Hydrodefluorination of non-activated C-F bonds by diisobutyl-aluminiumhydride via the aluminium cation [i-Bu2Al]+

Article abstract of DOI:10.1016/j.tetlet.2007.10.056

A novel system for the hydrodefluorination (HDF) of non-activated C-F bonds at room-temperature is described. The reaction of i-Bu₂AlH with [Ph₃C][B(C₆F₅)₄] (1), [Ph₃C][Al(C₆F₅)₄] (2) and [Ph₃C][Al{OC(CF₃)₃}₄] (3) as precatalysts leads under formation of triphenylmethane to the aluminium cation [i-Bu₂Al]⁺ and the non-coordinating anions [M(C₆F₅)₄]^- (M = B, Al) and [Al{OC(CF₃)₃}₄]^-. The formed aluminium cation is very reactive towards C-F bonds and easily forms i-Bu₂AlF releasing a carbocation that abstracts the hydride of excess i-Bu₂AlH and yields the corresponding hydrocarbon. Thereby, the active species [i-Bu₂Al]⁺ is regenerated and can realize a catalytic cycle. For 1-fluorohexane as an example including non-activated C-F bonds different activities were found (TON: 1: 20; 2: 12; 3: 30) in cyclohexane as solvent.

Full text of DOI:10.1016/j.tetlet.2007.10.056

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 8900–8903
Hydrodefluorination of non-activated C–F bonds by
diisobutyl-aluminiumhydride via the aluminium cation [i-Bu Al]
+
2
a
a
a
Marcus Klahn, Christine Fischer, Anke Spannenberg,
a,
b
*
Uwe Rosenthal and Ingo Krossing
a
Leibniz-Institut f u¨ r Katalyse e.V. an der Universit a¨ t Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany
b
Lehrstuhl f u¨ r Molek u¨ l- und Koordinationschemie, Institut f u¨ r Anorganische und Analytische Chemie,
Albert-Ludwigs-Universit a¨ t Freiburg, Albert Str. 21, D-79104 Freiburg i. Br., Germany
Received 2 August 2007; revised 4 October 2007; accepted 10 October 2007
Available online 14 October 2007
Abstract—A novel system for the hydrodefluorination (HDF) of non-activated C–F bonds at room-temperature is described. The
reaction of i-Bu AlH with [Ph C][B(C F ) ] (1), [Ph C][Al(C F ) ] (2) and [Ph C][Al{OC(CF ) } ] (3) as precatalysts leads under
2
3
6
5 4
3
6
5 4
3
3 3 4
+
ꢀ
formation of triphenylmethane to the aluminium cation [i-Bu
2
Al] and the non-coordinating anions [M(C
F
6 5
)
4
]
(M = B, Al)
ꢀ
and [Al{OC(CF
3
)
3
}
4
] . The formed aluminium cation is very reactive towards C–F bonds and easily forms i-Bu
2
AlF releasing a
carbocation that abstracts the hydride of excess i-Bu AlH and yields the corresponding hydrocarbon. Thereby, the active species
2
+
[
i-Bu Al] is regenerated and can realize a catalytic cycle. For 1-fluorohexane as an example including non-activated C–F bonds
2
different activities were found (TON: 1: 20; 2: 12; 3: 30) in cyclohexane as solvent.
Ó 2007 Elsevier Ltd. All rights reserved.
0
0
The activation of C–F bonds is one of the great chal-
lenges of modern organometallic chemistry and catalysis.
This is due to the fact that the C–F bond is the strongest
by various complexes Cp MCl
2
ðM ¼ Ti; Zr; Hf; Cp ¼
2
ꢁ
11
Cp; Cp Þ in combination with reducing agents. Jones
and co-workers described in a series of papers the activa-
tion of several types of C–F bonds in alkanes, arenes and
olefins by using the zirconocene hydride complex
1
single bond which carbon can form. The special strength
results from the high bond energy that arises from the
small size and the high electronegativity of the fluorine
atom. Several reviews and papers described the C–F
ꢁ
12–17
Cp ZrH .
2
2
2
–8
bond activation by transition metal complexes. Both,
stoichiometric and catalytic C–F bond activations for
aromatization reactions of cyclic perfluorocarbons were
achieved by using titanocene and zirconocene fragments.
These are formed from Cp MCl (M = Ti, Zr) with Mg/
We studied the C–F bond activation firstly by stoichio-
metric reactions and found consequences for the olefin
polymerisation and hydrodefluorination (HDF) reac-
1
8–22
tions.
2
2
9
HgCl or Cp ZrCl with Al/HgCl . Zirconocene gener-
2
2
2
2
ating systems, such as Cp ZrPh or Cp ZrCl /2 n-BuLi,
2
2
2
2
are able to effectively defluorinate perfluorodecaline to
perfluoronaphthalene.
1
0
R-H
R-F
2
-Fluoropyridine and 3-fluoropyridine as compounds
with activated C–F bond were catalytically defluorinated
Zr-F
Zr-H
Keywords: Bond activation; Hydrodefluorination; Organoaluminium
compounds; Homogeneous catalysis; Boranates; Aluminates.
Al-H
Al-F
*
Corresponding author. Tel.: +49 381 1281 176; fax: +49 381 1281
1176; e-mail: uwe.rosenthal@catalysis.de
5
Figure 1.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.056

M. Klahn et al. / Tetrahedron Letters 48 (2007) 8900–8903
8901
0
0
The Cp ZrH
2
catalysts ðCp ¼ Cp ; rac-ðebthiÞÞ were
This reaction proceeds in a similar way compared to
R SiH and [Ph C][M(C F ) ], in which the silylium cat-
ion is formed. The initially formed aluminium cation is
very reactive towards C–F bonds and easily forms
2
2
2
used for the catalytic HDF of activated C–F bonds.
Mixtures of zirconocene difluorides Cp ZrF as pre-cat-
alyst and diisobutyl-aluminiumhydride i-Bu AlH as an
3
3
6 5 4
0
2
2
2
activator were found to be active catalysts in the HDF
of activated C–F bonds (Fig. 1). Complexes such as
rac-(ebthi)ZrF and Cp ZrF together with i-Bu AlH
i-Bu AlF releasing a carbocation. This carbocation
2
reacts with additional i-Bu AlH, which is used in excess,
2
to the corresponding hydrocarbon (Scheme 2).
2
2
2
2
were established as systems for the room-temperature
HDF of pentafluoropyridine (TON = 67; 24 h). The
turnover number (TON) is defined as mol product
formed per mol catalyst. The in situ formed hydrides
(i
-Bu) Al
+
R-F
-Bu) AlH
(
i
-Bu)
2
AlF
+
+
R
2
R
+ (
i
(i
-Bu) Al
RH
[
rac-(ebthi)ZrH(l-H)]2
and
[Cp ZrH(l-H)]
are
2
2
2
2
assumed as the active species for this conversion
and one can speculate about ate complexes such
Scheme 2.
0
22
½
i ꢀ Bu
2
Alꢂ½Cp ZrH
3
ꢂ as intermediates. Similar reac-
+
2
The active species [i-Bu Al] is regenerated and the cycle
is closed which leads to the catalytic cycle shown in
Scheme 3.
2
tions with dichlorides gave no results because the chlo-
ride ligands are not replaced by the hydrides of
2
3
i-Bu AlH. An analogous system, that consists of LFeF
2
and Et SiH, was introduced by Holland and co-workers
3
R-F
(
i
-Bu) AlF
2
4
2
(
TON = 3.5; 12 h; 45 °C).
Ozerov and co-workers established an approach for the
2
5
HDF without transition metals. In a recent paper, they
described a system for the HDF of non-activated C–F
bonds consisting of Et SiH and [Ph C][B(C F ) ], in
(i
-Bu) Al
R
2
3
3
6 5 4
+
which the Et Si acts as the catalytically active species.
3
M u¨ ller and co-workers took a similar way to hydro-
defluorinate non-activated C–F bonds but with a differ-
R-H
(i-Bu) AlH
2
2
6
ent silicon-based compound.
They applied
a
hydrogen-bridged disilyl cation with a 1,8-naphthal-
enediyl backbone (Fig. 2).
Scheme 3.
The driving force for this reaction is the higher bond
energy of the Al–F bond (665 kJ/mol) compared to
2
7,28
the C–F bond (485 kJ/mol).
The existence of the cat-
alytically active species was proven by single crystals
B(C F )
6
5 4
which were obtained by the reaction of i-Bu AlH with
2
Me Si
SiMe2
2
[
Ph C][B(C F ) ] (1) in THF. However, the crystal qual-
3 6 5 4
H
ity of the complex [i-Bu Al(THF) ][B(C F ) ] (4) was
2
2
6 5 4
Figure 2.
insufficient to discuss the structural data in detail. Nev-
ertheless the structural principle could be verified. The
X-ray structure of 4 is shown in Figure 3. The alumin-
ium cation is stabilised by two molecules of THF, acting
as donor ligands.
On the basis of our recently published results22 with
i-Bu AlH this knowledge was extended to new systems
2
without zirconium for hydrodefluorination reactions.
These results are reported in this Letter.
For the HDF experiments different types of C–F bonds
2
were tested. One can assume that C–F bonds with a sp -
The reaction of i-Bu AlH with the precatalysts
2
hybridised carbon atom are hardly accessible for HDF
reactions (Table 1, entries 1–5). In contrast to this obser-
[
[
Ph C][B(C F ) ] (1), [Ph C][Al(C F ) ] (2) and
3 6 5 4 3 6 5 4
Ph C][Al{OC(CF ) } ] (3) leads under formation of
3
3
3 3 4
vation the C–F bond of a sp -hybridised carbon atom is
triphenylmethane Ph CH to the aluminium cation
3
cleaved more easily (entries 6–17). A possible explana-
tion can be found in the different stability of the formed
carbocation, and therefore the different reactivity. In a
+
[
[
i-Bu Al]
and the weakly coordinating anions
2
ꢀ
5 4
ꢀ
M(C F ) ] (M = B, Al) and [Al{OC(CF3)3}4] which
6
remain as counterparts (Scheme 1).
2
sp -system the positive charge can be better delocalised
and, therefore, the carbocation is less reactive. More-
over, the first C–F bond in C F is very strong
6
6
[
Ph C] [M(C F ) ] +
i-Bu AlH
3
6 5 4
2
ꢀ1
(
D = 650 kJ mol ), further reducing the likelihood
0
of the desired transformation. These could be reasons
for the lower TONs of the HDF of sp -C–F bonds,
(
1) M= B
2) M= Al
2
(
for example, in the case of hexafluorobenzene and fluoro-
benzene. The HDF of 1,1,1-trifluorotoluene did not
yield toluene as the desired product. The reaction
stopped with mono- and dihydrodefluorinated products,
Ph CH + [
i
-Bu Al] [M(C F ) ]
2
6 5 4
3
Scheme 1.
even though sufficient equivalents of i-Bu AlH were
2

8902
M. Klahn et al. / Tetrahedron Letters 48 (2007) 8900–8903
Figure 3.
a
Table 1. Hydrodefluorination of substrates with non-activated C–F bonds
Entry
RF
RH
Pre-cat.
Solvent
TON
Conversion (%)
b
1
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
F
F
F
F
H
H
H
H
6
6
6
6
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
F
F
F
F
H
H
H
H
5
5
5
5
H
H
H
H
A 1
A 1
Toluene
Toluene
Toluene
Toluene
Toluene
n-Heptane
o-Dichlorobenzene
Benzene
o-Dichlorobenzene
Toluene
0
2
3
3
4
8
9
9
12
5
c
2
3
4
A 1
A 1
d
5
6
5
5
5
5
F
6
A 1
A 1
5
CF
CF
CF
3
5
5
5
CH
CH
CH
3
3
3
Does not react to desired product toluene
60
19
28
e
7
3
B 1
C Disilylcation
100
f
8
e
3
9
n-C
n-C10
5
H
11
F
n-C
n-C10
5
H
12
B 1
C Disilylcation
100
f
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
H
21
F
H
22
45
n-C
n-C
n-C
n-C
n-C
n-C
n-C
6
6
6
6
6
6
6
H
H
H
H
H
H
H
13
13
13
13
13
13
13
F
F
F
F
F
F
F
n-C
n-C
n-C
n-C
n-C
n-C
n-C
6
6
6
6
6
6
6
H
14
H
14
H
14
H
14
H
14
H
14
H
14
A 1
A 1
A 1
A 1
A 1
A 2
A 3
Toluene
Alkylation products of toluene
3
9
14
20
12
30
o-Dichlorobenzene
o-Difluorobenzene
Diethyl ether
Cyclohexane
Cyclohexane
Cyclohexane
95
95
99
100
100
99
A: Pre-catalyst: [Ph
Reaction conditions: 5 mol % pre-catalyst, C–F bond to i-Bu
3
C][B(C
6
F
5
)
4
] (1), [Ph
3
C][Al(C
6
F
5
)
4
] (2), [Ph
3 3 3 4
C][Al{OC(CF ) } ] (3).
a
2
AlH ratio at 1:1.1, T = 25 °C, t = 24 h; analysis via GC.
b
c
0
1
.5 mol % pre-cat.
mol % pre-cat.
d
1
0 mol % pre-cat.
e
25
B: Results from Ozerov and co-workers : 3.35 mol %, 8 equiv Et
C: Results from M u¨ ller and co-workers : entry 8: 2.9 mol % pre-cat.; entry 10: 2.2 mol % pre-cat.; 1 equiv Et SiH.
3
3
SiH.
f
26
TON: turnover number.
Conversion of substrate in %.
used. The reason for the incomplete conversion is
unknown.
used as the solvent, but only an extremely low TON
1
9
was achieved. Neither
F NMR, showing signals
which could not be assigned to a certain species, nor
the GC–MS gave any hints. The occurrence of the side
reactions of 1-fluorohexane can also be seen from the
consumption of 1-fluorohexane but it gives only a low
yield of n-hexane. This is a great disadvantage of
o-dichlorobenzene and leads to the consideration that
it is an inappropriate solvent for our investigations.
The o-difluorobenzene was tested because it provides a
relatively high polarity. So it is possible to dissolve the
precursor completely. The results were slightly better
but not outstanding. Toluene, the most common aro-
matic solvent, was used in the HDF investigations too.
The most convincing results were achieved with 1-fluoro-
hexane. Therefore, several investigations were done with
this substrate. At first we tested precursors 1 and 2, this
showed that 1 (TON = 20; entry 15 in Table 1) is more
reactive than 2 (TON = 12; entry 16 in Table 1). Based
on this information further investigations were per-
formed with 1 as the precursor. The main aspect was
to change the solvent: on the one hand to achieve a high-
er solubility and on the other hand to control the reactiv-
ity of the intermediate by interaction with the solvent.
According to the Ozerov system o-dichlorobenzene was

M. Klahn et al. / Tetrahedron Letters 48 (2007) 8900–8903
8903
But here a side reaction became prevailing. The carbo-
cation reacted with toluene to give only alkylated prod-
ucts of toluene and no formation of n-hexane was
observed. The different behaviour of the reagents and
the thereby limited application is demonstrated by com-
parison of the systems (see entries 10 with 11 and entries
3. Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. A. Chem.
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2
69.
. Mazurek, U.; Schwarz, H. Chem. Commun. 2003, 1321–
We also tested some aliphatic solvents though these pro-
vide a lower solubility, but they should be more inert in
our reactions. n-Heptane could not be used because the
reactions are analysed via gas chromatography and the
difference in the retention time of n-heptane and 1-fluoro-
hexane is too small for a proper signal integration to
make an accurate quantitative conclusion.
1326.
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1
1
1
The use of ethers combines a higher polarity with a good
inertness. The simplest example for this solvent class is
diethyl ether. The results with it were slightly worse than
with cyclohexane. This maybe due to the strong coordi-
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1
1
1
1
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3 3 4
[
Ph C][M(C F ) ] (M = B, Al) the highest turnover num-
3 6 5 4
bers were achieved in our investigations (TON = 30;
2
9
entry 17 in Table 1). Comparing the counteranions
seems to reveal a trend in which the decreasing coordi-
nating power is responsible for the different results of
the used precursors.
In conclusion, we have shown that a system consisting
of i-Bu AlH and [Ph C][B(C F ) ] (1), [Ph C][Al(C F ) ]
2
3
6
5 4
3
6 5 4
(
2) or [Ph C][Al{OC(CF ) } ] (3) is potentially useful for
3 3 3 4
2
005, 14, 2842–2849.
the room-temperature hydrodefluorination of non-acti-
2
0. Rosenthal, U.; Burlakov, V. V.; Arndt, P.; Baumann, W.;
vated C–F bonds. Nevertheless, the formed aluminium
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2
2
4, 4739–4749.
the required properties like triethylsilane for the HDF.
Possibly, the reactivity of the aluminium cation as an
intermediate is too unselective and not only C–F bonds
are activated, this is shown in the experiments by entries
2
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A deeper discussion and comparison of the three sys-
+
25
26
tems: Et SiH/[Et Si] , the disilyl cation and the i-
3
3
+
Bu AlH/[i-Bu Al] (this work) is not justified by the
2
2
partly different reaction conditions and the limited data.
2
004, 23, 4792–4795.
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Acknowledgements
This work was supported by the Leibniz-Gemeinschaft,
the Deutsche Forschungsgemeinschaft (SPP 1118 and
GRK 1213) and the Land Mecklenburg-Vorpommern.
We thank our technical staff, in particular Petra Bartels
and Regina Jesse, for assistance.
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