchenko, A.V., and Trofimov, B.A., Magn.
Reson. Chem., 1990, vol. 28, p. 580.
10. Afonin, A.V., Sigalov, M.V., Voronov, V.K.,
Shmidt, E.Yu., and Trofimov, B.A., Izv. Akad. Nauk
SSSR, Ser. Khim., 1987, p. 1418.
11. Trofimov, B.A., Tarasova, O.A., Shemetova, M.A., Afo-
nin, A.V., Klyba, L.V., Baikalova, L.V., and Mikhale-
va, A.I., Russ. J. Org. Chem., 2003, vol. 39, p. 408.
12. Ionin, B.I., Ershov, B.A., and Kol’tsov, A.I., YaMR-
spektroskopiya v organicheskoi khimii (NMR Spectros-
copy in Organic Chemistry), Leningrad: Khimiya, 1983.
13. Afonin, A.V., Ushakov, I.A., Kuznetsova, S.Yu., Petro-
va, O.V., Schmidt, E.Yu., and Mikhaleva, A.I., Magn.
Reson. Chem., 2002, vol. 40, p. 114.
530, 501 w, 450 w. Found, %: C 78.78; H 7.06; N 6.50.
C14H16NO. Calculated, %: C 78.84; H 7.09; N 6.57.
1-[(E)-2-Methoxy-1-methylvinyl]-1H-pyrrole
(XI) was synthesized as described above for com-
pounds VIII–X from pyrrole and 3-methoxypropyne.
Compound XI was formed as the minor product;
according to the GLC data, its fraction in the reaction
mixture was 25% (yield 14%). The major product was
1-(1-methoxymethylvinyl)-1H-pyrrole (66%); 1H NMR
spectrum, δ, ppm: 3.55 s (3H, OMe), 4.38 d (2H, CH2),
4.95 m (1H, HA), 6.15 t (2H, 3-H, 4-H), 6.57 d (1H,
HB), 6.68 t (2H, 2-H, 5-H); 13C NMR spectrum, δC,
ppm: 47.82 (OMe), 56.00 (CH2), 98.94 (Cα), 108.09
(C3, C4), 120.03 (C2, C5), 150.63 (Cβ); also, the mix-
ture contained 9% of unidentified products. Compound
XI was isolated from the mixture as a light yellow
liquid, nD20 = 1.5155, purity 99%. IR spectrum, ν, cm–1:
3132 w, 3102 w, 3051 w, 2999, 2935, 2841, 2830,
1722, 1689, 1658, 1548 w, 1511 w, 1488 s, 1451,
1389 s, 1358 w, 1321 s, 1262, 1227 s, 1141 s, 1084,
1045, 1008, 970, 911, 840, 822, 725 s, 625, 505 w.
Found, %: C 69.88; H 8.06; N 10.11. C8H11NO. Cal-
culated, %: C 70.04; H 8.08; N 10.21.
14. Günther, H., NMR Spectroscopy: an Introduction,
Chichester: Wiley, 1980.
15. Afonin, A.V., Andriyankov, M.A., Pertsikov, B.Z., and
Voronov, V.K., Zh. Org. Khim., 1986, vol. 22, p. 2451.
16. Afonin, A.V., Vashchenko, A.V., Andriyankov, M.A.,
Voronov, V.K., Enikeeva, E.I., and Dmitrieva, L.L., Izv.
Akad. Nauk SSSR. Ser. Khim., 1990, p. 1539.
17. Afonin, A.V., Ushakov, I.A., Petrova, O.V., Sobeni-
na, L.N., Mikhaleva, A.I., Voronov, V.K., and Trofi-
mov, B.A., Russ. J. Org. Chem., 2000, vol. 36, p. 1043.
18. Tarasova, O.A., Shmidt, E.Yu., Albanov, A.I., Mikha-
leva, A.I., Brandsma, L., and Trofimov, B.A., Russ. J.
Org. Chem., 1999, vol. 35, 1499.
19. Tarasova, O.A., Mal’kina, A.G., Mikhaleva, A.I.,
Brandsma, L., and Trofimov, B.A., Synth. Commun.,
1994, vol. 24, p. 2035.
20. Trofimov, B.A., Tarasova, O.A., Mikhaleva, A.I., Kali-
nina, N.A., and Sinegovskaya, L.M., Synthesis, 2000,
p. 1585.
21. Trofimov, B.A., Pyrroles. Part Two: The Synthesis,
Reactivity and Physical Properties of Substituted Pyr-
roles, Jones, A., Ed., New York: Wiley, 1992, p. 131.
22. Domnina, E.S., Skvortsova, G.G., Glazkova, N.P., and
Shostakovskii, M.F., Khim. Geterotsikl. Soedin., 1966,
p. 390.
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