Tetrakis{3,5-bis(perfluorohexyl)phenyl}borate: A highly fluorous anion

Article abstract of DOI:10.1016/S0040-4039(01)01716-6

In search of new possibilities for rendering catalysts soluble in fluorous solvents, the preparation of a sodium derivative of tetrakis{3,5-bis(perfluorohexyl)phenyl}borate, a new and highly fluorous anion, is described. This weakly coordinating anion exhibits substantial affinity for perfluorinated solvents and is therefore expected to make a broad range of cationic transition-metal catalysts compatible with fluorous biphasic recycling techniques.

Full text of DOI:10.1016/S0040-4039(01)01716-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8085–8087
Tetrakis{3,5-bis(perfluorohexyl)phenyl}borate:
a highly fluorous anion
Joep van den Broeke,a Berth-Jan Deelmana,b,* and Gerard van Kotena
aDepartment of Metal-Mediated Synthesis, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
bATOFINA Vlissingen B.V., PO Box 70, 4380 AB Vlissingen, The Netherlands
Received 7 August 2001; accepted 12 September 2001
Abstract—In search of new possibilities for rendering catalysts soluble in fluorous solvents, the preparation of a sodium derivative
of tetrakis{3,5-bis(perfluorohexyl)phenyl}borate, a new and highly fluorous anion, is described. This weakly coordinating anion
exhibits substantial affinity for perfluorinated solvents and is therefore expected to make a broad range of cationic transition-metal
catalysts compatible with fluorous biphasic recycling techniques. © 2001 Elsevier Science Ltd. All rights reserved.
In recent years, economic and environmental consider-
ations have led to the development of a number of
advanced procedures for the recycling of homogeneous
catalysts.1 Fluorous Biphasic Catalyst Separation
(FBCS) as a mild method for catalyst recovery and an
alternative to aqueous biphasic separation2, was first
reported in the open literature in 1994 and has received
increasing attention ever since.1a A well established
method for adapting a catalyst to the requirements of
FBCS is the attachment of perfluoroalkyl substituents
to its ligand system.1a,3,4 In our search for new ways of
rendering ionic catalysts compatible with FBCS, we
decided to investigate the perfluoroalkyl- substitution
of a weakly-coordinating anion, instead of substituting
a coordinated ligand.5 The major advantage of this
approach is the absence of interference of the
perfluoroalkyl- substitution with the properties of the
often very carefully optimised set of spectator ligands.
Furthermore, this approach offers the possibility of
making a broad range of catalysts available for use in
FBCS by developing a single fluorous anion. Another
advantage is the stabilisation of the anion by the elec-
tron withdrawing fluorinated alkyl groups, whereas in
strongly-coordinated ligands this effect is often disad-
vantageous and requires insulating spacers to prevent a
negative influence on the catalytic activity.1a
As catalytically relevant tetrakis{3,5-bis(trifluoro-
methyl)phenyl}borate (TFPB)6 is known to increase the
solubility of metal complexes in scCO2,7 we set out to
prepare a related anion with a significantly increased
proportion of fluorine. Empirically, approximately 60
wt% of fluorine is required to achieve preferential solu-
bility of a compound in the fluorous phase of a fluorous
biphasic solvent system.1a,4 When charge separation is
neglected, this implies for a typical catalyst such as
[Rh(L2)(COD)]X, in which L2 is a diphosphine with a
molecular weight up to ca 400, e.g. dimethyl-phospho-
lanobenzene (Me-DuPhos) and
X represents the
fluorous tetraphenylborate anion, a requirement of at
least 70 wt% of fluorine in the anion. Therefore, we
decided to replace the trifluoromethyl groups in TFPB
with perfluorohexyl groups (Scheme 1), yielding a tetra-
phenylborate with the desired level of fluorous charac-
ter.
Using copper-mediated cross coupling,8 1,3-bis(per-
fluorohexyl)benzene (1) was prepared from 1,3-
diiodobenzene and a slight excess of IC6F13.9 The
relatively mild conditions employed here, yield the
desired product in 83% yield, with only the 1,3-product
being observed. This is in contrast to the previously
described synthesis of 1,3-bis(perfluorohexyl)benzene,
in which 1 was isolated as an isomerisation product in
the synthesis of 1,2-bis(perfluorohexyl)benzene.10
Electrophilic aromatic bromination of 1 is complicated
by the presence of the strongly electron withdrawing
perfluorohexyl substituents. The use of a powerful
bromination mixture, NBS/trifluoroacetic acid/sul-
phuric acid,11 resulted, after optimisation of reaction
Keywords: tetraphenylborate; fluorous biphasic catalysis; fluorous
anion.
* Corresponding author. Fax: +31 (0)30 2523615; e-mail: b.j.deelman
@chem.uu.nl
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01716-6

8086
J. 6an den Broeke et al. / Tetrahedron Letters 42 (2001) 8085–8087
C6F13
I
I
C6F13
NBS
Br
+ 2 IC6F13 + 4 Cu
H2SO4
CF3COOH
C6F13
C6F13
1
2
1) 2 tBuLi, Et2O, - 80 oC
2) BCl3
3) NaCl (aq)
C6F13
Na
B
C6F13
4
3
Scheme 1. Synthesis of sodium tetrakis{3,5-bis(perfluorohexyl)phenyl}borate.
As we have previously observed that transition-metal
complexes of related perfluoroalkylated tetra-
phenylborates are more soluble in apolar solvents in
comparison with their corresponding sodium salts,5 the
tetrakis{3,5-bis(perfluorohexyl)phenyl}borate appears
to offer significant potential in FBCS and possibly for
catalysis in supercritical solvents as well.7 Studies
directed at using this anion in FBCS are currently
underway.
conditions, in a mixture of 3,5-bis(perfluorohexyl)-
bromobenzene (2) and unreacted 1 (4:1 molar ratio,
54% non isolated yield of 2).12 Since the separation of 1
and 2 was difficult, the mixture was used without
further purification.
Reaction of 3,5-bis(trifluoromethyl)bromobenzene with
magnesium, followed by coupling with a boron reagent,
is the preferred way of preparing TFPB.13 As 2 could
not be converted into the corresponding Grignard, the
use of tert-butyllithium was explored. 2 was reacted
In summary, using existing synthetic procedures, new
and highly fluorinated sodium tetrakis{3,5-bis(pe-
rfluorohexyl)phenyl}borate has been prepared, which
shows substantial solubility in the perfluorinated sol-
vent FC-75. At 71 wt% of fluorine, it will introduce a
fluorous character which is likely to be sufficient to
render cationic transition-metal catalysts compatible
with fluorous biphasic recycling techniques.
t
with 2 equiv. of BuLi in Et2O at −80°C, followed by
the addition of BCl3 to the aryllithium intermediate in
situ (Caution: perfluoroalkyllithium compounds are
potentially explosive and should not be isolated or
warmed to room temperature).13 Work-up with brine
yielded the desired sodium tetrakis{3,5-bis(perfluoro-
hexyl)phenyl}borate (3) as a spectroscopically and ana-
lytically pure compound.14 The 11B NMR spectrum is
especially indicative of the formation of 3, as the
observed resonance corresponds well with those
observed for tetraarylborates.5,15
Acknowledgements
This work was financially supported by ATOFINA
Vlissingen B.V. and the Dutch Ministry of Economic
Affairs (Senter/BTS).
The sodium salt 3 proved soluble in perfluorinated
butyltetrahydrofuran (FC-75),16 which demonstrates
the high fluorophilicity of this salt despite the presence
of a highly positively charged sodium ion. Stabilisation
of such a cation in solution could in principle take
place through interaction with the anion or solvation.
1H and 19F NMR analysis of 3 in FC-75 showed no
indication for interaction of the sodium with either the
phenyl groups or the perfluorohexyl groups of the
anion.17 As FC-75 contains an ether function, this
appears to solvatate the cation. Addition of 12-crown-4
(1.5 equiv.) to the solution did not result in any changes
in the chemical shifts (1H, 11B and 19F NMR) of the
anion, which corresponds with the above interpretation.
References
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York, 1999; (d) Hovestad, N. J.; Ford, A.; Jastrzebski, J.
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J. 6an den Broeke et al. / Tetrahedron Letters 42 (2001) 8085–8087
8087
2. See e.g.: van Leeuwen, P. W. N. M.; Kamer, P. C. J.;
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7.43 g of a white solid was obtained from the fluorous
phase. This proved to be a mixture 1,3-bis(perfluoro-
hexyl)benzene and 3,5-bis(perfluorohexyl)bromobenzene
1
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54 % non-isolated yield of 2. 1H NMR (CDCl3:C6F6,
1:1): l 7.97 (s, 2H), 7.76 (s, 1H). 19F NMR (CDCl3:C6F6
1:1): l −92.9 (m, 3F), −123.1 (m, 2F), −133.2 (m, 2F),
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14. To a mixture consisting of 2 and 1 (0.83 g, molar ratio
83:17) containing 0.88 mmol of 2 and Et2O (50 mL) was
t
added BuLi (1.2 mL, 1.5 M in pentane, 1.8 mmol) at
−78°C followed by BCl3 (0.17 mL, 1.0 M in hexane,
0.17 mmol) after 60 min. After warming to room temper-
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water layer saturated with NaCl. After extraction with
Et2O (3×25 mL), drying over MgSO4 and removal of all
volatiles, a residue was obtained which was taken up in
FC-72 (2 mL) and extracted with acetone (5 mL). Evapo-
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2,2%-bipyridine, 15 mL of DMSO and 32 mL of C6F6 was
stirred at 50°C for 6 days. After addition of H2O (50 mL)
the mixture was extracted with CH2Cl2 (2×50 mL) and
C6F6 (5 mL) After removal of all volatiles, the residue
was taken up in acetone (50 mL) and extracted with
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1
Arp). 13C{19F} (acetone-d6): l 162.7 (q, Ar-B, JB,C=50.2
Hz), 130.4 (d, Aro, 1JC,H=159.2 Hz), 129.2 (s, Arm),
122.3 (d, Arp, 1JC.H=165.4 Hz), 120.5 (CF3), 117.7 (CF2),
113.0 (CF2), 112.6 (CF2), 112.1 (CF2), 110.6 (CF2).
1
of a light yellow oil. H NMR (CDCl3:C6F6, 1:1): l 7.88
(d, J=7.2 Hz, 2H), 7.87 (s, 1H), 7.76 (t, J=7.2 Hz, 1H).
1
13C{19F} (CDCl3:C6F6, 1:1): l 130.5 (s), 130.4 (d, JC,H
=
=
1
11B{1H} NMR (acetone-d6): l −11.9 (s, JBC=49.4 Hz).
1
1
165.0 Hz), 129.2 (d, JC,H=165.8 Hz), 125.8 (d, JC,H
19F NMR (acetone-d6): l −88.6 (m, 3F), −118.2 (m, 2F),
−128.5 (m, 2F), −129.0 (m, 2F), −129.4 (m, 2F), −129.9
(m, 2F), −133.5 (m, 2F). Anal. calcd for C72H12BF104Na:
C, 27.55; H, 0.43; F, 70.81. Found: C, 27.28; H, 0.52; F,
70.32.
158.5 Hz), 119.1 (CF3), 115.5 (CF2), 111.5 (CF2), 111.0
(CF2), 110.6 (CF2), 108.8 (CF2). 19F NMR (CDCl3:C6F6,
1:1): l −93.5 (m, 3F), −123.6 (m, 2F), −133.6 (m, 2F),
−134.1 (m, 2F), −135.0 (m, 2F), −138.5 (m, 2F). Anal.
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12. A mixture of 17.73 mmol of 1,3-bis(perfluorohexyl)-
benzene, 10 mL of trifluoroacetic acid and 6.6 mL of
H2SO4 (98 %) was stirred at 25°C for 7 days while 54.4
mmol N-bromosuccinimide was added in 3 portions at
intervals of 2 days. After addition of ice, extraction with
CH2Cl2 (2×20 mL), washing of the combined organic
16. Using gravimetric methods, the solubility of 3 in FC-75
was determined to be 37 mmol per L of solvent at 25°C.
17. 3: 1H NMR (FC-75): l 7.72 (s, 8H, Aro), 7.47 (s, 4H,
Arp). 11B NMR (FC-75): l −12.9 (s). 3+12-crown-4 (1.5
equiv.): 1H NMR (FC-75): l 7.65 (s, 8H, Aro) 7.44 (s,
4H, Arp), 3.28 (s, 16 H, 12-crown-4). 11B NMR (FC-75):
l −12.9 (s). Spectra were externally referenced against
acetone-d6 (1H NMR) and BF3·Et2O (11B NMR) by using
sealed capillaries.