β-Aminovinylphosphonium salts - A novel synthesis, properties, and structure

Article abstract of DOI:10.1080/10426500701642971

The deacylation of easily accessible β-(N-acylamino)vinylphosphonium salts with methanol or some other nucleophiles gives β- aminovinylphosphonium salts in good yields. As indicated by the spectroscopic properties and by the results of single crystal X-ray diffraction studies the N-alkyl derivatives of the investigated compounds should be considered as strongly resonance stabilized β-iminium ylides rather than β-aminovinylphosphonium salts. In the case of N-aryl derivatives, the β-iminium ylide resonance structures are probably less pronounced. Copyright Taylor & Francis Group, LLC.

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β-Aminovinylphosphonium
Salts—A Novel Synthesis,
Properties, and Structure
Roman Mazurkiewicz a , Beata Fryczkowska b , Rafał
Gabański a , Mirosława Grymel a & Justyna Libera a
a Department of Organic Chemistry, Biochemistry
and Biotechnology , The Silesian University of
Technology , Gliwice, Poland
b Department of Textile Engineering and
Environmental Sciences , The Technical-Humanistic
Academy, Bielsko-Biała , Bielsko-Biała, Poland
Published online: 04 Jun 2008.
To cite this article: Roman Mazurkiewicz , Beata Fryczkowska , Rafał Gabański ,
Mirosława Grymel & Justyna Libera (2008) β-Aminovinylphosphonium Salts—A Novel
Synthesis, Properties, and Structure, Phosphorus, Sulfur, and Silicon and the Related
Elements, 183:6, 1365-1378, DOI: 10.1080/10426500701642971
To link to this article: http://dx.doi.org/10.1080/10426500701642971
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Phosphorus, Sulfur, and Silicon, 183:1365–1378, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701642971
β-Aminovinylphosphonium Salts—A Novel Synthesis,
Properties, and Structure
Roman Mazurkiewicz,1 Beata Fryczkowska,2 Rafalꢀ
1
1
1
´
Gabanski, Miroslꢀawa Grymel, and Justyna Libera
1Department of Organic Chemistry, Biochemistry and Biotechnology,
The Silesian University of Technology, Gliwice, Poland
2Department of Textile Engineering and Environmental Sciences, The
Technical-Humanistic Academy, Bielsko-Bialꢀa, Bielsko-Bialꢀa, Poland
The deacylation of easily accessible β-(N-acylamino)vinylphosphonium salts with
methanol or some other nucleophiles gives β-aminovinylphosphonium salts in good
yields. As indicated by the spectroscopic properties and by the results of single crystal
X-ray diffraction studies the N-alkyl derivatives of the investigated compounds
should be considered as strongly resonance stabilized β-iminium ylides rather than
β-aminovinylphosphonium salts. In the case of N-aryl derivatives, the β-iminium
ylide resonance structures are probably less pronounced.
Keywords β-aminovinylphosphonium salts; β-iminium ylides; β-(N-acylamino)-
vinylphosphonium salts; deacylation; imine-enamine tautomerism
INTRODUCTION
β-Aminovinylphosphonium salts 1 were obtained first by Schweizer et
al. in 1977, by the nucleophilic addition of primary aromatic amines or
hydrazine derivatives to propargyltriphenylphosphonium bromide in
boiling acetonitrile (Scheme 1).1 A very similar method for the synthesis
of these phosphonium salts was extensively used later by Palacios et
al.2,3
As follows from Scheme 1, Schweizer’s method, which starts from
the commercially available propargyltriphenylphosphonium bromide,
allows to obtain only 2-amino-1-propenylphosphonium salts.1−3
Despite the fact that β-aminovinylphosphonium salts have been
known for only about 30 years, they have found many important ap-
plications in organic synthesis. They were used for the synthesis of
Received 20 July 2007; accepted 21 August 2007.
Address correspondence to Roman Mazurkiewicz, Department of Organic Chemistry,
Biochemistry and Biotechnology, The Silesian University of Technology, Krzywoustego 4,
PL 44-100 Gliwice, Poland. E-mail: roman.mazurkiewicz@polsl.pl
1365

1366
R. Mazurkiewicz et al.
SCHEME 1
quinoline derivatives,1 as well as of secondary E-allylamines and op-
tically active γ -amino acid derivatives via 1-aza-1,3-dienes.2 Both al-
lylamines and γ -amino acids represent two important classes of com-
pounds due to their occurrence in natural products and application
in medical chemistry as chemotherapeutic agents2−allylamines and 1-
aza-1,3-dienes are useful building blocks in organic synthesis, as well.2
Recently, we have communicated the simple synthesis of β-(N-acyl-
N-alkylamino)vinylphosphonium salts 2 by imidoylation of β-carbonyl
phosphorus ylides 3 with imidoyl halides 4 or similar imidoylating
agents (Scheme 2).4 Two other very complicated multi-step syntheses
of β-(N-acylamino)vinylphosphonium salts were described earlier by
Brovarets et al.5 In this paper, we describe a simple and effective
method for the synthesis of β-aminovinylphosphonium salts 1 by
deacylation of β-(N-acyl-N-alkylamino)vinylphosphonium salts 2 with
methanol or other nucleophiles, as well as the results of our studies on
their structure.
SCHEME 2
RESULTS AND DISCUSSION
Treatment of β-(N-acyl-N-alkylamino)vinylphosphonium salts in
methanol with DBU at room temperature for 24 h results in the forma-
tion of β-(N-alkylamino)vinylphosphonium salts 1 as stable, crystalline
compounds, which are obtained in good to very good yields (Scheme 3,
Table I). The starting β-(N-acyl-N-alkylamino)vinylphosphonium salts
can be used as pure compounds (procedure A) or can be prepared in
situ from the corresponding β-carbonyl phosphorus ylides 3 and imidoyl
chlorides 4 (procedure B). We also showed that other nucleophiles than
methanol can be used for the deacylation of β-(N-acyl-N-alkylamino)

β-Aminovinylphosphonium Salts
1367
SCHEME 3
vinylphosphonium salts, e.g., phenol or benzyl mercaptan in THF (pro-
cedures C and D, respectively); methanol is the most versatile reagent,
however. The work-up of the reaction mixture was very simple: after
evaporation of the solvent, the residue was crystallized from acetoni-
trile.
The structure of the β-aminovinylphosphonium salts was confirmed
1
by their spectroscopic properties (IR, H and 13C NMR) and satisfac-
tory elemental analyses (Tables I and II). Only in the case of com-
1
pound 1c did we observe two sets of H and 13C NMR signals, prob-
ably due to the formation of a mixture of the Z and the E isomer. For
comparison, we synthesised two β-aminovinylphosphonium salts us-
ing Schweizer’s method (Table I, procedure E, compounds 1e and 1f).
Their spectroscopic properties were very similar to the properties of the
β-aminovinylphosphonium salts, obtained by deacylation of the corre-
sponding β-(N-acyl-N-alkylamino)vinylphosphonium salts.
In the case of compounds 1a, 1d, and 1f a structure determination
by single crystal X-ray diffraction was performed, which revealed the
E-configuration at the C C double bond (Figures 1–3).
Using 1H and 31P NMR spectroscopy Schweizer et al. observed
the presence of an enamine-imine tautomeric equilibrium for some of
the synthesized β-aminovinylphosphonium salts.1 In the case of our
1
compounds, we have not noticed in the H NMR spectra any traces of
signals of a CH2 group connected to phosphorus (Scheme 4). Moreover,
SCHEME 4

1368

1369

1370
R. Mazurkiewicz et al.
FIGURE 1 ORTEP-Plot of the molecular structure of 2-(N-methylamino)-
vinyltriphenyphosphonium chloride 1a in the crystal. Thermal ellipsoids are
drawn at 50% probability level.
our attempts to exchange mobile protons of the investigated compounds
in MeCN-D2O solution at room temperature revealed, when monitored
by 1H NMR, an immediate exchange of the NH proton and no exchange
of the proton of the PCH group even after 24 h (Table III). This means,
that in practice the enamine-imine tautomeric transformation does not
occur under these conditions. Only after addition of DBU we observed
a fast exchange of the PCH group proton; it should be mentioned, how-
ever, that the exchange of these protons in the presence of a strong base
does not necessarily need an enamine-imine equilibrium. It seems, that
the enamine-imine tautomeric equilibrium claimed by Schweizer can
exist only in the case of β-aminovinylphosphonium salts with special
structure, e.g. derived from hydrazine.
The presence of a strong electron-donating amino group and a
strong electron-withdrawing phosphonium group at opposite sides of
the vinylic double bond of β-aminovinylphosphonium salts implicates
the strong push-pull resonance stabilization of these compounds. Con-
sequently, apart from the enamine structure, the β-iminium ylide reso-
nance structures must be taken into account (Scheme 4). We have tried

β-Aminovinylphosphonium Salts
1371
FIGURE 2 ORTEP-Plot of the molecular structure of 2-(N-benzylamino)-l-
propenyltriphenyphosphonium chloride 1d in the crystal. Thermal ellipsoids
are drawn at 50% probability level.
FIGURE 3 ORTEP-Plot of the molecular structure of 2-(N-phenylamino)-l-
propenyltriphenyphosphonium bromide 1f in the crystal. Thermal ellipsoids
are drawn at 50% probability level.

1372
R. Mazurkiewicz et al.
TABLE III H/D Exchange of the Mobile Protons of
β-Aminovinylphosphonium Salts in MeCN-D2O Solution
Time for the full H/D exchange (min)
Without base
In the presence of DBU
N-H
Cα-H
Cα-H
a
1a
1b
1d
<5
<4
<3
−
<5
<2
<2
a
−
b
−
aNo isotopic exchange after 1 h; and bno isotopic exchange after 24 h.
to evaluate the contribution of enamine and β-iminium ylide structures
basing on the spectroscopic properties of β-aminovinylphosphonium
salts as well as on the X-ray data.
A comparison of the chemical shifts of the proton at the α-carbon atom
of β-aminovinylphosphonium salts 1a, 1b, and 1d (Table II) with those
of the parent β-(N-benzoylamino)vinylphosphonium chlorides4 reveals,
that in the case of the β-aminovinylphosphonium salts the considered
protons are shielded at least by 2.65–3.42 ppm. Using the well-known
equation for the chemical shifts δ of olefinic protons postulated by Pas-
cual et al.7,8
ꢀ
δ = 5.25 +
Zi,
(1)
i
where Zi are the respective shielding increments of the substituents
in gem, cis, and trans position with respect to the olefinic protons,
the following expression for the expected difference ꢀ of the chemi-
cal shifts of the α-protons in β-(N-acyloamino)vinylphosphonium salts
and β-aminovinylphosphonium salts is obtained:
cis
cis
RNH
ꢀE,E = Z
− Z
,
(2)
RC O NH
for both phosphonium salts of E-configuration, and
trans
cis
Alkyl
cis
RNH
trans
,
Alkyl
ꢀZ,E = Z
+ Z
− Z
− Z
(3)
RC O NH
for the β-(N-acyloamino)vinylphosphonium salt of Z-configuration and
the β-aminovinylphosphonium salt of E-configuration. Substituting for
trans
cis
RNH
cis
Alkyl
trans
the corresponding increment
Alkyl
ZcRiCs O NH , Z
, Z
, Z
and Z
RC O NH
values (−0.57, −0.72, −1.26, −0.22, −0.28),9 the values 0.69 and 0.60
ppm are obtained as the expected shielding effect caused by exchange
of the acylamino group for the amino group. Therefore, the observed
strong shielding effect (2.65–3.42 ppm) cannot be simply explained

β-Aminovinylphosphonium Salts
1373
as the result of substitution of the less electron-donating acylamino
group by a more electron-donating amino group; to account for such
a strong shielding effect, we have to assume a high contribution of
the β-iminium ylide structures, and the strong shielding effect typi-
cal for ylides due to the increased electron density at the ylidic carbon
atom. Moreover, the range of the chemical shifts for the protons at the
ylidic carbon atom for the investigated compounds (3.56–4.64 ppm) is
typical for resonance-stabilized phosphonium ylides, e.g., Ph3P
CHC≡N (3.71 ppm), Ph3P CHCOMe (3.19 ppm) or Ph3P CHCOPh
(4.43 ppm).10
The shielding effect observed for the α-carbon atom in the 13C
NMR spectra of β-aminovinylphosphonium salts 1a, 1b, and 1d
is even more striking if compared with that of the parentβ-(N-
benzoylamino)vinylphosphonium chlorides4 and amounts to 28.6–47.0
ppm. The increase of the 1 J coupling constant from about 95–100
PC
Hz for β-(N-acyloamino)vinylphosphonium salts4 to more than 120 Hz
for the corresponding β-aminovinylphosphonium salts (Table II) also
indicates a change of the phosphonium salt structure to the ylide
structure. The range of chemical shifts for the α-carbon atom of β-
aminovinylphosphonium salts (52.4–58.5 ppm) as well as the range of
one bond C-P coupling constants (120.4–125.9 Hz) is close to the cor-
responding values for other resonance-stabilized phosphonium ylides,
e.g. Ph3P CHCOPh (50.4 ppm/112 Hz).10
Also the P-Cα bond distance determined by single crystal X-ray
diffraction can be used as an indication of ylide or phosphonium salt
structure of the investigated compounds (Table IV). The typical P-Cα
10
˚
bond distance for phosphonium salts is about 1.80 A. An inspec-
tion of the data in Table IV shows the P-Cα bond lengths to be in
˚
the range 1.725–1.730 A, which is close to the typical range of this
10
˚
bond for resonance-stabilized ylides, e.g., Ph3P CHCHO (1.709 A),
Ph3P CHCOPh (1.711 A) or Ph3P CH(CN)2(1.753 A).10 On the other
11
˚
˚
TABLE IV Lengths of Some Bonds of the β-Aminovinylphosphonium
Salts 1a, d, and f
˚
Bond lengths (A)
P
Cα
Cα Cβ Cβ
N
N
R2
1a
1d
1f
1.725 (3)
1.729 (3)
1.730 (13)
1.352 (4)
1.371 (4)
1.367 (16)
1.325 (3)
1.336 (4)
1.380 (16)
1.429 (4)
1.453 (4)
1.391 (16)

1374

β-Aminovinylphosphonium Salts
1375
hand, the Cβ N bond lengths for compounds 1a and 1d (1.325 and
˚
1.336 A, respectively) are much shorter if compared with a typical
12
˚
C N single bond (1.47 A) and with the Cβ N bond in 2-(N-benzoyl-
4
˚
N-methylamino)-1-propenyltriphenylphosphonium chloride (1.415 A),
which indicates a considerable contribution of the β-iminium ylide res-
onance structure. The situation is somewhat different in the case of
compound 1f; the Cβ N bond is in this case considerably longer (1.380
˚
˚
A), whereas the N-C bond to the phenyl ring is shorter (1.391 A) than the
corresponding N-Me bonds in compounds 1a and 1d (1.429 and 1.453
˚
A, respectively). It seems that this phenomenon can be explained tak-
ing into account the competitive resonance structures with the electron
pair of the nitrogen atom partly spread over the benzene ring.
The Cα-Cβ bonds of the investigated compounds are much shorter
13
˚
than a typical Csp2- Csp2 single bond (1.48 A), but markedly longer
13
˚
than a Csp2-Csp2 double bond (1.32 A). It is noteworthy, that the
lengths of the Cα-Cβ bonds in resonance-stabilized β-keto ylides range
10
˚
˚
from 1.35 to 1.48 A, with most being near 1.40 A.
CONCLUSIONS
Deacylation of easily accessible β-(N-acylamino)vinylphosphonium
salts with methanol offers a convenient way for the synthesis of β-
aminovinylphosphonium salts. It seems, that at least the N-alkyl
derivatives of the investigated compounds should be considered
as strongly resonance-stabilized β-iminium ylides rather than β-
aminovinylphosphonium salts. In the case of N-aryl derivatives, the
β-iminium ylide structures are probably less pronounced.
EXPERIMENTAL
General
Melting points are determined in capillary tubes with a Stuart Scien-
tific SMP3 melting point apparatus, and are uncorrected. IR spectra
were recorded with a Zeiss Specord M 80 spectrophotometer; the mea-
surements were carried out in CHCl3 (0.2 M) using cells of 0.105 mm.
1H and 13C NMR spectra were recorded in CDCl3 on a Varian UNITY
INOVA-300 spectrometer at operating frequencies of 300 and 75.5 MHz,
respectively, in the FT mode using TMS as an internal standard.
Single crystals of 1a, 1b, and 1d, suitable for X-ray measurements,
were obtained by slow evaporation of the solvent from solutions of these
compounds in acetonitrile. Data collection was performed on a KUMA
KM4 four-circle diffractometer, Zr foil filtered MoKα radiation, ω/2ꢁ

1376
R. Mazurkiewicz et al.
scan mode, ꢁ range 2.5–25.5◦; temperature of the measured crystals:
293 K (Table V). The structures were solved by direct methods using
the SHELX-97 program14 and refined by full-matrix least squares with
the SHELXL97 program.15 All the non-hydrogen atoms were refined
anisotropically. The positions of the hydrogen atoms were calculated
according to the standard geometry, and refined as a riding model with
isotropic thermal parameters.15 Software used to prepare material for
publication: Ortep-3.16 Crystallographic data for the structures 1a, 1d,
and 1f, were deposited with the Cambridge Crystallographic Data Cen-
tre as supplementary publications number CCDC 279686, 279687 and
279688, respectively.1
Starting Materials
Commercial grade acetonitrile and CH2Cl2 were distilled and dried
˚
over molecular sieves (4 A). The following reagents were of commer-
cial quality (Aldrich): triphenylphosphoranylideneacetaldehyde, triph-
enylphosphoranylideneacetone and propargyltriphenylphosphonium
bromide. The following compounds were synthesized as described in
the literature: N-methylbenzimidoyl chloride,17 N-benzylbenzimidoyl
chloride,18 triphenylphosphoranylideneacetophenone,19 and 2-(N-
benzoyl-N-methylamino)-1-propenyltriphenylphosphonium chloride.4
Synthesis of 2-(N-Methylamino)-1-
propenyltriphenylphosphonium Chloride (1b) from 2-(N-
Benzoyl-N-methylamino)-1-propenyltriphenylphosphonium
Chloride (2b) and Methanol (Procedure A)
To a solution of 2-(N-benzoyl-N-methylamino)-1-propenyltripheny
lphosphonium chloride 1b (0.943 g, 2.0 mmol) in methanol (5.0 mL)
DBU (0.08 mL, 0.5 mmol) was added and the mixture was left at room
temperature for 24 h. After evaporation of the methanol the residue
was extracted with benzene (4 × 4 mL), and the crude product was
recrystallized from acetonitrile (20 mL).
Synthesis of β-Aminovinylphosphonium Chlorides 1 from
Ylides 3, Imidoyl Chlorides 4, and Methanol (Procedure B)
To a solution of the imidoyl chloride 4 (2.4 mmol) in MeCN (3.6 mL)
the ylide 3 (2 mmol) was added and the mixture was left at room
1A complete listing of the atomic coordinates can be obtained free of charge, on request,
from the Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: (+int) 44-1223 336 033; e-mail: deposit@ccdc.cam.ac.uk], on quoting
the depository numbers, the names of the authors, and the journal citation.

β-Aminovinylphosphonium Salts
1377
temperature for 24 h. The solvent was evaporated, the residue was
dissolved in methanol (5.0 mL), DBU (0.08 mL, 0.5 mmol) was added,
and the mixture was left at room temperature for 24 h. The reaction
mixture was worked up as described above (Procedure A).
Synthesis of β-(N-Methylamino)-1-
propenyltriphenylphosphonium Chloride (1b) from 2-(N-
Benzoyl-N-methylamino)-1-propenyltriphenylphosphonium
Chloride (2b) and Phenol (Procedure C) or Benzylmercaptan
(Procedure D)
To a solution of 2-(N-benzoyl-N-methylamino)-1-propenyltripheny
lphosphonium chloride 2b (0.943 g, 2.0 mmol) in THF (5.0 mL) phenol
or benzylmarcaptan (4 mmol) and DBU (0.08 mL, 0.5 mmol) was added
and the mixture was left at room temperature for 24 h. After evapora-
tion of THF the residue was extracted with benzene (4 × 4 mL), and
the crude product was recrystallized from acetonitrile (20 mL).
Synthesis of β-Aminovinylphosphonium Bromides 1e and 1f
from 2-Propynyltriphenylphosphonium Bromide (Procedure E)
To a solution of propargyltriphenylphosphonium bromide (0.602 g, 2
mmol) in acetonitrile (20 mL), aniline or benzylamine (2 mmol) was
added, the mixture was heated under reflux for 3 h, and left at room
temperature for 24 h. The precipitated crystals were filtered and re-
crystallized from acetonitrile (35 and 45 mL, respectively).
H/D Exchange of Mobile Protons in
β-Aminovinylphosphonium Salts 1
A solution of the β-aminovinylphosphonium salt (0.1 mmol) in CD3CN
(0.5 mL) was added to a mixture of CD3CN (0.45 mL) and D2O (0.05
mL) in a NMR tube at room temperature. In some experiments, DBU
(0.024 g, 0.16 mmol) was added to this mixture. The progress of H/D
exchange was monitored by means of 1H NMR spectroscopy.
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