Lewis acid-catalyzed synthesis of functionalized pyrroles

Article abstract of DOI:10.1002/adsc.200800807

The synthesis of highly functionalized pyrroles is described. The sequence involves the preliminary preparation of α-aminohydrazones by Michael addition of primary amines to 1,2-diaza-1,3-butadienes. The treatment of these compounds with dialkyl acetylene

Full text of DOI:10.1002/adsc.200800807

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DOI: 10.1002/adsc.200800807
Lewis Acid-Catalyzed Synthesis of Functionalized Pyrroles
Orazio A. Attanasi,a Stefano Berretta,a Lucia De Crescentini,a Gianfranco Favi,a
Gianluca Giorgi,b and Fabio Mantellinia,*
a
Istituto di Chimica Organica, Universitꢀ degli Studi di Urbino “Carlo Bo”, Via I Maggetti 24, 61029 Urbino (PU), Italy
Fax : (+39)-0722-303441; e-mail: fabio.mantellini@uniurb.it
Dipartimento di Chimica, Universitꢀ degli Studi di Siena, Via Aldo Moro, 53100 Siena, Italy
b
Received: December 23, 2008; Revised: March 2, 2009; Published online: March 17, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800807.
Abstract: The synthesis of highly functionalized
pyrroles is described. The sequence involves the
preliminary preparation of a-aminohydrazones by
Michael addition of primary amines to 1,2-diaza-
1,3-butadienes. The treatment of these compounds
with dialkyl acetylenedicarboxylates produces a-(N-
enamino)-hydrazones that were converted into the
corresponding pyrroles by Lewis acid-catalyzed ring
closure. A screening of several Lewis/Brønsted acid
catalysts was also performed.
rather than carbonyl compounds.[10] In our methodol-
ogy, the a-amino ketones are replaced with a-amino-
hydrazones. Their easy and flexible preparation in-
volves the 1,2-diaza-1,3-butadienes 1a–d that readily
react with different primary amines 2a, b in tetrahy-
drofuran at room temperature in the case of 1a–c, or
under reflux for 1d producing the desired a-aminohy-
drazones 3a–f. The reaction takes place by means of
1,4-hydroamination (Michael-type) of the amino de-
rivatives 2a, b to the azo-ene system of the 1,2-diaza-
1,3-butadienes 1a–d. (Scheme 1, Table 1).[8b,11]
Keywords: alkynes; 1,2-diaza-1,3-butadienes; hydro-
amination; Lewis acids; Michael addition; pyrroles
It is noteworthy that a-aminohydrazones are solid
compounds and are appreciably more stable to stor-
age and handling than a-amino ketones. In fact, no
self-condensation of compounds 3 was observed.
In turn, the a-aminohydrazones 3a–f reacted with
dialkyl acetylenedicarboxylates 4a–c in ethanol under
Pyrroles are among the most studied heterocyclic ring reflux to give a-(N-enamino)-hydrazones 5a–i in 2–
systems due to their diverse biological activities and
applications in materials science.[1,2] As a conse-
quence, much attention has been paid to their prepa-
ration by classical methods such as the Knorr,[3]
Hantzsch,[4] and Paal–Knorr[5] syntheses. However,
these approaches usually present significant limita-
tions in terms of substituents that can be introduced,
the substitution pattern, or regioselectivity. Several
recent variations in the formation of pyrrole rings are
based on metal-catalyzed reactions[6] and catalytic
multicomponent coupling methodologies[7] which can
improve usefully the classical synthetic approaches.
We have demonstrated that the reactions between
1,2-diaza-1,3-butadienes and carbonyl compounds[8] or
enol silyl derivatives[9] represent useful and conven-
ient entries to 1-aminopyrroles. Here, we report a
new and flexible Knorr-related strategy for the con-
struction of amply functionalized pyrroles. The typical
Knorr approach utilizes a-amino ketones and carbon-
yl derivatives containing an activated methylene
Scheme 1. Synthesis of the a-aminohydrazones 3a–f, and a-
(N-enamino)-hydrazones 5a–i. i: THF, room temperature
group as starting materials.[3] A variation of this syn-
thesis provides for the use of alkynes as reagents for 1a–c; THF, reflux for 1d. ii: EtOH, reflux.
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Orazio A. Attanasi et al.
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Yield [%][d] Time [h]
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Table 1. Yields of the a-aminohydrazones 3a–f, a-(N-enamino)-hydrazones 5a–i and pyrroles 8a–f.
1
R1 R2
2
R3
3
Yield [%][a]
4
R4
5
E isomer
Z isomer
Yield [%][b,c] Yield [%][b,c]
1a Me
1a Me
1b Me
1b Me
1b Me
1b Me
1b Me
H
H
2a
2b
2a
2a
2a
2b
2b
3a 83
3b 91
3c 97
3c 97
3c 97
3d 81
3d 81
4a Et
4a Et
4a Et
5a 75
5b 55
5c 76
0
8a 87
8b 90
8a 91
8c 86
8d 88
8e 93
8b 89
0.5
0.5
0.6
0.5
0.6
0.6
0.5
19
0
4b Me 5d 74
4c t-Bu 5e 66
4b Me 5f 68
0
0
25
14
4a Et
5g 80
1c Me
2a
2b
3e 74
3f 69
4b Me 5h 56
4b Me 5i 57
0
8c 93
8f 92
0.5
0.6
1d Et
H
28
[a]
Yield of the isolated purified compounds 3a–f based on the 1,2-diaza-1,3-butadienes 1a–d.
Yield of the isolated purified compounds (E/Z) 5a–i based on the hydrazones 3a–f.
[b]
[c]
The E and Z geometry was assigned on the basis of the chemical shifts of the vinylic proton and of the vicinal heteronu-
1
clear coupling constant between the esteric carbon and the vinylic H nucleus.
[d]
Yield of the isolated purified compounds 8a–f based on a-(N-enamino)-hydrazones 5a–i.
4 h as E/Z mixtures in good yields (Scheme 1, The compound 5d was chosen to test the catalytic ac-
Table 1). The C=C bond geometry of compounds 5 tivity by different types of Lewis/Brønsted acids in
was determined by NMR measurements, considering different solvents and temperatures (see Supporting
the vicinal heteronuclear coupling constant between Information). Among the different types of the tested
the ester carbon and the vinylic proton. For the com- acids, only the Lewis acids exhibited remarkable cata-
pound 5f, chosen as a representative example, the E lytic activity, and this occurrence is probably due to
isomer was predominant (68%) showing a coupling
constant of 9.3 Hz, while for the Z isomer (25%) the
coupling constant value was 3.2 Hz, in good agree-
ment with data reported in the literature.[12] Also the
chemical shift of the vinylic proton can be diagnostic:
4.93 ppm for the (Z)-5f versus 5.20 ppm for the (E)-5f
in complete agreement with the NMR data reported
by Dolfini.[13] The same behaviour was observed for
all compounds 5b, g, i derived from the a-benzylami-
nohydrazones 3b, d, f. The X-ray diffraction study of
(E)-5f unambiguously supports the assigned structure.
The compound 5f crystallizes as an N-methylpyrroli-
dine solvate (Figure 1).[14] In the case of the a-(N-en-
amino)-hydrazones 5a, c–e, h, derived from the a-cy-
clohexylaminohydrazones 3a, c, e, the formation of
only one isomer was observed. The heteronuclear
coupling constant values between the ester carbon
and the vinylic proton, that were in the range 10.4–
12.0 Hz, confirmed the E configuration for these com-
pounds.[12]
We then explored the conversion of the a-(N-enam-
ino)-hydrazones 5a–i into the corresponding pyrroles
Figure 1. X-ray structure of compound (E)-5f·C5H11N. Ellip-
soids for non-hydrogen atoms enclose 50% probability. N-
8a–f by acid-catalyzed intramolecular ring closure. Methylpyrrolidine has been omitted for clarity.
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Lewis Acid-Catalyzed Synthesis of Functionalized Pyrroles
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Scheme 3. Different approaches for the construction of pyr-
roles starting from 1,2-diaza-1,3-butadienes: Route A with
primary amines and acetylenedicarboxylates. Route B with
carbonyl compounds. Route C with enol silyl derivatives.
Scheme 2. Synthesis of the pyrroles 8a–f. i: CH2Cl2, reflux,
ZnACHTUNGTRENNUNG(TfO)2.
permits an additional diversification site that can be
easily introduced using a variety of amines.
the chelation effect of the N-carbonylhydrazones to-
In conclusion, this paper describes a smooth proce-
wards metal catalysts (Scheme 2).[15] In particular, dure to prepare a-(N-enamino)-hydrazones and then
ZnCl2, Zn
(TfO)3 have furnished the best results in terms of use of 1,2-diaza-1,3-butadienes as building blocks in
yields (75–95%). Our choice of Zn(TfO)2 was based the modelling of azaheterocycles is the stability and
ACHTUNGTRENNUNG(TfO)2, InCl3, InBr3, InCAHTUNGTREN(NUNG TfO)3 and Yb- amply functionalized pyrroles. The advantage of the
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
on the lower reaction time (0.5 h) together with lower accessibility of the starting materials, as well as the
cost with respect to other tested catalysts. The choice mild conditions of the experimental procedures. Fur-
of the solvent and temperature also played a crucial ther investigations are presently in progress in order
role. We have found that the use of dichloromethane to improve the utility and application of this synthetic
under reflux is determinant in the Lewis acid-cata- methodology.
lyzed ring closure. Thus, the use of these optimal con-
ditions was extended to the conversion of the a-(N-
enamino)-hydrazones 5a–i into the corresponding pyr-
roles 8a–f. The reaction proceeds by means of the in-
Experimental Section
tramolecular nucleophilic attack of the ene-amino
carbon to the hydrazone moiety of the intermediate
6, activated by coordination with the Lewis acid. This
process produces the 4,5-dihydro-1H-pyrrole inter-
mediates 7 that furnish the final aromatic 5-amidopyr-
roles 8, by elimination of the hydrazino moiety
(Scheme 2, Table 1). In this particular case, moreover,
the Knorr equivalent starting a-amino-a-amido ke-
tones are both not commercially available products
and difficult to prepare by other methods.
Some differences in the construction of the pyrrole
skeleton can be observed with respect to our previous
synthesis. In fact, starting from 1,2-diaza-1,3-buta-
dienes and carbonyl compounds[8] (Route B,
Scheme 3) or enol silyl derivatives[9] (Route C,
Scheme 3), we obtained N-substituted-aminopyrroles
in which the pyrrole nitrogen is originally situated in
the position two of the azo-ene system, while in this
case the pyrrole nitrogen derives from the substituted
General Procedure for the Synthesis of a-
Aminohydrazones 3a–f
To a solution of the 1,2-diaza-1,3-butadiene 1a–d as a mix-
ture of E/Z isomers (1.0 mmol) in tetrahydrofuran (10 mL),
the cyclohexylamine 2a or the benzylamine 2b (1 mmol) was
added. The reaction was allowed to proceed under magnetic
stirring at room temperature for 2–4 h in the case of 1a–c,
or under reflux for 4 h in the case of 1d, until the disappear-
ance of the reagents (monitored by TLC). The solvent was
then evaporated under reduced pressure. The products 3a–f
were purified by silica gel chromatography (elution mixture:
ethyl acetate/cyclohexane) and they were crystallized from
ethyl acetate (see Supporting Information).
3-[(Aminocarbonyl)hydrazono]-2-(cyclohexylamino)-N,N-
dimethylbutanamide (3a): mp 124–1268C; 1H NMR
(400 MHz, CDCl3, 258C): d=0.77–0.94 (m, 6H), 1.23–1.29
(m, 1H), 1.35–1.41 (m, 2H), 1.46–1.52 (m, 1H), 1.54 (s, 3H),
1.93–1.99 (m, 1H), 2.65 (s, 3H), 2.72 (s, 3H), 2.83 (brs, 1H),
3.99 (s, 1H), 5.86 and 6.21 (2 brs, 2H), 9.13 (s, 1H);
13C NMR (100 MHz, CDCl3, 258C): d=12.4 (q), 24.7 (t),
amino reagents 2a, b (Route A, Scheme 3). This fact 24.8 (t), 26.0 (t), 33.1 (t), 33.4 (t), 36.0 (q), 36.9 (q), 54.7 (d),
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Orazio A. Attanasi et al.
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61.8 (d), 147.9 (s), 159.0 (s), 170.6 (s) ppm; IR (nujol): nmax
=
(71), 233 (100), 188 (67), 159 (100); anal. calcd. for
C20H30N2O5 (378.46): C 63.47, H 7.99, N 7.40; found: C
63.29, H 8.04, N 7.51.
3422, 3374, 3265, 3204, 1776, 1732, 1676 cmÀ1; MS: m/z
(%)=283 (M+) (16), 238 (74), 211 (100), 167 (31), 152 (11);
anal calcd. for C13H25N5O2 (283.37): C 55.10, H 8.89, N
24.71; found: C 55.21, H 8.94, N 24.59.
Supporting Information
Experimental details and spectroscopic characterization of
all compounds are given in the Supporting Information.
General Procedure for the Synthesis a-(N-Enamino)-
hydrazones 5a–i
To a solution of the a-aminohydrazones 3a–f (1 mmol) in
ethanol (20 mL) a stoichiometric amount of the dialkyl ace-
tylenedicarboxylates 4a–c (1 mmol) was added. The reaction
mixture was refluxed for 2–4 h until the complete disappear-
ance of the reagents (monitored by TLC). The solvent was
then evaporated under reduced pressure. The products 5a–i
were purified by silica gel chromatography (elution mixture:
ethyl acetate/cyclohexane) and they were crystallized from
ethyl acetate. In the case of the a-(N-enamino)-hydrazones
5b, f, g, i derived from the benzylamine 2b, it was possible
to separate the E and Z isomers, while for 5a, c–e, h, de-
rived from the cyclohexylamine 2a, we have observed the
exclusive formation of the E isomer (see Supporting Infor-
mation).
Diethyl (2E)-2-({2-[(aminocarbonyl)hydrazono]-1-[(dime-
thylamino)carbonyl]propyl}(cyclohexyl)amino)but-2-ene-
dioate (5a): mp 161–1638C; 1H NMR (400 MHz, CDCl3,
258C): d=0.97–1.23 (m, 9H), 1.28 (t, J=7.2 Hz, 3H), 1.47–
1.54 (m, 2H), 1.71–1.81 (m, 2H), 1.90 (s, 3H), 2.98 (s, 3H),
2.93 (s, 3H), 3.23–3.29 (m, 1H), 3.89–4.06 (m, 2H), 4.18–
4.32 (m, 2H), 4.77 (s, 1H), 5.13 (s, 1H), 5.58 and 5.86 (2 brs,
2H), 9.18 (s, 1H); 13C NMR (100 MHz, CDCl3, 258C): d=
13.8 (q), 14.2 (q), 25.4 (t), 25.8 (t), 26.0 (t), 30.9 (t), 31.1 (t),
35.9 (t), 37.0 (t), 59.1 (t), 61.4 (d), 61.8 (t), 64.0 (d), 91.2 (d),
144.0 (s), 153.1 (s), 157.8 (s), 166.0 (s), 167.4 (s), 167.8 (s);
IR (nujol): nmax =3424, 3200, 1741, 1727, 1704, 1685,
1658 cmÀ1; MS: m/z (%)=453 (M+ +1) (1), 408 (3), 378
(48), 349 (11), 334 (41), 306 (8), 262 (100), 239 (22), 193
(53); anal. calcd. for C21H35N5O6 (453.53): C 55.61, H 7.78, N
15.44; found: C 55.72, H 7.91, N 15.51.
Acknowledgements
The authors thank the Ministero dell’Universitꢀ, dell’Istru-
zione e della Ricerca (MIUR) – Roma and the Universitꢀ
degli Studi di Urbino “Carlo Bo” for support of this work.
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General Procedure for the Synthesis of Pyrroles 8a–f
To
(1.0 mmol) in dichloromethane (15 mL), a catalytic amount
of Zn(OTf)2 (0.2 mmol) was added. The reaction mixture
a solution of the a-(N-enamino)-hydrazones 5a–i
ACHTUNGTRENNUNG
was refluxed for 0.5–0.6 h, until the disappearance of the
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1-cyclohexyl-5-[(dimethylamino)carbonyl]-4-
methyl-1H-pyrrole-2,3-dicarboxylate (8a): mp 143–1458C;
1H NMR (400 MHz, CDCl3, 258C): d=1.12–1.26 (m, 4H),
1.29 (t, J=7.2 Hz, 3H), 1.35 (t, J=7.2 Hz, 3H), 1.58–1.65
(m, 1H), 1.73–1.82 (m, 3H), 1.89–1.96 (m, 2H), 2.07 (s, 3H),
2.94 (s, 3H), 3.10 (s, 3H), 4.06–4.15 (m, 1H), 4.23 (q, J=
7.2 Hz, 2H), 4.32 (q, J=7.2 Hz, 2H); 13C NMR (100 MHz,
CDCl3, 258C): d=10.2 (q), 13.9 (q), 14.2 (q), 24.9 (t), 26.0
(t), 26.3 (t), 31.3 (t), 33.2 (t), 34.7 (q), 38.1 (q), 59.8 (d), 60.1
(t), 61.6 (t), 116.1 (s), 118.8 (s), 126.7 (s), 127.7 (s), 162.9 (s),
164.4 (s), 164.5 (s) ppm; IR (nujol): nmax =1745, 1730,
1640 cmÀ1; MS: m/z (%)=378 (M+) (45), 306 (100), 260
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Lewis Acid-Catalyzed Synthesis of Functionalized Pyrroles
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