Aromatic perfluoroalkylation with metal complexes in electrocatalytic conditions

Article abstract of DOI:10.1016/j.jorganchem.2012.06.032

A one-step catalytic method for aromatic perfluoroalkylation with electrochemically reduced metal complexes under mild conditions with decisive role of the sacrificial anode metal ion has been developed. Cross-coupling is successful with bromo- and iodo-benzene and perfluoroalkyl iodide and number of cobalt and nickel complexes.

Full text of DOI:10.1016/j.jorganchem.2012.06.032

Journal of Organometallic Chemistry 718 (2012) 101e104
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Aromatic perfluoroalkylation with metal complexes in electrocatalytic conditions
*
Mikhail Khrizanforov, Tatyana Gryaznova, Oleg Sinyashin, Yulia Budnikova
A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of Russian Academy of Sciences, 8, Arbuzov str., 420088 Kazan, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
A one-step catalytic method for aromatic perfluoroalkylation with electrochemically reduced metal
complexes under mild conditions with decisive role of the sacrificial anode metal ion has been devel-
oped. Cross-coupling is successful with bromo- and iodo-benzene and perfluoroalkyl iodide and number
of cobalt and nickel complexes.
Received 12 April 2012
Received in revised form
14 June 2012
Accepted 26 June 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Nickel complexes
Co complexes
Electrosynthesis
Fluoroalkylation
Co-catalyst
1. Introduction
the reaction by microwave or elevated temperatures are described,
for example, in Ref. [14]. Compounds containing highly fluorinated
The increasingly broad realization of the key role of organo-
fluorine compounds in numerous areas has recently attracted many
new scientists to the field. The development of new methods for
the selective introduction of fluorine and fluorine containing
groups into organic molecules for biologically active and other
useful materials has become hot area [1e14].
chains e(CH2)z(CF2)yF are receiving great current interest as stoi-
chiometric reagents and as catalysts for selective chemical trans-
formations. The unique solubility properties of highly fluorinated
compounds open possibilities to apply these materials to organic
syntheses in innovative reaction media like “fluorous biphasic
systems” or in supercritical carbon dioxide. The most general and
widely used method is perfluoroalkylation with RfCu reagents
generated in situ from perfluoroalkyl iodides and copper. Penta-
fluoroethylation can also be achieved via decarboxylation of
sodium or methyl perfluoropropionate in the presence of Cu(I) or
via C2F5 transfer from C2F5SiMe3 to copper [13].
Other methods are also implemented in conditions of high
temperature and usually a large excess of fluoroalkylemetal
reagents [13]. The only electrochemical trifluoromethylation
method was described by Perichon [16]. This is a two stage process,
trifluoromethylcopper (I) complex was prepared utilizing a copper
anode in the presence of CF3Br excess, TMEDA (4e5 equiv) on the
first stage. The cross-coupling reaction proceeds in DMF at
90e100 ꢀC. Ligand was essential to avoid the precipitation of cop-
per(I) bromide. Using different from TMEDA ligands was less
successful.
Extensive efforts have led to new methods for the introduction
of the trifluoromethyl and perfluoroalkyl groups onto aromatic and
heteroaromatic ring systems. The most synthetically useful method
of effecting this transformation is the coupling of the corresponding
aryl and heteroaryl halides with a fluoromethylcopper species.
These protocols are typically implemented at high temperatures
and large excess of copper or fluoromethylcopper agents, some-
times with the use of derivatives of mercury or zinc [detailed
review, for example, in [13,14]]. Aromatic trifluoromethylation
reactions that are catalytic in metal are still rare. In 1982, Kitazume
and Ishikawa [15] briefly communicated Pd-catalyzed reductive
coupling of perfluoroalkyl iodides with some organic halides in the
presence of Zn dust under sonication. No full report has followed.
Interestingly, although [(Ph3P)2Pd(Cl)2] was reported [15] to be
active added catalyst, more recently it was demonstrated that Ph3P-
stabilized trifluoromethyl Pd aryls do not undergo AreCF3 reduc-
tive elimination. Other examples of using copper or palladium
catalysts or many metallic systems (PdeNieCu or Pd, Zn) to initiate
Catalytic methods to incorporate the trifluoromethyl group into
aryl bromides and chlorides have yet to be developed. There is
a rich history of stoichiometric trifluoromethylations of aryl halides
using copper [17e22], however the first example of a process
catalytic in copper to couple the trifluoromethyl group with aryl
iodides was only recently reported in 2009 [23].
* Corresponding author.
E-mail address: yulia@iopc.ru (Y. Budnikova).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.06.032

102
M. Khrizanforov et al. / Journal of Organometallic Chemistry 718 (2012) 101e104
Table 1
The purpose of this study is to develop a new one-step method
The yield of cross-coupling product in catalytic reactions in DMF of iodobenzene and
hexafluorohexyl iodide PhI þ C6F13I ¼ C6F13ePh, room temperature.
of aromatic perfluoroalkylation with metal complexes in electro-
chemical catalytic mild conditions and discovery the regularities of
this process. Unsaturated complexes of Ni, Co, Cu with “non-
innocent” ligands were selected as catalysts. Apart from the lower
price compared to the precious platinum metals Pd or Pt, the use of
Ni or Co is mainly motivated by the fact that lasts possesses a very
favourable combination of properties to meet the requirement for
an organic reaction to take place via coordination. It is known the
sufficiently low ionization potential to favour oxidative addition for
nickel; sufficiently weak metalecarbon bonds; tendency to form
square-planar complexes and to reach penta-coordination to allow
insertion; sufficiently high electron affinity to allow reductive
elimination. Some properties are conflicting and a compromise has
to be reached [24e26].
L
Metal of complex
Anode
Yield, %
[catalyst:RFX]
1:100
1:10
1:1
bpy
bpy
bpy
Ni
Zn
Mg
Pt
25
>10
>10
25
20
20
30
20
20
tpy
bpy
tpy
bpy
dppe
bpy
dtbpy
dtbpy
dmphen
dmphen
bpy
Mg
Cu
Cu
Zn
Zn
Zn
Zn
Cu
Zn
Cu
Cu
15
75
70
e
70
60
e
78
73
e
Cu
Co
e
e
e
30
40
84
35
82
80
20
42
5
44
60
80 (40 ꢀC)
2. Results and discussion
80
78
75
77
To achieve our goals we have attempted to solve a set of inter-
related problems, namely, the rationale for the choice of effective
and affordable catalysts e nickel, copper, cobalt complexes with
different ligands, improvement of processes in cell with different
soluble anode, identifying patterns of cross-coupling, the assump-
tion of key stages of the catalytic cycle.
Model reaction with simple alkyl halide (more available and
cheaper) as the substrate used has been investigated in order to
select conditions for cross-coupling aryl and perfluoroalkyl halides.
Results of electrocatalytic cross-coupling of iodobenzene and
perfluorohexyl iodide are given in Table 1. It was found that the
most effective catalysts are complexes of nickel and cobalt with bpy
or dtbubpy ligands in the presence of copper anode.
We have discovered the joint electrolysis of some metal
complexes MX2L (M ¼ Ni, Co, Cu; L ¼ 2,20-bipyridine (bpy),
2,20:60,200-terpyridine (tpy), 1,2-bis(diphenylphosphino)ethane
(dppe), 2,9-dimethyl-1,10-phenanthroline (dmphen), 4,40-di-tert-
butylpyridine (dtbpy)) and fluoroalkyl iodide in dimethylforma-
mide in the presence of bromobenzene in undivided electrolyzer
yields to cross-coupling product PheRF with good yield in some
cases. Reaction proceed with catalytic regeneration of M(I)L
complex under first reduction peak potential of catalyst:
formation RFCuLX by transmetalation, which further reacts with
PhI under conditions of electroreduction of formation of the
desired cross-coupling products, or stage of the transmetalation is
later, at the stage bi-organic sigma complex RF(Ar)ML. It is known
that nickel complexes of the type Ni(Ar)CF3L are very stable and do
not lead to cross-coupling products AreCF3 [27,28]. Therefore
transmetalation of intermediate sigma complexes with copper
favours the catalytic reaction and provides a reductive elimination
in the formation of cross-coupling product.
To justify the ways of reacting and study of the catalytic cycle,
the method of cyclic voltammetry was used in comparative terms.
It was shown that copper complexes (reduced form of CuCl2bpy,
CuCl2dppe, CuCl2dmphen) do not react with the RFI with a notice-
able rate in CVA conditions, their reduction picture is not changed
by addition of RFI. In contrast, CVA complexes of nickel and cobalt
additives are sensitive to RFI. The reduction peak of M(II) to M(I)
increases by about 2-fold with the loss of reversibility (for NiBr2bpy
reduced 2-fold, as earlier shown [29]), which indicates the occur-
rence of oxidative addition RFX to M(I)L without the cyclic regen-
eration of the catalyst, which should have led to the increase in
catalytic current of the catalyst in the presence of substrate [for
example, Ref. [30,31]]. Rather, the observed changes in CVA can be
described by the ECE scheme on this stage.
MX2L, 2F
PhBr + RFI
RF - Ph
Plausible catalytic cycle of cross-coupling can be represented as
follows (for example with NiX2tpy, Scheme 1).
anode: Cu, Zn, Mg
2,2'-bipyridine
;
terpyridine
2,9-dimethyl-1,10-phenanthroline
4,4'-di-tert-butyl-2,2'-bipyridine
1,2-bis(diphenylphosphino)ethane
L :
The Scheme 1 includes the traditional stage for the catalytic
cycle of oxidative addition and reductive elimination as the key
steps [27e31]. The cycle starts by reaction of the active form of the
catalyst Ni (I) with RfX (reduction potential is less negative than one
for ArX partner, so the rate of interaction with RfX should be
higher), the intermediate sigma complexes through several stages
of recovery and the reaction with a coupling partner ArX give
;
;
;
It is found that the most effective catalysts are complexes of
nickel and cobalt with bpy, dtbpy or dmphen ligands in the pres-
ence of copper anode (Table 1). The determining influence of the
choice of pair of metal-complex catalysts and metal soluble anode
is found.
Interestingly, the success of the catalytic process depends on the
potential recovery of the catalyst (Table 2). The more negative
reduction potential is the more effective catalyst.
A key role of transmetalation during the catalytic cycle has been
assumed. Catalyst begins catalytic cycle for the oxidative addition
reactions of M(I)L (M ¼ Ni, Co) with RFX, but the resulting inter-
mediate e sigma-complex RFMLX then either reacts with the
Table 2
First peak reduction potentials of investigated cata-
lysts (ref. Ag/AgCl, 10ꢁ2 M) in DMF.
Complex
ꢁE1pred, V
CuCl2dppe
CuCl2bpy
CoCl2bpy
NiBr2bpy
CoCl2dtbpy
NiBr2tpy
dmphen
0.024
0.050
1.089
1.125
1.221
1.291
1.389

M. Khrizanforov et al. / Journal of Organometallic Chemistry 718 (2012) 101e104
103
Cathode
Anod e
RFX
NiIL
RFNiIIILX
2e
RFNiIL
+e
ArX
RF
Ar
LXNiIII
e
Ar-RF
CuX2
-2e
NiIIL
RF
Ar
LNiII
Ar-RF
Fig. 1. Cyclic voltammograms of NiBr2bpy (5
ꢂ
10ꢁ3 M) in the presence of
CuCl2(5 ꢂ 10ꢁ3 M) and C6F12HBr (5 ꢂ 10ꢁ3 M) on GC electrode in DMF (1, red);
NiBr2bpy (5 ꢂ 10ꢁ3 M) (2, blue); NiBr2bpy (5 ꢂ 10ꢁ3 M) in the presence of C6F12HBr
(5 ꢂ 10ꢁ3 M) (3, dark blue). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
-NiIIX2
RF
Ar
LCuII
Cu0L
Future efforts will focus on modelling the individual stages of
the cycle and the isolation of intermediates to confirm the
assumptions made, and the expansion of the electrochemical
approach to cross-coupling of heteroaryl halides and perfluoroalkyl
halides.
Ar-RF
Scheme 1. Catalytic cycle of aromatic perfluoroalkylation with electrochemically
reduced metal complexes.
3. Experimental section
biorganonickel sigma complexes, which is known to be stable and
do not lead to cross-coupling products [32,33]. At this stage, the
overall reaction would be to stop, because the catalyst is bound in
a stable compound and is not regenerated. However, the presence
of copper ions Cu2þ leads to the successful completion of the cycle,
cross-coupling product and regenerate the catalyst. We can assume
that this is due to the reaction transmetalation, with the conversion
biorganonickel sigma-complexes (one or all possible) to the cor-
responding more reactive copper complexes and lead to the desired
product and the release of Ni (II) L, the closure of the cycle.
It should be noted that the Cu (I) L at room temperature and
electrolysis conditions does not react at an appreciable rate with
either RfX, nor with ArX, CVA of catalyst do not feel the addition of
substrates, as preparative synthesis prevents products from organic
halides.
3.1. General considerations
All reactions were carried out under dry argon atmosphere. All
solvents employed were purified and dried prior to use. N,N-
Dimethylformamide was purified by double fractionation distilla-
tion over melting potash. Perfluoroiodohexane was purchased from
P&M Invest and used without further purification. Bromobenzene
was procured from Acros Organics. Tetrabutylammonium bromide
was purchased from Aldrich and recrystallized from diethylether.
CVA of the catalyst in the presence of Cu2þ and RfX additives
confirm the importance of copper for the regeneration of the
catalyst. For example, in the absence of copper RfX addition to the
solution of the NiBr2bpy complex reduces the catalyst reduction
current almost twice (Fig. 1), and in the presence of copper the
increase in current observed ik/d ¼ 2 ([RfX] ¼ [Ni]), that the current
is 4-fold greater.
Similarly, for example, for CoBr2bpy complex without copper ik/d
is 1.2, and in their presence ik/d ¼ 4.8 ([RfX] ¼ [Co]) (Fig. 2). So, the
current increases approximately in 4-fold, the catalyst regeneration
rate grows.
Thus, we have developed an efficient electrocatalytic aryl- and
fluoroalkyl cross-coupling technique which was tested with various
catalysts and anode metal. Some laws of this reaction discovered.
The advantages of this method are as follows: the cross-
coupling is successful not only with iodobenzene, but bromo-
benzene and perfluoroalkyl iodide; it is a one-step catalytic method
under mild conditions, room temperature.
Fig. 2. Cyclic voltammograms of CoCl2bpy (5 ꢂ 10ꢁ3 M) in the presence of CuCl2
(5 ꢂ 10ꢁ3 M) and C6F12HBr (5 ꢂ 10ꢁ3 M) on GC electrode in DMF (1, red), CoCl2bpy
(5 ꢂ 10ꢁ3 M) (2, blue), CoCl2bpy (5 ꢂ 10ꢁ3 M) in the presence of C6F12HBr (5 ꢂ 10ꢁ3 M)
(3, green). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)

104
M. Khrizanforov et al. / Journal of Organometallic Chemistry 718 (2012) 101e104
Catalysts were prepared according reported procedure [32].
Preparative electrolyses were performed by means of the direct
current source B5-49 in thermostatically controlled cylindrical
individed 70 ml electrolyzer (a three-electrode cell). Platinum with
surface areas of 20 cm2 was used as a cathode; metal (Cu, Mg, Zn or
Pt) rod was used as an anode. The working electrode potential was
determined using reference electrode Ag/AgCl. During electrolysis,
the electrolytewas stirredwith a magneticstirrer. Mass spectrawere
recorded in EI mode using ThermoQuest TRACE MS. Chromatograms
of reaction mixtures obtained by gas chromatographyemass spec-
trometer Perkin Elmer «Turbo Mass Gold».
Acknowledgements
This work was supported by the RFBR grants 11-03-92662-
MCX_a, 10-03-00335 and 11-03-12152 ofi-m.
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