Substituted pyrrole synthesis from nitrodienes

Article abstract of DOI:10.1016/j.tetlet.2017.04.077

Though the Cadogan-Sundberg approach has been employed to synthesize a variety of indole and carbazole derivatives from nitroarenes, surprisingly, very little is reported for making pyrroles using the same approach from non-arene nitrodienes. Herein, we report a general method to synthesize substituted pyrroles, in one step with modest yields, from nitrodienes using triphenylphosphine in the presence of an Mo catalyst, bis(acetylaceto)dioxomolybdenum (VI). To shed light on the mechanism of this reaction, we performed DFT calculations using uB3LYP/6-31+G(d) basis set and observed that the reaction favors a path through a nitrene intermediate.

Full text of DOI:10.1016/j.tetlet.2017.04.077

Accepted Manuscript
Substituted pyrrole synthesis from nitrodienes
Sasan Karimi, Shuai Ma, Yanan Liu, Keith Ramig, Edyta M. Greer, Kitae Kwon,
William F. Berkowitz, Gopal Subramaniam
PII:
S0040-4039(17)30531-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.04.077
TETL 48869
Reference:
Tetrahedron Letters
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Received Date:
Revised Date:
Accepted Date:
25 January 2017
19 April 2017
24 April 2017
Please cite this article as: Karimi, S., Ma, S., Liu, Y., Ramig, K., Greer, E.M., Kwon, K., Berkowitz, W.F.,
Subramaniam, G., Substituted pyrrole synthesis from nitrodienes, Tetrahedron Letters (2017), doi: http://dx.doi.org/
10.1016/j.tetlet.2017.04.077
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Substituted pyrrole synthesis from nitrodienes
Sasan Karimi,a Shuai Ma,a Yanan Liu,a Keith Ramig,b Edyta M. Greer,b Kitae
Kwon,b William F. Berkowitz,c Gopal Subramaniam c
a Department of Chemistry, Queensborough Community College of the City University of New York, 222-05 56th Ave., Bayside,
NY 11364, USA
b Department of Natural Sciences, Baruch College of the City University of New York, 17 Lexington Ave., New York, NY 10010,
USA
c Department of Chemistry and Biochemistry, Queens College of the City University of New York, 65-30 Kissena Blvd., Flushing,
NY 11367, USA
Abstract: Though the Cadogan-Sundberg approach has been employed to synthesize a variety of
indole and carbazole derivatives from nitroarenes, surprisingly, very little is reported for making
pyrroles using the same approach from non-arene nitrodienes. Herein, we report a general
method to synthesize substituted pyrroles, in one step with modest yields, from nitrodienes using
triphenylphosphine in the presence of an Mo catalyst, bis(acetylaceto)dioxomolybdenum (VI).
To shed light on the mechanism of this reaction, we performed DFT calculations using
uB3LYP/6-31+G(d) basis set and observed that the reaction favors a path through a nitrene
intermediate.
Key words: Cadogan-Sundberg, nitrodienes, Mo catalyst, nitrene intermediate, DFT calculations
Introduction
The importance of pyrrole, a simple heterocycle, in medicinal and pharmaceutical
chemistry is well documented.1 The pyrrole skeleton is found in many natural products such as
chlorophyll, hemoglobin, and indigo. Some of its substituted derivatives have biological
activities and a trisubstituted derivative is a biosynthetic precursor to many natural products such
as heme.2 Pyrrole is also found in some marine natural products such as nakamuric acid3 and
chiral marinopyrroles which show activity against Staphylococcus aureus strains.4 It is
noteworthy that some of the derivatives of pyrrole are intermediates to many heterocyclic
compounds such as chiral indolizidines.5
1

The Cadogan-Sundberg synthesis of indoles and carbazoles involves
deoxygenation/cyclization of o-nitrostyrenes or o-nitrobiphenyls with a trialkyl phosphite.6 For
example, o-nitrostyrene is converted to indole (Scheme 1a),6a and o-nitrobiphenyl is converted to
carbazole (Scheme 1b),6c using a reducing agent which produces a nitrene and/or a nitroso
intermediate. A recent study on the mechanism of the Cadogan-Sundberg synthesis of indole
from a nitroarene illustrates the presence of a nitroso intermediate through a labeling
experiment.6d In a typical experiment, one dissolves the nitro compound in triethylphosphite
and refluxes the mixture until the starting material is consumed, usually within 2-24 hrs. This
method of indole synthesis is similar to that of the Leimgruber-Batcho approach except there is
no need to prepare a dinitrostyrene before reduction, thus avoiding an extra step.7 Reductive
cyclization of o-nitrostyrenes to indoles has also been observed with many different reducing
agents.8
R
P(OEt)3
R
a)
b)
150 oC
N
H
NO2
R
R
P(OEt)3
150 oC
N
H
NO2
Scheme 1. Synthesis of indoles and carbazoles using the Cadogan-Sundberg reaction
Another approach for making indoles and carbazoles involves reduction of nitroarenes
with carbon monoxide in the presence of a metal catalyst (e.g. Pd(OAc)2).9 Of the two likely
mechanisms of this reaction, one involving a nitroso derivative and the other a nitrene, the
nitrene mechanism appears to be favored.10
Thermal decomposition of an organic azide has also been employed to synthesize indoles
(from o- or β-azidostyrenes, as in Scheme 2a or Scheme 2b), carbazoles, and in particular,
pyrroles from azido-dienes (Scheme 2c).11 As noted, many pyrroles have been made from the
2

appropriate azides, but the corresponding reactions of structurally similar nitro compounds have
not been shown to produce simple pyrroles from nitrobutadienes, even though they are readily
made! One must note that organic azides are potential explosives,12 nitrodienes are safer to use.
Scheme 2. Synthesis of indoles and pyrrole from azido-dienes
Results and Discussion
3

Although the Cadogan-Sundberg approach has been extensively used to synthesize
indoles and carbazoles, to date, there is only one example found in the literature which uses this
approach to convert a simple non-arene nitrodiene to a pyrrole derivative (Scheme 3a).13
Therefore, we sought a general method to synthesize a series of pyrrole derivatives that would
add to the lacking literature. With the traditionally used triethylphosphite as the reducing agent,
our yield was unacceptably low (10-12%) for making the pyrrole derivatives from the
corresponding nitrodienes. However, with triphenylphosphine in the presence of an Mo catalyst,
bis-(acetylacetonato)dioxo-molybdenum (VI),14 we were able to synthesize a number of known
pyrrole derivatives from the corresponding nitrodienes. The nitrodiene precursors leading to
compounds 1-9 were synthesized using the aldol condensation of an aldehyde with nitromethane,
nitroethane, and nitropropane.15 The nitrodiene 10 (mixture of E and Z), leading to pyrrole 8,
was the only unknown diene synthesized (29%) using the above procedure. The yields for the
cyclization reactions for compounds 1,16 2,17 3,18 4,19 5,20 6,21 7,22 8,23 and 924 are listed in
Scheme 3. To observe if the substituents on the aromatic ring have an effect on the yield of the
reaction, we examined the effect of electron donating (CH3, OCH3) versus the electron
withdrawing (NO2) groups in the p-position. The yield of the cyclization reaction remained
primarily unaffected. The low yield in 5, however, is not surprising since the catalyst can also
reduce the p-nitro group. It is noteworthy that the yields for the 2-ethyl substituted pyrroles (7-
9) were actually higher compared to the 2-methyl substituted (2 and 3) or the unsubstituted
derivatives at position 2 (1, 4, 5, and 6).
4

Scheme 3. Cyclization reactions of nitrodienes
The use of dichlorodioxomolybdenum (VI) catalyst for the oxidation of thiols to disulfides with
dimethylsulfoxide has been reported.25 Based on this report, the authors extended their methodology to
include synthesis of indoles and carbazole using the same catalyst.14 They proposed that the mechanism
of the cyclization of nitroaromatic compounds to the corresponding indoles and carbazoles involves a
rapid oxidation of triphenylphosphine to triphenylphosphine oxide and reduction of Mo (VI) to Mo (IV).
Subsequently, the Mo (IV) species reacts with the remaining Mo (VI) to form a dinuclear Mo (V)
complex.26 They further postulate that either the mononuclear Mo (IV) or the dinuclear Mo (V) can
convert the nitroaromatic compound to the corresponding nitroso and then to the nitrene intermediate
which then undergoes C-H insertion to afford indoles or carbazoles. Similarly, we suggest that our
nitrodiene 11 undergoes reduction first to the corresponding nitroso-diene intermediate 12 (Scheme 4).
Repeat of this step will convert 12 to the nitrene-diene intermediate 13. C-H insertion in 13 produces 14
followed by 1,5-H shift to 1 (Path A). Alternatively, intermediate 12 can cyclize to give 15 (Path B).
5

Aromatization of 15 will produce the pyrrole-1-ol 16 which gets further reduced to 1. We performed
calculations to shed light on the mechanisms for the cyclization reactions via both the nitroso 12 and
nitrene 13 intermediates, vide infra.
Scheme 4. Possible reaction paths for the cyclization of nitrodiene to pyrrole
Computational Methods. Gaussian 09 package27 was employed to perform all the calculations. The
calculations for optimizing the geometries were performed at the uB3LYP28 functional and 6-31+G(d)29
level of theory using broken symmetry wave function using the command Guess=(mix,always), and extra
quadratic convergence for self-consistent field method (scf=xqc). The uB3LYP/6-31+G(d) level of theory
has been used to study nitrenes and nitroso species alike.30 The wavefunction of singlet nitrene was found
to be unstable, and stability optimization calculation was performed (stable=opt). The energies from the
stabilized wavefunction were obtained and used to calculate the energetics of the nitrene pathway (Path
A, Scheme 4). The natures of all minima and first-order saddle points were verified by harmonic
vibrational analyses. All first-order saddle points were confirmed via intrinsic reaction coordinates
calculations.31
Computational Results. The energetics of Path A and Path B (Scheme 4) were computed at the
uB3LYP/6-31+G(d) level of theory to determine which pathway is more favorable for the formation of
pyrrole. The results are summarized in Figure 1. In Path A, the first step is the ring closure between N and
the benzylic carbon to form 14. The energy barrier (∆G‡) was calculated to be 2.7 kcal/mol, and the ∆G
for the ring closure was calculated to be -42.1 kcal/mol. The [1,5]H-shift from 14 to 1 has a ∆G‡ = -13.6
kcal/mol with respect to the 13, or 28.5 kcal/mol from 14, and the overall ∆G= -60.7 kcal/mol.
6

The alternative Path B, involving the ring closure of the nitroso intermediate 12 to 15, requires
∆G‡ = 10.3 kcal/mol, and the ∆G = -9.9 kcal/mol. Intramolecular proton transfer followed by
aromatization of 15 to 16 required ∆G‡ = 43.8 kcal/mol with respect to the nitroso intermediate 12 (or
53.7 kcal/mol with respect to 15), and the overall ∆G = -19.8 kcal/mol.
Path A
• G‡ = 2.7 kcal/mol
TS1
13
• G‡ = -13.6 kcal/mol
TS2
14
• G = -42.1 kcal/mol
1
• G = -60.7 kcal/mol
Path B
• G‡ = 43.8 kcal/mol
TS4
• G‡ = 10.3 kcal/mol
TS3
12
15
16
• G = -19.8 kcal/mol
• G = -9.9 kcal/mol
7

Figure 1. uB3LYP/6-31+G(d) energetics of cyclizations involving intermediates 12 and 13.
Conclusion
We have shown that the Cadogan-Sundberg approach towards the synthesis of indoles and
carbazoles can be applied to synthesize pyrroles. Though the reaction works with both P(OEt)3 and PPh3
in poor yields (12-14%), the yield improves considerably (50-60%) in the presence of the Mo catalyst.
Based on the computational results, the pathway involving the nitrene intermediate is found to be more
energetically favored compared to the nitroso intermediate, consistent with the widely accepted nitrene
path leading to the indole and carbazole derivatives.
Typical experimental procedure
In a typical experiment, Mo catalyst (5 mol%, 0.014 mmol), bis(acetylaceto)dioxomolybdenum (VI), was
added to a solution of nitrodiene (0.28 mmol) and Ph3P (0.67 mmol) in toluene (3 mL). The mixture was
refluxed (1.5-2.5 hrs) until the starting material was consumed as indicated by TLC. After completion of
the reaction, solvent was removed. The residue was chromatographed on silica gel using hexanes first to
remove unreacted PPh3 and later by 1% EtOAc/hexanes to obtain the substituted pyrrole product. All
pyrrole compounds gave satisfactory spectroscopic and analytical data as compared to the literature.
Compound 10: ((1E,3E)-2-methyl-4-nitrohexa-1,3-dien-1-yl)benzene and ((1E,3Z)-2-methyl-4-
nitrohexa-1,3-dien-1-yl)benzene. To a mixture of commercially available (Aldrich) α-methyl-trans-
cinnamaldehyde (3.2063 g, 22 mmol) and nitropropane (30mL) was added ammonium acetate (0.6 g).
The solution was heated to 110 oC for 3hr. Evaporation of excess nitropropane followed by purification of
the crude on silica gel (2% EtOAc/hexane) gave 1.3793 g (29%) of compound 10 as an oil as a mixture of
E and the Z isomers.
1H NMR: (CDCl3, 400 MHz) δ isomer 1: 0.95 (3H, t, J = 7.4 Hz), 2.08 (3H,s), 2.81 (2H, q, =
7.4 Hz), 6.65 (1H, s), 7.13 (2H, d, J = 8.0 Hz), 7.21 (1H, t, J = 7.2 Hz), 7.24 (2H, t, J =7.2 Hz),
7.70 (1H, s) & isomer 2: 1.14 (3H, t, J = 7.4 Hz), 2.08 (3H,s), 2.59 (2H, q, = 7.4 Hz), 6.76 (1H,
s), 7.25 (1H, t, J = 7.2 Hz), 7.26 (2H, d, J = 7.2 Hz), 7.32 (2H, t, J =7.2 Hz), 7.61 (1H, s)
8

13C NMR: (CDCl3, 100 MHz) δ isomer 1: 12.4, 21.2, 23.5, 128.0, 128.49 (2C), 129.2 (2C),
129.7 (q), 133.1, 136.4 (q). 136.5, 153.3 (q) & isomer 2: 13.4, 17.4, 21.1, 128.1, 128.47 (2C),
129.5 (2C), 131.3 (q), 136.2 (q), 138.0, 138.9, 152.1 (q)
ESI-HRMS (m/z) for C13H15NO2 (exact mass 217.1103): [M+H]+ calc. 218.1176, obs. 218.1176.
Acknowledgement
The Professional Staff Congress of the City University of New York is acknowledged for financial
support of this work. E. M. G. and K. K. acknowledge support from the donors of the Petroleum
Research Fund of the American Chemical Society, Grant No. 54244-UR4. Computational support was
provided by the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported
by the National Science Foundation Grant No. ACI01053575.
References and notes
9

• A general method to make pyrroles from nitrodienes, in one step, is reported.
• The yield of cyclization is increased with the use of Mo catalyst.
•
DFT calculations supports a reaction path through a nitrene intermediate.
10

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