Intramolecular cyclization of 1-(ω-Phenylalkyl)-2-(nitromethylene) pyrrolidines in triflic acid

Article abstract of DOI:10.1080/00397911.2014.898307

1-(ω-Phenylalkyl)-2-(nitromethylene)pyrrolidines in triflic acid undergo a C,O-diprotonation, followed by loss of water, to form conjugated iminium-hydroxynitrilium dications, which react with the tethered phenyl ring by electrophilic aromatic substitution to afford tricyclic iminium compounds as triflate salts. The scope and mechanism of this reaction are discussed. Copyright

Full text of DOI:10.1080/00397911.2014.898307

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Intramolecular Cyclization
of 1-(ω-Phenylalkyl)-2-
(nitromethylene)pyrrolidines in Triflic
Acid
Bamba Fanté a , Yaya Soro b , Sorho Siaka b , Jérôme Marrot c &
Jean-Marie Coustard d
a Laboratoire de Chimie Organique Structurale , Université Félix
Houphouët-Boigny , Abidjan , Côte d'Ivoire
b Laboratoire des Procédés Industriels de Synthèse, de
l'Environnement et des Énergies Nouvelles, Institut National
Polytechnique Félix Houphouët-Boigny , Yamoussoukro , Côte
d'Ivoire
c Laboratoire ILV-UVSQ-UMR 8180 CNRS , Versailles , France
d Laboratoire IC2MP-UMR 7285 CNRS , Poitiers , France
Accepted author version posted online: 15 Apr 2014.Published
online: 13 Jun 2014.
To cite this article: Bamba Fanté , Yaya Soro , Sorho Siaka , Jérôme Marrot & Jean-Marie Coustard
(2014) Intramolecular Cyclization of 1-(ω-Phenylalkyl)-2-(nitromethylene)pyrrolidines in Triflic Acid,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 44:16, 2377-2385, DOI: 10.1080/00397911.2014.898307
To link to this article: http://dx.doi.org/10.1080/00397911.2014.898307
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Synthetic Communications1, 44: 2377–2385, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.898307
INTRAMOLECULAR CYCLIZATION OF
1-(x-PHENYLALKYL)-2-(NITROMETHYLENE)
PYRROLIDINES IN TRIFLIC ACID
1
Bamba Fante, Yaya Soro, Sorho Siaka, Jerome Marrot,
2
2
3
´
´ ˆ
and Jean-Marie Coustard4
1
´
Laboratoire de Chimie Organique Structurale, Universite Felix
´
ˆ
Houphoue¨t-Boigny, Abidjan, Cote d’Ivoire
2
´ ´
Laboratoire des Procedes Industriels de Synthese, de l’Environnement et des
Energies Nouvelles, Institut National Polytechnique Felix Houphoue¨t-Boigny,
`
´
´
ˆ
Yamoussoukro, Cote d’Ivoire
3Laboratoire ILV-UVSQ-UMR 8180 CNRS, Versailles, France
4Laboratoire IC2MP-UMR 7285 CNRS, Poitiers, France
GRAPHICAL ABSTRACT
Abstract 1-(x-Phenylalkyl)-2-(nitromethylene)pyrrolidines in triflic acid undergo
a
C,O-diprotonation, followed by loss of water, to form conjugated iminium–hydroxynitrilium
dications, which react with the tethered phenyl ring by electrophilic aromatic substitution to
afford tricyclic iminium compounds as triflate salts. The scope and mechanism of this reac-
tion are discussed.
Keywords 1-(x-Phenylalkyl)-2-(nitromethylene)pyrrolidines; cation; intramolecular
cyclization; triflate salts; triflic acid
INTRODUCTION
x-Phenylalkylaminonitroethene derivatives are useful synthons in the field of
heterocyclic synthesis,[1] as they readily undergo a variety of condensation reac-
tions.[2,3] Several compounds having a nitroenamine unit (Fig. 1) can be found on
the market as drugs (ranitidine) or pesticides (nitenpyrame).[4]
Received November 6, 2013.
´
Address correspondence to Bamba Fante, Laboratoire de Chimie Organique Structurale,
Universite´ Fe´lix Houphoue¨t-Boigny, 22 BP 582 Abidjan 22, Coˆte d’Ivoire. E-mail: Fante_bamba1@
yahoo.fr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
2377

´
B. FANTE ET AL.
2378
Figure 1. Some nitroenamine derivatives.
One of the most common methods of preparing nitroenamines is by the
reaction of amide acetals with nitromethane. Thus dimethylformamide diethylacetal
gives 1-dimethylamino-2-nitroethylene.[5] A variety of substituted amide and lactam
acetals have been successfully used in this reaction (Scheme 1).[6] A simplified
procedure, in some cases, makes use of the amide–dimethylsulfate complex.
It has been shown that 1-methyl-2-(nitromethylene)pyrrolidine in triflic acid
formed a hydroxynitrilium ion, which can be trapped in situ with either triflate
anion or benzene.[7] Following our work on the reactivity of various chemicals in
superacids, the current study describes the reactivity of 1-(x-phenylalkyl)-2-
(nitromethylene)pyrrolidines in triflic acid.
RESULTS AND DISCUSSION
Starting Material
1-(x-Phenylalkyl)pyrrolidin-2-one derivatives 1a–d were prepared in a conven-
tional way by alkylation[8] of pyrrolidin-2-one with the appropriate 1-bromo-x-phe-
nylalcane derivatives. Yields are reported in Scheme 2 without optimization.
Further treatment with dimethylsulfate followed by sodium ethoxide led to the
corresponding 1-x-phenylalkyl-2,2-diethoxypyrrolidines 2,[9] which upon reaction
with nitromethane[6] afforded the expected 1-(x-phenylalkyl)-2-(nitromethylene)
pyrrolidine derivatives 3a–d (Scheme 2). Yields are reported in Scheme 2 without
optimization.
A single set of signals was observed by NMR, indicating the presence of a sole
stereoisomer in solution, presumably the (E)-isomer, as previously reported for
1-methyl-2-(nitromethylene)pyrrolidine.[10]
Reaction in Triflic Acid
The reaction of the 1-(x-phenylalkyl)-2-(nitromethylene)pyrrolidine derivatives
3a–d in triflic acid was generally clean, with yields varying from 19 to 98% (Scheme 3).
The yield of isolated cyclization products increases with the size of the formed ring
from 85% for the seven-membered ring 4a to 98% for the nine-membered ring 4c.
Scheme 1. Nitrovinylamines synthesis by condensation of lactam acetals with nitromethane.

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
2379
Scheme 2. Synthesis of 1-(x-phenylalkyl)-2-(nitromethylene)pyrrolidine derivatives 3.
As expected for reasons of entropy,[11] the yield in intramolecular cyclization drops to
19% for the 10-membered ring product 4d.
The structure of the molecule was further confirmed by the X-ray crystallo-
graphic analysis of 4a[12–14] (Fig. 2). In this crystal, the hydroxyl group of the newly
formed hydroxyimino group adopts a cis conþfiguration relative to the aromatic ring.
=
=
The torsion angle between the iminium C N and the C N of the oxime is close to
2ꢀ, indicating that these functions are in the same plane; meanwhile, the torsion angle
between the aromatic ring and the oxime is about 57ꢀ, which indicates a lack of
electronic conjugation at this level.
NMR Study and Reaction Mechanism
To have a better understanding of the mechanism of the reaction, an NMR
study was carried out with 1-phenethyl-2-(nitromethylene)pyrrolidine 3a in triflic
acid. The resulting solution was observed by NMR spectroscopy at 255 K. The start-
ing material 3a was rapidly protonated to afford the observable dication 5a with a
deshielded methylene (dC 71.4 and dH 4.37 ppm) next to the protonated nitro
group (Figs. 3 and 4). These values are close to those previously observed for C,O-
diprotonated nitroethylene derivatives in triflic acid.[7,15,16]
The arrows denote the signals of the initially formed dication, which will dis-
appear over time in favor of other signals corresponding to the final dication. Thus,
after 1 h of reaction time at 2 ꢀC, intermediate dication 5a was fully transformed into
a stable observable dication 7a, characterized by the presence of the hydroxyl proton
at dH ¼ 12.99 ppm (Fig. 5) and the presence of iminium and hydroxyiminium groups
respectively at dC ¼ 173.2 ppm and dC ¼ 153.6 ppm (Fig. 6).
Scheme 3. Reaction of 1-(x-phenylalkyl)-2-(nitromethylene)pyrrolidine derivatives 3 in triflic acid.

´
B. FANTE ET AL.
2380
Figure 2. X-ray crystallographic analysis of 4a.
The mechanism for the formation of cation 5a is probably similar to the one
previously reported for nitroketene aminals.[17] At the beginning of the reaction,
1-(x-phenylalkyl)-2-(nitromethylene)pyrrolidine derivatives 3 undergo a double pro-
tonation: a C-protonation on the carbon bearing the nitro group and an O-protona-
tion of the nitro group. The O-protonation occurs through a fast proton exchange
process with the acidic medium, as observed in fluorosulfonic acid at ꢁ80 ꢀC.[18]
In this medium, the nitro group is known to be protonated but the proton is not
observed because of its very rapid exchange rate with the medium.[19] Prototropic
exchange and the formal loss of a molecule of water lead to the formation of the
conjugated hydroxynitrilium ion 6.[1,20] As soon as it is formed as indicated in
Scheme 4, 6 reacts with the tethered phenyl ring to afford the observed stable
dication 7a.
Figure 3. 1H NMR spectrum of transient dication 5a in triflic acid at 255 K (reference TMS in [D4]
methanol as external lock).

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
2381
Figure 4. DEPT 135-NMR spectrum of transient dication 5a in triflic acid at 255 K (reference TMS in [D4]
methanol as external lock).
Figure 5. 1H NMR spectrum of dication 7a in triflic acid at 278 K (reference TMS in [D4] methanol as
external lock).
Figure 6. 13C NMR spectrum of dication 7a in triflic acid at 278 K (reference TMS in [D4] methanol as
external lock).
At the end of the reaction, the acid was quenched, and compounds 4 were iso-
lated as triflate salts because of the presence of quaternary nitrogen in the azamacro-
cyclic compound structure.
The observed configuration of the oxime of 4a is consistent with the mech-
anism of addition to a triple bond, as expected from a theoretical point of view[21]
and in agreement with previously reported results.[15,16,17]

´
B. FANTE ET AL.
2382
Scheme 4. Suggested mechanism for the formation of products 4.
NMR spectroscopy shows a single (E)-oxime isomer for the seven- to nine-
membered ring products 4a–c, but two sets of signals are observed for the 10-
membered ring product 4d. These two sets of signals can be due to either (i) (E)
and (Z) isomers of the oxime or (ii) to a mixture of two conformers, whose intercon-
version would be hampered by steric and geometric constraints of both the phenyl
ring and pyrrolidine ring.[22]
CONCLUSION
This study extends previous work on the formation of hydroxynitrilium ion
from heterosubstituted nitroethene derivatives and their use in the field of hetero-
cyclic synthesis. In triflic acid, 1-(x-phenylalkyl)-2-(nitromethylene)pyrrolidine
derivatives lead to conjugated hydroxynitrilium ions, which undergo an electrophilic
aromatic substitution to afford tricyclic compounds as triflate salts.
EXPERIMENTAL
Melting points were determined with a Buchi melting-point B545 apparatus
¨
using capillary tubes (temperature rate 2 ꢀC=mn) and were not corrected. A Bruker
1
DPX 300 spectrometer, equipped with a low-temperature probe, was used for H,
9F, and 13C NMR spectra recorded at 300.13, 282.37, and 75.47 MHz, respectively.
NMR spectra of cations were recorded in neat triflic acid at low temperature in the
presence of methanol-d4 contained in a sealed capillary tube placed inside the NMR
tube. NMR spectra of starting compounds and products were recorded at room
temperature and chemical shifts are reported relative to Me4Si or hexafluorobenzene
for fluorine. The reproducibility of 13C NMR shift was about ꢂ0.05 ppm, depending
on cell and concentration. Chemical assignments were made using distortionless
enhancement polarization technique (DEPT) 135 and usual chemical shift assignments
rules. Electron-impact ionization (70 eV) mass spectra were obtained with a Finnigan
Incos 500 Instrument. High-resolution mass spectrometry was performed at the
´
´
Faculte des Sciences–Universite de Picardie Jules Verne, France, or Rennes. Flash
chromatography was achieved on silica gel (20- to 45-mm particle size). Triflic acid

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
2383
was purchased from Across and 1,1-bis(methylthio)-2-nitroethene was obtained from
Lancaster, and both were used without further purification. No attempt was made to
optimize the yields. The yields and extractions were not optimized.
Starting Material
1-(4-Phenylbutyl)pyrrolidin-2-one (1c). NaH (0.95 g, 40 mmol.) in 60% sus-
pension in paraffin oil (1.58 g suspension) was added to a solution of pyrrolidin-2-
one (3 mL; 40 mmol) in 80 mL of anhydrous toluene. After stopping the hydrogen
release, the reaction mixture was heated to reflux with stirring under nitrogen atmos-
phere for 10 h. Then x-phenylalkylbromide (80 mmol) dissolved in anhydrous xylene
(20 mL) was added. The mixture was then refluxed for 4 h under nitrogen atmos-
phere. After cooling and filtration, the solvent was eliminated under reduced press-
ure. The viscous residue was purified by flash chromatography using CH2Cl2=MeOH
1
(99=1) to give compound 1c (7.88 g, 91%) as oil. H NMR (CDCl3): d (ppm) ¼ 1.57
(m, 4H, H-20 and H-30), 1.96 (q, J ¼ 7.7 Hz, 2H, H-4), 2.35 (t, J ¼ 7.9 Hz, H-3), 2.63
(t, J ¼ 7.1 Hz, 2H, H-40), 3.18–3.25 (quadruplet, J ¼ 7.1 Hz and 14.2 Hz, 4H, H-5 and
H-10), 7.14–7.18 (m, 3H, aromatic o-H and p-H), 7.23–7.29 (m, 2H, aromatic m-H).
13C NMR (CDCl3): d (ppm) ¼ 18.21 (C-4), 27.07 (C-20), 28.90 (C-30), 31.42
(C-3), 35.76 (C-40), 42.50 (C-5), 47.32 (C-10), 126.10 (aromatic CH), 128.64 (2 aro-
matic CH), 128.74 (2 aromatic CH), 142.43 (aromatic C), 175.20 (C-2). MS (electro-
spray) C14H19NONa ([MþNa]þ): calcd. 240; found 240.
Compounds 1 NMR spectra were characterized by the absence of signal
=
corresponding to N-H and the presence of signal assigned to C O at dC ¼ 175 ppm.
Compounds 1c and d have not been described in the literature.
(E)-1-Phenethyl-2-(nitromethylene)pyrrolidine 3a. 1-Phenethylpyrrolidin-
2-one 1a (5.00 g, 26.45 mmol) and dimethylsulfate (2.60 mL, 27.40 mmol) were
heated at 80 ꢀC for 15 h under nitrogen atmosphere. The reaction mixture was then
cooled at 50=60 ꢀC and a solution of sodium ethoxide (0.742 g of sodium in 14 mL of
ethanol) was added dropwise. The resulting mixture was stirred under nitrogen for
4 h at 50=60 ꢀC. Then nitromethane, dried over CaCl2 (3 mL, 55.0 mmol), was added,
and the resulting solution was refluxed for 10 min. After cooling, the solvent was
eliminated under reduced pressure. The residue was dissolved in CH2Cl2 and filtered,
and the solvent evaporated under reduced pressure. The resulting oil was purified by
column chromatography on silica gel (CH2Cl2=AcOEt, 99=1). Further crystallization
from CH2Cl2=petroleum ether afforded compound 3a (1.60 g, 26%) as crystals.
1
Mp 113.7 ꢀC (CH2Cl2=petroleum ether). H NMR (CDCl3): d (ppm) ¼ 1.95 (q, J ¼
7.5 Hz, 2H, H-4), 2.89 (t, J ¼ 7.4 Hz, 2H, H-3), 3.33 (t, J ¼ 7.4 Hz, 2H, H-20), 3.45 (m,
4H, H-5 and H-10), 6.76 (s, 1H, vinylic H), 7.16–7.19 (m, 2H, aromatic o-H), 7.22–7.35
(m, 3H, aromatic p-H and m-H). 13C NMR (CDCl3): d (ppm) ¼ 20.83 (C-4), 32.62
(C-3), 35.03 (C-20), 49.29 (C-10), 55.21 (C-5, 109.14 (vinylic CH), 127.45 (aromatic
CH), 129.00 (2 aromatic CH), 129.25 (2 aromatic CH), 137.94 (aromatic C), 164.26
(C-2). HRMS for C13H16N2O2Na ([MþNa]þ): calcd. 255.1109; found 255.1109
Starting materials 3a–d were characterized from their NMR spectra, with the
vinylic protons in the range dH 6.69–6.87 ppm, the carbons bearing the nitromethylene
=
group ¼CH-NO2 at dC 109.7–109.9 ppm, and the other ethylenic carbons >C C
at 164.5–164.7 ppm.

´
B. FANTE ET AL.
2384
Tricyclic Compounds
Triflic acid (5 mL, 56.2 mmol) was placed under nitrogen in a 10-mL,
round-bottom flask equipped with a three-way valve and a Teflon-coated magnetic
stirrer. Caution: Triflic acid is a strong corrosive acid that may react violently with
water. The system was cooled to ice-water bath temperature, and then the 1-(x-
phenylalkyl)-2-(nitromethylene)pyrrolidine 3 (1 mmol) was added.
11-Hydroxyimino-1,2,3,5,6,11-hexahydrobenzo[d]pyrrolo[1,2-a]azepiny-
lium trifluoromethanesulfonate 4a. (E)-1-Phenethyl-2-(nitromethylene)pyrroli-
dine 3a (232 mg, 1 mmol) was dissolved in triflic acid (5 mL, 56.2 mmol) at 2 ꢀC
under nitrogen atmosphere. The reaction was monitored by thin-layer chromato-
graphy (TLC; CH2Cl2=MeOH, 97=3). After the disappearance of the starting com-
pound, the acidic solution was poured into 50 mL of a mixture of CH2Cl2=MeOH
(90:10) at ꢁ60 to ꢁ40 ꢀC and let warm. When the temperature was close to 0 ꢀC,
brine (10 mL) and anhydrous Na2CO3 (6 g) were added. The extraction was carried
out promptly with CH2Cl2=MeOH, 95=5 (4 ꢃ 20 mL). The organic phase was dried
over MgSO4, and the solvent was evaporated under reduced pressure. The resulting
product was purified by column chromatography using CH2Cl2=MeOH, 95=5 as elu-
ent, followed by precipitation from a mixture of dichloromethane=petroleum ether,
to yield 4a (309 mg, 85%) as crystals. Mp 190.9 ꢀC (CH2Cl2=MeOH (95=5)=pet-
1
roleum ether). H NMR (acetone-d6): d (ppm) 2.29 (q, J ¼ 7.8 Hz, 2H, H-2), 3.31
(m, 2H, H-1), 3.68 (m, 2H, H-6), 4.15 (s, 2H, H-5), 4.51 (t, J ¼ 7.33 Hz, 2H, H-3),
7.33–7.45 (m, 3H, H-7, H-8 and H-9), 7.66–7.69 (d, J ¼ 7.49 Hz, 1H, H-10). 13C
NMR (acetone-d6) : d (ppm) 18.96 (C-2), 32.15 (C-1), 40.41 (C-6), 54,79 (C-5),
66.14 (C-3), 122.34 (qd, JCF ¼ 320.5 Hz, CF3SOꢁ3 ), 127.96 (C-9), 129.46 (C-10a),
129.96 (C-7), 132.07 (C-10), 132.35 (C-8), 137.98 (C-6a), 149.43 (C-11), 179.98
19
(C-11a). F NMR (282.37 MHz, acetone-d6): d (ppm) ꢁ72.45 (CF3SOꢁ3 ). HRMS
for C13H15N2O [M-(CF3SOꢁ3 )]: calcd. 215.1184; found 215.1184.
The compounds 4a–d were characterized by the presence of four CH aromatic
carbons (DEPT 135), an oxime group (dC 150 ppm), and an iminium carbon (dC
180 ppm). The presence of a deshielded ortho-aromatic proton (dH 7.46 to
7.69 ppm) indicates that the oxime is (E) configured with the phenyl ring and the
oxime OH in a cis configuration. The trifluoromethanesulfonate anion was charac-
terized by its CF3 group that resonates at dC 122.3 and dF ꢁ 74.3 ppm.
DEDICATION
This publication is dedicated in memory of Professor Jean-Marie Coustard,
who supervised this work and passed away at the end of August 2013.
FUNDING
`
The authors acknowledge CNRS for financial support; Ministere des Affaires
Etrangere, France, for fellowship funds (S. S. and S. Y.); and Ministere de
`
`
´
l’Enseignement Superieur, Ivory Coast (B. F.).

INTRAMOLECULAR CYCLIZATION IN TRIFLIC ACID
2385
SUPPORTING INFORMATION
1
Full experimental detail, H and 13C NMR spectra, and X-ray data can be
found via the Supplementary Content section of the publisher’s website.
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