An efficient route to 1-vinylpyrrole-2-carbaldehydes

Article abstract of DOI:10.1055/s-0028-1083312

1-Vinylpyrroles are formylated by the N-N dimethyl- formamide/oxalyl chloride reagent system (CH₂Cl₂, r.t., 40 min) to give the corresponding 1-vinylpyrrole-2-carbaldehydes in yields up to 97%. Georg imeme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083312

PAPER
587
An Efficient Route to 1-Vinylpyrrole-2-carbaldehydes
A
n
E
fficient Route
l
to1-V
’
inylpyr
r
b
ole-2-carbald
i
ehyde
n
a I. Mikhaleva, Andrey V. Ivanov, Elena V. Skital’tseva, Igor’ A. Ushakov, Alexander M. Vasil’tsov,
Boris A. Trofimov*
A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Street, 664033 Irkutsk,
Russian Federation
Fax +7(3952)419346; E-mail: boris_trofimov@irioch.irk.ru
Received 9 September 2008; revised 12 September 2008
since such substituents decrease the sensitivity of the vi-
Abstract: 1-Vinylpyrroles are formylated by the N,N-dimethyl-
nyl group towards electrophilic reagents. However, in the
formamide/oxalyl chloride reagent system (CH2Cl2, r.t., 40 min) to
case of 1-vinylpyrroles containing donor (alkyl) substitu-
give the corresponding 1-vinylpyrrole-2-carbaldehydes in yields up
ents, the yields of the target aldehydes were significantly
reduced. For instance, from 1-vinyl-4,5,6,7-tetrahydroin-
dole (1b) the corresponding formyl derivative 2b was ob-
tained in only 56% yield (Table 1, entry 2),16 and the
formylation of 3-ethyl-2-propyl-1-vinylpyrrole (1a) gave
2a in 28% yield only (entry 1). To increase the efficiency
of this approach in a series of 1-vinyl-2-alkyl- and 1-vi-
nyl-2,3-dialkylpyrroles, we have tried to replace the N,N-
dimethylformamide/phosphoryl chloride system by a
milder formylating agent.
to 97%.
Key words: aldehydes, pyrroles, formylation, oxalyl chloride,
Vilsmeier–Haack reaction
Pyrroles bearing functional groups have been finding
ever-increasing application in the synthesis of pharma-
ceuticals and analogues of natural compounds.1 New anti-
biotics, pheromones, toxins, inhibitors of cell division and
immunomodulating agents containing the pyrrole core
have been discovered.2 Pyrrole-2-carbaldehydes have
been used as intermediates in the synthesis of diverse oli-
gopyrrole systems,3 anion receptors in biomedical analy-
sis,4 porphyrins,5 models for the investigation of multiple
sclerosis6 and expansion of the genetic alphabet,7 ligands
for metallocomplexes,8 and conjugated polymers.9 Pyr-
role-2-carbaldehydes are also used in the modification of
natural structures such as proteins10 and lipids.11 Chemical
transformation of pyrrole-2-carbaldehydes has allowed
valuable compounds such as carbolines,12 cyanopyr-
roles,13 and divinylpyrroles14 to be obtained. The synthetic
potential of the aldehyde moiety combined with the bio-
logical importance of pyrroles has attracted the continu-
ous attention of chemists all over the world to this class of
compounds.
R2
R1
R2
R1
1. DMF, (COCl)2, CH2Cl2, r.t., 40 min
2. NaOAc, H2O, r.t., 30 min
CHO
N
N
2a–g
1a–g
Scheme 1
It is known that the formylation of pyrroles, in particular
dipyrromethanes sensitive to the action of acidic reagents,
sometimes requires the use of an N,N-dimethyl-
formamide/benzoyl chloride system instead of the classic
Vilsmeier reagent.17 However, our experiments have
shown that 1-vinylpyrroles are not formylated by the N,N-
dimethylformamide/benzoyl chloride reagent under the
conditions reported earlier for this class of compounds16
(–78 °C, 3 h): the conversion of 1-vinyl-4,5-dihy-
drobenz[g]indole (1d) into the corresponding formyl de-
rivative 2d was only 3.5% (GLC analysis). At a higher
reaction temperature (0 °C), the conversion of 1-vinylpyr-
role 1d to aldehyde 2d increased to 11.7%, while under re-
flux (DCE, 57 °C, 3 h) of the same reactants the
conversion of pyrrole 1d did not exceed 33%. The N,N-
dimethylformamide/benzoyl chloride complex appears to
be unsuitable for introduction of the formyl group to 1-vi-
nylpyrroles.
In the last years, 1-vinylpyrroles, readily available in one
step from ketoximes and acetylenes by the Trofimov reac-
tion,15 have been efficiently applied in the synthesis of
functionalized pyrroles. Recently, we16 have developed a
general method for the synthesis of the previously un-
known 1-vinylpyrrole-2-carbaldehydes 2, based on the
Vilsmeier–Haack reaction modified for 1-vinylpyrroles 1.
A modification was required because of the sensitivity of
the N-vinyl group towards electrophilic attack from the
N,N-dimethylformamide/phosphoryl chloride reagent that
reduced the chemoselectivity of the reaction. As a result,
1-vinylpyrrole-2-carbaldehydes containing aryl and
hetaryl substituents, including fused polycyclic systems,
The use of oxalyl chloride (among a number of other
formylating reagents) in the Vilsmeier–Haack reaction
was briefly mentioned in only a few papers18 and pat-
ents.19 In one of these papers, the influence of the substit-
uent in an amide component was studied,18c showing that
the chloro anhydride structure had ‘only a very small ef-
fect’ on the reaction results. Oxalyl chloride was used in-
e.g.
1-vinyl-4,5-dihydrobenz[g]indole-2-carbaldehyde
(2d), were obtained in good yields (Table 1, entry 4),
SYNTHESIS 2009, No. 4, pp 0587–0590
Advanced online publication: 12.01.2009
DOI: 10.1055/s-0028-1083312; Art ID: Z20808SS
x
x
.
x
x
.2
0
0
8
© Georg Thieme Verlag Stuttgart · New York

588
A. I. Mikhaleva et al.
PAPER
This method was successfully applied to various 1-vi-
nylpyrroles 1 containing aliphatic, aromatic, condensed
aromatic, and heteroaromatic substituents, as well as to 1-
vinylpyrroles condensed with cycloaliphatic and dihy-
dronaphthalene systems (see Table 1). As shown in
Table 1, the yields of 1-vinylpyrrole-2-carbaldehydes
2a,b containing donor (alkyl) substituents were increased
approximately by 20% (entries 1 and 2). Moreover, the
yields of 1-vinylpyrroles with aryl and hetaryl substitu-
ents 2c,e–g were also increased (by 9–27%) (entries 3 and
5–7).
Table 1 Comparison of the N,N-Dimethylformamide/Phosphoryl
Chloride and N,N-Dimethylformamide/Oxalyl Chloride Reagent Sys-
tems in the Formylation of 1-Vinylpyrroles
Entry
Product
Isolated yielda (%)
DMF/
POCl3
DMF/
(COCl)2
Et
Pr
1
2a
28b
48
CHO
CHO
N
It may be concluded that, in general, oxalyl chloride pro-
vides the best results in the formylation of 1-vinylpyr-
roles, being a less aggressive reagent than phosphoryl
chloride, more active than benzoyl chloride, and safer and
more convenient to handle than other formylating re-
agents.21
2
3
4
2b
2c
2d
5616
6616
9116
72
93
89
N
CHO
CHO
The reaction proceeds at room temperature (instead of
–78 °C16) and needs a shorter time (40 min instead of 3 h16).
The use of the new method of 1-vinylpyrrole formylation
by the N,N-dimethylformamide/oxalyl chloride reagent
apparently suppresses oligomerization side reactions and
the removal of N-vinyl groups that occurs in the case of
the N,N-dimethylformamide/phosphoryl chloride reagent.
N
N
Me
b
Melting points were measured on a Kofler micro hot stage appara-
tus. IR spectra were measured on a Bruker IFS-25 as KBr pellets or
5
2e
–
83
CHO
N
Et
1
films. H NMR and 13C NMR spectra were recorded on a Bruker
DPX spectrometer. Oxalyl chloride, DMF and benzoyl chloride are
commercial products. The 1-vinylpyrroles were prepared according
to a published procedure.15a
CHO
6
7
N
2f
7220
81
97
1-Vinylpyrrole-2-carbaldehydes 2; General Procedure
(COCl)2 (1.40 g, 11.0 mmol) was added dropwise (2–3 min) to
DMF (0.80 g, 11.0 mmol) at ~10 °C (cold H2O), and the white crys-
tals obtained were stirred for 15 min without cooling. Then CH2Cl2
(10.0 mL) was added, and a soln of 1-vinylpyrrole 1 (10 mmol) in
CH2Cl2 (15.0 mL) was added dropwise over 10 min at 25 °C. The
resulting mixture was stirred for 0.5 h at r.t. Then a soln of NaOAc
(4.10 g, 50 mmol) in H2O (45 mL) was added and the stirring was
continued for 0.5 h at r.t. The lower (organic) layer was separated.
The aqueous layer was extracted with Et2O (5 × 30 mL). The com-
bined organic phases were washed with sat. aq NaHCO3 (3 × 30
mL) and H2O (3 × 30 mL), and dried (K2CO3). The residue obtained
after evaporation of the Et2O was purified on basic alumina (hex-
ane–Et2O, 2:1); this gave formylpyrrole 2.
S
2g
8816
CHO
N
a The products were identified by IR and 1H and 13C NMR spectros-
copy and elemental analysis.
b Previously unknown.
stead of phosphorus oxychloride18a to afford an iminium
salt (the precursor of pyrrole-2-carbaldehydes), since the
formation of phosphorus-containing side products was
thus avoided. However, no evidence that oxalyl chloride
was a ‘softer’ reagent was presented.
4-Ethyl-5-propyl-1-vinylpyrrole-2-carbaldehyde (2a)
From 1-vinylpyrrole 1a (1.63 g, 10 mmol), 2a was obtained.
Here we report that the replacement of phosphoryl chlo- Yield: 0.92 g (48%); yellow oily liquid.
ride by oxalyl chloride in the formylation of 1-vinylpyr-
roles 1 makes it possible to increase the yields of 1-
vinylpyrrole-2-carbaldehydes 2 to almost quantitative
with complete conversion of the starting 1-vinylpyrroles
1; it also makes it possible to perform all stages of the re-
action at room temperature (instead of at –78 °C, as the
former protocol16 required) (Scheme 1, Table 1). Stirring
of the reaction mixture in dichloromethane for 40 minutes
resulted in full consumption of the starting 1-vinylpyr-
roles 1 (GLC analysis).
IR (film, KBr): 3111, 3049, 2943, 2933, 2867, 2862, 2809, 2727,
1670, 1641, 1580, 1475, 1418, 1375, 1318, 1285, 1250, 1212, 1150,
1125, 1075, 1030, 969, 915, 880, 855, 800, 775, 745, 710, 684, 596,
515, 487 cm–1.
1H NMR (400 MHz, CDCl3): d = 9.43 (s, 1 H, CHO), 7.19 (dd,
3JB–X = 15.8 Hz, 3JA–X = 8.7 Hz, 1 H, HX), 6.84 (s, 1 H, H-3), 5.26
(d, 3JA–X = 8.7 Hz, 1 H, HA), 5.21 (d, 3JB–X = 15.8 Hz, 1 H, HB), 2.61
[m, 2 H, CH2 (Pr)], 2.41 [q, 3J = 7.5 Hz, 2 H, CH2 (Et)], 1.53 [m, 2
3
H, CH2 (Pr)], 1.17 [t, J = 7.5 Hz (Et), 3 H, CH3], 0.93 [t, 3J = 7.3
Hz, 3 H, CH3 (Pr)].
Synthesis 2009, No. 4, 587–590 © Thieme Stuttgart · New York

PAPER
An Efficient Route to 1-Vinylpyrrole-2-carbaldehydes
589
13C NMR (100 MHz, CDCl3): d = 178.2 (C=O), 140.3 (C-5), 131.6
(Ca), 131.2 (C-2), 126.4 (C-4), 123.3 (C-3), 111.3 (Cb), 26.6 [CH2
(Pr)], 22.5 [CH2 (Pr)], 18.9 [CH2 (Et)], 15.0 [CH3 (Et)], 14.0 [CH3
(Pr)].
Anal. Calcd for C15H13NO: C, 80.69; H, 5.87; N, 6.27. Found: C,
80.74; H, 5.94; N 6.09.
5-(4-Ethylphenyl)-4-methyl-1-vinylpyrrole-2-carbaldehyde
(2e)
From 1-vinylpyrrole 1e (2.11 g, 10 mmol), 2e was obtained.
Anal. Calcd. for C12H17NO: C, 75.35; H, 8.96; N, 7.32. Found: C,
75.39; H, 8.91; N, 7.40.
Yield: 1.98 g (83%); red viscous oil.
4,5,6,7-Tetrahydro-1-vinylindole-2-carbaldehyde (2b)
From 1-vinylpyrrole 1b (1.47 g, 10 mmol), 2b was obtained.
IR (film, KBr): 3099, 3062, 3039, 2932, 2888, 2840, 1637, 1538,
1508, 1463, 1419, 1372, 1331, 1294, 1193, 1165, 1128, 1092, 1029,
976, 918, 876, 835, 763, 724, 669, 634, 606, 469, 432 cm–1.
Yield: 1.26 g (72%); yellowish oil.
IR (film, KBr): 3120, 3085, 2933, 2850, 2786, 2715, 1658, 1641,
1569, 1477, 1460, 1440, 1424, 1387, 1323, 1288, 1255, 1236, 1212,
1157, 1133, 1110, 1087, 1060, 966, 945, 869, 831, 815, 747, 705,
686, 636, 550, 489 cm–1.
1H NMR (400 MHz, CDCl3): d = 9.40 (s, 1 H, CHO), 7.54 (dd,
3JA–X = 9.1 Hz, 3JB–X = 16.1 Hz, 1 H, HX), 6.70 (s, 1 H, H-3), 5.05
(d, 3JB–X = 16.1 Hz, 1 H, HB), 5.03 (d, 3JA–X = 9.1 Hz, 1 H, HA), 2.67
(t, 3J6–7 = 6.0 Hz, 2 H, H-7), 2.51 (t, 3J4–5 = 6.0 Hz, 2 H, H-4), 1.78
(m, 2 H, H-6), 1.72 (m, 2 H, H-5).
1H NMR (400 MHz, CDCl3): d = 9.69 (s, 1 H, CHO), 7.43 (dd,
3
3JB–X = 15.8 Hz, JA–X = 8.8 Hz, 1 H, HX), 7.38 (m, 2 H, Hm), 7.33
(m, 2 H, Ho), 7.03 (s, 1 H, H-4), 5.04 (d, 3JA–X = 8.8 Hz, 1 H, HA),
4.85 (d, 3JB–X = 15.8 Hz, 1 H, HB), 2.82 (q, 3J = 7.6 Hz, 2 H, CH2),
2.14 (s, 3 H, CH3), 1.40 (t, 3J = 7.6 Hz, 3 H, CH3).
13C NMR (100 MHz, CDCl3): d = 178.8 (C=O), 143.6 (Cp), 139.9
(C-5), 132.2 (C-2), 131.4 (Cb), 130.9 (Co), 130.1 (Ci), 127.8 (Cm),
123.9 (C-3), 120.2 (C-4), 110.6 (Ca), 28.7 (CH2, Et), 15.5 (CH3, Et),
12.2 (CH3, Me).
13C NMR (100 MHz, CDCl3): d = 178.0 (C=O), 139.8 (C-2), 131.3
(Ca), 131.1 (C-7a), 124.5 (C-3), 122.2 (C-3a), 106.4 (Cb), 24.3 (C-
4), 23.2 (C-7), 22.7 (C-5, C-6).
Anal. Calcd for C16H17NO: C, 80.30; H, 7.16; N, 5.85. Found: C,
80.42; H, 7.25; N, 5.93.
5-(2-Naphthyl)-1-vinylpyrrole-2-carbaldehyde (2f)
From 1-vinylpyrrole 1f (2.19 g, 10 mmol), 2f was obtained.
Anal. Calcd for C11H13NO: C, 75.40; H, 7.48; N, 7.99. Found: C,
75.34; H, 7.53; N, 8.01.
Yield: 2.00 g (81%); crimson oily liquid.
5-Phenyl-1-vinylpyrrole-2-carbaldehyde (2c)
From 1-vinylpyrrole 1c (1.69 g, 10 mmol), 2c was obtained.
IR (film, KBr): 3290, 3121, 3059, 2959, 2921, 2835, 2800, 2788,
2725, 1661, 1604, 1539, 1488, 1447, 1424, 1366, 1323, 1317, 1294,
1277, 1250, 1218, 1133, 1096, 1040, 955, 898, 864, 838, 825, 787,
767, 752, 696, 677, 666, 632, 478 cm–1.
Yield: 1.83 g (93%); yellowish oil.
IR (film, KBr): 3121, 3055, 3020, 2987, 2929, 2800, 2727, 1655,
1630, 1593, 1559, 1533, 1492, 1450, 1440, 1428, 1401 1359, 1324,
1285, 1222, 1059, 1032, 951, 919, 888, 865, 830, 746, 680, 660,
606, 537, 462 cm–1.
1H NMR (400 MHz, CDCl3): d = 9.62 (s, 1 H, CHO), 7.40 (m, 5 H,
Ph), 7.35 (dd, 3JB–X = 15.8 Hz, 3JA–X = 8.7 Hz, 1 H, HX), 7.05 (d,
3J3–4 = 3.9 Hz, 1 H, H-3), 6.35 (d, 3J3–4 = 3.9 Hz, 1 H, H-4), 5.08 (d,
3JA–X = 8.7 Hz, 1 H, HA), 4.87 (d, 3JB–X = 15.8 Hz, 1 H, HB).
1H NMR (400 MHz, CDCl3): d = 9.71 (s, 1 H, CHO), 7.99 (s, 1 H,
HAr), 7.87 (m, 3 H, HAr), 7.55 (m, 3 H, HAr), 7.49 (dd, 3JB–X = 15.7
Hz, 3JA–X = 8.6 Hz, 1 H, HX), 7.17 (d, 1 H, J = 4.1 Hz, H-3), 6.55 (d,
3
1 H, J = 4.1 Hz, H-4), 5.11 (d, JA–X = 8.6 Hz, 1 H,
3
HA), 4.90 (d, JB–X = 15.7 Hz, 1 H, HB,).
13C NMR (100 MHz, CDCl3): d = 179.5 (C=O), 142.5 (C-5), 133.3
(C-2), 133.2 (CAr), 132.8 (CAr), 131.5 (Ca), 129.0–126.7 (8C, CAr)
124.6 (C-3), 113.3 (C-4), 112.9 (Cb).
13C NMR (100 MHz, CDCl3): d = 180.0 (C=O), 142.4 (C-2), 133.3
(Ci), 131.0 (C-5), 131.4 (Cp), 130.1 (Cm), 128.1 (Co), 128.3 (C-3),
124.4 (C-4), 112.6 (Ca), 112.4 (Cb).
Anal. Calcd for C17H13NO: C, 82.57; H, 5.30; N, 5.66. Found: C,
82.65; H, 5.43; N, 5.55.
Anal. Calcd for C13H11NO: C, 79.17; H, 5.62; N, 7.10. Found: C,
79.08; H, 5.57; N, 7.21.
5-(2-Thienyl)-1-vinylpyrrole-2-carbaldehyde (2g)
From 1-vinylpyrrole 1g (1.75 g, 10 mmol), 2g was obtained.
1-Vinyl-4,5-dihydrobenzo[g]indole-2-carbaldehyde (2d)
From 1-vinylpyrrole 1d (1.95 g, 10 mmol), 2d was obtained.
Yield: 1.97 g (97%); cherry-colored crystals; mp 35–37 °C (Lit.16
34–35 °C).
Yield: 1.99 g (89%); beige crystals; mp 136–138 °C (Lit.16 133–
135 °C).
IR (KBr): 3100, 3076, 2992, 2841, 2819, 2794, 2727, 1669, 1655,
1640, 1592, 1563, 1511, 1470, 1435, 1415, 1361, 1330, 1313, 1292,
1233, 1199, 1111 1094, 1076, 1038, 1011, 958, 906, 851, 704, 677,
634, 616, 581, 511, 464 cm–1.
IR (KBr): 3095, 3020, 2987, 2957, 2899, 2841, 1643, 1616, 1533,
1511, 1462, 1438, 1417, 1370, 1335, 1289, 1236, 1180, 1155, 1136,
1097, 1044, 1029, 948, 921, 915, 877, 842, 777, 765, 738, 710, 685,
666, 637, 600, 575, 539, 524, 477, 436 cm–1.
1H NMR (400 MHz, CDCl3): d = 9.59 (s, 1 H, CHO), 7.75 (d,
3J6–7 = 8.1 Hz, 1 H, H-6), 7.49 (dd, 3JB–X = 15.7 Hz, 3JA–X = 8.3 Hz,
1 H, HX), 7.27 (d, 3J8–9 = 9.1 Hz, 1 H, H-9), 7.20 (m, 2 H, H-7, H-
8), 6.91 (s, 1 H, H-3), 5.41 (d, 3JA–X = 8.3 Hz, 1 H, HA), 5.38 (d,
3JB–X = 15.7 Hz, 1 H, HB), 2.91 (t, 3J4–5 = 7.2 Hz, 2 H, H-4), 2.69 (t,
3J4–5 = 7.2 Hz, 2 H, H-5).
13C NMR (100 MHz, CDCl3): d = 179.1 (C=O), 138.7 (C-5a), 136.1
(C-9b), 133.8 (C-2), 132.1 (Ca), 128.2 (C-7), 128.0 (C-9a), 127.5
(C-6), 126.5 (C-8), 125.4 (C-3a), 124.2 (C-9), 121.1 (C-3), 113.9
(Cb), 30.5 (C5), 22.2 (C-4).
1H NMR (400 MHz, CDCl3): d = 9.65 (s, 1 H, CHO), 7.41 (dd,
3
3
3J3¢–4¢ = 3.6 Hz, J3¢–5¢ = 1.1 Hz, 1 H, H-3¢), 7.30 (dd, JB–X = 15.7
Hz, 3JA–X = 8.4 Hz, 1 H, HX), 7.22 (dd, 3J3¢–4¢ = 3.6 Hz, 3J4¢–5¢ = 5.1
Hz, 1 H, H-4¢), 7.11 (dd, 3J4¢–5¢ = 5.1 Hz, 3J3¢–5¢ = 1.1 Hz, 1 H, H-5¢),
7.00 (d, 3J3–4 = 4.0 Hz, 1 H, H-3), 6.54 (d, 3J3–4 = 4.0 Hz, 1 H, H-4),
5.39 (d, 3JA–X = 8.4 Hz, 1 H, HA), 5.17 (d, 3JB–X = 15.7 Hz, 1 H, HB).
13C NMR (100 MHz, CDCl3): d = 179.3 (C=O), 135.3 (C-2¢), 133.7
(C-2), 132.3 (C-5), 130.6 (Ca), 128.1 (C-5¢), 127.7 (C-4¢), 127.0 (C-
3¢), 123.4 (C-3), 114.4 (Cb), 112.9 (C-4).
Anal. Calcd for C11H9NOS: C, 65.00; H, 4.46; N, 6.89; S, 15.77.
Found: C, 65.10; H, 4.38; N, 6.98; S, 15.81.
Synthesis 2009, No. 4, 587–590 © Thieme Stuttgart · New York

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A. I. Mikhaleva et al.
PAPER
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Acknowledgment
This work has been carried out under financial support of leading
scientific schools by the President of the Russian Federation (grant
NSH-263.2008.3), of the Russian Foundation of Basic Research
(Project RFBR-NNSF No 06-03-39003) and the National Natural
Science Foundation of China (20711120177, 20573122), and the
Presidium of Russian Academy of Sciences (Project No 8.20).
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