Catalytic C-F activation via cationic group IV metallocenes

Article abstract of DOI:10.1016/j.jorganchem.2014.12.011

The catalytic cleavage of sp³ C-F bonds of 3,3,3-trifluoropropene (TFP) can be performed using cationic group IV metallocenes and an excess of triisobutylaluminum. The isobutyl adduct 1,1-difluoro-5-methyl-hex-1-ene (DFMH) as well as 3,3-(diflu

Full text of DOI:10.1016/j.jorganchem.2014.12.011

Journal of Organometallic Chemistry 778 (2015) 21e28
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Catalytic CeF activation via cationic group IV metallocenes
Dominik Lanzinger, Ignaz M. H o€ hlein, Sebastian B. Weiß, Bernhard Rieger*
WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universit a€ t München, Lichtenbergstrasse 4, 85748 Garching bei München, Germany
a r t i c l e i n f o
a b s t r a c t
The catalytic cleavage of sp3 CeF bonds of 3,3,3-trifluoropropene (TFP) can be performed using cationic
group IV metallocenes and an excess of triisobutylaluminum. The isobutyl adduct 1,1-difluoro-5-methyl-
hex-1-ene (DFMH) as well as 3,3-(difluoroallyl)aromates (DFAArs) are formed in different ratios,
depending on reaction conditions. The FriedeleCrafts type reaction of TFP and the aromatic solvent
represents a new catalytic route toward DFAArs with different substituents (especially electron donors),
such as alkyl groups. In-situ FTIR and 19F NMR spectroscopy were used to gain closer insight into the
different defluorination reactions. The influence of the central metal, the ligand structure, the aromatic
solvent and the concentration of the reactants was investigated and mechanistic conclusions were
drawn.
Article history:
Received 1 November 2014
Received in revised form
8
December 2014
Accepted 10 December 2014
Available online 16 December 2014
Keywords:
C-F activation
Metallocene
Catalysis
© 2014 Elsevier B.V. All rights reserved.
Friedel-Crafts
Introduction
toward the formation of the corresponding alkyl chloride or alkyl
N
bromide, respectively. Thereby the CeF bond is cleaved via S 1-
Fluorinated organic molecules are widely spread in various
fields such as polymers, refrigerants, pharmaceuticals, agrochemi-
cals and many more [1,2]. Beneficial properties of most of these
substances are based on extremely stable carbonefluorine bonds,
type abstraction of fluoride by the Lewis acid. As a consecutive
reaction, FriedeleCrafts alkylation of aromatic compounds is
possible [4,6]. Terao et al. [14] enhanced this principle and devel-
3
oped a general method for the conversion of sp CeF bonds of alkyl
ꢀ
1
3
having homolytic dissociation energies up to (500 ± 50) kJ mol
3]. Synthesis of fluorinated organic molecules is a challenging task
fluorides to sp C-X (X ¼ Cl, C, H, O, S, Se, Te, N) bonds using
i
[
organoaluminum reagents of the composition R
2
AlY (R ¼ Et, Bu;
n
n
and can be achieved by introducing fluorinated groups as well as by
selective fluorination or defluorination reactions [3e5]. Defluori-
nation of easy to obtain, highly fluorinated molecules is a versatile
pathway toward complex, partially fluorinated hydrocarbons [6].
Various strategies for the activation of CeF bonds have been
developed and recent reviews give an excellent overview over the
state of the art in this field [5e10].
Y ¼ Cl, Et, CH ¼ CH Hex, C^C- Hex, H, OPh, SPh, SePh, TePh, NEt
They were able to show that the reaction does not proceed via a S
but via a S 2 mechanism [14].
2
).
1
N
N
3
Anexample ofcatalytichydrodefluorinationofsp CeFbondswas
reported by Ozerov et al. [15] An excess of triethylsilane was reacted
with 1-fluoropentane in the presence of trityl tetrakis(penta-
fluorophenyl)borate or triethylsilyl tetrakis(pentafluorophenyl)
borate to form pentane. Further investigations showed that the re-
action can be performed with dialkylaluminum hydrides or trialkyl
aluminum in a similar way, whereby trityl carboranes are used as
long lived catalyst, which enable much higher turnover numbers
(TONs) than the tetrakis(pentafluorophenyl)borates [16,17]. If the
reaction is conducted in aromatic solvents or if aromatic substrates
were used, FriedeleCrafts type alkylation products are formed as
well. Similar results were reported by Mülleret al. [18] and Rosenthal
et al. [19], who performed catalytic hydrodefluorination reactions
with silylium borates and aluminum borates, respectively.
Focusing on defluorination reactions, two major trends should
be kept in mind concerning the stability of CeF bonds. Firstly, the
higher the number of fluorine atoms attached to one C-unit, the
shorter and stronger are all bonds to this C-species. This effect is
3
2
more pronounced for sp than for sp carbon atoms [11]. Secondly,
2
sp CeF bonds of aromatic and olefinic fluorocarbons are weaker
than sp CeF bonds of the aliphatic analogs [12,13].
3
3
Lewis acids are known to be able to activate sp CeF bonds [7].
In general boron and aluminum containing Lewis acids, especially
AlCl and BBr are capable of activating CeF bonds of alkyl fluorides
3 3
CeF bond activation by transition metal complexes via various
mechanisms is a well established field although it is mainly limited
2
*
Corresponding author. Tel.: þ49 89 289 13571; fax: þ49 89 289 13562.
E-mail address: Rieger@tum.de (B. Rieger).
to sp CeF bonds [8]. Especially for group IV metal complexes
http://dx.doi.org/10.1016/j.jorganchem.2014.12.011
022-328X/© 2014 Elsevier B.V. All rights reserved.
0

22
D. Lanzinger et al. / Journal of Organometallic Chemistry 778 (2015) 21e28
2
several stoichiometric and also some catalytic strategies for sp CeF
bond activation are known [20e30]. Kiplinger and Richmond [28]
reported that low valent Zr (II) or Ti (II) species, which are gener-
Experimental section
General considerations
ated by treating Cp
2
ZrCl
2 2 2
or Cp TiCl with reducing agents (Al or
Mg together with HgCl
2
) can perform reductive defluorination e.g.
Unless otherwise stated, all manipulations were performed
either under standard Schlenk conditions using dry argon or in an
argon filled Glovebox (MBraun Unibox). Benzene was distilled over
Na/benzophenone and stored over molecular sieves (4 Å), HPLC
grade TCB was degassed and stored over molecular sieves (4 Å),
toluene was obtained from an MBraun MB-SPS solvent purification
of perfluorodecaline to octafluoronaphthalene. Furthermore the Zr
systems can mediate hydrodefluorination reactions of fluorinated
aromatic compounds via oxidative addition of the aromatic CeF
bond followed by homolytic cleavage of ZreC bond. Rear-
omatization takes place by radical hydrogen abstraction from the
solvent THF [28,29]. Similar reactions were observed for Ti (II), Zr
system. Bis(cyclopentadienyl) zirconium(IV) dichloride (Cp
bis(pentamethylcyclopentadienyl) zirconium(IV) dichloride
(Cp* ZrCl ), bis(cyclopentadienyl) hafnium(IV) dichloride (Cp HfCl
2 2
ZrCl ),
(
II) and Hf (II) species generated by other strategies [9,24,30,31].
However, not exclusively low valent group IV metal complexes
2
2
2
2
)
can activate CeF bonds. Jones et al. [12,23,32e37] contributed
comprehensive work on CeF activation by zirconocene and haf-
nocene dihydrides [12,23,32e37]. Depending on the substrate,
various mechanisms were suggested involving the formation of Zr
and 3,3,3-trifluoropropene (TFP) were purchased from ABCR, trii-
sobutylaluminum (TIBA) was purchased from Aldrich and all were
used without further purification. Trityl tetrakis(pentafluorophenyl)
5
borate (borate) [2] and [1-(9-
h
-fluorenyl)-2-(5,6-cyclopenta-2-
eZrCl ) [1] were prepared as
described in literature. Solution NMR spectra were recorded on a
5
(
III) radicals, Zr (II) species, or in case of fluorinated olefins insertion
into ZreH bonds followed either by - or -fluoride elimination
12,23,32e37]. For the stoichiometric reaction of bis(pentamethyl
cyclopentadienyl)zirconium(IV) dihydride (Cp* ZrH ) with 3,3,3-
trifluoropropene (TFP) both insertion regioisomers were observed
methyl-1-
h
-indenyl)ethane]ZrCl
2
(C
1
2
b
a
1
13
[
Bruker AV-500CRYO or a Bruker AV-500 spectrometer. H-, C-, and
F NMR spectroscopic chemical shifts
19
2
2
d
are reported in ppm relative
1
to tetramethylsilane.
d( H) is calibrated to the residual proton signal,
ꢁ
ꢁ
13
19
at ꢀ90 C and decomposed upon warming to ꢀ70 and ꢀ10 C,
d(
C) to the carbon signal of the deuterated solvent.
brated to the signal of hexafluorobenzene (C
, ꢀ164.9 ppm) or
trichloro-fluoro-methane (CFCl , 0 ppm). GCeMS analysis was
conducted using a Varian 3900 GC (Varian FactorFour VF-5 ms
d( F) is cali-
respectively [12].
6 6
F
Although very stable metal-fluorine bonds are formed during
the CeF activation, highly fluorophilic aluminum compounds
enable the regeneration of both, neutral and cationic Zr (IV) hy-
drides from the corresponding fluorides [25,38]. Rosenthal et al.
3
capillary column 0.25 mm Å ~ 30 m Å ~ 0,25
m
m; injector temper-
ꢁ
ꢁ
ature: 210 C; carrier gas: He (1 ml/min); column temperature 80 C
ꢁ
[25] published the first example for the hydrodefluorination of
(3 min) -> 250 C (10 K/min) combined with a Varian Saturn 2100T
fluorinated pyridines catalyzed by neutral Zr (IV) hydrides. Crim-
min et al. [20] extended this concept to non-heterocyclic fluo-
roarenes. Lentz et al. [22] published quite similar results for the
system (EI: 70 eV; m/z: 20e650).
Procedure for defluorination reactions
hydrodefluorination of fluorinated olefins with Cp
2 2
TiF as a catalyst
and silanes as fluorine trap. Ti (III) hydrides are stated to be the
Defluorination reactions were performed using a React-FTIR/
MultiMax four-autoclave system (Mettler-Toledo). One of the
50 ml stainless steel reactors with a diamond window, a mechanical
stirring and a heating device was dried and put under argon at-
mosphere prior to use. The reactor was heated to 40 C and pres-
surized with 1.4 bar argon. Unless otherwise stated, all
active species and the reaction is assumed to proceed both, via a
s
b
-
-
bond metathesis mechanism as well as via a -hydride insertion/
b
fluoride elimination pathway. Radical mechanisms are assumed to
ꢁ
be unlikely to occur [21]. Using TFP as a substrate even catalytic
3
cleavage of sp CeF bonds was observed, albeit with low TOF
ꢀ1
(
0.04 min ) and TON (43) values. Under the reaction conditions
defluorination reactions were performed using benzene as aromatic
which were applied, the conversion of TFP is rather unselective and
not only 1,1-difluoropropene, but also 1,1,1-trifluoropropane and 1-
fluoropropene were formed [21,22].
solvent, 8 mmol of metallocene, 8 mmol of TIBA, 32 mmol of borate
and a TFP partial pressure of 1.1 bar. The amount of aromatic solvent
was adapted to yield a total volume of the reaction solution of 23 ml.
If a metallocene dichloride was used, the cationic species was
generated in-situ in two steps from neutral metallocene dichlorides.
Firstly, the metallocene dichloride was placed in a 25 ml Schlenk
tube and dissolved in the aromatic solvent (benzene, toluene or
1,2,4-trichlorobenzene). 200 equivalents of TIBA were added to give
Whereas there are some reports on CeF bond activation by
neutral group IV metallocene complexes, only few reports on the
interaction of cationic metallocene complexes with fluorocarbons
exist [39,40]. Cationic group IV metallocenes are well studied sys-
tems and find broad application especially in olefin polymerization
reactions [41]. Attractive metal-fluorine interactions have been
ꢁ
a total volume of 3 ml. The preactivation mixture was kept at 60 C
19
detected in crystal structures and low temperature F NMR studies
for 60 min. The preactivation solution was then transferred into the
reactor, which was previously equipped with the corresponding
amounts of aromatic solvent and TIBA. In a second step, 5 ml of a
solution of borate and the aromatic solvent were added. This acti-
vation procedure is well known from olefin polymerization re-
actions using metallocene dichlorides as precatalysts [44].
[
[
40,42,43]. Marks et al. [39] reported the decomposition of
(CH CpZrCH ][B(C ] in benzene solution after several weeks
3
)
2
3
6 5 4
F )
forming a fluorine bridged binuclear complex. Hessen et al. could
prove the coordination of fluorobenzene to the cationic Ti(III)
complex [Cp*
not cleave it [40]. In contrast to fluorobenzene,
fluorotoluene reacts rapidly with [Cp* Ti][B(C
Cp* TiF and 1,2-diphenyl-1,1,2,2-tetrafluoroethane as main
products.
Formation of Cp*
complexes with borate counterions, which contain benzylic fluo-
rine atoms, such as in [Cp* Ti][B(3,5-(CF ].
2
Ti][B(C
6
5
H )
4
], which weakens the CeF bond but does
-tri-
to form
a,a,
a
The in-situ FTIR data collection was started and after three mi-
ꢁ
2
6
H
5
)
4
]
nutes of stirring at 40 C and 500 rpm, TFP was added. The addition
2
2
of TFP was controlled with an EL PRESS pressure meter controller
unit (Bronkhorst High-Tech). Within seconds, the desired total
pressure was reached. The reaction was stopped after 60 min by
depressurizing the reactor. The reaction solution was immediately
2
2
TiF was also observed for the corresponding
ꢁ
2
3
)
2
C
6
H
3
)
4
transferred to sealable glass vials and stored at ꢀ20 C. Samples for
19
Up to now, no attempts have been made to use cationic group IV
metallocene complexes for selective defluorination of fluorocar-
bons, although there is a great potential.
F NMR spectroscopy were taken directly from the reaction solu-
tion and a sealed glass capillary filled with hexafluorobenzene in
19
benzene-d8 (1 mL/ml) was added. For the F NMR spectra taken

D. Lanzinger et al. / Journal of Organometallic Chemistry 778 (2015) 21e28
23
directly from the reaction solutions, 19F NMR shifts were deter-
mined relative to the signal of hexafluorobenzene, which was
referenced to ꢀ164,9 ppm. The ratio of product integrals to the
hexafluorobenzene integral was used for quantification, whereby
the observed concentration of hexafluorobenzene in the NMR-
carefully quenched with ice water and washed with 2 M hydro-
chloric acid. The organic layer was dried with MgSO and distilled
under reduced pressure. A fraction containing DFMH in benzene
4
ꢁ
was obtained at 650 mbar and 55 C.
1
H NMR (500 MHz, C
6
D
6
,
d
): 3.81 (dtd, J ¼ 25.8, 8.0, 2.5 Hz, 1H),
19
samples was previously determined by F NMR spectroscopy of a
sample of -trifluorotoluene in benzene (1 L/ml) containing
1.74 (q, J ¼ 7.8 Hz, 2H), 1.31 (dh, J ¼ 13.3, 6.6 Hz, 1H), 0.99 (q,
19
a,a
,a
m
J ¼ 7.3 Hz, 2H), 0.76 (d, J ¼ 6.6 Hz, 6H). F NMR (471 MHz, C
6 6
D ,
the same capillary.
d
): ꢀ91.9 (d, J ¼ 50 Hz, 1F), ꢀ94.6 (ddt, J ¼ 50, 26, 2 Hz, 1F). IR (thin
ꢀ
1
film):
n
max 1756 cm (C]CF
2
). GCeMS m/z (% relative intensity,
), 99 (10, -CH e HF), 91
).
Determination of max. rate, TON* and TOF values
ion): 135 (5, M), 134 (31, M), 119 (11, -CH
15, -C ), 83 (16, -CHF ), 77 (95, -C4H
3
3
(
3
H
7
2
9
Max. rates were determined from the maximum slope of the FTIR
ꢀ
1
signal traces (1740e1760 cm ) and the sum of the yields of 3,3-
1,1-Difluoropropene (DFP)
DFP was identified by 19NMR spectroscopy using literature data
(
difluoroallyl)aromates (DFAAr), 1,1 difluoro-5-methyl-hex-1-ene
DFMH) and 1,1-difluoropropene (DFP) determined by F NMR
19
(
spectroscopy. As even in absence of metallocene, certain amounts of
DFAArs, DFMH and DFP are formed, for the calculation of corrected
TON (TON*) and TOF values, FTIR traces were corrected by subtracting
the FTIR trace of the corresponding metallocene free experiment.
[45].
Results and discussion
Metallocene catalyzed CeF activation of TFP
3,3-(Difluoroallyl)benzene (DFAB)
In contrast to most of the studies on the activation of CeF bonds
with metallocene catalysts, we did not use neutral but cationic
species with weakly coordinating counterions in the presence of an
excess of trialkyl aluminum [46].
The reaction solutions of several metallocene catalyzed re-
actions, which were performed in benzene were combined, care-
fully quenched with ice water and washed with 2 M hydrochloric
acid. The organic layer was dried with MgSO
4
and distilled under
As depicted in Scheme 1, we observed that the defluorination
reaction of TFP is much faster in the presence of preactivated group
reduced pressure. The sump containing mainly DFAB was purified
by column chromatography in n-pentane. Removal of pentane
under reduced pressure yielded DFAB as a colorless liquid.
Analytical data is in accordance to literature [27].
ꢀ1
IV metallocene dichlorides (max. rate of 758
mmol min
for
Cp ZrCl ) compared to the reaction of TFP in a solution of TIBA and
2
2
ꢀ1
borate without metallocene (max. rate of 35 mmol min ) or a so-
1
1
13
19
H NMR, C NMR, F NMR, FT-IR, GCeMS.
H NMR (500 MHz, CDCl ): 7.35e7.27 (m, 2H), 7.26e7.16 (m,
lution of TIBA in benzene in absence of borate and metallocene
ꢀ1
3
,
d
(max. rate of 7.5
m
mol min ).
3
H), 4.40 (dtd, J ¼ 24.8, 8.0, 2.2 Hz, 1H), 3.33 (dt, J ¼ 8.2, 1.8 Hz, 2H).
The addition of preactivated metallocene did not affect the re-
action in absence of borate as no cation forming agent was present.
This indicates that under the given conditions not neutral metal-
locene species but cationic metallocene complexes are the cata-
lytically active species.
1
6 6
H NMR (500 MHz, C D , d): 7.10e7.06 (m, 2H), 7.05e7.01 (m, 1H),
6
.91e6.87 (m, 2H), 3.98 (dtd, J ¼ 25.1, 8.0, 2.3 Hz, 1H), 2.93 (dt,
1
3
J ¼ 8.1, 1.7 Hz, 2H). C NMR (126 MHz, CDCl
3
, d): 156.7 (dd,
J ¼ 287.6, 285.9 Hz), 139.6 (t, 2.3 Hz), 128.7, 128.2, 126.6, 77.8 (dd,
19
2
(
.3, 2.2 Hz), 28.5 (d, 4.7 Hz). F NMR (471 MHz, C
dd, J ¼ 46, 2 Hz), -94.33 (ddt, J ¼ 45, 25, 2 Hz). GCeMS m/z (%
relative intensity, ion): 155 (10, M), 154 (100, M), 134 (30, MꢀHF),
6
D
6
,
d): ꢀ91.28
Two fluorinated main products, 3,3-(difluoroallyl)benzene
(DFAB) and 1,1-difluoro-5-methyl-hex-1-ene (DFMH) were identi-
fied by NMR and GCeMS analyses. 1,1-Difluoropropene (DFP) was
formed as a minor product. All three of these products have a
ꢀ
1
1
33 (46, MꢀHF). IR (thin film):
nmax 1749 cm (C]CF ).
2
ꢀ1
characteristic IR absorption band between 1740 and 1760 cm
corresponding to F C CHR (R CH eC H , CH3,
3
,3-(Difluoroallyl)toluene (DFAT)
a
¼
¼
2
2
6 5
DFAT (mixture of isomers) was isolated in analogy to DFAB from
the solution of the reaction with 8 mol of Cp ZrCl , 32 mol
m
2
2
m
borate, 8 mmol TIBA and a TFP partial pressure of 1.1 bar in toluene.
DFAT was obtained as a colorless liquid.
1
6 6
H NMR (500 MHz, C D , d): 7.25e6.80 (m, 4H), 4.07e3.85 (m,
19
1
d
H), 3.00e2.85 (m, 2H), 2.2e1.95 (m, 3H). F NMR (471 MHz, C
): ꢀ91.3 (d, J ¼ 46 Hz, 1F Isomer 1), ꢀ91.4 (d, J ¼ 46 Hz, 1F Isomer
), ꢀ91.5 (d, J ¼ 46 Hz, 1F Isomer 2), ꢀ93.9 (ddt, J ¼ 46, 25, 2 Hz, 1F
Isomer 1), ꢀ94.3 to ꢀ94.5 (m, 1F Isomer 3), ꢀ94.5 (dd, J ¼ 46, 25 Hz,
F Isomer 2). 19F NMR (471 MHz, CDCl
6 6
D ,
3
1
3
,
d): ꢀ88.8 (d, J ¼ 46 Hz, 1F
Isomer 1), ꢀ88.9 (d, J ¼ 46 Hz, 1F Isomer 3), ꢀ89.0 (d, J ¼ 46 Hz, 1F
Isomer 2), ꢀ91.3 (ddt, J ¼ 46, 25, 2 Hz, 1F Isomer 1), ꢀ91.8 (ddt,
J ¼ 46, 25, 2 Hz, 1F Isomer 3), ꢀ91.9 (ddt, J ¼ 46, 25, 2 Hz, 1F Isomer
ꢀ
1
2
). IR (thin film):
n
max 1749 cm (C]CF
2
). GCeMS m/z (% relative
), 153 (66,
intensity, ion): 169 (11, M), 168 (100, M), 154 (5, MꢀCH
3
MꢀCH ), 134 (6, MꢀCH eHF), 133 (59, MꢀCH - HF).
3
3
3
1,1-Difluoro-5-methyl-hex-1-ene (DFMH)
The reaction solutions of several metallocene catalyzed re-
actions, which were performed in benzene were combined,
Scheme 1. Formation of DFAB and DFMH from TFP (partial pressure 1.1 bar with a)
TIBA, b) TIBA and borate, c) TIBA, borate and Cp ZrCl
2
2
.

24
D. Lanzinger et al. / Journal of Organometallic Chemistry 778 (2015) 21e28
place at the same time, we performed several sets of experiments
whereby reaction parameters, such as concentration of TIBA, TFP,
borate and metallocene, as well as the type of solvent were varied
in order to gain insight into the different reaction pathways.
Influence of the central metal
Defluorination reactions of TFP were performed using TIBA
ꢀ
1
(
“TIBA”), TIBA and borate (“borate”) (max. rate: 35
and TIBA, borate and 8 mol of either Cp TiCl
Cp HfCl in the presence of TIBA and borate (“Cp
and “Cp HfCl ”) (see Figs. 1 and 2). Both, Cp
58
max. rate 750
m
mol min ),
, Cp ZrCl , or
TiCl ”, “Cp ZrCl ”,
ZrCl (max. rate:
HfCl
mol min ; max. TOF: 78 min , TON*: 95) strongly
m
2
2
2
2
2
2
2
2
2
2
2
2
2
2
ꢀ
1
ꢀ1
7
(
m
mol min ; max. TOF: 80 min ; TON*: 117) and Cp
2
2
ꢀ1
ꢀ1
m
Fig. 1. Yields of DFAB and DFMH for defluorination reactions of TFP determined by 19F
NMR spectroscopy. Reactions were performed without metallocene using TIBA
accelerate the defluorination reaction during the first minutes after
TFP addition compared to the metallocene free control experiment
(
“TIBA”), TIBA and borate (“borate”), and with TIBA, borate and 8
mmol of either
ꢀ1
ꢀ1
(
borate). Cp
2
TiCl
2
(max. rate: 300 min ; max. TOF: 26 min
;
Cp TiCl , Cp HfCl , or Cp ZrCl , respectively (“Cp2TiCl2”, “Cp2HfCl2”, and “Cp2ZrCl2”).
2
2
2
2
2
2
TON*: 25) enhances the reaction as well, although the effect is
19
much weaker. F NMR product analysis (Fig. 1) reveals that the
addition of Cp ZrCl , Cp HfCl , and Cp TiCl increase the formation
2 2 3 3
CH eCH eCH(CH ) ) double bond stretching vibration. Due to the
2
2
2
2
2
2
of DFAB by 242%, 191%, and 23% respectively, compared to the
metallocene free control experiment. Meanwhile, the production of
DFMH barely changes.
The higher catalytic activities found for the zirconocene and the
hafnocene complex compared to the corresponding titanocene
complex are in accordance with higher catalytic activities being
found for zirconocene and hafnocene complexes in well studied
metallocene catalyzed olefin polymerization reactions [1,44,47].
The lower activity, which is observed for titanocenes is mostly
contributed to their tendency toward reduction of Ti (IV).
lack of further intense and characteristic IR-bands it is not possible
to differentiate between these three compounds in in-situ FTIR
spectra. Therefore, 19F NMR spectroscopy was applied for the
determination of product yields, product ratios and TONs, while in-
situ FTIR signal traces (1740e1760 cm ) reflect the reaction
progress toward DFAB, DFMH, and DFP and were used to calculate
max. rates and max. TOFs.
TON is defined as number of TFP molecules that are converted
into DFAB, DFMH or DFP by one molecule of metallocene in the
investigated time span (57 min). As mentioned above, significant
amounts of DFAB, DFMH and DFP are produced even in absence of
metallocene in the reaction mixture (see “borate” in Fig. 1 Fig. 2).
Therefore, for the calculation of corrected TON values (TON*) we
subtracted the total yield of DFMH, DFAB and DFP of the metal-
locene free experiment from the corresponding value of the met-
allocene catalyzed reaction.
ꢀ
1
Influence of the ligand structure
Different zirconocene dichlorides were used as precatalysts in
order to investigate effects of the ligand structure on the defluori-
nation reaction (see Fig. 3).
The comparison of L
ethylcyclopentadienyl (Cp*) ligands lead to a lower catalytic activity
2
ZrCl
2
(L ¼ Cp, Cp*) shows that pentam-
The max. TOF is defined to be the max. number of turnovers per
minute per molecule of metallocene. For the calculation of max.
ꢀ1
ꢀ1
(
max. rate 196
mmol min ; max. TOF 13 min ), during the first
ꢀ
1
TOF values, the FTIR signal trace (1740e1760 cm ) of the metal-
locene free experiment is subtracted from the FTIR signal traces of
the corresponding metallocene catalyzed reactions. Product yields
obtained from 19F NMR spectroscopy were used to calculate the
max. TOFs from the max. slopes of these corrected FTIR signal
traces.
minutes of the reaction compared to the cyclopentadienyl-system
ꢀ1
ꢀ1
(
Cp
Fig. 4). This can be contributed to a higher sterical demand of the
Cp*. The well studied ethylene-bridged, C -symmetric metallocene
2 2
ZrCl ) (max. rate 758 mmol min ; max. TOF 80 min ) (see
1
5
5
[
1-(9-
ethane]ZrCl
21
2
than Cp ZrCl [44,48] Figs. 5,6.
h -fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-h -indenyl)
) gives a lower catalytic activity (max. rate
mol min ; max. TOF 13 min ) and a longer catalytic lifetime
2
(C
1
eZrCl
2
In order to investigate effects of the catalyst composition, met-
allocene dichlorides with Ti, Zr and Hf as central metal and different
ligand structures were tested. As there are multiple reactions taking
ꢀ1
ꢀ1
1
m
2
Fig. 2. FTIR signal traces (1740e1760 cmꢀ1
defluorination reactions with TIBA, borate, and 8
Cp TiCl (“Cp ZrCl ”, “Cp HfCl ”, and “Cp TiCl ”), control experiments without met-
allocene using TIBA and borate (“borate”), or only TIBA (“TIBA”), respectively.
)
of DFAB, DFMH and DFP for TFP
mol of either Cp ZrCl , Cp HfCl , or
Fig. 3. FTIR signal traces (1740e1760 cm
defluorination reactions with TIBA, borate and either Cp
respectively (“Cp ZrCl ”, “C eZrCl ”, and “Cp* ZrCl ”) and the control experiment
without metallocene using TIBA and borate (borate).
ꢀ1
)
of DFAB, DFMH and DFP for TFP
m
2
2
2
2
2
ZrCl , C eZrCl , or Cp* ZrCl ,
2
1
2
2
2
2
2
2
2
2
2
2
2
2
2
1
2
2
2

D. Lanzinger et al. / Journal of Organometallic Chemistry 778 (2015) 21e28
25
Fig. 4. Long term measurement of the FTIR signal traces (1740e1760 cmꢀ1) of DFAB,
2 2 2 2
DFMH and DFP for TFP defluorination reactions with Cp* ZrCl , Cp ZrCl and reference
experiment without any metallocene (“borate”).
Scheme 2. Reaction of benzene (a), toluene (b) or other aromatic substances (c) with
TFP in the presence of TIBA, borate, and Cp ZrCl .
2 2
toluene and TCB not DFAB, but isomers of the corresponding 3,3-
difluoroallyl)aromates (DFAArs) 3,3-(difluoroallyl)-toluene (DFAT)
(
and 3,3-(difluoroallyl)-trichlorobenzene (DFATCB) are expected.
While DFAT could be isolated, DFATCB was not identified unam-
biguously. The comparison of the experiments conducted in ben-
zene and toluene indicates that higher DFAAr yields are reached for
more electron rich aromates. The fact that no, or only low amounts
of DFATCB are formed also supports this conclusion, although steric
effects might also play a role in case of TCB. In order to prove that
the catalytically active species is not decomposed rapidly in TCB, 1-
hexene was added to the reaction solution.1-hexene is known to be
Fig. 5. Sum of the yields of DFAAr, DFMH and DFP for TFP defluorination reactions with
oligomerized by cationic metallocenes generated from Cp
2 2
ZrCl
TIBA, borate and Cp
2
ZrCl
2
in benzene, toluene and TCB calculated from FTIR signal
[
26]. Consumption of 1-Hexene was observed, as the FTIR absor-
ꢀ
1
19
traces (1740e1760 cm ) and F NMR spectra.
ꢀ
1
bance of the C]C bending vibration at 910 cm decreased with
time. While the yields of DFAArs vary strongly, the yields of DFMH
are less affected by the solvent.
We could not only show that the rate of the formation of DFAArs
depends on the electron density in the
p-system of the aromatic
solvent but also established a new catalytic route toward DFAArs
with different substituents, especially electron donors, such as alkyl
groups (see Scheme 2). These might be of interest e.g. for phar-
maceutical applications [49].
Reaction of TFP with TIBA in absence of borate and metallocene
Trialkyl aluminum reagents are known to react with fluo-
3
3
Fig. 6. Yields of DFAArs (DFAB, DFAT and DFATCB) and DFMH for defluorination re-
roalkanes by activating sp CeF bonds and forming sp CeC bonds
instead [14]. In accordance to literature, we observed a reaction of
TFP with the trialkylaluminum compound TIBA in benzene as sol-
vent [14]. The conversion of TFP was extremely low compared to
experiments conducted in the presence of borate or metallocene
and borate. DFAB, DFP and DFMH are formed, whereby DFMH is the
main product.
actions of TFP with TIBA, borate and Cp
by F NMR spectroscopy (DFATCB < 150 mol).
2
ZrCl
m
2
in TCB, benzene and toluene determined
19
In contrast to the Cp system, the Cp* system stays catalytically
ꢀ1
active even after more than one hour (TOF of 1.2 min
0 min). The deactivation of the catalytically active species is sup-
pressed by the sterically demanding methyl groups in Cp* ZrCl
after
5
2
2
.
Depending on the concentration of TIBA in the reaction mixture,
the product ratios changed (Fig. 7). With increasing TIBA concen-
tration, the yield of DFMH rises while the amount of DFAB does not
change significantly. Variation of the TFP partial pressure resulted
in no significant change in the product composition (Fig. 8).
Reliable kinetic measurements could not be performed due to
very low yields. However, the data indicate that the reaction toward
DFMH is of first order with respect to TIBA.
This result shows that long catalyst lifetimes are possible for the
catalytic defluorination of TFP with cationic metallocenes if the
ligand system is properly substituted.
Influence of the solvent
Three aromatic solvents: benzene, toluene, and 1,2,4-
trichlorobenzene (TCB) were tested under similar conditions us-
ing Cp
2
ZrCl
2
as metallocene precatalyst.
In absence of borate and metallocene, DFMH is formed by the
reaction of TIBA with TFP (see Scheme 3). Insertion reactions of
olefins into AleC bonds in so called Aufbau reactions are known for
In-situ FTIR studies and 19F NMR analyses reveal that the con-
version of TFP strongly depends on the aromatic solvent. In case of

26
D. Lanzinger et al. / Journal of Organometallic Chemistry 778 (2015) 21e28
Fig. 7. Yields of DFAB and DFMH for defluorination reactions of TFP in absence of
borate and metallocene at different TIBA concentrations.
Scheme 3. Proposed mechanism for the formation of DFMH in one reaction step via a
six membered transition state (TS) (counterclockwise) or in a sequence of two reaction
steps via an insertion elimination mechanism (clockwise).
many years and usually take place at elevated temperatures and
high olefin concentrations [50]. The electron withdrawing eCF
substituent is expected to change the selectivity of this insertion
reation from 1,2-, which is usually preferred for -olefins to 2,1-
insertion [12,47,50,51]. A consecutive -fluoride elimination leads
to DFMH and fluoro diisobutylaluminum ( Bu
3
a
b
i
2
AlF). In contrast, the
reaction via a six membered transition state (TS) leads to the re-
action products in a one step reaction.
Fig. 8. Yields of DFAB and DFMH for defluorination reactions of TFP with TIBA in
absence of borate and metallocene at different TFP partial pressures.
As shown in Fig. 7, the formation of DFAB is not enhanced at
higher TIBA concentrations. We assume the formation of DFAB to
take place via a cationic mechanism (Scheme 4) even in absence of
a cation forming agent, such as borate. In a first step, the reaction of
i
ꢀ
TIBA with TFP leads to the formation of anionic Bu
3
AlF and
þ
cationic C
H
3 3
F
2
. This carbocation subsequently reacts either with
the aromatic solvent benzene to form DFAB in a FriedeleCrafts type
reaction or with TIBA to form DFMH. The formation of DFP is most
likely caused by DIBAL-H impurities [52]. All three reaction
Fig. 9. Yields of DFAB and DFMH for defluorination reactions of TFP with TIBA, borate
at different C eZrCl concentrations.
1 2
Fig. 10. Double logarithmic plot of the corrected max. rates (max. rate*) [
versus the C eZrCl concentrations [ mol/ml] for TFP defluorination reactions con-
ducted with 2, 4 and 8 mol C eZrCl
mmol/min]
1
2
m
2
.
Scheme 4. Proposed mechanisms for the formation of DFAB, DFMH and DFP via a
difluoroallyl cation.
m
1

D. Lanzinger et al. / Journal of Organometallic Chemistry 778 (2015) 21e28
27
pathways lead to the formation of a diisobutylaluminium cation
defluorination reactions, which are not attributed to metallocene
catalysis.
The data presented in Fig. 10 indicates that the reaction toward
DFAB is approximately of first order with respect to the
metallocene.
i
þ
þ
3 2
F car-
(
Bu
2
Al ), which can react with TFP to form another C
3
H
i
bocation and Bu
is closed [15,16].
2
AlF. Thus, the reaction cycle of this chain reaction
Reaction of TFP with TIBA in the presence of borate
Influence of the TFP partial pressure
A set of experiments at varying TFP pressures in the presence
and absence of metallocene C eZrCl was performed. The com-
1 2
parison of these results indicates that under the given conditions
the metallocene catalyzed reaction (e.g. the difference between
metallocene containing and metallocene free reactions) is inde-
pendent of the TFP pressure.
In accordance to literature, we found the reaction of TIBA and
TFP in the presence of borate to give higher yields of DFAB and
DFMH compared to borate free reactions, whereby DFAB is pro-
duced as the main product (Figs. 7 and 11) [15e17]. The higher
yields of DFMH compared to borate free experiments give another
hint that DFMH is not exclusively formed by the reaction of TIBA
and TFP (Scheme 3) but also via a mechanism involving cationic
species similar to the one depicted in Scheme 4. Upon addition of
Influence of the TIBA concentration
In order to investigate the influence of the TIBA concentration
on the product composition, experiments were conducted using
variable TIBA concentrations (see Fig. 12). As expected from the
data obtained for the reaction in absence of borate and metallocene
borate, the yield in DFAB is increased by a factor of 18 (22.83
> 408.5 mol) compared to a factor of only 3.6 (62.87 mol ->
25.8 mol) for DFMH. This strongly indicates that DFMH is formed
mmol
-
m
m
2
m
via two different mechanisms, whereby only one of them involves a
cationic species. This cationic mechanism only plays a minor role in
absence of borate, but becomes dominant if borate is present.
(
see Fig. 7) the production of DFMH increases with increasing TIBA
concentrations. However, to our surprise, less DFAB was produced
at higher TIBA concentrations. This leads us to the conclusion that
high TIBA concentrations lead to a faster decomposition of the
catalytically active, cationic metallocene species.
Fig. 11. Yields of DFAB and DFMH for defluorination reactions of TFP with TIBA and
borate at different TFP partial pressures a) in the presence and b) in absence of
1 2
C eZrCl .
Fig. 12. Yields of DFAB and DFMH for defluorination reactions of TFP with different
1 2
amounts of TIBA in the presence of borate and C eZrCl .
Reaction of TFP with TIBA in the presence of borate and metallocene
For the following experiments [1-(9-h5-fluorenyl)-2-(5,6-
Reaction mechanism for the metallocene catalyzed formation of
DFAB
cyclopenta-2-methyl-1-h5-indenyl)ethane]ZrCl
2
1 2
(C eZrCl ) was
used as metallocene compound because of its moderate catalytic
activity and life time.
Based on our experimental data we suggest the following
mechanism for the metallocene-catalyzed defluorination of TFP to
form the FriedeleCrafts adduct DFAB (see Scheme 5).
Influence of the metallocene concentration
If catalytic amounts of preactivated C
reaction mixture, the defluorination reaction is accelerated
1
eZrCl
2
are added to the
The cationic metallocene species is stabilized by the weakly
coordinating borate anion. The coordination of TFP to the metal
compared to the control experiment with TIBA and borate. Exper-
center is assumed to take place both, via the p-system of the C]C
iments with different amounts of C
mol) were performed under similar reaction conditions (see
Fig. 9).
1
eZrCl
2
(4
m
mol, 8
m
mol and
double bond, as well as via agostic interaction of one of the fluorine
atoms and the metal center. We assume the fluorine atom to be
coordinated in the central equatorial site and the double bond, as
well as the isobutyl group to be coordinated in the lateral sites.
Therefore, the spatial separation of the isobutyl group and the C1-
atom of the olefin inhibit an intramolecular insertion reaction,
16 m
The analysis of the reaction products reveals that mainly the
reaction toward DFAB is accelerated by the addition of metallocene,
while the amount of DFMH does not change significantly.
In analogy to the max. TOF values, so called “corrected max.
which would be followed by a b-fluoride elimination to give DFMH.
19
2
rates” (max. rate*) were determined from in-situ FTIR and F NMR
This coordination activates the terminal sp carbon atom toward
the electrophilic attack of aromates, which simultaneously leads to
the formation of a metalecarbon bond. The resulting carbocationic
data. For the determination of max. rate*s, the FTIR signal trace
ꢀ
1
(
1740e1760 cm ) of the metallocene free experiment is subtracted
i
þ
from the FTIR signal traces of the corresponding metallocene
catalyzed reactions prior to the determination of the maximum
slope. This correction of the data is necessary to omit the share of
intermediate [L
both lead to the formation of DFAB, [L
methylpropane. Firstly, it can undergo a
2
M Bu DFAB] can react in two manners, which
i
i
þ
2
M BuF], BuAl and 2-
-fluorine elimination to
b

28
D. Lanzinger et al. / Journal of Organometallic Chemistry 778 (2015) 21e28
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yield the monofluorinated metallocene [L
2
M BuF] and the 6-(3,3,3-
trifluoropropyl)cyclohexadienylium cation. This cation conse-
quently reacts with TIBA to form DFAB, iBu
Al , and 2-
þ
þ
2
i
methylpropane. As a second option, [L
2
M Bu DFAB] can first
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3
8
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3
8
0 min were achieved in case of the highly active metallocene
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*
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[
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7
Appendix A. Supplementary data
1
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.12.011.
1
[
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