Titanium-catalyzed C-F activation of fluoroalkenes

Article abstract of DOI:10.1002/anie.200907162

(Figure Presented) Detox: Air-stable titanocene difluoride efficiently catalyzes the chemoselective hydrodefluorination of fluoroalkenes at room temperature leading to hydrofluoroalkenes in high yields (see scheme: Cp = cyclopentadienyl). This is a rare example of the catalyzed conversion of fluoroalkenes into less-fluorinated compounds, which have a lower climatic impact, and is a potential method for breaking down toxic perfluoroalkenes.

Full text of DOI:10.1002/anie.200907162

Angewandte
Chemie
DOI: 10.1002/anie.200907162
À
Catalytic C F Activation
Titanium-Catalyzed C–F Activation of Fluoroalkenes**
Moritz F. Kꢀhnel and Dieter Lentz*
The thermodynamic and kinetic inertness of carbon–fluorine
bonds has proven to be a mixed blessing to mankind. Ideally
suited for practical applications, organofluorine compounds
have found their way into everyday life in chemically stable
polymers like polytetrafluoroethene (PTFE) and in modern
pharmaceutical chemistry, where fluorine substitution has
beneficial effects on the physicochemical and physiological
properties of organic molecules.[1] However, halofluorocar-
bons have also found their way into the upper atmosphere,
where their long-time persistence has had a deleterious effect
on the ozone layer and has contributed to global warming.[2]
Consequently, there is a long-standing interest in selectively
activating the inert carbon–fluorine bond, although the
number of catalytically active systems for such applications
is still very sparse.[3,4] Most of these catalysts are based on late
transition metals, but recently they have been competing with
first-row transition metals as well as with Lewis acidic main-
group species.[5,6]
As reported previously,[13] treatment of a solution of air-
stable titanocene difluoride (1) with silanes 2a–c results in the
formation of the titanium(III) hydride species 3. Upon
addition of hexafluoropropene (4), the green color of the
metal complex changes to red within minutes and 1,2,3,3,3-
pentafluoropropene (5a, b) is obtained in high yields
(Scheme 1).
Scheme 1. Hydrodefluorination of hexafluoropropene (4) to give (Z)-
pentafluoropropene (5a) and (E)-pentafluoropropene (5b);
R3SiH=Ph2SiH2 (2a), PhSiH3 (2b), poly(methylhydrosiloxane) (PMHS,
2c).
The few examples of catalytic activation of fluoroalkenes
have a common drawback, as they are either costly or slow
and inefficient in terms of turnover frequencies (TOFs) and
turnover numbers (TONs), not to mention that the high
sensitivity of most catalysts limits possible practical applica-
tions.[7] Complexes of the less expensive Group 4 metals are
reactive towards fluoroalkenes, as shown by Jones et al. in
stoichiometric hydrodefluorination (HDF) employing zirco-
nium and hafnium hydrides.[8] However, C–F activation
catalyzed by Group 4 metals is known only for fluoroarenes.[9]
We were surprised to find that titanium, despite its broad
application in homogeneous catalysis, has rarely been used in
C–F activation.[10] In fact only two examples of titanium-
catalyzed C–F activation can be found in the literature.[11]
Richmond et al. have shown that titanocene dihalides under
reducing conditions can catalytically defluorinate perfluori-
nated cycloalkanes to give perfluoroarenes and their HDF
products. Herein, we report the first titanium-catalyzed HDF
of fluoroalkenes to give hydrofluoroalkenes at room temper-
ature as part of our studies on the hydrometalation of
fluorinated substrates.[12]
The reaction is very fast: TOFs of up to 26 minÀ1 have
been observed at 208C; it even proceeds at À258C with a
TOF of 0.1 minÀ1, and more than 125 turnovers are possible
(Table 1). These values dramatically exceed the scarce
published data on comparable reactions:
A TOF of
0.05 minÀ1 at 1008C and a TON of less than 10 have been
reported for the HDF of 4 employing a b-diketiminate
iron(II) fluoride catalyst;[7g] a TOF of 0.2 minÀ1 at 358C and a
TON of 8 were observed for the related HDF of fluoro-
ethylene using Wilkinsonsꢀs catalyst.[7e] However, the recently
published rhodium-catalyzed functionalization of 4 to give
fluoroalkyl boronates achieves a TOF of 12.5 minÀ1 at room
temperature with a TON of up to 250.[7a] The high chemo-
selectivity of the titanium catalyst is demonstrated by the
absence of any 1,1,3,3,3-pentafluoropropene (6) in the
reaction mixture.
To expand the scope of titanium-catalyzed C–Factivation,
we subjected the commercially relevant 1,1,3,3,3-pentafluor-
opropene (6) and 3,3,3-trifluoropropene (8) to similar reac-
tion conditions (Scheme 2). Hydrodefluorination of 6 pro-
ceeded smoothly at room temperature, leading to 1,3,3,3-
tetrafluoropropene (7a,b) and 1,1,3,3-tetrafluoropropene
(7c). The reaction was significantly slower with a TOF of
0.69 minÀ1, but highly selective leading to predominantly the
E-isomer 7a (90%) along with small amounts of the Z-isomer
7b (6%) and 7c (4%; Scheme 2).
Although 8 does not contain any olefinic fluorine
substituents, HDF is possible, but the reaction proceeds less
smoothly. A drastically lowered TOF of 0.04 minÀ1, formation
of large amounts of the hydrogenation product 9b, and the
generation of the secondary HDF products 9c,d prior to
complete consumption of the starting material make this
[*] M. F. Kꢀhnel, Prof. Dr. D. Lentz
Freie Universitꢁt Berlin
Institut fꢀr Chemie und Biochemie, Anorganische Chemie
Fabeckstrasse 34–36, 14195 Berlin (Germany)
Fax: (+49)30-838-52440
E-mail: lentz@chemie.fu-berlin.de
[**] We thank Solvay Germany for donating hexafluoropropene and
Hoechst for donating trifluoropropene. This work was supported
financially by the Deutsche Forschungsgemeinschaft as part of the
graduate school program GRK 1582/1 “Fluor als Schlꢀsselelement”.
We thank S. Matthies, D. Nitsch, and. M. Sparenberg for their
contributions.
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Table 1: Catalytic hydrodefluorination of fluoroalkenes with titanocene difluoride (1).[a]
[1]
[mmol]
Substrate
(mmol)
Solvent
(mL)
t [min]
Silane
(mmol)
Yield after workup[b]
Recovered
TON[c]
substrate[b] [%]
6.9
6.0
6.5
4 (1.09)
4 (0.98)
4 (1.02)
4 (1.05)
4 (1.14)
4 (0.49)
4 (0.96)
6 (1.05)
8 (1.05)
diglyme (2.0)
diglyme (2.0)
diglyme (2.0)
diglyme (2.0)
diglyme (2.0)
diglyme (0.5)
toluene (2.0)
diglyme (2.0)
diglyme (2.0)
15
15
15
3600
75
3
15
60
1130
2a (1.08)
2b (1.11)
2c (1.21)
2a (1.07)
2a (1.06)
2a (0.54)
2a (1.09)
2a (1.11)
2a (1.07)
5a: 47%, 5b: 32%
5a: 8%, 5b: 6%
5a: 8%, 5b: 6%
4 (0)
125
22
22
4 (61)
4 (60)
4 (91)
4 (60)
4 (0)
4 (76)
6 (46)
8 (7)
0.0
0%
13.9[d]
2.8
5a: 5%, 5b: 3%
5a: 26%, 5b: 18%
5a, 5b: traces
7a: 46%, 7b: 3%, 7c: 2%
9a: 39%, 9b: 25%, 9c: 1%, 9d: 1%
6
79
<1
42
6.5
13.0
16.2
43
[a] Reactions conducted at room temperature. [b] Yields were determined by integration of 19F NMR resonances in the product mixture (versus internal
fluorobenzene) after workup. [c] TON is defined as the total moles of product divided by the moles of precatalyst. [d] Reaction conducted at À258C.
hydride (or silyl hydride) 3a, an intermediate previously
postulated in the context of imine hydrosilylation and silane
dehydrocoupling.[13] Insertion of 4 into the metal–hydride
bond yields a titanium fluoroalkyl species 3b, which is prone
to undergo b-fluoride elimination[14] to form the strong
titanium–fluorine bond in 3c and the hydrodefluorination
products 5a,b. The fluoride 3c can then be reconverted into
the active species 3a by reaction with silane 2, since
fluorosilane formation provides a thermodynamic sink.[13f]
These assumptions are supported by the fact that besides
the main hydrodefluorination products 5a,b, trace amounts of
the isomeric 1,1,2,3,3-pentafluoropropene (5c) were identi-
fied in the reaction mixture by their 19F NMR spectra. This
Scheme 2. Catalytic hydrodefluorination of 6 and 8.
compound is formed by fluoride elimination from the less
reactive CF3 group rather than the more reactive CF2H group
reaction suitable for further optimization. The hydrogenation
can be explained by the presence of hydrogen resulting from
the competing silane dehydrocoupling also catalyzed by
1.[13c–e]
in 3b (Scheme 4).
We assume the reaction mechanism to follow the well-
established sequence of olefin insertion followed by b-
fluoride elimination (Scheme 3), which was previously de-
scribed for the stoichiometric reaction of zirconium hydrides
with 4 to give 5b.[8b–e] In the activation step, the precatalyst 1
reacts with silane 2 forming catalytically active titanium (III)
Scheme 4. Formation of 5c during HDF of 4 to give 5a,b.
The formation of 1,1-difluoropropene (9a) from 8 must
involve an addition step to explain the formation of a methyl
group (Scheme 5). Subsequent b-fluoride elimination from
the CF3 group in the intermediate, again, is expected to be
slow. In contrast, HDF of the primary product 9a should be
significantly faster as it involves an elimination step from a
more reactive CF2H group. This is in good agreement with the
observed formation of monofluoropropenes 9c,d prior to the
complete consumption of 8.
In summary, we have shown that air-stable titanocene
difluoride can act as a highly efficient catalyst for the
hydrodefluorination of fluoroalkenes. The HDF of
hexafluoropropene proceeds at room temperature in high
yields and outstanding turnover frequencies of up to 26 minÀ1
and turnover numbers of up to 125. 1,1,3,3,3-Pentafluoropro-
Scheme 3. Proposed mechanism of the hydrodefluorination of 4 cata-
lyzed by 1.
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Scheme 5. Formation of 9a and 9c,d from 8.
À
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pene and 3,3,3-trifluoropropene can also be converted
effectively to tetrafluoropropene and difluoropropene,
respectively. These results represent a rare example of the
catalytic activation of fluoroalkenes leading to less fluori-
nated compounds, which have a negligible global-warming
potential.[15] This type of reaction may also provide a
promising means of detoxifying highly toxic perfluoroalkenes
like perfluoroisobutene and perfluorocyclobutene.[16] Further
studies to elucidate the underlying mechanism as well as to
expand the scope of the titanium-catalyzed C–Factivation are
currently in progress.
Experimental Section
All preparations were performed using standard vacuum-line tech-
niques or by working in an argon-filled glove box. Diglyme and
toluene were distilled from sodium/benzophenone; diphenylsilane
(2a) and phenylsilane (2b) were distilled from calcium hydride;
PMHS (2c) was dried over molecular sieves. Titanocene difluoride
(1) was prepared following Roeskyꢀs method;[17] hexafluoropropene
(4) (Solvay), 1,1,3,3,3-pentafluoropropene (6) (SynQuest Labs), and
3,3,3-trifluoropropene (8) (Hoechst) were used as received.
Reaction conditions are listed in Table 1. In a 50 mL single-
necked flask equipped with a PTFE valve, a solution of silane 2a–c
and titanocene difluoride (1) was heated until its color changed to
green (1–5 min). After repeated degassing, fluoropropene 4, 6, or 8
was condensed into the reaction vessel, which was subsequently
stirred at room temperature for the designated period of time. The
crude reaction mixture was purified by fractional condensation under
vacuum through two subsequent traps kept at À808C and À1968C,
respectively. The contents of the second trap were condensed into an
NMR tube containing a standard CDCl3 solution of fluorobenzene.
Hydrodefluorination products were identified by their characteristic
NMR spectra.[18,19]
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Received: December 18, 2009
Published online: March 12, 2010
À
Keywords: C F activation · alkenes · homogeneous catalysis ·
hydrodefluorination · titanium
.
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3
[19] 5c: 1H (400 MHz, CDCl3): d = 6.37 ppm (dddt, 1H, JHF
=
4
4
2
14.9 Hz, JHF = 2.7 Hz, JHF = 1.3 Hz, JHF = 51.1 Hz, CF2H); 19F
(376 MHz, CDCl3): d = À93.7 (dddt, 1F, 2JFF = 63.5 Hz, JFF
=
3
4
4
=
34.9 Hz, JFH = 1.3 Hz, JFF = 12.2 Hz, CF2), À113.8 (dddt, 1F,
3JFF = 118.1 Hz, 2JFF = 63.5 Hz, 4JFH = 2.8 Hz, 4JFF = 4.8 Hz,
=
CF2), À123.0 (dddd, 2F, 2JFH = 51.0 Hz, 3JFF = 17.7 Hz, JFF
=
=
4
12.2 Hz, 4JFF = 4.8 Hz, CF2H), À196.5 ppm (dddt, 1F, JFF
3
118.2 Hz, 3JFF = 34.9 Hz, 3JFF = 17.6 Hz, 4JFH = 14.8 Hz, -CF ).
7c 1H (400 MHz, CDCl3): d = 4.69 (dq br, 1H, 3JHF = 23.1 Hz,
=
3JHF = 7 Hz, 3JHH = 7 Hz, 4JFH = 0.9 Hz, -CH ), 6.46 ppm (dtt,
=
1H, 2JFH = 54.5 Hz, 3JHH = 7.3 Hz, CF2H); 19F (376 MHz, CDCl3):
À78.7 (ddt, 1F, 2JFF = 22.0 Hz, 4JFF = 0.8 Hz, 4JFF = 14.3 Hz, 2JFF
=
22 Hz, CF2), À79.5 (ddtt, 1F, 2JFF = 22.0 Hz, 3JFH = 23.0 Hz,
=
4JFF = 3.7 Hz, 4JFH = 0.9 Hz, CF2), À108.6 ppm (dddd, 2F, JFH
=
2
=
54.5 Hz, 4JFF = 14.3 Hz, 3JFH = 7.0 Hz, 4JFF = 3.7 Hz, CF2H).
2936 www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 2933 –2936