Nonmetallic Wurtz coupling reaction of perfluorohexyl iodide

Article abstract of DOI:10.1016/j.tetlet.2018.05.091

It is shown for the first time that the Wurtz reaction can be realized by the action of an organic substance (triphenylphosphine) on an alkyl halide (perfluoroalkyl iodide) without the use of metals. It was found that when trying to prepare the bis(3,3′-aminophenyl)(fluoroalkyl)phosphine oxide by four-step synthesis, in the first stage of the reaction of perfluoro-1-iodohexane with triphenylphosphine does not proceed towards the formation of an intermediate quaternary phosphonium salt. Instead, the carbon chain of perfluoroalkyl iodide dimerizes to form perfluorododecane – Wurtz reaction product. We have proposed a new pathway for homocoupling of perfluoroalkylhalides into even-numbered perfluoroalkanes.

Full text of DOI:10.1016/j.tetlet.2018.05.091

Accepted Manuscript
Nonmetallic Wurtz coupling reaction of perfluorohexyl iodide
Anton V. Kolotaev, Derenik S. Khachatryan
PII:
S0040-4039(18)30729-9
https://doi.org/10.1016/j.tetlet.2018.05.091
TETL 50038
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
14 March 2018
24 May 2018
31 May 2018
Please cite this article as: Kolotaev, A.V., Khachatryan, D.S., Nonmetallic Wurtz coupling reaction of perfluorohexyl
iodide, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.05.091
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Tetrahedron Letters
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Nonmetallic Wurtz coupling reaction of perfluorohexyl iodide
a
Anton V. Kolotaev , Derenik S. Khachatryan
a*
a
The Federal State Unitary Enterprise «Institute of Chemical Reagents and High Purity Chemical Substances of National Research Centre «Kurchatov
Institute», Bogorodsky val str. 3, Moscow 107076, Russia
ARTICLE INFO
ABSTRACT
Article history:
Received
It is shown for the first time that the Wurtz reaction can be realized by the action of an organic
substance (triphenylphosphine) on an alkyl halide (perfluoroalkyl iodide) without the use of
metals. It was found that when trying to prepare the bis (3,3'-aminophenyl) (fluoroalkyl)
phosphine oxide by four-step synthesis, in the first stage of the reaction of perfluoro-1-
iodohexane with triphenylphosphine does not proceed towards the formation of an intermediate
quaternary phosphonium salt. Instead, the carbon chain of perfluoroalkyl iodide dimerizes to
form perfluorododecane - Wurtz reaction product. We have proposed a new pathway for
homocoupling of perfluoroalkylhalides into even-numbered perfluoroalkanes.
Received in revised form
Accepted
Available online
Keywords:
Nonmetallic
Wurtz reaction
perfluoroalkyl iodides
triphenylphosphine
perfluoroalkanes
2
009 Elsevier Ltd. All rights reserved.
Is known that diarylphosphine oxide derivatives containing
fluorinated alkyl substituents are useful as a monomer in
preparation of polymers with improved properties such as
chemical resistance and electrical insulating property as well as
1. Reaction of perfluoroalkyl substituted ethyl iodide 1a with
triphenylphosphine. Synthesis of phosphine oxide 6a
When 1,1,1,2-tetrafluoro-4-iodo-2-(trifluoromethyl)-butane
1a) is used, intermediate phosphonium iodide 3a is first obtained
1
(
with a high yield according to known procedure (see supporting
adhesiveness and flame retardancy. The introduction of
fluoroalkyl side chain has several advantages such as low
dielectric constants, optical clarity, high permeability and high
selectivity for gas separation, hydrophobicity and oleophobicity,
1
information). Should be noted that the using of solvents
1
(
toluene) significantly reduces the reaction rate and yield.
2-4
besides the increased solubility. Polymers with phosphorus
Next, obtained phosphonium salt 3a is undergone Michaelis-
Arbuzov rearrangement using 30 % alkali gives two products:
target diphenyl(3,4,4,4-tetrafluoro-3-(trifluoromethyl)butyl)
phosphine oxide (4a) and impurity triphenylphosphine oxide (4c)
in ratio 9:1. This ratio is constant varying reaction conditions and
can be explained on the electron withdrawing nature of the
perfluoroalkyl group attached to phosphorus via ethyl spacer
group. In the third step phosphine oxide 4a is converted to bis(3-
nitrophenyl)(3,4,4,4-tetrafluoro-3-(trifluoromethyl)-
butyl)phosphine oxide (5a) via a typical procedure of nitration by
a mixture of concentrated nitric acid (4.4 eq.) in concentrated
sulfuric acid. In the final step phosphine oxide 5a is reduced to
bis(3-aminophenyl)(3,4,4,4-tetrafluoro-3-(trifluoromethyl)-
5-7
moieties have several advantages, such as good adhesion to
various substrates, excellent solubility in organic solvents, high
thermal oxidative stability and radiation resistance and resistant
8
,9
to atomic oxygen exposure. This study reports a four-stage
synthesis about four-step synthesis of bis(3,3’-
aminophenyl)fluoroalkylphosphine oxides 6 according to a
1
0
modified method. The supposed synthetic route is illustrated in
scheme 1 and the details are as follows. As a fluoro-containing
alkylators are chosen having an ethyl spacer group which
separate perfluoroalkyl group from the phosphine oxide group
1,1,1,2-tetrafluoro-4-iodo-2-(trifluoromethyl)butane (1a) and 1-
iodoperfluorohexane (1b).
2 2
butyl)phosphine oxide (6a) with Pd/C and NH NH . The
procedure of preparation of amine with required purity has been
optimized to goal the least losses at the purification stage. The
best total yield (65 %) and purity (≥98 %) are obtained with only
one column purification after nitration stage and no further
purification after reduction of phosphine oxide 5a.
Scheme 1. The supposed synthetic route for synthesis of bis(3,3’-
aminophenyl)fluoroalkylphosphine oxides 6

2
Tetrahedron Letters
2. Unexpected reaction of perfluoroalkyl iodide 1b with
triphenylphosphine
by the radical pathway and the ion pathway is not even
considered because of the presence of strong electrostatic
14
repulsion between like charges. On the other hand, in work by
NMR spectroscopy, the study of the behavior of
triphenylphosphine in the reaction with electrophiles revealed the
While attempting to prepare the bis (3,3'-aminophenyl)-
fluoroalkyl) phosphine oxide (6b) using perfluorinated alkyl
(
iodide 1b in the first stage of the synthesis, the expected
phosphonium salt 3b isn’t detected by ESI-MS (Scheme 2).
IV
existence of an equilibrium between the ionic (P ) and covalent
V
P ) forms of the phosphorus compounds (e.g., [Ph
+
-
(
Ph PI ). According to the X-ray structural analysis compound 8
3
P I] I =
3
2
1
5
IV
exists in the ionic (P ) form. We performed quantum-chemical
calculations with Gaussian 09W program using the Semi-
V
empirical method PM3, and found that covalent (P ) form 3b is
unlikely. A comparison of the enthalpies of the transition state
(TS) 11 (65.79 kcal/mol) and form 3b (83.42 kcal/mol) shows a
greater possibility of TS 11.
Rf
I
Ph
Ph
Ph
P
P+
Ph
Ph
F
F
Rf
I-
I
1b
Ph
Rf
F
F
F
F
3b
11
Ph
P
F
Ph
Ph
Scheme 2. Reaction of perfluoroalkyl iodide 1b with
triphenylphosphine
F
I
Rf
F
Rf
Rf
I
- Ph3PI2
F
A thorough study of compounds in reaction mixture by ESI-
MS has shown a presence of perfluorododecane (7) – a product
of carbon chain doubling of 1-iodoperfluorohexane 1b with m/z
F
8
F
RF
F
1
F
7
2
1
+
1+
ratio 661.78 (M+Na) . Also signals 279.20 (M+H) and 557.27
2
2M+H) are detected which could confirm formation of
+
Scheme 5. Proposed pathway of formation compound 7
(
triphenylphosphine oxide 4c. However expected signal at 27.9
From all the above, we assume that the Wurtz dimerization of
perfluoroalkyl iodide under the action of triphenylphosphine can
proceed through a "push-pull" pathway. In case a three-molecular
act, which is statistically unlikely, or stepwise, through the
formation of a quaternary phosphonium salt 3b, by interaction of
the transition form (intermediate 11) with perfluoroalkyl iodide
(1b) by synchronous 4-centered intermediate 12 (see Scheme 5).
3
1
p.p.m. ( P NMR, CDCl
3
) produced by triphenylphosphine oxide
c is absent, the P NMR spectrum is contained only signal at
7.5 p.p.m. (CDCl ) which confirms the presence of only
3
1
4
4
3
11-12
triphenyliodophosphonium iodide 8. The absent of the signals
of this compound in the conditions of ESI-MS can be explained
+
-
3
by the hydrolysis of [Ph P I]I adduct to triphenylphosphine
oxide 4c. The coupling product perfluorododecane (7) was
obtained with excellent yield (89%).
Conclusion
Thus, we managed to get the product in quantitative yield. The
found method opens the way to a simple and effective synthesis
of perfluorinated hydrocarbons, excluding the use of metals and
flammable explosive techniques for carrying out the reactions of
Wurtz, Grignard. We have proposed a probable pathway for the
formation of compound 7 by homocoupling of compound 1b, but
the establishment of a reaction mechanism requires further
investigation.
It was also found that prolonged boiling (100 °C) of iodide 1b
in the dark in the absence of triphenyl phosphine did not lead to
1
3
dimer 7. Along with this, on the other hand, the literature
describes the interaction of perfluorodecyl iodide with
triphenylphosphine under the irradiation of the Xe lamp light
resulting in the formation of compound 10 by substitution of the
phenyl group with a perfluoroalkyl group (Scheme 3).
Ph
h?(Xe lamp)
2
RI
+
PPh3
R
P
Acknowledgements
CDCl3, r.t.
Ph
9
2
10
We appreciate the Ministry of Education of Russia
R = perfluorodecyl
(
Agreement on granting subsidies No. 14.625.21.0037 of October
3, 2016. Unique identifier for Applied Scientific Researches
project) RFMEFI62516X0037) for financial support. We thank
Scheme 3. The supposed synthetic route for synthesis of bis(3,3’-
0
(
aminophenyl)fluoroalkylphosphine oxides 6
E. E. Anisimova for active participation in the experiments.
1
4
Authors based on experimental results (formation of
intermediate perfluoroalkyl and diarylphosphonium radicals)
proposed a radical mechanism for the formation of compound 10
according to the following scheme 4.
Supplementary Material
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/
*Rf
hv
Ar*
References and notes
Ar2PRf
Ar3P
Ar2P*
Ar2PPAr2
-
-
Ar2P*
1
2
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Scheme 4. Proposed pathway of formation compound 10
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Publishers: New York, 2002, pp. 233-276.
The absence of the analog of compound 10 - (HPLC-ESI-
MS/MS) in the products of our reaction gives grounds to assume
that the dimerization of the perfluoroalkyl group does not occur

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Graphical Abstract

4
Tetrahedron Letters
A new pathway of dimerization of perfluoroalkyl
halides to perfluoroalkanes was found.
Perfluoroalkyl halides under the action of
triphenylphosphine undergo homocoupling.
Not of forming of the intermediate quaternary
phosphonium salt.