A solvent-free facile synthesis of (E)-bis(phosphonium)ethylenes from organo-phosphines and TfOCH2CF2H reagent

Article abstract of DOI:10.1016/j.tetlet.2015.09.092

(E)-Bis(phosphonium)ethylenes were synthesized from aryl-, alkyl-, and arylalkylphosphines under solvent-free conditions using TfOCH₂CF₂H as reagent. The reaction allows for a convenient access to vinylenebis(trialkylphosphonium) sal

Full text of DOI:10.1016/j.tetlet.2015.09.092

Tetrahedron Letters xxx (2015) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A solvent-free facile synthesis of (E)-bis(phosphonium)ethylenes
from organo-phosphines and TfOCH2CF2H reagent
⇑
⇑
Shi-Meng Wang, Jia-Bin Han, Cheng-Pan Zhang , Hua-Li Qin
School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, 205 Luoshi Road, Wuhan 430070, China
a r t i c l e i n f o
a b s t r a c t
Article history:
(E)-Bis(phosphonium)ethylenes were synthesized from aryl-, alkyl-, and arylalkylphosphines under sol-
vent-free conditions using TfOCH2CF2H as reagent. The reaction allows for a convenient access to vinyle-
nebis(trialkylphosphonium) salts in good to high yields.
Received 9 August 2015
Revised 18 September 2015
Accepted 21 September 2015
Available online xxxx
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
(E)-Bis(phosphonium)ethylene
Phosphine
2,2-Difluoroethyl triflate
Nucleophilic substitution
Substituent effect
Quaternary aryl and alkyl phosphonium salts have found a wide
range of applications.1 Some of them are the precursors of phos-
phonium ylides in the Wittig reactions.2 Some can be utilized as
phase transfer catalysts (PTC) and Lewis acids for a large number
of chemical transformations.3 Others provide excellent counterca-
tions to stabilize unusual and complex anions, providing very
organic-soluble and easily crystallized salts.1,4 The utilization of
phosphonium cations to furnish ionic liquids as reaction media
and materials is well in agreement with the principle of green
chemistry.5 To satisfy the needs of phosphonium salts from many
research areas, the synthesis of various phosphonium-containing
compounds is particularly important.6,7
Bis(phosphonium)ethylenes (e.g., [Ar3PCH@CHPAr3]X2) bearing
two phosphonium cations and a vinyl linkage are very intriguing
intermediates and reagents.8–11 The easy hydrolysis or alcoholysis
of [Ar3PCH@CHPAr3]X2 in the presence of bases, the nucleophilic
vinylic substitution of the phosphonium group by t-BuÀ anion, the
decisive application of bis(phosphonio)diaminoethene for the syn-
thesis of 4,5-bis(dimethylamino)imidazolium species, and the
incredible use of [Ph3PCH@CHPPh3]Y2 (Y = IÀ, Ph4BÀ) as fire-resis-
tant additives indicated the rich chemistry of bis(phosphonium)
ethylenes.8a–d,h,i Several approaches for the synthesis of these
interesting salts from tertiary phosphines have been
reported.8–11 At the beginning, bis(triarylphosphonium)ethylenes
[Ar3PCH@CHPAr3]X2 (X = BrÀ, ClÀ) with E-configuration were con-
structed by the reaction of Ar3P with Ac2O/HBr, AcBr, or AcCl.8e–g
Later, (E)-vinylenebis(trialkylphosphonium)salt [Bu3PCH@CHPBu3]
{[CH3CO2H]2CH3COÀ2 }2 was verified to form in the reaction of Ac2O
and alcohol when Bu3P was used as catalyst for the acetylation.9
(E)-Bis(triphenylphosphonium)ethylene
CHPPh3][OTf]2) was then produced by the reaction of diiodonium
acetylene triflate and excess Ph3P in wet CH3CN at low
triflate
([Ph3PCH@
a
temperature (À35 °C).10 Vinylenebis(alkylarylphosphonium) salts
[(E)-R1R2MePCH@CHPMeR1R2]I2 were eventually achieved by
treatment of (E)-R1R2PCH@CHPR1R2 with MeI (1:2).11 Although
these methods can supply some bis(phosphonium)ethylenes, the
disadvantages such as the narrow range of substrates, the use
of air- and moisture-sensitive reagents, and the preparation of
the tedious starting materials like diiodonium acetylene and
(E)-R1R2PCH@CHPR1R2 restrain their applications.
⇑
Corresponding authors.
E-mail addresses: cpzhang@whut.edu.cn (C.-P. Zhang), qinhuali@whut.edu.cn
(H.-L. Qin).
http://dx.doi.org/10.1016/j.tetlet.2015.09.092
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, S.-M.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.09.092

2
S.-M. Wang et al. / Tetrahedron Letters xxx (2015) xxx–xxx
previous synthetic methods
R3P
O
I
2Br or 2AcO
PR3
R3P
or
+
+
Ac2O
PPh3
Br
Ph
I
Ph3P
-35 o
C
2
OTf
wet CH3CN
Ph
PPh3
2
OTf
R2
P
R3
P
R1
R1
R1
R2
R2
Cl
R3X
EtOH, reflux
Figure 1. ORTEP diagrams of 3a (left) and 3o (right). Ellipsoids are shown at the
50% probability level. Hydrogens, anions, and solvents are omitted for clarity.
reflux
THF
+
PLi
R1
P
P
R1
R2
2X
Cl
R2
R3
resonances at 6.62 ppm (tdt, J = 54.4, 7.7, 4.4 Hz) and 4.64 ppm
(m) in 1H NMR and À108.6 ppm (dm, J = 54.5 Hz) in 19F NMR, while
[Ph3PCH@CHPPh3]X2 (3a) has a characteristic signal of 8.71 ppm
(t, J = 20.7 Hz) in 1H NMR, which can easily differentiate them.
To our delight, the reaction is also amenable to other tri-
arylphosphines (entries 1–12, Table 2). Substrates either with elec-
tron-donating groups or with moderate electron-withdrawing
groups on the phenyl rings are all transformed under the standard
reaction conditions. As is the case of 1a, the reaction temperature
has great influence on the reaction (entries 1 and 2, 3 and 4, 16
and 17, Table 2). For instance, treatment of tris(meta-methylphe-
nyl)phosphine (1c) with 2a at room temperature for 12 h provided
4c in 5% yield (entry 3, Table 2), and the same reaction mixture
stirred at 120 °C for 6 h afforded 3c in 80% yield (entry 4, Table 2).
The position of substituents on phenyl rings of phosphines also
severely impacts on the transformation. Phosphines bearing para-
and meta-electron-donating groups favored the reaction (entries
2, 4, 7, 8, 11, and 12, Table 2). Particularly, the reaction of tris
this work
R1 R3
P
O
O
R1
P
R2
F
S
solvent-free
120 oC
+
O
CF3
R3
R1
R3
R2
P
2
OTf
F
R2
1
2a
Herein we report a concise synthesis of bis(phosphonium)ethy-
lene salts from tertiary aryl and/or alkyl phosphines with the mild
and bench-stable TfOCH2CF2H reagent under solvent-free condi-
tions. TfOCH2CF2H is a powerful difluoroethylation reagent, which
can incorporate the CH2CF2H group into bioactive molecules on the
oxygen and nitrogen sites via SN2 reactions.12 However, the reac-
tions of 2,2-difluoroethyl triflate with other types of nucleophiles
are rarely known. It was reported that 2,2,2-trifluoroethyl triflate
(TfOCH2CF3) reacting with triphenylphosphine (1a) in dry toluene
at 100 °C for 2 days afforded [Ph3PCH2CF3][OTf] in 85% yield.13
Nevertheless, treatment of TfOCH2CF2H (2a) with PPh3 (1a) at
room temperature for 12 h under solvent-free condition, after crys-
tallization, gave [Ph3PCH2CF2H][OTf] (4a) in 76% yield (entry 1,
Table 1). Increasing the reaction temperature to 80 °C, it was sur-
prising that, the reaction even with excess 2a yielded a mixture
of 3a and 4a (entry 2, Table 1). Further elevating the reaction tem-
perature to 120 °C led to the formation of 3a in 78% yield with only
E-configuration (entry 3, Table 1). Using excess PPh3 did not
improve the yield of 3a but promoted the availability of 2a (entry
4, Table 1). The exact molecular structure of compound 3a was
determined by means of NMR spectroscopy and single crystal
X-ray diffraction (Fig. 1).14 [Ph3PCH2CF2H][OTf] (4a) has distinct
Table 2
Synthesis of symmetric [(E)-R1R2R3PCH@CHPR1R2R3][OTf]2 from R1R2R3P and
TfOCH2CF2H
R1 R3
P
O
O
R1
P
R1
P
R3
R2
CF2H
OTf
F
S
solvent-free
CF3
conditions
R2
and/or
+
O
R3
R2
R3
R1
P
2 OTf
F
R2
1
2a
4
3
Entry
R1R2R3P
Ratioa
Conditions (°C/h)
Yieldb (%)
3c or 4
1
2
3
4
5
6
7
8
(p-MeC6H4)3P (1b)
(p-MeC6H4)3P (1b)
(m-MeC6H4)3P (1c)
(m-MeC6H4)3P (1c)
(o-MeC6H4)3P (1d)
(o-MeOC6H4)3P (1e)
(p-MeOC6H4)3P (1f)
(m-MeOC6H4)3P (1g)
(p-ClC6H4)3P (1h)
(p-FC6H4)3P (1i)
(Bipheyl)PPh2 (1j)
(p-MeOC6H4)2PPh(1k)
Ph2PCy (1l)
1:1
1:1
1:1
1:1
1:1
rt/12
120/6
rt/12
6 (3b)
70 (3b)
5 (4c)
80 (3c)
49 (4d)d
94 (4e)d
74 (3f)
51 (3g)
41 (3h)
79 (3i)
Table 1
120/6
120/6
120/6
120/6
120/6
120/24
120/24
120/24
120/6
120/15
120/8
rt/72
The solvent-free reactions between Ph3P and TfOCH2CF2H
Ph Ph
1:1
1:1.2
O
O
Ph
P
Ph
Ph
Ph
P
CF2H
OTf
Ph
F
S
solvent-free
CF3
conditions
and/or
+
P
O
1:1.2
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
2:1
2:1
Ph
Ph
Ph
P
2
OTf
F
9
Ph
3a
1a
2a
Ph
10
11
12
13
14
15
16
17
18
4a
72 (3j)
91 (3k)
66 (4l)d
72 (3m)
67 (4n)e
55 (4o)
63 (3o)
Complicated
Entry
Ratioa
Conditions (°C/h)
Yieldb (%)
3ac or 4a
Ph2PCH3 (1m)
PhP(CH3)2 (1n)
Cy3P (1o)
Cy3P (1o)
P(n-Bu)3 (1p)
1
2
3
4
1:4.4
1:1.2
1:1.2
2:1
rt/12
76 (4a)d
80/12
120/24
120/15
80e
rt/12
120/8
120/6
78 (3a)
50 (3a)
a
b
c
a
b
c
The molar ratio of Ph3P and HCF2CH2OTf.
Isolated yield.
Trace of fluorides was observed in the isolated products, which was determined
The molar ratio of R1R2R3P and HCF2CH2OTf.
Isolated yield.
Trace of fluorides was observed in most cases, which was determined by 19F
by 19F NMR (see SI).15 Efforts to remove these byproducts by recrystallization failed.
NMR (see SI).15 Efforts to remove these byproducts by recrystallization failed. All
3a was obtained in E-configuration.
the products (3) were obtained in E-configuration.
d
d
23% yield of 4a was obtained when 1.2 equiv of HCF2CH2OTf was employed.
See Ref. 16.
e
e
Yield of the crude product. The molar ratio of 3a/4a is 25:8, which was deter-
Crude product. Bis(phosphonium)ethylene was not formed in this reaction,
mined by NMR spectroscopy.
according to NMR spectroscopy.
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S.-M. Wang et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
products by another molecule of R3P eventually led to mixtures
of symmetric and asymmetric bis(phosphonium)ethylenes.8
HCF2CH2I and other difluoroethyl esters (like TsOCH2CF2H and
AcOCH2CF2H) were also employed to synthesize bis(phosphonium)-
ethylene salts. The negative results (such as the incomplete trans-
formation of the starting materials and the formation of massive
unknown byproducts) indicated that 2a may be the best reagent
for this reaction.
Based on the results above, we envisioned a reaction mecha-
nism in Scheme 1, which might involve SN2-type substitution,
nucleophilic addition, and b-F elimination. First, nucleophilic sub-
Figure 2. ORTEP diagrams of 4d. Ellipsoids are shown at the 50% probability level.
Hydrogens and anions are omitted for clarity.
stitution of 2a by 1 at the a-carbon gives monophosphonium salt
4. Intermediate 4 is susceptible to b-F elimination providing 5
and 6 in the presence of a second phosphine (or FÀ) because of
the favorable leaving ability of fluoride.13,16 Compound 5 would
be easily dissociated at high temperature to release 1 and HF. Inter-
mediate 6 is nucleophilically attacked by 1 to form 7, which under-
goes b-F elimination to generate 8. Reaction of 8 with excess 2a
finally affords bis(phosphonium)ethylene 3. When R1, R2, or R3 is
(para-methylphenyl)phosphine (1b) with 2a afforded 3b even at
room temperature, albeit in a low yield (entry 1, Table 2). However,
phosphines with ortho-substitution on the phenyl rings frustrated
the transformation, which gave only monophosphonium salts (4d,
4e, entries 5 and 6, Table 2).16 The structure of 4d was strictly con-
firmed by X-ray crystallography (Fig. 2).14 We speculate that the
steric hindrance of the ortho-substituents on phenyl rings might
cause the failure of the conversion.16
Using arylalkylphosphines and alkylphosphines instead of
arylphosphines as substrates, the reactions with 2a became com-
plicated (entries 13–18, Table 2). For example, treatment of 1l with
2a at 120 °C for 15 h gave monophosphonium salt 4l in 66% yield
(entry 13, Table 1),16 whereas 1m reacting with 2a at 120 °C for
8 h provided bis(phosphonium)ethylene 3m in 72% yield (entry
14, Table 2). Dimethyl(phenyl)phosphine (1n) reacted with 2a at
room temperature for 72 h, after standard workup, affording crude
4n in a moderate yield (entry 15, Table 2). Heating the reaction
mixture to 120 °C, however, gave a chaotic mixture of salts. Inter-
estingly, the reaction of tricyclohexylphosphine (1o) with 2a at
room temperature for 12 h furnished 4o in 55% yield, while the
similar mixture reacted at 120 °C for 8 h provided 3o in 63% yield
(entries 16 and 17, Table 2). In the case of P(n-Bu)3 (1p), no pure bis
(phosphonium)ethylene product was obtained under the standard
reaction conditions according to NMR spectroscopy analysis (entry
18, Table 2). The elusive electronic and steric effects that arose
from the small variations in alkyl groups made the reactions a little
difficult to be sought out.
an alkyl group, the tendency of a-deprotonation of phosphoniums
at R1, R2, or R3 site, competing with CH2CF2H group, is rationalized,
which might illustrate the frustrated transformation of alky-
larylphosphines and alkylphosphines. Nonetheless, the whole
mechanism of the reaction is still unclear.
In conclusion, we have developed a facile method to the synthe-
sis of vinylenebis(trialkylphosphonium) salts from aryl-, alkyl-,
and arylalkylphosphines by using TfOCH2CF2H as reagent. The
reactions proceeded at 120 °C under solvent-free conditions to
afford (E)-bis(phosphonium)ethylenes in good to high yields. The
reaction temperature and the substituents on the phenyl rings of
phosphines greatly influence the reaction. Phosphines bearing
para- and meta-substituents on the phenyl rings favored the for-
mation of bis(phosphonium)ethylenes, whereas the substrates
with ortho-substitution on phenyl rings gave only monophospho-
nium salts.16 The reactions of arylalkylphosphines and alkylphos-
phines are elusive but some of them can also provide the desired
products. Further mechanistic study and the application of these
vinylenebis(trialkylarylphosphonium) salts are ongoing in our lab.
Acknowledgements
In addition, extensive efforts were directed to the synthesis of
asymmetric [Ph3PCH@CHPR3]2+ꢀ2XÀ from [Ph3PCH2CF2H][OTf]
and R3P (R = p-MeC6H4, m-MeC6H4, p-MeOC6H4, Cy). The reactions
did occur, but unfortunately, no pure asymmetric vinylenebis(tri-
alkylarylphosphonium) salts were isolated. The possible nucle-
ophilic vinylic substitution of the phosphonium groups of the
Financial supports of this research by Wuhan University of
Technology, the Natural Science Foundation of Hubei Province
(China) (2015CFB176), and the ‘Chutian Scholar’ Program from
Department of Education of Hubei Province (China) are gratefully
acknowledged.
R1
P
R3
R1
P
R3
H
R2
F
5
F
R2
R1
P
R1
P
R1
P
R3
O
O
R1
P
R3
F
F
R2
F
R2
S
R2
R2
+
O
CF3
R3
H
F
R3
2a
OTf
OTf
1
6
4
R1
R2
R3
R1
F
R1
R2
R3
2 OTf ( with trace fluorides)
R1
P
R3
R1
P
R3
R1
P
R3
P
P
R2
P
2a
R2
R2
R3
F
R2
OTf
,
OTf
3
7
8
R1
P
R1
P
R3
heat
R2
+
HF
H
R2
F-
R3
5
Scheme 1. Proposed mechanism for the solvent-free synthesis of bis(phosphonium)ethylenes from phosphines (1) and 2a.
Please cite this article in press as: Wang, S.-M.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.09.092

4
S.-M. Wang et al. / Tetrahedron Letters xxx (2015) xxx–xxx
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092.
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16. The addition of KF into the reaction mixture can facilitate the production of
vinylenebis(trialkylarylphosphonium) triflate. It was found that the reaction of
1d (1 equiv) with TfOCH2CF2H (2a, 1 equiv) at 120 °C for 6 h in the presence of
2 equiv of KF gave 3d in 48% yield, while treatment of 1e or 1l (1 equiv) with 2a
(1 equiv) and KF (2 equiv) at 120 °C for 6 h or 15 h, respectively, afforded a
mixture of monophosphonium and vinylenebis(phosphonium) salts. We
appreciate the reviewer for the suggest.
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Please cite this article in press as: Wang, S.-M.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.09.092